In vivo ATP production during free-flow and ischaemic muscle contractions in humans

Minimum velocity threshold in response to the free-weight back squat: reliability and validity of different submaximal loading schemes

PURPOSE

To explore if mean concentric velocity (MCV) of the last repetition before set failure differs between free-weight back squat protocols with greater emphasis on metabolic accumulation vs. mechanical loading. The between-set and between-day reliability of terminal MCV obtained with these different loading schemes was also determined.

METHODS

Fifteen healthy male participants (18-30 years) were included. They all were required to exhibit a relative strength ≥ 1.5 times their body mass. MCVs were obtained at one-repetition maximum (1RM) and with two submaximal protocols (metabolic emphasis: three sets of 40%1RM with blood-flow restriction vs. mechanical emphasis: three sets 80%1RM without blood-flow restriction). Participants were instructed to reach maximal intended concentric velocity in each repetition up to failure.

RESULTS

Set failure was achieved at a faster MCV with the metabolic protocol (p < 0.05). The reliability of MCV at failure reached higher values for the metabolic loading scheme. However, while the MCV achieved at failure during the metabolic protocol was systematically higher than the MCV at 1RM (p < 0.05), this was not entirely the case for the mechanical protocol (similar to 1RM MCV during the last sets in both testing days). Finally, the absolute error derived from estimating the MCV at 1RM based on the MCV obtained at set failure with the mechanical protocol was considerably high (≥ 0.05 m/s).

CONCLUSION

This study indicates that MCV obtained at set failure is dependent on the specificity of the physiological demands of exercise. Thus, MCVs obtained at failure with submaximal loads should not be used to estimate 1RM MCV.

Lactate and hydrogen ions play a predominant role in evoking the exercise pressor reflex during ischaemic contractions but not during freely perfused contractions

We investigated the role played by lactate and hydrogen in evoking the exercise pressor reflex (EPR) in decerebrated rats whose hindlimb muscles were either freely perfused or ischaemic. Production of lactate and hydrogen by the contracting hindlimb muscles was manipulated by knocking out the myophosphorylase gene (pygm). In knockout rats (pygm-/-; n = 13) or wild-type rats (pygm+/+; n = 13), the EPR was evoked by isometrically contracting the triceps surae muscles. Blood pressure, tension, blood flow, renal sympathetic nerve activity and blood lactate concentrations were measured. Intramuscular metabolites and pH changes induced by the contractions were quantified by 31P-magnetic resonance spectroscopy (n = 5). In a subset of pygm-/- rats (n = 5), contractions were evoked with prior infusion of lactate (pH 6.0) in an attempt to restore the effect of lactate and hydrogen ions. Contraction of freely perfused muscles increased blood lactate and decreased muscle pH in pygm+/+ rats only. Despite these differences, the reflex pressor and sympathetic responses to freely perfused contraction did not differ between groups (P = 0.992). During ischaemia, contraction increased muscle lactate and hydrogen ion production in pygm+/+ rats (P < 0.0134), whereas it had no effect in pygm-/- rats (P > 0.783). Likewise, ischaemia exaggerated the reflex pressor, and sympathetic responses to contraction in pygm+/+ but not in pygm-/- rats. This exaggeration was restored when a solution of lactate (pH 6.0) was infused prior to the contraction in pygm-/- rats. We conclude that lactate and hydrogen accumulation in contracting myocytes play a key role in evoking the metabolic component of the EPR during ischaemic but not during freely perfused contractions. KEY POINTS: Conflicting results exist about the role played by lactate and hydrogen ions in evoking the exercise pressor reflex. Using CRISP-Cas9, we rendered the myophosphorylase gene non-functional to block the production of lactate and hydrogen ions. The exercise pressor reflex was evoked in decerebrated rats by statically contracting the triceps surae muscles with or without muscle ischaemia. Static contraction elevated the concentration of lactate and hydrogen ions in pygm+/+ but not in pygm-/- rats. Despite these differences, the exercise pressor reflex was not different between groups. Acute muscle ischaemia exaggerated the concentration of lactate and hydrogen ions in pygm+/+ but not in pygm-/- rats. Likewise, acute muscle ischaemia exaggerated the exercise pressor reflex in pygm+/+ but not in pygm-/- rats. We conclude that lactate and hydrogen play a key role in evoking the exercise pressor reflex during ischaemic but not during freely perfused contractions.

