Influence of duty cycle on the power-duration relationship: observations and potential mechanisms

A Moderate Blood Flow Restriction Pressure Does Not Affect Maximal Strength or Neuromuscular Responses

ABSTRACT

Lubiak, SM, Lawson, JE, Gonzalez Rojas, DH, Proppe, CE, Rivera, PM, Hammer, SM, Trevino, MA, Dinyer-McNeely, TK, Montgomery, TR, Olmos, AA, Sears, KN, Bergstrom, HC, Succi, PJ, Keller, JL, and Hill, EC. A moderate blood flow restriction pressure does not affect maximal strength or neuromuscular responses. J Strength Cond Res 38(12): e727-e734, 2024-The purpose of this study was to examine the acute effects of blood flow restriction (BFR) applied at 60% of total arterial occlusion pressure (AOP) on maximal strength. Eleven college-aged female subjects completed two testing sessions of maximal unilateral concentric, isometric, and eccentric leg extension muscle actions performed with and without BFR. Separate 3 (mode [isometric, concentric, eccentric]) × 2 (condition [BFR, no BFR]) × 2 (visit [2, 3]) repeated-measures analysis of variances were used to examine mean differences in maximal strength, neuromuscular function, rating of perceived exertion (RPE), and pain. For maximal strength (collapsed across condition and visit), isometric (128.5 ± 22.7 Nm) and eccentric (114.5 ± 35.4 Nm) strength were greater than concentric maximal strength (89.3 ± 22.3 Nm) ( p < 0.001-0.041). Muscle excitation relative (%) to isometric non-BFR was greater during the concentric (108.6 ± 31.5%) than during the eccentric (86.7 ± 29.2%) ( p = 0.045) assessments but not different than isometric (93.4 ± 17.9%) ( p = 0.109) assessments, collapsed across condition and visit. For RPE, there was an interaction such that RPE was greater during non-BFR (4.3 ± 1.7) than during BFR (3.7 ± 1.7) ( p = 0.031) during the maximal concentric strength assessments. Furthermore, during maximal strength assessments performed with BFR, isometric RPE (5.8 ± 1.9) was greater than concentric (3.7 ± 1.7) ( p = 0.005) and eccentric (4.6 ± 1.9) ( p = 0.009) RPE. Finally, pain was greater during the isometric (2.8 ± 2.1 au) than during the concentric (1.8 ± 1.5 au) ( p = 0.016), but not eccentric, maximal strength assessments (2.1 ± 1.6 au) ( p = 0.126), collapsed across condition and visit. The application of BFR at 60% AOP did not affect concentric, isometric, or eccentric maximal strength or neuromuscular function. Trainers, clinicians, and researchers can prescribe exercise interventions relative to a restricted (when using a moderate AOP) or nonrestricted assessment of maximal strength.

Muscle microvascular oxygen delivery limitations during the contraction phase of intermittent maximal effort contractions

PURPOSE

The end-test torque (ETT) during intermittent maximal effort contractions reflects the highest contraction intensity at which a muscle metabolic steady-state can be attained. This study determined if ETT is the highest intensity at which the contraction phase of intermittent exercise does not limit the matching of microvascular oxygen delivery to muscle oxygen demand.

METHODS

Microvascular oxygenation characteristics of the biceps brachii muscle were measured in sixteen young, healthy individuals (8M/8F, 22 ± 3 years, 80.9 ± 20.3 kg) by near-infrared spectroscopy during maximal effort elbow flexion under control conditions (CON) and with complete circulatory occlusion (OCC).

RESULTS

Increases in total-[heme] were blunted during OCC compared to CON (225 ± 87 vs. 264 ± 88 μM, p < 0.001) but OCC did not elicit a compensatory increase in deoxygenated-[heme] at any timepoint (108 ± 62 vs. 101 ± 61 μM, p > 0.05). Deoxygenated-[heme] was significantly elevated during contraction, relative to relaxation, above ETT (107 ± 60 vs. 98.8 ± 60.5 μM, p < 0.001), but not at ETT (91.7 ± 54.1 vs. 98.4 ± 62.2 μM, p = 0.174). Total-[heme] was significantly reduced during contraction, relative to relaxation, at all contraction intensities during CON (p < 0.05) and OCC (p < 0.05).

CONCLUSION

These data suggest that ETT may reflect the highest contraction intensity at which contraction-induced increases in intramuscular pressures do not limit muscle perfusion to a degree that requires further increases in fractional oxygen extraction (i.e., deoxygenated-[heme]) despite limited microvascular diffusive conductance (i.e., total-[heme]).

Influence of acute dietary nitrate supplementation on oxygen delivery/consumption and critical impulse during maximal effort forearm exercise in males: a randomized crossover trial

Beetroot juice supplementation (BRJ) should increase nitric oxide bioavailability under conditions of muscle deoxygenation and acidosis that are a normal consequence of the maximal effort exercise test used to identify forearm critical impulse. We hypothesized BRJ would improve oxygen delivery:demand matching and forearm critical impulse performance. Healthy males (20.8 ± 2.4 years) participated in a randomized crossover trial between October 2017 and May 2018 (Queen's University, Kingston, ON). Participants completed 10 min of rhythmic maximal effort forearm handgrip exercise 2.5 h post placebo (PL) vs. BRJ (9 completed PL/BRJ vs. 4 completed BRJ/PL) within a 2 week period. Data are presented as mean ± SD. There was a main effect of drink (PL > BRJ) for oxygen extraction (P = 0.033, ηp2 = 0.351) and oxygen consumption/force (P = 0.017, ηp2 = 0.417). There was a drink × time interaction (PL > BRJ) for oxygen consumption/force (P = 0.035, ηp2 = 0.216) between 75 and 360 s (1.25-6 min) from exercise onset. BRJ did not influence oxygen delivery (P = 0.953, ηp2 = 0.000), oxygen consumption (P = 0.064, ηp2 = 0.278), metabolites ((lactate) (P = 0.196, ηp2 = 0.135), pH (P = 0.759, ηp2 = 0.008)) or power-duration performance parameters (critical impulse (P = 0.379, d = 0.253), W' (P = 0.733, d = 0.097)). BRJ during all-out handgrip exercise does not influence oxygen delivery or exercise performance. Oxygen cost of contraction with BRJ is reduced as contraction impulse is declining during maximal effort exercise resulting in less oxygen extraction.

Measuring critical force in sport climbers: a validation study of the 4 min all-out test on finger flexors

PURPOSE

The critical force (CF) concept, differentiating steady and non-steady state conditions, extends the critical power paradigm for sport climbing. This study aimed to validate CF for finger flexors derived from the 4 min all-out test as a boundary for the highest sustainable work intensity in sport climbers.

METHODS

Twelve participants underwent multiple laboratory visits. Initially, they performed the 4 min intermittent contraction all-out test for CF determination. Subsequent verification visits involved finger-flexor contractions at various intensities, including CF, CF -2 kg, CF -4 kg, and CF -6 kg, lasting for 720 s or until failure, while monitoring muscle-oxygen dynamics of forearm muscles.

RESULTS

CF, determined from the mean force of last three contractions, was measured at 20.1 ± 5.7 kg, while the end-force at 16.8 ± 5.2 kg. In the verification trials, the mean time to failure at CF was 440 ± 140 s, with only one participant completing the 720 s task. When the load was continuously lowered (-2 kg, -4 kg, and -6 kg), a greater number of participants (38%, 69%, and 92%, respectively) successfully completed the 720 s task. Changes of muscle-oxygen dynamics showed a high variability and could not clearly distinguish between exhaustive and non-exhaustive trials.

CONCLUSIONS

CF, based on the mean force of the last three contractions, failed to reliably predict the highest sustainable work rate. In contrast, determining CF as the end-force of the last three contractions exhibited a stronger link to sustainable work. Caution is advised in interpreting forearm muscle-oxygen dynamics, lacking sensitivity for nuanced metabolic responses during climbing-related tasks.

Motor unit discharge rate modulation during isometric contractions to failure is intensity- and modality-dependent

The physiological mechanisms determining the progressive decline in the maximal muscle torque production capacity during isometric contractions to task failure are known to depend on task demands. Task-specificity of the associated adjustments in motor unit discharge rate (MUDR), however, remains unclear. This study examined MUDR adjustments during different submaximal isometric knee extension tasks to failure. Participants performed a sustained and an intermittent task at 20% and 50% of maximal voluntary torque (MVT), respectively (Experiment 1). High-density surface EMG signals were recorded from vastus lateralis (VL) and medialis (VM) and decomposed into individual MU discharge timings, with the identified MUs tracked from recruitment to task failure. MUDR was quantified and normalised to intervals of 10% of contraction time (CT). MUDR of both muscles exhibited distinct modulation patterns in each task. During the 20% MVT sustained task, MUDR decreased until ∼50% CT, after which it gradually returned to baseline. Conversely, during the 50% MVT intermittent task, MUDR remained stable until ∼40-50% CT, after which it started to continually increase until task failure. To explore the effect of contraction intensity on the observed patterns, VL and VM MUDR was quantified during sustained contractions at 30% and 50% MVT (Experiment 2). During the 30% MVT sustained task, MUDR remained stable until ∼80-90% CT in both muscles, after which it continually increased until task failure. During the 50% MVT sustained task the increase in MUDR occurred earlier, after ∼70-80% CT. Our results suggest that adjustments in MUDR during submaximal isometric contractions to failure are contraction modality- and intensity-dependent. KEY POINTS: During prolonged muscle contractions a constant motor output can be maintained by recruitment of additional motor units and adjustments in their discharge rate. Whilst contraction-induced decrements in neuromuscular function are known to depend on task demands, task-specificity of motor unit discharge behaviour adjustments is still unclear. In this study, we tracked and compared discharge activity of several concurrently active motor units in the vastii muscles during different submaximal isometric knee extension tasks to failure, including intermittent vs. sustained contraction modalities performed in the same intensity domain (Experiment 1), and two sustained contractions performed at different intensities (Experiment 2). During each task, motor units modulated their discharge rate in a distinct, biphasic manner, with the modulation pattern depending on contraction intensity and modality. These results provide insight into motoneuronal adjustments during contraction tasks posing different demands on the neuromuscular system.

Sex differences in muscle contraction-induced limb blood flow limitations

PURPOSE

To determined sex differences in absolute- and %-reductions in blood flow during intermittent muscular contractions as well as relationships between blood flow reductions and time to task failure (TTF).

METHODS

Thirteen males (25 ± 4 years) and 13 females (22 ± 5 years) completed intermittent isometric trapezoidal forearm flexion at 50% maximal voluntary contraction until task failure. Doppler ultrasound was used to measure brachial artery blood flow (BABF) during the 12-s plateau phase and 12-s relaxation phase.