Postcontraction [acetylcarnitine] reflects interindividual variation in skeletal muscle ATP production patterns in vivo

In addition to its role in substrate selection (carbohydrate vs. fat) for oxidative metabolism in muscle, acetylcarnitine production may be an important modulator of the energetic pathway by which ATP is produced. A combination of noninvasive magnetic resonance spectroscopy measures of cytosolic acetylcarnitine and ATP production pathways was used to investigate the link between [acetylcarnitine] and energy production in vivo. Intracellular metabolites were measured in the vastus lateralis muscle of eight males (mean: 28.4 yr, range: 25-35) during 8 min of incremental, dynamic contractions (0.5 Hz, 2-min stages at 6%, 9%, 12%, and 15% maximal torque) that increased [acetylcarnitine] approximately fivefold from resting levels. ATP production via oxidative phosphorylation, glycolysis, and the creatine kinase reaction was calculated based on phosphorus metabolites and pH. Spearman rank correlations indicated that postcontraction [acetylcarnitine] was positively associated with both absolute (mM) and relative (% total ATP) glycolytic ATP production (rs = 0.95, P = 0.001; rs = 0.93, P = 0.002), and negatively associated with relative (rs = -0.81, P = 0.02) but not absolute (rs = -0.14, P = 0.75) oxidative ATP production. Thus, acetylcarnitine accumulated more when there was a greater reliance on "nonoxidative" glycolysis and a relatively lower contribution from oxidative phosphorylation, reflecting the fate of pyruvate in working skeletal muscle. Furthermore, these data indicate striking interindividual variation in responses to the energy demand of submaximal contractions. Overall, the results of this preliminary study provide novel evidence of the coupling in vivo between ATP production pathways and the carnitine system.NEW & NOTEWORTHY Production of acetylcarnitine from acetyl-CoA and free carnitine may be important for energy pathway regulation in contracting skeletal muscle. Noninvasive magnetic resonance spectroscopy was used to investigate the link between acetylcarnitine and energy production in the vastus lateralis muscle during dynamic contractions (n = 8 individuals). A positive correlation between acetylcarnitine accumulation and "nonoxidative" glycolysis and an inverse relationship with oxidative phosphorylation, provides novel evidence of the coupling between ATP production and the carnitine system in vivo.

On the role of skeletal muscle acidosis and inorganic phosphates as determinants of central and peripheral fatigue: A 31 P-MRS study