RESULTS

Target torque was less in females than males (24 ± 5 vs. 42 ± 7 Nm; p < 0.001); however, TTF was not different between sexes (F: 425 ± 187 vs. M: 401 ± 158 s; p = 0.72). Relaxation-phase BABF at end-exercise was less in females than males (435 ± 161 vs. 937 ± 281 mL/min; p < 0.001) but contraction-phase BABF was not different (127 ± 46 vs. 190 ± 99 mL/min; p = 0.42). Absolute- and %-reductions in BABF by contraction were less in females than males (309 ± 146 vs. 747 ± 210 mL/min and 69 ± 10 vs. 80% ± 6%, respectively; both p < 0.01) and were associated with target torque independent of sex (r = 0.78 and 0.56, respectively; both p < 0.01). Absolute BABF reduction per target torque (mL/min/Nm) and TTF were positively associated in males (r = 0.60; p = 0.031) but negatively associated in females (r = - 0.61; p = 0.029).

CONCLUSIONS

This study provides evidence that females incur less proportional reduction in limb blood flow from muscular contraction than males at a matched relative intensity suggesting females may maintain higher levels of muscle oxygen delivery and metabolite removal than males across the contraction-relaxation cycle of intermittent exercise.

Vascular dysfunction and the age-related decline in critical power

Ageing results in lower exercise tolerance, manifested as decreased critical power (CP). We examined whether the age-related decrease in CP occurs independently of changes in muscle mass and whether it is related to impaired vascular function. Ten older (63.1 ± 2.5 years) and 10 younger (24.4 ± 4.0 years) physically active volunteers participated. Physical activity was measured with accelerometry. Leg muscle mass was quantified with dual X-ray absorptiometry. The CP and maximum power during a graded exercise test (PGXT ) of single-leg knee-extension exercise were determined over the course of four visits. During a fifth visit, vascular function of the leg was assessed with passive leg movement (PLM) hyperaemia and leg blood flow and vascular conductance during knee-extension exercise at 10 W, 20 W, slightly below CP (90% CP) and PGXT . Despite not differing in leg lean mass (P = 0.901) and physical activity (e.g., steps per day, P = 0.735), older subjects had ∼30% lower mass-specific CP (old = 3.20 ± 0.94 W kg-1 vs. young = 4.60 ± 0.87 W kg-1 ; P < 0.001). The PLM-induced hyperaemia and leg blood flow and/or conductance were blunted in the old at 20 W, 90% CP and PGXT (P < 0.05). When normalized for leg muscle mass, CP was strongly correlated with PLM-induced hyperaemia (R2  = 0.52; P < 0.001) and vascular conductance during knee-extension exercise at 20 W (R2  = 0.34; P = 0.014) and 90% CP (R2  = 0.39; P = 0.004). In conclusion, the age-related decline in CP is not only an issue of muscle quantity, but also of impaired muscle quality that corresponds to impaired vascular function.

Sex differences in the intensity-duration relationships of the severe- and extreme-intensity exercise domains

Extreme-intensity exercise is described by W'ext (analogous to J' for isometric exercise) that is smaller than W' of severe-intensity exercise (W'sev) in males. Sex differences in exercise tolerance appear to diminish at near-maximal exercise, however, there is evidence of greater contributions of peripheral fatigue (i.e. potentiated twitch force; Qpot) in males during extreme-intensity exercise. Therefore, the current study tested the hypotheses that J'ext would not be different between males and females, however, males would exhibit a greater reduction in neuromuscular function (i.e. maximal voluntary contraction, MVC; Qpot) following extreme-intensity exercise. Seven males and 7 females completed three severe- (Tlim: 2-4 min, S3; 5-8 min, S2; 9-15 min, S1) and three extreme-intensity (70, 80, 90%MVC) knee-extension bouts. MVC and Qpot relative to baseline were compared at task failure and at 150 s of recovery. J'ext was significantly less than J'sev in males (2.4 ± 1.2kJ vs 3.9 ± 1.3kJ; p = 0.03) and females (1.6 ± 0.8kJ vs 2.9 ± 1.7kJ; p = 0.05); however, there were no sex differences in J'ext or J'sev. MVC (%Baseline) was greater at task failure following extreme-intensity exercise (76.5 ± 20.0% vs 51.5 ± 11.5% in males, 75.7 ± 19.4% vs 66.7 ± 17.4% in females), but was not different at 150 s of recovery (95.7 ± 11.8% in males, 91.1 ± 14.2% in females). Reduction in Qpot, however, was greater in males (51.9 ± 16.3% vs 60.6 ± 15.5%) and was significantly correlated with J'ext (r2 = 0.90, p < 0.001). Although there were no differences in the magnitude of J'ext, differences in MVC and Qpot are evidence of sex-specific responses and highlight the importance of appropriately characterizing exercise intensity regarding exercise domains when comparing physiological responses in males and females.Highlights We have previously shown evidence that extreme-intensity dynamic exercise is described by W'ext in males and smaller than W'sev. We currently tested for potential sex differences in J'ext (isometric analogue to W') and neuromuscular responses (i.e. maximal voluntary contraction, MVC; potentiated twitch force, Qpot) during extreme-intensity exercise.J'ext and extreme-intensity exercise tolerance was not different between males and females. The reduction in MVC was not different across extreme-intensity exercise across males and females, whereas the reduction in Qpot was greater in males following all extreme-intensity exercises, although not after exercise at 90%MVC.Together, although extreme-intensity exercise tolerance is not different, these data highlight differences in the contributing mechanisms of fatigue during severe- and extreme-intensity exercise between males and females.

Interaction of Factors Determining Critical Power

The physiological determinants of high-intensity exercise tolerance are important for both elite human performance and morbidity, mortality and disease in clinical settings. The asymptote of the hyperbolic relation between external power and time to task failure, critical power, represents the threshold intensity above which systemic and intramuscular metabolic homeostasis can no longer be maintained. After ~ 60 years of research into the phenomenon of critical power, a clear understanding of its physiological determinants has emerged. The purpose of the present review is to critically examine this contemporary evidence in order to explain the physiological underpinnings of critical power. Evidence demonstrating that alterations in convective and diffusive oxygen delivery can impact upon critical power is first addressed. Subsequently, evidence is considered that shows that rates of muscle oxygen utilisation, inferred via the kinetics of pulmonary oxygen consumption, can influence critical power. The data reveal a clear picture that alterations in the rates of flux along every step of the oxygen transport and utilisation pathways influence critical power. It is also clear that critical power is influenced by motor unit recruitment patterns. On this basis, it is proposed that convective and diffusive oxygen delivery act in concert with muscle oxygen utilisation rates to determine the intracellular metabolic milieu and state of fatigue within the myocytes. This interacts with exercising muscle mass and motor unit recruitment patterns to ultimately determine critical power.

Impact of Interrepetition Rest on Muscle Blood Flow and Exercise Tolerance during Resistance Exercise

Background and Objectives: Muscle blood flow is impeded during resistance exercise contractions, but immediately increases during recovery. The purpose of this study was to determine the impact of brief bouts of rest (2 s) between repetitions of resistance exercise on muscle blood flow and exercise tolerance. Materials and Methods: Ten healthy young adults performed single-leg knee extension resistance exercises with no rest between repetitions (i.e., continuous) and with 2 s of rest between each repetition (i.e., intermittent). Exercise tolerance was measured as the maximal power that could be sustained for 3 min (PSUS) and as the maximum number of repetitions (Reps80%) that could be performed at 80% one-repetition maximum (1RM). The leg blood flow, muscle oxygenation of the vastus lateralis and mean arterial pressure (MAP) were measured during various exercise trials. Alpha was set to p ≤ 0.05. Results: Leg blood flow was significantly greater, while vascular resistance and MAP were significantly less during intermittent compared with continuous resistance exercise at the same power outputs (p < 0.01). PSUS was significantly greater during intermittent than continuous resistance exercise (29.5 ± 2.1 vs. 21.7 ± 1.2 W, p = 0.01). Reps80% was also significantly greater during intermittent compared with continuous resistance exercise (26.5 ± 5.3 vs. 16.8 ± 2.1 repetitions, respectively; p = 0.02), potentially due to increased leg blood flow and muscle oxygen saturation during intermittent resistance exercise (p < 0.05). Conclusions: In conclusion, a brief rest between repetitions of resistance exercise effectively decreased vascular resistance, increased blood flow to the exercising muscle, and increased exercise tolerance to resistance exercise.

Neuromuscular recovery from severe- and extreme-intensity exercise in men and women

Maximal voluntary contraction force (MVC), potentiated twitch force (Q_pot), and voluntary activation (%VA) recover to baseline within 90 s following extreme-intensity exercise. However, methodological limitations mask important recovery kinetics. We hypothesized reductions in MVC, Q_pot, and %VA at task failure following extreme-intensity exercise would be less than following severe-intensity exercise, and Q_pot and MVC following extreme-intensity exercise would show significant recovery within 120 s but remain depressed following severe-intensity exercise. Twelve subjects (6 men) completed 2 severe-intensity (40, 50% MVC) and 2 extreme-intensity (70, 80% MVC) isometric knee-extension exercise bouts to task failure (T_lim). Neuromuscular function was measured at baseline, T_lim, and through 150 s of recovery. Each intensity significantly reduced MVC and Q_pot compared with baseline. MVC was greater at T_lim (p < 0.01) and at 150 s of recovery (p = 0.004) following exercise at 80% MVC compared with severe-intensity exercise. Partial recovery of MVC and Q_pot were detected within 150 s following T_lim for each exercise intensity; Q_pot recovered to baseline values within 150 s of recovery following exercise at 80% MVC. No differences in %VA were detected pre- to post-exercise or across recovery for any intensity. Although further analysis showed sex-specific differences in MVC and Q_pot, future studies should closely examine sex-dependent responses to extreme-intensity exercise. It is clear, however, that these data reinforce that mechanisms limiting exercise tolerance during extreme-intensity exercise recover quickly. Novelty: Severe- and extreme-intensity exercise cause independent responses in fatigue accumulation and the subsequent recovery time courses. Recovery of MVC and Q_pot occurs much faster following extreme-intensity exercise in both men and women.

The acute effects of passive heating on endothelial function, muscle microvascular oxygen delivery, and expression of serum HSP90α

Passive heating has been a therapeutic tool used to elevate core temperature and induce increases in cardiac output, blood flow, and shear stress. We aimed to determine the effects of a single bout of passive heating on endothelial function and serum heat shock protein 90α (HSP90α) levels in young, healthy subjects. 8 healthy subjects were recruited to participate in one bout of whole-body passive heating via immersion in a 40 °C hot tub to maintain a 1 °C increase in rectal temperature for 60 min. Twenty-four hours after heating, shear-rate corrected endothelium-dependent dilation increased (pre: 0.004 ± 0.002%SR_AUC; post: 0.006 ± 0.003%SR_AUC; p = 0.034) but serum [HSP90α] was not changed (pre: 36.7 ± 10.3 ng/mL; post: 40.6 ± 15.9 ng/mL; p = 0.39). Neither resting muscle O₂ utilization (pre: 0.17 ± 0.11 ml O₂ min^-1 (100 g)^-1; post: 0.14 ± 0.09 ml O₂ min^-1 (100 g)^-1); p = 0.28) nor mean arterial pressure (pre: 74 ± 11 mmHg; post: 73 ± 11 mmHg; p = 0.79) were influenced by the heating intervention. Finally, time to peak after cuff release was significantly delayed for % O₂ sat (TTP_pre = 39 ± 8.9 s and TTP_post = 43.5 ± 8.2 s; p = 0.007) and deoxy-[heme] (TTP_pre = 41.3 ± 18.1 s and TTP_post = 51.4 ± 16.3 s; p = 0.018), with no effect on oxy-[heme] (p = 0.19) and total-[heme] (p = 0.41). One bout of passive heating improved endothelium-dependent dilation 24 h later in young, healthy subjects. This data suggests that passive heat treatments may provide a simple intervention for improving vascular health.

The W' Balance Model: Mathematical and Methodological Considerations

Since its publication in 2012, the W' balance model has become an important tool in the scientific armamentarium for understanding and predicting human physiology and performance during high-intensity intermittent exercise. Indeed, publications featuring the model are accumulating, and it has been adapted for popular use both in desktop computer software and on wrist-worn devices. Despite the model's intuitive appeal, it has achieved mixed results thus far, in part due to a lack of clarity in its basis and calculation. Purpose: This review examines the theoretical basis, assumptions, calculation methods, and the strengths and limitations of the integral and differential forms of the W' balance model. In particular, the authors emphasize that the formulations are based on distinct assumptions about the depletion and reconstitution of W' during intermittent exercise; understanding the distinctions between the 2 forms will enable practitioners to correctly implement the models and interpret their results. The authors then discuss foundational issues affecting the validity and utility of the model, followed by evaluating potential modifications and suggesting avenues for further research. Conclusions: The W' balance model has served as a valuable conceptual and computational tool. Improved versions may better predict performance and further advance the physiology of high-intensity intermittent exercise.

Influence of muscular contraction on vascular conductance during exercise above versus below critical power

We tested the hypothesis that limb vascular conductance (LVC) would increase during the immediate recovery phase of dynamic exercise above, but not below, critical power (CP) indicating a threshold for muscular contraction-induced impedance of limb blood flow (LBF). CP (115 ± 26 W) was determined in 7 men and 7 women who subsequently performed ∼5 min of near-supine cycling exercise both below and above CP. LVC demonstrated a greater increase during immediate recovery and remained significantly higher following exercise above, compared to below, CP (all p < 0.001). Power output was associated with the immediate increases in LVC following exercise above, but not below, CP (p < 0.001; r = 0.85). Additionally, variance in percent LBF impedance was significantly lower above (CV: 10.7 %), compared to below (CV: 53.2 %), CP (p < 0.01). CP appears to represent a threshold above which the characteristics of LBF impedance by muscular contraction become intensity-dependent. These data suggest a critical level of LBF impedance relative to contraction intensity exists and, once attained, may promote the progressive metabolic and neuromuscular responses known to occur above CP.

The role of vascular function on exercise capacity in health and disease

Three sentinel parameters of aerobic performance are the maximal oxygen uptake ( V ̇ O 2 max ), critical power (CP) and speed of the V ̇ O 2 kinetics following exercise onset. Of these, the latter is, perhaps, the cardinal test of integrated function along the O2 transport pathway from lungs to skeletal muscle mitochondria. Fast V ̇ O 2 kinetics demands that the cardiovascular system distributes exercise-induced blood flow elevations among and within those vascular beds subserving the contracting muscle(s). Ideally, this process must occur at least as rapidly as mitochondrial metabolism elevates V ̇ O 2 . Chronic disease and ageing create an O2 delivery (i.e. blood flow × arterial [O2 ], Q ̇ O 2 ) dependency that slows V ̇ O 2 kinetics, decreasing CP and V ̇ O 2 max , increasing the O2 deficit and sowing the seeds of exercise intolerance. Exercise training, in contrast, does the opposite. Within the context of these three parameters (see Graphical Abstract), this brief review examines the training-induced plasticity of key elements in the O2 transport pathway. It asks how structural and functional vascular adaptations accelerate and redistribute muscle Q ̇ O 2 and thus defend microvascular O2 partial pressures and capillary blood-myocyte O2 diffusion across a ∼100-fold range of muscle V ̇ O 2 values. Recent discoveries, especially in the muscle microcirculation and Q ̇ O 2 -to- V ̇ O 2 heterogeneity, are integrated with the O2 transport pathway to appreciate how local and systemic vascular control helps defend V ̇ O 2 kinetics and determine CP and V ̇ O 2 max in health and how vascular dysfunction in disease predicates exercise intolerance. Finally, the latest evidence that nitrate supplementation improves vascular and therefore aerobic function in health and disease is presented.

Effect of Hyperoxia on Critical Power and V˙O2 Kinetics during Upright Cycling

INTRODUCTION/PURPOSE

Critical power (CP) is a fundamental parameter defining high-intensity exercise tolerance; however, its physiological determinants are incompletely understood. The present study determined the impact of hyperoxia on CP, the time constant of phase II pulmonary oxygen uptake kinetics (τV˙O2), and muscle oxygenation (assessed by near-infrared spectroscopy) in nine healthy men performing upright cycle ergometry.

METHODS

Critical power was determined in normoxia and hyperoxia (fraction of inspired O2 = 0.5) via four severe-intensity constant load exercise tests to exhaustion on a cycle ergometer, repeated once in each condition. During each test, τV˙O2 and the time constant of muscle deoxyhemoglobin kinetics (τ[HHb]), alongside absolute concentrations of muscle oxyhemoglobin ([HbO2]), were determined.

RESULTS

Critical power was greater (hyperoxia, 216 ± 30 W vs normoxia, 197 ± 29 W; P < 0.001), whereas W' was reduced (hyperoxia, 15.4 ± 5.2 kJ; normoxia, 17.5 ± 4.3 W; P = 0.037) in hyperoxia compared with normoxia. τV˙O2 (hyperoxia, 35 ± 12 s vs normoxia, 33 ± 10 s; P = 0.33) and τ[HHb] (hyperoxia, 11 ± 5 s vs normoxia, 14 ± 5 s; P = 0.65) were unchanged between conditions, whereas [HbO2] during exercise was greater in hyperoxia compared with normoxia (hyperoxia, 73 ± 20 vs normoxia, 66 ± 15 μM; P < 0.001).

CONCLUSIONS

This study provides novel insights into the physiological determinants of CP and by extension, exercise tolerance. Microvascular oxygenation and CP were improved during exercise in hyperoxia compared with normoxia. Importantly, the improved microvascular oxygenation afforded by hyperoxia did not alter τV˙O2, suggesting that microvascular O2 availability is an independent determinant of the upper limit for steady-state exercise, that is, CP.

Strength-Endurance: Interaction Between Force-Velocity Condition and Power Output

Context

Strength-endurance mainly depends on the power output, which is often expressed relative to the individual’s maximal power capability (P_max). However, an individual can develop the same power, but in different combinations of force and velocity (force-velocity condition). Also, at matched power output, changing the force-velocity condition results in a change of the velocity-specific relative power (P_maxv), associated with a change in the power reserve. So far, the effect of these changing conditions on strength-endurance remains unclear.

Purpose

We aimed to test the effects of force-velocity condition and power output on strength-endurance.

Methods

Fourteen sportsmen performed (i) force- and power-velocity relationships evaluation in squat jumps and (ii) strength-endurance evaluations during repeated squat jump tests in 10 different force-velocity-power conditions, individualized based on the force- and power-velocity relationships. Each condition was characterized by different (i) relative power (%P_max), (ii) velocity-specific relative power (%P_maxv), and (iii) ratio between force and velocity (R_Fv). Strength-endurance was assessed by the maximum repetitions (SJ_Rep), and the cumulated mechanical work (W_tot) performed until exhaustion during repeated squat jump tests. Intra and inter-day reliability of SJ_Rep were tested in one of the 10 conditions. The effects of %P_max, %P_maxv, and R_Fv on SJ_Rep and W_tot were tested via stepwise multiple linear regressions and two-way ANOVAs.

Results

SJ_Rep exhibited almost perfect intra- and inter-day reliability (ICC=0.94 and 0.92, respectively). SJ_Rep and W_tot were influenced by %P_maxv and R_Fv (R² = 0.975 and 0.971; RSME=0.243 and 0.234, respectively; both p < 0.001), with the effect of R_Fv increasing with decreasing %P_maxv (interaction effect, p = 0.03). %P_max was not considered as a significant predictor of strength-endurance by the multiple regressions analysis. SJ_Rep and W_tot were higher at lower %P_maxv and in low force-high velocity conditions (i.e., lower R_Fv).

Conclusion

Strength-endurance was almost fully dependent on the position of the exercise conditions relative to the individual force-velocity and power-velocity relationships (characterized by %P_maxv and R_Fv). Thus, the standardization of the force-velocity condition and the velocity-specific relative power should not be overlooked for strength-endurance testing and training, but also when setting fatiguing protocols.

Physiological sex differences affect the integrative response to exercise: acute and chronic implications

NEW FINDINGS

What is the topic of this review? We review sex differences within physiological systems implicated in exercise performance; specifically, how they integrate to determine metabolic thresholds and fatigability. Thereafter, we discuss the implications that these sex differences might have for long-term adaptation to exercise. What advances does it highlight? The review collates evidence from recent physiological studies that have investigated sex as a biological variable, demonstrating that the physiological response to equivalent 'dosages' of exercise is not the same in males and females; thus, highlighting the need to research diversity in physiological responses to interventions.

ABSTRACT

The anatomical and physiological differences between males and females are thought to determine differences in the limits of human performance. The notion of studying sex as a biological variable has recently been emphasized in the biosciences as a vital step in enhancing human health. In this review, we contend that the effects of biological sex on acute and chronic responses must be studied and accounted for when prescribing aerobic exercise, much like any intervention targeting the optimization of physiological function. Emerging evidence suggests that the response of physiological systems to exercise differs between males and females, potentially mediating the beneficial effects in healthy and clinical populations. We highlight evidence that integrative metabolic thresholds during exercise are influenced by phenotypical sex differences throughout many physiological systems. Furthermore, we discuss evidence that female skeletal muscle is more resistant to fatigue elicited by equivalent dosages of high-intensity exercise. How the different acute responses affect the long-term trainability of males and females is considered, with discussion about tailoring exercise to the characteristics of the individual presented within the context of biological sex. Finally, we highlight the influence of endogenous and exogenous sex hormones on physiological responses to exercise in females. Sex is one of many mediating influences on the outcomes of exercise, and with careful experimental designs, physiologists can advance the collective understanding of diversity in physiology and optimize outcomes for both sexes.