Intramuscular hydrogen ion (H⁺ ) and inorganic phosphate (Pi) concentrations were dissociated during exercise to challenge their relationships with peripheral and central fatigue in vivo. Ten recreationally active, healthy men (27 ± 5 years; 180 ± 4 cm; 76 ± 10 kg) performed two consecutive intermittent isometric single-leg knee-extensor trials (60 maximal voluntary contractions; 3 s contraction, 2 s relaxation) interspersed with 5 min of rest. Phosphorus magnetic resonance spectroscopy (³¹ P-MRS) was used to continuously quantify intramuscular [H⁺ ] and [Pi] during both trials. Using electrical femoral nerve stimulation, quadriceps twitch force (Q_tw ) and voluntary activation (VA) were quantified at rest and throughout both trials. Decreases in Q_tw and VA from baseline were used to determine peripheral and central fatigue, respectively. Q_tw was strongly related to both [H⁺ ] (β coefficient: -0.9, P < 0.0001) and [Pi] (-1.1, P < 0.0001) across trials. There was an effect of trial on the relationship between Q_tw and [H⁺ ] (-0.5, P < 0.0001), but not Q_tw and [Pi] (0.0, P = 0.976). This suggests that, unlike the unaltered association with [Pi], a given level of peripheral fatigue was associated with a different [H⁺ ] in Trial 1 vs. Trial 2. VA was related to [H⁺ ] (-0.3, P < 0.0001), but not [Pi] (-0.2, P = 0.243), across trials and there was no effect of trial (-0.1, P = 0.483). Taken together, these results support intramuscular Pi as a primary cause of peripheral fatigue, and muscle acidosis, probably acting on group III/IV muscle afferents in the interstitial space, as a contributor to central fatigue during exercise. KEY POINTS: We investigated the relationship between intramuscular metabolites and neuromuscular function in humans performing two maximal, intermittent, knee-extension trials interspersed with 5 min of rest. Concomitant measurements of intramuscular hydrogen (H⁺ ) and inorganic phosphate (Pi) concentrations, as well as quadriceps twitch-force (Q_tw ) and voluntary activation (VA), were made throughout each trial using phosphorus magnetic resonance spectroscopy (³¹ P-MRS) and electrical femoral nerve stimulations. Although [Pi] fully recovered prior to the onset of the second trial, [H⁺ ] did not. Q_tw was strongly related to both [H⁺ ] and [Pi] across both trials. However, the relationship between Q_tw and [H⁺ ] shifted leftward from the first to the second trial, whereas the relationship between Q_tw and [Pi] remained unaltered. VA was related to [H⁺ ], but not [Pi], across both trials. These in vivo findings support the hypotheses of intramuscular Pi as a primary cause of peripheral fatigue, and muscle acidosis, probably acting on group III/IV muscle afferents, as a contributor to central fatigue.

Acute high-intensity exercise and skeletal muscle mitochondrial respiratory function: role of metabolic perturbation

Recently it was documented that fatiguing, high-intensity exercise resulted in a significant attenuation in maximal skeletal muscle mitochondrial respiratory capacity, potentially due to the intramuscular metabolic perturbation elicited by such intense exercise. With the utilization of intrathecal fentanyl to attenuate afferent feedback from group III/IV muscle afferents, permitting increased muscle activation and greater intramuscular metabolic disturbance, this study aimed to better elucidate the role of metabolic perturbation on mitochondrial respiratory function. Eight young, healthy males performed high-intensity cycle exercise in control (CTRL) and fentanyl-treated (FENT) conditions. Liquid chromatography-mass spectrometry and high-resolution respirometry were used to assess metabolites and mitochondrial respiratory function, respectively, pre- and postexercise in muscle biopsies from the vastus lateralis. Compared with CTRL, FENT yielded a significantly greater exercise-induced metabolic perturbation (PCr: -67% vs. -82%, Pi: 353% vs. 534%, pH: -0.22 vs. -0.31, lactate: 820% vs. 1,160%). Somewhat surprisingly, despite this greater metabolic perturbation in FENT compared with CTRL, with the only exception of respiratory control ratio (RCR) (-3% and -36%) for which the impact of FENT was significantly greater, the degree of attenuated mitochondrial respiratory capacity postexercise was not different between CTRL and FENT, respectively, as assessed by maximal respiratory flux through complex I (-15% and -33%), complex II (-36% and -23%), complex I + II (-31% and -20%), and state 3_CI+CII control ratio (-24% and -39%). Although a basement effect cannot be ruled out, this failure of an augmented metabolic perturbation to extensively further attenuate mitochondrial function questions the direct role of high-intensity exercise-induced metabolite accumulation in this postexercise response.

The Ryanodine Receptor as a Sensor for Intracellular Environments in Muscles

The ryanodine receptor (RyR) is a Ca²⁺ release channel in the sarcoplasmic reticulum of skeletal and cardiac muscles and plays a key role in excitation-contraction coupling. The activity of the RyR is regulated by the changes in the level of many intracellular factors, such as divalent cations (Ca²⁺ and Mg²⁺), nucleotides, associated proteins, and reactive oxygen species. Since these intracellular factors change depending on the condition of the muscle, e.g., exercise, fatigue, or disease states, the RyR channel activity will be altered accordingly. In this review, we describe how the RyR channel is regulated under various conditions and discuss the possibility that the RyR acts as a sensor for changes in the intracellular environments in muscles.