A model-based estimation of critical torques reduces the experimental effort compared to conventional testing

Purpose

Critical torque (CT) is an important fatigue threshold in exercise physiology and can be used to analyze, predict, or optimize performance. The objective of this work is to reduce the experimental effort when estimating CTs for sustained and intermittent isometric contractions using a model-based approach.

Materials and methods

We employ a phenomenological model of the time course of maximum voluntary isometric contraction (MVIC) torque and compute the highest sustainable torque output by solving an optimization problem. We then show that our results are consistent with the steady states obtained when simulating periodic maximum loading schemes. These simulations correspond to all-out tests, which are used to estimate CTs in practice. Based on these observations, the estimation of CTs can be formulated mathematically as a parameter estimation problem. To minimize the statistical uncertainty of the parameter estimates and consequently of the estimated CTs, we compute optimized testing sessions. This reduces the experimental effort even further.

Results

We estimate CTs of the elbow flexors for sustained isometric contractions to be 28% of baseline MVIC torque and for intermittent isometric contractions consisting of a 3 s contraction followed by 2 s rest to be 41% of baseline MVIC torque. We show that a single optimized testing session is sufficient when using our approach.

Conclusions

Our approach reduces the experimental effort considerably when estimating CTs for sustained and intermittent isometric contractions.

Exercise tolerance through severe and extreme intensity domains

The power‐duration relationship accurately predicts exercise tolerance for constant power exercise performed in the severe intensity domain. However, the accuracy of the prediction of time to task failure (T_lim) is currently unclear for work rates (WR) above severe intensities; that is, within the extreme intensity domain (T_lim < 2 min). We hypothesized that T_lim would be shorter for WRs within the extreme intensity domain than predicted from the linear 1/time relationship of the severe intensity domain which would suggest mechanisms limiting exercise are different between intensity domains. Six men completed 7 knee‐extension tests. T_lim of extreme intensity exercise (60%, 70%, 80%, and 90% 1RM; T_lim < 2 min) were compared to the predicted T_lim from the slope of the S1–S3 (T_lim ≥ 2–15 min) regression. Twitch force (Q_tw) and maximal voluntary contraction (MVC) were measured on the right vastus lateralis before and after each test. T_lim at 70–90% 1RM were shorter than the T_lim predicted by the severe domain 1/time model (P < 0.05); however, T_lim at 60% 1RM was not different than the predicted severe T_lim, suggesting the mechanisms limiting extreme exercise manifest ≥60% 1RM. A significant linear relationship for 60–90% 1RM was observed which suggested a curvature constant unique to the extreme domain (Wext′) that was smaller than the W ′ of the severe domain (1.5 ± 0.6 vs. 5.9 ± 1.5 kJ, P < 0.001). Q_tw and MVC were significantly decreased following severe exercise, however, Q_tw and MVC were not significantly decreased following 80% and 90% 1RM, giving evidence that mechanisms causing task failure were recovered by the time post‐exercise measurements were made (~90 sec).

Peripheral Determinants of Oxygen Utilization in Heart Failure With Preserved Ejection Fraction: Central Role of Adiposity

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ΔAVo₂ during exercise is a complex metric that incorporates into its calculation skeletal muscle blood flow and DmO₂ across the skeletal muscle capillary membrane.

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Although ΔAVo₂ was reduced in patients with HFpEF during both systemic and local (forearm) exercise, there was no difference in forearm DmO₂ among subjects with HFpEF, those with hypertension, and healthy control subjects; therefore, abnormalities in forearm DmO₂ cannot explain the reduced forearm ΔAVo₂ seen in subjects with HFpEF.

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Local forearm exercise performance predicted about one-third of the variability in systemic aerobic capacity, demonstrating that peripheral factors are important in determining whole-body exercise tolerance.

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Degree of adiposity strongly correlated with ΔAVo₂ during both local and whole-body exercise, suggesting that adipose tissue may play an active role in limiting exercise capacity in subjects with HFpEF.

The aim of this study was to determine the arteriovenous oxygen content difference (ΔAVo₂) in adult subjects with and without heart failure with preserved ejection fraction (HFpEF) during systemic and forearm exercise. Subjects with HFpEF had reduced ΔAVo₂. Forearm diffusional conductance for oxygen, a lumped conductance parameter that incorporates all impediments to the movement of oxygen from red blood cells in skeletal muscle capillaries into the mitochondria within myocytes, was estimated. Forearm diffusional conductance for oxygen was not different among adults with HFpEF, those with hypertension, and healthy control subjects; therefore, diffusional conductance cannot explain the reduced forearm ΔAVo₂. Instead, adiposity was strongly associated with ΔAVo₂, suggesting an active role for adipose tissue in reducing exercise capacity in patients with HFpEF.

Limb blood flow and muscle oxygenation responses during handgrip exercise above vs. below critical force

This study compared the brachial artery blood flow (Q̇_BA) and microvascular oxygen delivery responses during handgrip exercise above vs. below critical force (CF; the isometric analog of critical power). Q̇_BA and microvascular oxygen delivery are important determinants of oxygen utilization and metabolite accumulation during exercise, both of which increase progressively during exercise above CF. However the Q̇_BA and microvascular oxygen delivery responses above vs. below CF remain unknown. We hypothesized that Q̇_BA, deoxygenated-heme (deoxy-[heme]; an estimate of microvascular fractional oxygen extraction), and total-heme concentrations (total-[heme]; an estimate of changes in microvascular hematocrit) would demonstrate physiological maximums above CF despite increases in exercise intensity. Seven men and six women performed 1) a 5-min rhythmic isometric-handgrip maximal-effort test (MET) to determine CF and 2) two constant target-force tests above (severe-intensity; S1 and S2) and two constant target-force tests below (heavy-intensity; H1 and H2) CF. CF was 189.3 ± 16.7 N (29.7 ± 1.6%MVC). At end-exercise, Q̇_BA was greater for tests above CF (S1: 418 ± 147 mL/min; S2: 403 ± 137 mL/min) compared to tests below CF (H1: 287 ± 97 mL/min; H2: 340 ± 116 mL/min; all p < 0.05) but was not different between S1 and S2. Further, end-test Q̇_BA during both tests above CF was not different from Q̇_BA estimated at CF (392 ± 37 mL/min). At end-exercise, deoxy-[heme] was not different between tests above CF (S1: 150 ± 50 μM; S2: 155 ± 57 μM), but was greater during tests above CF compared to tests below CF (H1: 101 ± 24 μM; H2: 111 ± 21 μM; all p < 0.05). At end-exercise, total-[heme] was not different between tests above CF (S1: 404 ± 58 μM; S2: 397 ± 73 μM), but was greater during tests above CF compared to H1 (352 ± 58 μM; p < 0.01) but not H2 (371 ± 57 μM). These data suggest limb blood flow limitations exist and maximal levels of muscle microvascular oxygen delivery and extraction occur during exercise above, but not below, CF.

Vascular function is related to blood flow during high-intensity, but not low-intensity, knee extension exercise

While vascular function, assessed as the ability of the vasculature to dilate in response to a stimulus, is related to cardiovascular health, its relationship to exercise hyperemia is unclear. This study sought to determine if blood flow during submaximal and maximal exercise is related to vascular function. Nineteen healthy adults completed multiple assessments of vascular function specific to the leg, including passive leg movement (PLM), rapid onset vasodilation (ROV), reactive hyperemia (RH), and flow-mediated dilation (FMD). On a separate day, exercise blood flow (Doppler ultrasound) was assessed in the same leg during various intensities of single-leg, knee-extension (KE) exercise. Vascular function, determined by PLM, ROV, and RH, was related to exercise blood flow at high intensities, including maximum work rate (WRmax) ( r = 0.58–0.77, P < 0.001), but not low intensities, like ~21% WRmax ( r = 0.12–0.34, P = 0.12–0.62). Relationships between multiple indices of vascular function and peak exercise blood flow persisted when controlling for quadriceps mass and exercise work rate ( P < 0.05), indicating vascular function is independently related to the blood flow response to intense exercise. When divided into two groups based upon the magnitude of the PLM response, subjects with a lower PLM response exhibited lower exercise flow at several absolute work rates, as well as lower peak flow ( P < 0.05). In conclusion, leg flow during dynamic exercise is independently correlated with multiple different indices of microvascular function. Thus microvascular function appears to modulate the hyperemic response to high-intensity, but not low-intensity, exercise.

NEW & NOTEWORTHY While substantial evidence indicates that individuals with lower vascular function are at greater risk for cardiovascular disease, with many redundant vasodilator pathways present during exercise, it has been unclear if low vascular function actually impacts blood flow during exercise. This study provides evidence that vascular function, assessed by multiple noninvasive methods, is related to the blood flow response to high-intensity leg exercise in healthy young adults. Importantly, healthy young adults with lower levels of vascular function, particularly microvascular function, exhibit lower blood flow during high-intensity, and maximal knee extension exercise. Thus it appears that in addition to increasing one’s risk of cardiovascular disease, lower vascular function is also related to a blunted blood flow response during high-intensity exercise.

Near-infrared diffuse correlation spectroscopy tracks changes in oxygen delivery and utilization during exercise with and without isolated arterial compression

Near-infrared diffuse correlation spectroscopy (NIR-DCS) is an emerging technology for simultaneous measurement of skeletal muscle microvascular oxygen delivery and utilization during exercise. The extent to which NIR-DCS can track acute changes in oxygen delivery and utilization has not yet been fully established. To address this knowledge gap, 14 healthy men performed rhythmic handgrip exercise at 30% maximal voluntary contraction, with and without isolated brachial artery compression, designed to acutely reduce convective oxygen delivery to the exercising muscle. Radial artery blood flow (Duplex Ultrasound) and NIR-DCS derived variables [blood flow index (BFI), tissue oxygen saturation (StO2), and metabolic rate of oxygen (MRO2)] were simultaneously measured. During exercise, both radial artery blood flow (+51.6 ± 20.3 mL/min) and DCS-derived BFI (+155.0 ± 82.2%) increased significantly (P < 0.001), whereas StO2 decreased -7.9 ± 6.2% (P = 0.002) from rest. Brachial artery compression during exercise caused a significant reduction in both radial artery blood flow (-32.0 ± 19.5 mL/min, P = 0.001) and DCS-derived BFI (-57.3 ± 51.1%, P = 0.01) and a further reduction of StO2 (-5.6 ± 3.8%, P = 0.001) compared with exercise without compression. MRO2 was not significantly reduced during arterial compression (P = 0.83) due to compensatory reductions in StO2, driven by increases in deoxyhemoglobin/myoglobin (+7.1 ± 6.1 μM, P = 0.01; an index of oxygen extraction). Together, these proof-of-concept data help to further validate NIR-DCS as an effective tool to assess the determinants of skeletal muscle oxygen consumption at the level of the microvasculature during exercise.

Impact of high sodium intake on blood pressure and functional sympatholysis during rhythmic handgrip exercise

High dietary sodium intake is a risk factor for arterial hypertension; given that the ability to overcome sympathetically mediated vasoconstriction (functional sympatholysis) is attenuated in individuals with hypertension, we investigated the cardiovascular responses to high salt (HS) intake in healthy humans. We hypothesized that a HS intake of 15 g/day for 7 days would attenuate functional sympatholysis and augment the blood pressure response to handgrip exercise (HGE). Thirteen participants (6 males, 7 females) underwent 2 individual days of testing. Beat-by-beat blood pressure and heart rate were recorded throughout the trial on the non-exercising limb. Forearm blood flow was derived from ultrasonography on the brachial artery of the exercising limb. Participants then underwent a flow-mediated dilation (FMD) test. Next, a submaximal HGE was performed for 7 min with lower body negative pressure initiated during minutes 5-7. A single spot urine sample revealed a significant increase in sodium excretion during the HS conditions (p < 0.01). FMD was reduced during the HS condition. Mean arterial pressure was significantly higher during HS intake. No alteration to functional sympatholysis was found between conditions (p > 0.05). In summary, HS intake increases blood pressure without impacting functional sympatholysis or blood pressure responsiveness during HGE. These findings indicate that brachial artery dysfunction precedes an inefficient functional sympatholysis. Novelty Functional sympatholysis was not impacted by 1 week of high sodium intake. High sodium intake augmented the rate pressure product during handgrip exercise in healthy humans.

Critical skating intensity on a slide board: physiological and neuromuscular responses and correlation with performance on ice

The aim of this study was to assess the physiological and neuromuscular responses at critical skating intensity on a slide board and to investigate the correlations between critical cadence (CC) and skating performances on ice. Thirteen well-trained speed skaters (age,19.8 ± 4.2 years; weight, 69.6 ± 9.06 kg) performed a maximal skating incremental test (IT) on a slide board. CC was determined from 3 to 4 trials to exhaustion lasting from 3.1 ± 0.7 to 13.9 ± 3.1 min, using linear and hyperbolic mathematical fittings. A time to exhaustion test at CC (TTE-CC) was performed. CC values (55.3 ± 5.0 ppm) were significantly higher than cadence at the respiratory compensation point (RCP) (53.5 ± 4.0 ppm). Mean duration of TTE-CC was 22.9 ± 4.8 min. Peak values of oxygen uptake, heart rate (HR), ventilation, respiratory exchange ratio (RER), and ratings of perceived exertion (RPE) during TTE-CC were significantly lower (p < 0.05) than the peak values reached during the IT. Oxygen uptake, HR, ventilation, RER, and RPE significantly increased from 25% to 100% of TTE-CC. Muscle activity (integrated electromyography) significantly increased after 75% of TTE-CC for vastus lateralis and gluteus maximus muscles. Oxygen uptake at CC was better associated to skating performance on 500, 1000, 1500, and 5000 m than peak oxygen uptake at IT and oxygen uptake at RCP. Physiological responses indicate that critical skating intensity on slide board occurred within the heavy exercise domain where oxygen uptake increases but does not reach its maximum. Critical cadence could be used as a better indicator of performance and training prescription for long track speed skating distances.

Studies into the determinants of skeletal muscle oxygen consumption: novel insight from near-infrared diffuse correlation spectroscopy

KEY POINTS

Diffuse correlation spectroscopy (DCS) is emerging as a powerful tool to assess skeletal muscle perfusion. Near-infrared spectroscopy (NIRS) is an established technique for characterizing the transport and utilization of oxygen through the microcirculation. Here we compared a combined NIRS-DCS system with conventional measures of oxygen delivery and utilization during handgrip exercise. The data show good concurrent validity between convective oxygen delivery and DCS-derived blood flow index, as well as between oxygen extraction at the conduit and microvascular level. We then manipulated forearm arterial perfusion pressure by adjusting the position of the exercising arm relative to the position of the heart. The data show that microvascular perfusion can be uncoupled from convective oxygen delivery, and that tissue saturation seemingly compensates to maintain skeletal muscle oxygen consumption. Taken together, these data support a novel role for NIRS-DCS in understanding the determinants of muscle oxygen consumption at the microvascular level.

ABSTRACT

Diffuse correlation spectroscopy (DCS) is emerging as a powerful tool to assess skeletal muscle perfusion. Combining DCS with near-infrared spectroscopy (NIRS) introduces exciting possibilities for understanding the determinants of muscle oxygen consumption; however, no investigation has directly compared NIRS-DCS to conventional measures of oxygen delivery and utilization in an exercising limb. To address this knowledge gap, nine healthy males performed rhythmic handgrip exercise with simultaneous measurements by NIRS-DCS, Doppler blood flow and venous oxygen content. The two approaches showed good concurrent validity, with directionally similar responses between: (a) Doppler-derived forearm blood flow and DCS-derived blood flow index (BFI), and (b) venous oxygen saturation and NIRS-derived tissue saturation. To explore the utility of combined NIRS-DCS across the physiological spectrum, we manipulated forearm arterial perfusion pressure by altering the arm position above or below the level of the heart. As expected, Doppler-derived skeletal muscle blood flow increased with exercise in both arm positions, but with markedly different magnitudes (below: +424.3 ± 41.4 ml/min, above: +306 ± 12.0 ml/min, P = 0.002). In contrast, DCS-derived microvascular BFI increased to a similar extent with exercise, regardless of arm position (P = 0.65). Importantly, however, the time to reach BFI steady state was markedly slower with the arm above the heart, supporting the experimental design. Notably, we observed faster tissue desaturation at the onset of exercise with the arm above the heart, resulting in similar muscle oxygen consumption profiles throughout exercise. Taken together, these data support a novel role for NIRS-DCS in understanding the determinants of skeletal muscle oxygen utilization non-invasively and throughout exercise.

Classification of Intensity in Team Sport Activity

PURPOSE

This study aimed to assess the efficacy of critical metabolic power derived from variable-speed movement for classifying intensity in team sport activity.

METHODS

Elite male hockey players (n = 12) completed a series of time trials (100 yards, 400 yards, 1500 yards) and a 3-min all-out test to derive both critical speed (CS) and critical power (CP). Heart rate (HR), blood lactate, and rating of perceived exertion were measured during each protocol. Participants (n = 10) then played two competitive hockey matches. Time spent greater than 85% of maximum HR was compared with time spent above CS (from the time trials) and CP (from the 3-min test).

RESULTS

Between protocols, there was a moderate and nonsignificant association for CS (r = 0.359, P = 0.252) and a very large association for CP (r = 0.754, P = 0.005); the association was very large for peak HR (r = 0.866, P < 0.001), large for blood lactate (r = 0.506, P = 0.093), and moderate for rating of perceived exertion (rho = 0.441, P = 0.152). Time trials produced higher CS (4.3 vs 2.0 m·s, P < 0.001) and CP (18.3 vs 10.5 W·kg, P < 0.001) values than did the 3-min test. In matches, there was a very large association between time spent above 85% of maximum HR and time spent above both CS (r = 0.719, P < 0.001) and CP (r = 0.867, P < 0.001). This relationship was stronger for CP compared with CS (Z = 3.29, P = 0.0007).

CONCLUSIONS

Speed is not an appropriate parameter for the classification of team sport activity comprising continual changes in speed and direction; however, critical metabolic power derived from variable-speed activity seems useful for this purpose.

Understanding near infrared spectroscopy and its application to skeletal muscle research

Near infrared spectroscopy (NIRS) is a powerful noninvasive tool with which to study the matching of oxygen delivery to oxygen utilization and the number of new publications utilizing this technique has increased exponentially in the last 20 yr. By measuring the state of oxygenation of the primary heme compounds in skeletal muscle (hemoglobin and myoglobin), greater understanding of the underlying control mechanisms that couple perfusive and diffusive oxygen delivery to oxidative metabolism can be gained from the laboratory to the athletic field to the intensive care unit or emergency room. However, the field of NIRS has been complicated by the diversity of instrumentation, the inherent limitations of some of these technologies, the associated diversity of terminology, and a general lack of standardization of protocols. This Cores of Reproducibility in Physiology (CORP) will describe in basic but important detail the most common methodologies of NIRS, their strengths and limitations, and discuss some of the potential confounding factors that can affect the quality and reproducibility of NIRS data. Recommendations are provided to reduce the variability and errors in data collection, analysis, and interpretation. The goal of this CORP is to provide readers with a greater understanding of the methodology, limitations, and best practices so as to improve the reproducibility of NIRS research in skeletal muscle.

Vasoconstrictor responsiveness through alterations in relaxation time and metabolic rate during rhythmic handgrip contractions

Increasing the relaxation phase of the contraction-relaxation cycle will increase active skeletal muscle blood flow ( Q ˙ m ). However, it remains unknown if this increase in Q ˙ m alters the vasoconstriction responses in active skeletal muscle. This investigation determined if decreasing mechanical impedance would impact vasoconstriction of the active skeletal muscle. Eight healthy men performed rhythmic handgrip exercise under three different conditions; "low" duty cycle at 20% maximal voluntary contraction (MVC), "low" duty cycle at 15% MVC, and "high" duty cycle at 20% MVC. Relaxation time between low and high duty cycles were 2.4 sec versus 1.5 sec, respectively. During steady-state exercise lower body negative pressure (LBNP) was used to evoke vasoconstriction. Finger photoplethysmography and Doppler ultrasound derived diameters and velocities were used to measure blood pressure, forearm blood flow (FBF: mL min-1 ) and forearm vascular conductance (FVC: mL min-1  mmHg) throughout testing. The low duty cycle increased FBF and FVC versus the high duty cycle under steady-state conditions at 20% MVC (P < 0.01). The high duty cycle had the greatest attenuation in %ΔFVC (-1.9 ± 3.8%). The low duty cycle at 20% (-13.3 ± 1.4%) and 15% MVC (-13.1 ± 2.5%) had significantly greater vasoconstriction than the high duty cycle (both: P < 0.01) but were not different from one another (P = 0.99). When matched for work rate and metabolic rate ( V ˙ O 2 ), the high duty cycle had greater functional sympatholysis than the low duty cycle. However, despite a lower V ˙ O 2 , there was no difference in functional sympatholysis between the low duty cycle conditions. This may suggest that increases in Q ˙ m play a role in functional sympatholysis when mechanical compression is minimized.