Exercise Physiology From 1980 to 2020: Application of the Natural Sciences

The field of exercise physiology has enjoyed tremendous growth in the past 40 years. With its foundations in the natural sciences, it is an interdisciplinary field that is highly relevant to human performance and health. The focus of this review is on highlighting new approaches, knowledge, and opportunities that have emerged in exercise physiology over the last four decades. Key among these is the adoption of advanced technologies by exercise physiologists to address fundamental research questions, and the expansion of research topics to range from molecular to organismal, and population scales in order to clarify the underlying mechanisms and impact of physiological responses to exercise in health and disease. Collectively, these advances have ensured the position of the field as a partner in generating new knowledge across many scientific and health disciplines.

Muscle Ionic Shifts During Exercise: Implications for Fatigue and Exercise Performance

Exercise causes major shifts in multiple ions (e.g., K⁺ , Na⁺ , H⁺ , lactate^- , Ca²⁺ , and Cl^- ) during muscle activity that contributes to development of muscle fatigue. Sarcolemmal processes can be impaired by the trans-sarcolemmal rundown of ion gradients for K⁺ , Na⁺ , and Ca²⁺ during fatiguing exercise, while changes in gradients for Cl^- and Cl^- conductance may exert either protective or detrimental effects on fatigue. Myocellular H⁺ accumulation may also contribute to fatigue development by lowering glycolytic rate and has been shown to act synergistically with inorganic phosphate (Pi) to compromise cross-bridge function. In addition, sarcoplasmic reticulum Ca²⁺ release function is severely affected by fatiguing exercise. Skeletal muscle has a multitude of ion transport systems that counter exercise-related ionic shifts of which the Na⁺ /K⁺ -ATPase is of major importance. Metabolic perturbations occurring during exercise can exacerbate trans-sarcolemmal ionic shifts, in particular for K⁺ and Cl^- , respectively via metabolic regulation of the ATP-sensitive K⁺ channel (K_ATP ) and the chloride channel isoform 1 (ClC-1). Ion transport systems are highly adaptable to exercise training resulting in an enhanced ability to counter ionic disturbances to delay fatigue and improve exercise performance. In this article, we discuss (i) the ionic shifts occurring during exercise, (ii) the role of ion transport systems in skeletal muscle for ionic regulation, (iii) how ionic disturbances affect sarcolemmal processes and muscle fatigue, (iv) how metabolic perturbations exacerbate ionic shifts during exercise, and (v) how pharmacological manipulation and exercise training regulate ion transport systems to influence exercise performance in humans. © 2021 American Physiological Society. Compr Physiol 11:1895-1959, 2021.

High-Resolution Dynamic 31P-MR Spectroscopic Imaging for Mapping Mitochondrial Function

OBJECTIVE

To enable non-invasive dynamic metabolic mapping in rodent model studies of mitochondrial function using 31P-MR spectroscopic imaging (MRSI).

METHODS

We developed a novel method for high-resolution dynamic 31P-MRSI. The method synergistically integrates physics-based models of spectral structures, biochemical modeling of molecular dynamics, and subspace learning to capture spatiospectral variations. Fast data acquisition was achieved using rapid spiral trajectories and sparse sampling of (k, t, T)-space; image reconstruction was accomplished using a low-rank tensor-based framework.

RESULTS

The proposed method provided high-resolution dynamic metabolic mapping in rat hindlimb at spatial and temporal resolutions of 4[Formula: see text]2 mm3 and 1.28 s, respectively. This allowed for in vivo mapping of the time-constant of phosphocreatine resynthesis, a well established index of mitochondrial oxidative capacity. Multiple rounds of in vivo experiments were performed to demonstrate reproducibility, and in vitro experiments were used to validate the accuracy of the estimated metabolite maps.