Effect of dietary nitrate supplementation on conduit artery blood flow, muscle oxygenation, and metabolic rate during handgrip exercise

Dietary nitrate supplementation has positive effects on mitochondrial and muscle contractile efficiency during large muscle mass exercise in humans and on skeletal muscle blood flow (Q̇) in rats. However, concurrent measurement of these effects has not been performed in humans. Therefore, we assessed the influence of nitrate supplementation on Q̇ and muscle oxygenation characteristics during moderate- (40 %peak) and severe-intensity(85% peak) handgrip exercise in a randomized, double-blind, crossover design. Nine healthy men (age: 25 ± 2 yr) completed four constant-power exercise tests (2/intensity) randomly assigned to condition [nitrate-rich (nitrate) or nitrate-poor (placebo) beetroot supplementation] and intensity (40 or 85% peak). Resting mean arterial pressure was lower after nitrate compared with placebo (84 ± 4 vs. 89 ± 4 mmHg, P < 0.01). All subjects were able to sustain 10 min of exercise at 40% peak in both conditions. Nitrate had no effect on exercise tolerance during 85% peak (nitrate: 358 ± 29; placebo: 341 ± 34 s; P = 0.3). Brachial artery Q̇ was not different after nitrate at rest or any time during exercise. Deoxygenated [hemoglobin + myoglobin] was not different for 40% peak ( P > 0.05) but was elevated throughout 85% peak ( P < 0.05) after nitrate. The metabolic cost (V̇o2) was not different at the end of exercise; however, the V̇o2 primary amplitude at the onset of exercise was elevated after nitrate for the 85% peak work rate (96 ± 20 vs. 72 ± 12 ml/min, P < 0.05) and had a faster response. These findings suggest that an acute dose of nitrate reduces resting blood pressure and speeds V̇o2 kinetics in young adults but does not augment Q̇ or reduce steady-state V̇o2 during small muscle mass handgrip exercise.

NEW & NOTEWORTHY We show that acute dietary nitrate supplementation via beetroot juice increases the amplitude and speed of local muscle V̇o2 on kinetics parameters during severe- but not moderate-intensity handgrip exercise. These changes were found in the absence of an increased blood flow response, suggesting that the increased V̇o2 was attained via improvements in fractional O2 extraction and/or spatial distribution of blood flow within the exercising muscle.

The 'Critical Power' Concept: Applications to Sports Performance with a Focus on Intermittent High-Intensity Exercise

The curvilinear relationship between power output and the time for which it can be sustained is a fundamental and well-known feature of high-intensity exercise performance. This relationship 'levels off' at a 'critical power' (CP) that separates power outputs that can be sustained with stable values of, for example, muscle phosphocreatine, blood lactate, and pulmonary oxygen uptake ([Formula: see text]), from power outputs where these variables change continuously with time until their respective minimum and maximum values are reached and exercise intolerance occurs. The amount of work that can be done during exercise above CP (the so-called W') is constant but may be utilized at different rates depending on the proximity of the exercise power output to CP. Traditionally, this two-parameter CP model has been employed to provide insights into physiological responses, fatigue mechanisms, and performance capacity during continuous constant power output exercise in discrete exercise intensity domains. However, many team sports (e.g., basketball, football, hockey, rugby) involve frequent changes in exercise intensity and, even in endurance sports (e.g., cycling, running), intensity may vary considerably with environmental/course conditions and pacing strategy. In recent years, the appeal of the CP concept has been broadened through its application to intermittent high-intensity exercise. With the assumptions that W' is utilized during work intervals above CP and reconstituted during recovery intervals below CP, it can be shown that performance during intermittent exercise is related to four factors: the intensity and duration of the work intervals and the intensity and duration of the recovery intervals. However, while the utilization of W' may be assumed to be linear, studies indicate that the reconstitution of W' may be curvilinear with kinetics that are highly variable between individuals. This has led to the development of a new CP model for intermittent exercise in which the balance of W' remaining ([Formula: see text]) may be calculated with greater accuracy. Field trials of athletes performing stochastic exercise indicate that this [Formula: see text] model can accurately predict the time at which W' tends to zero and exhaustion is imminent. The [Formula: see text] model potentially has important applications in the real-time monitoring of athlete fatigue progression in endurance and team sports, which may inform tactics and influence pacing strategy.

Influence of group III/IV muscle afferents on small muscle mass exercise performance: a bioenergetics perspective

KEY POINTS

This investigation assessed the influence of group III/IV muscle afferents on small muscle mass exercise performance from a skeletal muscle bioenergetics perspective. Group III/IV muscle afferent feedback was attenuated with lumbar intrathecal fentanyl during intermittent isometric single-leg knee-extensor all-out exercise, while ³¹ P-MRS was used to assess skeletal muscle bioenergetics. Attenuation of group III/IV muscle afferent feedback improved exercise performance during the first minute of exercise, due to an increase in total ATP production with no change in the ATP cost of contraction. However, exercise performance was not altered during the remainder of the protocol, despite a sustained increase in total ATP production, due to an exacerbated ATP cost of contraction. These findings reveal that group III/IV muscle afferents directly limit exercise performance during small muscle mass exercise, but, due to their critical role in maintaining skeletal muscle contractile efficiency, with time, the benefit of attenuating the muscle afferents is negated.

ABSTRACT

The direct influence of group III/IV muscle afferents on exercise performance remains equivocal. Therefore, all-out intermittent isometric single-leg knee-extensor exercise and phosphorous magnetic resonance spectroscopy (³¹ P-MRS) were utilized to provide a high time resolution assessment of exercise performance and skeletal muscle bioenergetics in control conditions (CTRL) and with the attenuation of group III/IV muscle afferent feedback via lumbar intrathecal fentanyl (FENT). In both conditions, seven recreationally active men performed 60 maximal voluntary quadriceps contractions (MVC; 3 s contraction, 2 s relaxation), while knee-extensor force and ³¹ P-MRS were assessed during each MVC. The cumulative integrated force was significantly greater (8 ± 6%) in FENT than CTRL for the first minute of the all-out protocol, but was not significantly different for the second to fifth minutes. Total ATP production was significantly greater (16 ± 21%) in FENT than CTRL throughout the all-out exercise protocol, due to a significantly greater anaerobic ATP production (11 ± 13%) in FENT than CTRL with no significant difference in oxidative ATP production. The ATP cost of contraction was not significantly different between FENT and CTRL for the first minute of the all-out protocol, but was significantly greater (29 ± 34%) in FENT than in CTRL for the second to fifth minutes. These findings reveal that group III/IV muscle afferents directly limit exercise performance during small muscle mass exercise, but, due to their critical role in maintaining skeletal muscle contractile efficiency, with time, the benefit from muscle afferent attenuation is negated.

Estimation of critical end-test torque using neuromuscular electrical stimulation of the quadriceps in humans

Characterization of critical power/torque (CP/CT) during voluntary exercise requires maximal effort, making difficult for those with neuromuscular impairments. To address this issue we sought to determine if electrically stimulated intermittent isometric exercise resulted in a critical end-test torque (ETT) that behaved similar to voluntary CT. In the first experiment participants (n = 9) completed four bouts of stimulated exercise at a 3:2 duty cycle, at frequencies of 100, 50, 25 Hz, and a low frequency below ETT (Sub-ETT; ≤ 15 Hz). The second experiment (n = 20) consisted of four bouts at a 2:2 duty cycle-two bouts at 100 Hz, one at an intermediate frequency (15-30 Hz), and one at Sub-ETT. The third experiment (n = 12) consisted of two bouts at 50 Hz at a 3:2 duty* cycle with proximal blood flow occlusion during one of the bouts. ETT torque was similar (p ≥ 0.43) within and among stimulation frequencies in experiment 1. No fatigue was observed during the Sub-ETT bouts (p > 0.05). For experiment 2, ETT was similar at 100 Hz and at the intermediate frequency (p ≥ 0.29). Again, Sub-ETT stimulation did not result in fatigue (p > 0.05). Altering oxygen delivery by altering the duty cycle (3:2 vs. 2:2; p = 0.02) and by occlusion (p < 0.001) resulted in lower ETT values. Stimulated exercise resulted in an ETT that was consistent from day-to-day and similar regardless of initial torque, as long as that torque exceeded ETT, and was sensitive to oxygen delivery. As such we propose it represents a parameter similar to voluntary CT.

The noninvasive simultaneous measurement of tissue oxygenation and microvascular hemodynamics during incremental handgrip exercise

Limb blood flow increases linearly with exercise intensity; however, invasive measurements of muscle microvascular blood flow during incremental exercise have demonstrated submaximal plateaus. We tested the hypotheses that 1) brachial artery blood flow (Q̇BA) would increase with increasing exercise intensity until task failure, 2) blood flow index of the flexor digitorum superficialis (BFIFDS) measured noninvasively via diffuse correlation spectroscopy would plateau at a submaximal work rate, and 3) muscle oxygenation characteristics (total-[heme], deoxy-[heme], and percentage saturation) measured noninvasively with near-infrared spectroscopy would demonstrate a plateau at a similar work rate as BFIFDS. Sixteen subjects (23.3 ± 3.9 yr, 170.8 ± 1.9 cm, 72.8 ± 3.4 kg) participated in this study. Peak power (Ppeak) was determined for each subject (1.8 ± 0.4 W) via an incremental handgrip exercise test. Q̇BA, BFIFDS, total-[heme], deoxy-[heme], and percentage saturation were measured during each stage of the exercise test. On a subsequent testing day, muscle activation measurements of the FDS (RMSFDS) were collected during each stage of an identical incremental handgrip exercise test via electromyography from a subset of subjects ( n = 7). Q̇BA increased with exercise intensity until the final work rate transition ( P < 0.05). No increases in BFIFDS or muscle oxygenation characteristics were observed at exercise intensities greater than 51.5 ± 22.9% of Ppeak. No submaximal plateau in RMSFDS was observed. Whereas muscle activation of the FDS increased until task failure, noninvasively measured indices of perfusive and diffusive muscle microvascular oxygen delivery demonstrated submaximal plateaus.

NEW & NOTEWORTHY Invasive measurements of muscle microvascular blood flow during incremental exercise have demonstrated submaximal plateaus. We demonstrate that indices of perfusive and diffusive microvascular oxygen transport to skeletal muscle, measured completely noninvasively, plateau at submaximal work rates during incremental exercise, even though limb blood flow and muscle recruitment continued to increase.

Mechanisms for the age-related increase in fatigability of the knee extensors in old and very old adults

The mechanisms for the age-related increase in fatigability during high-velocity contractions in old and very old adults (≥80 yr) are unresolved. Moreover, whether the increased fatigability with advancing age and the underlying mechanisms differ between men and women is not known. The purpose of this study was to quantify the fatigability of knee extensor muscles and identify the mechanisms of fatigue in 30 young (22.6 ± 0.4 yr; 15 men), 62 old (70.5 ± 0.7 yr; 33 men), and 12 very old (86.0 ± 1.3 yr; 6 men) men and women elicited by high-velocity concentric contractions. Participants performed 80 maximal velocity contractions (1 contraction per 3 s) with a load equivalent to 20% of the maximum voluntary isometric contraction. Voluntary activation and contractile properties were quantified before and immediately following exercise (<10 s) using transcranial magnetic stimulation and electrical stimulation. Absolute mechanical power output was 97 and 217% higher in the young compared with old and very old adults, respectively. Fatigability (reductions in power) progressively increased across age groups, with a power loss of 17% in young, 31% in old, and 44% in very old adults. There were no sex differences in fatigability among any of the age groups. The age-related increase in power loss was strongly associated with changes in the involuntary twitch amplitude ( r = 0.75, P < 0.001). These data suggest that the age-related increased power loss during high-velocity fatiguing exercise is unaffected by biological sex and determined primarily by mechanisms that disrupt excitation contraction coupling and/or cross-bridge function. NEW & NOTEWORTHY We show that aging of the neuromuscular system results in an increase in fatigability of the knee extensors during high-velocity exercise that is more pronounced in very old adults (≥80 yr) and occurs similarly in men and women. Importantly, the age-related increase in power loss was strongly associated with the changes in the electrically evoked contractile properties suggesting that the increased fatigability with aging is determined primarily by mechanisms within the muscle for both sexes.