CONCLUSIONS

A new model-based method is proposed to achieve high-resolution dynamic 31P-MRSI. The proposed method's ability to delineate metabolic heterogeneity was demonstrated in rat hindlimb.

SIGNIFICANCE

Abnormal mitochondrial metabolism is a key cellular dysfunction in many prevalent diseases such as diabetes and heart disease; however, current understanding of mitochondrial function is mostly gained from studies on isolated mitochondria under nonphysiological conditions. The proposed method has the potential to open new avenues of research by allowing in vivo and longitudinal studies of mitochondrial dysfunction in disease development and progression.

Importance of Dietary Phosphorus for Bone Metabolism and Healthy Aging

Inorganic phosphate (Pi) plays a critical function in many tissues of the body: for example, as part of the hydroxyapatite in the skeleton and as a substrate for ATP synthesis. Pi is the main source of dietary phosphorus. Reduced bioavailability of Pi or excessive losses in the urine causes rickets and osteomalacia. While critical for health in normal amounts, dietary phosphorus is plentiful in the Western diet and is often added to foods as a preservative. This abundance of phosphorus may reduce longevity due to metabolic changes and tissue calcifications. In this review, we examine how dietary phosphorus is absorbed in the gut, current knowledge about Pi sensing, and endocrine regulation of Pi levels. Moreover, we also examine the roles of Pi in different tissues, the consequences of low and high dietary phosphorus in these tissues, and the implications for healthy aging.

ASIC1a does not play a role in evoking the metabolic component of the exercise pressor reflex in a rat model of peripheral artery disease

The role of the ASIC1a in evoking the exercise pressor reflex in rats with simulated peripheral artery disease is unknown. This prompted us to determine whether ASIC1a plays a role in evoking the exaggerated exercise pressor reflex in decerebrated rats with simulated peripheral artery disease. To simulate peripheral artery disease, we ligated the left femoral artery 72 h before the experiment. The right femoral artery was freely perfused and used as a control. To test our hypothesis, we measured the effect of injecting two ASIC1a blockers into the arterial supply of the triceps surae muscles with and without the femoral artery ligated on the reflex pressor responses to 1) static contraction of the triceps surae muscles, 2) calcaneal tendon stretch, and 3) intra-arterial injection of diprotonated phosphate (pH 6.0). We found that the ASIC1a blockers psalmotoxin-1 (200 ng/kg) and mambalgin-1 (6.5 μg/kg) decreased the pressor responses to static contraction as well as the peak pressor responses to injection of diprotonated phosphate when these responses were evoked from the freely perfused hindlimb. In contrast, ASIC1a blockers only decreased the peak pressor responses evoked by injection of diprotonated phosphate in the hindlimb circulation with simulated peripheral artery disease. This inhibitory effect was less than the one measured from the healthy hindlimb. Independently of the hindlimb of interest, ASIC1a blockers had no effect on the pressor responses to tendon stretch. Our results do not support the hypothesis that ASIC1a play a role in evoking the exercise pressor reflex arising from a hindlimb with simulated peripheral artery disease.NEW & NOTEWORTHY The role of ASIC1a in evoking the metabolic component of the exercise pressor reflex in peripheral artery disease is unknown. Using a within-rat experimental design, we found that the contribution of ASIC1a decreased in a rat model of peripheral artery disease. These results have key implications to help finding better treatments and improve morbidity, quality of life, and mortality in patients with peripheral artery disease.

Monitoring the training dose and acute fatigue response during elbow flexor resistance training using a custom-made resistance band

BACKGROUND

Home-based resistance training offers an alternative to traditional, hospital-based or rehabilitation center-based resistance training and has attracted much attention recently. However, without the supervision of a therapist or the assistance of an exercise monitoring system, one of the biggest challenges of home-based resistance training is that the therapist may not know if the patient has performed the exercise as prescribed. A lack of objective measurements limits the ability of researchers to evaluate the outcome of exercise interventions and choose suitable training doses.