Evidence of a metabolic reserve in the skeletal muscle of elderly people

The purpose of the present study was to determine whether mitochondrial function is limited by O₂ availability or the intrinsic capacity of mitochondria to synthesize ATP in elderly individuals. To this aim, we examined, in comparison to free-flow conditions (FF), the effect of superimposing reactive hyperemia (RH), induced by a period of brief ischemia during the last min of exercise, on O₂ availability and mitochondrial function in the calf muscle. 12 healthy, untrained, elderly subjects performed dynamic plantar flexion exercise and phosphorus magnetic resonance spectroscopy (³¹P-MRS), near-infrared spectroscopy (NIRS), and Doppler ultrasound were used to assess muscle metabolism and peripheral hemodynamics. Limb blood flow [area under the curve (AUC), FF: 1.5±0.5L; RH: 3.2±1.1L, P<0.01] and convective O₂ delivery (AUC, FF: 0.30±0.13L; RH: 0.64±0.29L, P<0.01) were significantly increased in RH in comparison to FF. RH was also associated with significantly higher capillary blood flow (P<0.05) and this resulted in a 33% increase in estimated peak mitochondrial ATP synthesis rate (FF: 24±11 mM.min⁻¹; RH: 31±7 mM.min⁻¹, P<0.05). These results document a hemodynamic reserve in the contracting calf muscle of the elderly accessible by superimposing reactive hyperemia. Furthermore, this increase in O₂ availability enhanced mitochondrial function thus indicating a skeletal muscle metabolic reserve despite advancing age and low level of physical activity.

W' expenditure and reconstitution during severe intensity constant power exercise: mechanistic insight into the determinants of W'

The sustainable duration of severe intensity exercise is well-predicted by critical power (CP) and the curvature constant (W'). The development of the W'BAL model allows for the pattern of W' expenditure and reconstitution to be characterized and this model has been applied to intermittent exercise protocols. The purpose of this investigation was to assess the influence of relaxation phase duration and exercise intensity on W' reconstitution during dynamic constant power severe intensity exercise. Six men (24.6 ± 0.9 years, height: 173.5 ± 1.9 cm, body mass: 78.9 ± 5.6 kg) performed severe intensity dynamic handgrip exercise to task failure using 50% and 20% duty cycles. The W'BAL model was fit to each exercise test and the time constant for W' reconstitution (τW') was determined. The τW' was significantly longer for the 50% duty cycle (1640 ± 262 sec) than the 20% duty cycle (863 ± 84 sec, P = 0.02). Additionally, the relationship between τW' and CP was well described as an exponential decay (r(2) = 0.90, P < 0.0001). In conclusion, the W'BAL model is able to characterize the expenditure and reconstitution of W' across the contraction-relaxation cycles comprising severe intensity constant power handgrip exercise. Moreover, the reconstitution of W' during constant power severe intensity exercise is influenced by the relative exercise intensity, the duration of relaxation between contractions, and CP.

Effect of adipose tissue thickness, muscle site, and sex on near-infrared spectroscopy derived total-[hemoglobin + myoglobin]

Craig JC, Broxterman RM, Wilcox SL, Chen C, Barstow TJ. Effect of adipose tissue thickness, muscle site, and sex on near-infrared spectroscopy derived total-[hemoglobin + myoglobin]. J Appl Physiol 123: 1571-1578, 2017. First published September 21, 2017; doi: 10.1152/japplphysiol.00207.2017 .-Adipose tissue thickness (ATT) attenuates signals from near-infrared spectroscopy (NIRS) and diminishes the absolute quantification of underlying tissues by contemporary NIRS devices. Based on the relationship between NIRS-derived total-[hemoglobin + myoglobin] (total-[Hb + Mb]) and ATT, we tested the hypotheses that the correction factor for ATT 1) is muscle site specific; 2) does not differ between men and women; and that 3) exclusion of the shortest source-detector distance from data analysis increases total-[Hb + Mb]. Fourteen healthy subjects (7 men) rested in a neutral body position (supine or prone) while measurements of total-[Hb + Mb] and ATT were taken at four muscles common to resting and exercise studies: vastus lateralis (VL), rectus femoris (RF), gastrocnemius (GS), and flexor digitorum superficialis (FDS). ATT averaged 6.0 ± 0.4 mm across all muscles. Every muscle showed a negative slope ( r²: 0.6-0.94; P < 0.01) for total-[Hb + Mb] as a function of ATT: VL (-34 μM/mm), RF (-26 μM/mm), GS (-54 μM/mm), and FDS (-33 μM/mm). The projected total-[Hb + Mb] at 0 mm ATT ( y-intercept) was 452, 372, 620, and 456 μM for VL, RF, GS, and FDS, respectively. No differences were found between the sexes within VL, RF, or FDS, but men had a greater projected total-[Hb + Mb] at 0 mm for GS (688 ± 44 vs. 552 ± 40 μM; P < 0.05). Exclusion of the shortest source-detector distance increased total-[Hb + Mb] by 12 ± 1 μM ( P < 0.05). The present findings demonstrate that total-[Hb + Mb] should be corrected for ATT using muscle site-specific factors which are not sex specific, except in the case of GS. NEW & NOTEWORTHY Near-infrared spectroscopy (NIRS) is an important tool for physiologists and clinicians. However, adipose tissue greatly attenuates the signals from these devices. Correcting for this attenuation has been suggested based on the strength of the relationship between NIRS-derived measurements and the adipose tissue thickness. We show that this relationship is unique to the muscle site of interest but may not be sex specific. Accurate quantification of underlying tissue mandates researchers correct for adipose tissue thickness.

Prior exercise speeds pulmonary oxygen uptake kinetics and increases critical power during supine but not upright cycling

NEW FINDINGS

What is the central question of this study? Critical power (CP) represents the highest work rate for which a metabolic steady state is attainable. The physiological determinants of CP are unclear, but research suggests that CP might be related to the time constant of phase II oxygen uptake kinetics (τV̇O2). What is the main finding and its importance? We provide the first evidence that τV̇O2 is mechanistically related to CP. A reduction of τV̇O2 in the supine position was observed alongside a concomitant increase in CP. This effect may be contingent on measures of oxygen availability derived from near-infrared spectroscopy. Critical power (CP) is a fundamental parameter defining high-intensity exercise tolerance and is related to the time constant of phase II pulmonary oxygen uptake kinetics (τV̇O2). To test the hypothesis that this relationship is causal, we determined the impact of prior exercise ('priming') on CP and τV̇O2 in the upright and supine positions. Seventeen healthy men were assigned to either upright or supine exercise groups, whereby CP, τV̇O2 and muscle deoxyhaemoglobin kinetics (τ_[HHb] ) were determined via constant-power tests to exhaustion at four work rates with (primed) and without (control) priming exercise at ∼31%Δ. During supine exercise, priming reduced τV̇O2 (control 54 ± 18 s versus primed 39 ± 11 s; P < 0.001), increased τ_[HHb] (control 8 ± 4 s versus primed 12 ± 4 s; P = 0.003) and increased CP (control 177 ± 31 W versus primed 185 ± 30 W, P = 0.006) compared with control conditions. However, priming exercise had no effect on τV̇O2 (control 37 ± 12 s versus primed 35 ± 8 s; P = 0.82), τ_[HHb] (control 10 ± 5 s versus primed 14 ± 10 s; P = 0.10) or CP (control 235 ± 42 W versus primed 232 ± 35 W; P = 0.57) during upright exercise. The concomitant reduction of τV̇O2 and increased CP following priming in the supine group, effects that were absent in the upright group, provide the first experimental evidence that τV̇O2 is mechanistically related to critical power. The increased τ_[HHb+Mb] suggests that this effect was mediated, at least in part, by improved oxygen availability.

Oxygen delivery and the restoration of the muscle energetic balance following exercise: implications for delayed muscle recovery in patients with COPD

Patients with chronic obstructive pulmonary disease (COPD) experience a delayed recovery from skeletal muscle fatigue following exhaustive exercise that likely contributes to their progressive loss of mobility. As this phenomenon is not well understood, this study sought to examine postexercise peripheral oxygen (O₂) transport and muscle metabolism dynamics in patients with COPD, two important determinants of muscle recovery. Twenty-four subjects, 12 nonhypoxemic patients with COPD and 12 healthy subjects with a sedentary lifestyle, performed dynamic plantar flexion exercise at 40% of the maximal work rate (WR_max) with phosphorus magnetic resonance spectroscopy (³¹P-MRS), near-infrared spectroscopy (NIRS), and vascular Doppler ultrasound assessments. The mean response time of limb blood flow at the offset of exercise was significantly prolonged in patients with COPD (controls: 56 ± 27 s; COPD: 120 ± 87 s; P < 0.05). In contrast, the postexercise time constant for capillary blood flow was not significantly different between groups (controls: 49 ± 23 s; COPD: 51 ± 21 s; P > 0.05). The initial postexercise convective O₂ delivery (controls: 0.15 ± 0.06 l/min; COPD: 0.15 ± 0.06 l/min) and the corresponding oxidative adenosine triphosphate (ATP) demand (controls: 14 ± 6 mM/min; COPD: 14 ± 6 mM/min) in the calf were not significantly different between controls and patients with COPD (P > 0.05). The phosphocreatine resynthesis time constant (controls: 46 ± 20 s; COPD: 49 ± 21 s), peak mitochondrial phosphorylation rate, and initial proton efflux were also not significantly different between groups (P > 0.05). Therefore, despite perturbed peripheral hemodynamics, intracellular O₂ availability, proton efflux, and aerobic metabolism recovery in the skeletal muscle of nonhypoxemic patients with COPD are preserved following plantar flexion exercise and thus are unlikely to contribute to the delayed recovery from exercise in this population.