OBJECTIVE

To create an automated and objective method for segmenting resistance force data into contraction phase-specific segments and calculate the repetition number and time-under-tension (TUT) during elbow flexor resistance training. A pilot study was conducted to evaluate the performance of the segmentation algorithm and to show the capability of the system in monitoring the compliance of patients to a prescribed training program in a practical resistance training setting.

METHODS

Six subjects (three male and three female) volunteered to participate in a fatigue and recovery experiment (5 min intermittent submaximal contraction (ISC); 1 min rest; 2 min ISC). A custom-made resistance band was used to help subjects perform biceps curl resistance exercises and the resistance was recorded through a load cell. The maximum and minimum values of the force-derivative were obtained as distinguishing features and a segmentation algorithm was proposed to divide the biceps curl cycle into concentric, eccentric and isometric contraction, and rest phases. Two assessors, who were unfamiliar with the study, were recruited to manually pick the visually observed cut-off point between two contraction phases and the TUT was calculated and compared to evaluate performance of the segmentation algorithm.

RESULTS

The segmentation algorithm was programmatically implemented and the repetition number and contraction-phase specific TUT were calculated. During isometric, the average TUT (3.75 ± 0.62 s) was longer than the prescribed 3 s, indicating that most subjects did not perform the exercise as prescribed. There was a good TUT agreement and contraction segment agreement between the proposed algorithm and the assessors.

CONCLUSION

The good agreement in TUT between the proposed algorithm and the assessors indicates that the proposed algorithm can correctly segment the contraction into contraction phase-specific parts, thereby providing clinicians and researchers with an automated and objective method for quantifying home-based elbow flexor resistance training. The instrument is easy to use and cheap, and the segmentation algorithm is programmatically implemented, indicating good application prospect of the method in a practical setting.

31 P magnetic resonance spectroscopy in skeletal muscle: Experts' consensus recommendations

Skeletal muscle phosphorus-31 31 P MRS is the oldest MRS methodology to be applied to in vivo metabolic research. The technical requirements of 31 P MRS in skeletal muscle depend on the research question, and to assess those questions requires understanding both the relevant muscle physiology, and how 31 P MRS methods can probe it. Here we consider basic signal-acquisition parameters related to radio frequency excitation, TR, TE, spectral resolution, shim and localisation. We make specific recommendations for studies of resting and exercising muscle, including magnetisation transfer, and for data processing. We summarise the metabolic information that can be quantitatively assessed with 31 P MRS, either measured directly or derived by calculations that depend on particular metabolic models, and we give advice on potential problems of interpretation. We give expected values and tolerable ranges for some measured quantities, and minimum requirements for reporting acquisition parameters and experimental results in publications. Reliable examination depends on a reproducible setup, standardised preconditioning of the subject, and careful control of potential difficulties, and we summarise some important considerations and potential confounders. Our recommendations include the quantification and standardisation of contraction intensity, and how best to account for heterogeneous muscle recruitment. We highlight some pitfalls in the assessment of mitochondrial function by analysis of phosphocreatine (PCr) recovery kinetics. Finally, we outline how complementary techniques (near-infrared spectroscopy, arterial spin labelling, BOLD and various other MRI and 1 H MRS measurements) can help in the physiological/metabolic interpretation of 31 P MRS studies by providing information about blood flow and oxygen delivery/utilisation. Our recommendations will assist in achieving the fullest possible reliable picture of muscle physiology and pathophysiology.

Dietary Nitrate and Physical Performance

Nitric oxide (NO) plays a plethora of important roles in the human body. Insufficient production of NO (for example, during older age and in various disease conditions) can adversely impact health and physical performance. In addition to its endogenous production through the oxidation of l-arginine, NO can be formed nonenzymatically via the reduction of nitrate and nitrite, and the storage of these anions can be augmented by the consumption of nitrate-rich foodstuffs such as green leafy vegetables. Recent studies indicate that dietary nitrate supplementation, administered most commonly in the form of beetroot juice, can ( a) improve muscle efficiency by reducing the O₂ cost of submaximal exercise and thereby improve endurance exercise performance and ( b) enhance skeletal muscle contractile function and thereby improve muscle power and sprint exercise performance. This review describes the physiological mechanisms potentially responsible for these effects, outlines the circumstances in which ergogenic effects are most likely to be evident, and discusses the effects of dietary nitrate supplementation on physical performance in a range of human populations.