Use of electromyography to detect muscle exhaustion in finishing barrows fed ractopamine HCl

The objectives of this study were to determine the effects of dietary ractopamine HCl (RAC) on muscle fiber characteristics and electromyography (EMG) measures of finishing barrow exhaustion when barrows were subjected to increased levels of activity. Barrows ( = 34; 92 ± 2 kg initial BW) were assigned to 1 of 2 treatments: a conventional swine finishing diet containing 0 mg/kg ractopamine HCl (CON) or a diet formulated to meet the requirements of finishing barrows fed 10 mg/kg RAC (RAC+). After 32 d on feed, barrows were individually moved around a track at 0.79 m/s until subjectively exhausted. Wireless EMG sensors were affixed to the deltoideus (DT), triceps brachii lateral head (TLH), tensor fasciae latae (TFL), and semitendinosus (ST) muscles to measure median power frequency (MdPF) and root mean square (RMS) as indicators of action potential conduction velocity and muscle fiber recruitment, respectively. After harvest, samples of each muscle were collected for fiber type, succinate dehydrogenase (SDH), and capillary density analysis. Speed was not different ( = 0.82) between treatments, but RAC+ barrows reached subjective exhaustion earlier and covered less distance than CON barrows ( < 0.01). There were no treatment × muscle interactions or treatment effects for end-point MdPF values ( > 0.29). There was a treatment × muscle interaction ( = 0.04) for end-point RMS values. The RAC diet did not change end-point RMS values in the DT or TLH ( > 0.37); however, the diet tended to decrease and increase end-point RMS in the ST and TFL, respectively ( < 0.07). There were no treatment × muscle interactions for fiber type, SDH, or capillary density measures ( > 0.10). Muscles of RAC+ barrows tended to have less type I fibers and more capillaries per fiber ( < 0.07). Type I and IIA fibers of RAC+ barrows were larger ( < 0.07). Compared with all other muscles, the ST had more ( < 0.01) type IIB fibers and larger type I, IIA, and IIX fibers ( < 0.01). Type I, IIA, and IIX fibers of the ST also contained less SDH compared with the other muscles ( < 0.01). Barrows fed a RAC diet had increased time to subjective exhaustion due to loss of active muscle fibers in the ST, possibly due to fibers being larger and less oxidative in metabolism. Size increases in type I and IIA fibers with no change in oxidative capacity could also contribute to early exhaustion of RAC+ barrows. Overall, EMG technology can measure real-time muscle fiber loss to help explain subjective exhaustion in barrows.

Skeletal muscle bioenergetics during all-out exercise: mechanistic insight into the oxygen uptake slow component and neuromuscular fatigue

Although all-out exercise protocols are commonly used, the physiological mechanisms underlying all-out exercise performance are still unclear, and an in-depth assessment of skeletal muscle bioenergetics is lacking. Therefore, phosphorus magnetic resonance spectroscopy (³¹P-MRS) was utilized to assess skeletal muscle bioenergetics during a 5-min all-out intermittent isometric knee-extensor protocol in eight healthy men. Metabolic perturbation, adenosine triphosphate (ATP) synthesis rates, ATP cost of contraction, and mitochondrial capacity were determined from intramuscular concentrations of phosphocreatine (PCr), inorganic phosphate (P_i), diprotonated phosphate ([Formula: see text]), and pH. Peripheral fatigue was determined by exercise-induced alterations in potentiated quadriceps twitch force (Q_tw) evoked by supramaximal electrical femoral nerve stimulation. The oxidative ATP synthesis rate (ATP_OX) attained and then maintained peak values throughout the protocol, despite an ~63% decrease in quadriceps maximal force production. ThusATP_OX normalized to force production (ATP_OX gain) significantly increased throughout the exercise (1st min: 0.02 ± 0.01, 5th min: 0.04 ± 0.01 mM·min^-1·N^-1), as did the ATP cost of contraction (1st min: 0.048 ± 0.019, 5th min: 0.052 ± 0.015 mM·min^-1·N^-1). Additionally, the pre- to postexercise change in Q_tw (-52 ± 26%) was significantly correlated with the exercise-induced change in intramuscular pH (r = 0.75) and [Formula: see text] concentration (r = 0.77). In conclusion, the all-out exercise protocol utilized in the present study elicited a "slow component-like" increase in intramuscular ATP_OX gain as well as a progressive increase in the phosphate cost of contraction. Furthermore, the development of peripheral fatigue was closely related to the perturbation of specific fatigue-inducing intramuscular factors (i.e., pH and [Formula: see text] concentration).NEW & NOTEWORTHY The physiological mechanisms and skeletal muscle bioenergetics underlying all-out exercise performance are unclear. This study revealed an increase in oxidative ATP synthesis rate gain and the ATP cost of contraction during all-out exercise. Furthermore, peripheral fatigue was related to the perturbation in pH and deprotonated phosphate ion. These findings support the concept that the oxygen uptake slow component arises from within active skeletal muscle and that skeletal muscle force generating capacity is linked to the intramuscular metabolic milieu.

Decreases in maximal oxygen uptake following long-duration spaceflight: Role of convective and diffusive O2 transport mechanisms

We have previously predicted that the decrease in maximal oxygen uptake (V̇o_2max) that accompanies time in microgravity reflects decrements in both convective and diffusive O₂ transport to the mitochondria of the contracting myocytes. The aim of this investigation was therefore to quantify the relative changes in convective O₂ transport (Q̇o₂) and O₂ diffusing capacity (Do₂) following long-duration spaceflight. In nine astronauts, resting hemoglobin concentration ([Hb]), V̇o_2max, maximal cardiac output (Q̇_Tmax), and differences in arterial and venous O₂ contents ([Formula: see text]-[Formula: see text]) were obtained retrospectively for International Space Station Increments 19-33 (April 2009-November 2012). Q̇o₂ and Do₂ were calculated from these variables via integration of Fick's Principle of Mass Conservation and Fick's Law of Diffusion. V̇o_2max significantly decreased from pre- to postflight (-53.9 ± 45.5%, P = 0.008). The significant decrease in Q̇_Tmax (-7.8 ± 9.1%, P = 0.05), despite an unchanged [Hb], resulted in a significantly decreased Q̇o₂ (-11.4 ± 10.5%, P = 0.02). Do₂ significantly decreased from pre- to postflight by -27.5 ± 24.5% (P = 0.04), as did the peak [Formula: see text]-[Formula: see text] (-9.2 ± 7.5%, P = 0.007). With the use of linear regression analysis, changes in V̇o_2max were significantly correlated with changes in Do₂ (R² = 0.47; P = 0.04). These data suggest that spaceflight decreases both convective and diffusive O₂ transport. These results have practical implications for future long-duration space missions and highlight the need to resolve the specific mechanisms underlying these spaceflight-induced changes along the O₂ transport pathway.NEW & NOTEWORTHY Long-duration spaceflight elicited a significant decrease in maximal oxygen uptake. Given the adverse physiological adaptations to microgravity along the O₂ transport pathway that have been reported, an integrative approach to the determinants of postflight maximal oxygen uptake is needed. We demonstrate that both convective and diffusive oxygen transport are decreased following ~6 mo International Space Station missions.

Critical Power: An Important Fatigue Threshold in Exercise Physiology

: The hyperbolic form of the power-duration relationship is rigorous and highly conserved across species, forms of exercise, and individual muscles/muscle groups. For modalities such as cycling, the relationship resolves to two parameters, the asymptote for power (critical power [CP]) and the so-called W' (work doable above CP), which together predict the tolerable duration of exercise above CP. Crucially, the CP concept integrates sentinel physiological profiles-respiratory, metabolic, and contractile-within a coherent framework that has great scientific and practical utility. Rather than calibrating equivalent exercise intensities relative to metabolically distant parameters such as the lactate threshold or V˙O2max, setting the exercise intensity relative to CP unifies the profile of systemic and intramuscular responses and, if greater than CP, predicts the tolerable duration of exercise until W' is expended, V˙O2max is attained, and intolerance is manifested. CP may be regarded as a "fatigue threshold" in the sense that it separates exercise intensity domains within which the physiological responses to exercise can (CP) be stabilized. The CP concept therefore enables important insights into 1) the principal loci of fatigue development (central vs. peripheral) at different intensities of exercise and 2) mechanisms of cardiovascular and metabolic control and their modulation by factors such as O2 delivery. Practically, the CP concept has great potential application in optimizing athletic training programs and performance as well as improving the life quality for individuals enduring chronic disease.

Acute supplementation of N-acetylcysteine does not affect muscle blood flow and oxygenation characteristics during handgrip exercise

N‐acetylcysteine (NAC; antioxidant and thiol donor) supplementation has improved exercise performance and delayed fatigue, but the underlying mechanisms are unknown. One possibility is NAC supplementation increases limb blood flow during severe‐intensity exercise. The purpose was to determine if NAC supplementation affected exercising arm blood flow and muscle oxygenation characteristics. We hypothesized that NAC would lead to higher limb blood flow and lower muscle deoxygenation characteristics during severe‐intensity exercise. Eight healthy nonendurance trained men (21.8 ± 1.2 years) were recruited and completed two constant power handgrip exercise tests at 80% peak power until exhaustion. Subjects orally consumed either placebo (PLA) or NAC (70 mg/kg) 60 min prior to handgrip exercise. Immediately prior to exercise, venous blood samples were collected for determination of plasma redox balance. Brachial artery blood flow (BABF) was measured via Doppler ultrasound and flexor digitorum superficialis oxygenation characteristics were measured via near‐infrared spectroscopy. Following NAC supplementaiton, plasma cysteine (NAC: 47.2 ± 20.3 μmol/L vs. PLA: 9.6 ± 1.2 μmol/L; P = 0.001) and total cysteine (NAC: 156.2 ± 33.9 μmol/L vs. PLA: 132.2 ± 16.3 μmol/L; P = 0.048) increased. Time to exhaustion was not significantly different (P = 0.55) between NAC (473.0 ± 62.1 sec) and PLA (438.7 ± 58.1 sec). Resting BABF was not different (P = 0.79) with NAC (99.3 ± 31.1 mL/min) and PLA (108.3 ± 46.0 mL/min). BABF was not different (P = 0.42) during exercise or at end‐exercise (NAC: 413 ± 109 mL/min; PLA: 445 ± 147 mL/min). Deoxy‐[hemoglobin+myoglobin] and total‐[hemoglobin+myoglobin] were not significantly different (P = 0.73 and P = 0.54, respectively) at rest or during exercise between conditions. We conclude that acute NAC supplementation does not alter oxygen delivery during exercise in men.

Inspiratory muscle performance in endurance-trained elderly males during incremental exercise

The aim of this study was to compare the inspiratory muscle performance during an incremental exercise of twelve fit old endurance-trained athletes (OT) with that of fit young athletes (YT) and healthy age-matched controls (OC). The tension-time index (TT0.1) was determined according to the equation TT0.1=P0.1/PImax×ti/ttot, where P0.1 is the mouth occlusion pressure, PImax the maximal inspiratory pressure and ti/ttot the duty cycle. For a given VCO2, OT group displayed P0.1, P0.1/PImax ratio, TT0.1 and effective impedance of the respiratory muscle values which were lower than OC group and higher than YT group. At maximal exercise, P0.1/PImax ratio and TT0.1 was still lower in the OT group than OC group and higher than YT group. This study showed lower inspiratory muscle performance attested by a higher (TT0.1) during exercise in the OT group than YT group, but appeared to be less marked in elderly men having performed lifelong endurance training compared with sedentary elderly subjects.