Bioenergetic basis of skeletal muscle fatigue

Energetic demand from high-intensity exercise can easily exceed ATP synthesis rates of mitochondria leading to a reliance on anaerobic metabolism. The reliance on anaerobic metabolism results in the accumulation of intracellular metabolites, namely inorganic phosphate (P_i) and hydrogen (H⁺), that are closely associated with exercise-induced reductions in power. Cellular and molecular studies have revealed several steps where these metabolites impair contractile function demonstrating a causal role in fatigue. Elevated P_i or H⁺ directly inhibits force and power of the cross-bridge and decreases myofibrillar Ca²⁺ sensitivity, whereas P_i also inhibits Ca²⁺ release from the sarcoplasmic reticulum (SR). When both metabolites are elevated, they act synergistically to cause marked reductions in power, indicating that fatigue during high-intensity exercise has a bioenergetic basis.

Modification of a three-compartment muscle fatigue model to predict peak torque decline during intermittent tasks

This study aimed to test whether adding a rest recovery parameter, r, to the analytical three-compartment controller (3CC) fatigue model (Xia and Frey Law, 2008) will improve fatigue estimates during intermittent contractions. The 3CC muscle fatigue model uses differential equations to predict the flow of muscle between three muscle states: Resting (MR), Active (MA), and Fatigued (MF). This model uses a feedback controller to match the active state to target loads and two joint-specific parameters: F, fatigue rate controlling flow from active to fatigued compartments) and R, the recovery rate controlling flow from the fatigued to the resting compartments. This model does well to predict intensity-endurance time curves for sustained isometric tasks. However, previous studies find when rest intervals are present that the model over predicts fatigue. Intermittent rest periods would allow for the occurrence of subsequent reactive vasodilation and post-contraction hyperemia. We hypothesize a modified 3CC-r fatigue model will improve predictions of force decay during intermittent contractions with the addition of a rest recovery parameter, r, to augment recovery during rest intervals, representing muscle re-perfusion. A meta-analysis compiling intermittent fatigue data from 63 publications reporting decline in peak torque (% torque decline) were used for comparison. The original model over-predicted fatigue development from 19 to 29% torque decline; the addition of a rest multiplier significantly improved fatigue estimates to 6-10% torque decline. We conclude the addition of a rest multiplier to the three-compartment controller fatigue model provides a physiologically consistent modification for tasks involving rest intervals, resulting in improved estimates of muscle fatigue.

Effects of ipsilateral and contralateral fatigue and muscle blood flow occlusion on the complexity of knee-extensor torque output in humans

NEW FINDINGS

What is the central question of this study? We addressed the question "what role do central and peripheral fatigue mechanisms play in the fatigue-induced loss of isometric torque complexity?" What is the main finding and its importance? When the contralateral limb is fatigued, the complexity of isometric torque output is unaffected even if the blood flow to the contralateral limb is occluded, which suggests that neither central fatigue nor afferent feedback from ischaemic muscle influences the complexity of torque output in an otherwise fresh muscle.

ABSTRACT

Neuromuscular fatigue reduces the temporal structure, or complexity, of torque output during muscular contractions. To determine whether the fatigue-induced loss of torque complexity could be accounted for by central or peripheral factors, nine healthy participants performed four experimental trials involving intermittent isometric contractions of the knee extensors at 50% of the maximal voluntary contraction torque. These trials involved: (i) two bouts of contractions to failure using the right leg separated by 3 min recovery (IPS); (ii) the same protocol but with cuff occlusion during the 3 min recovery (IPS-OCC); (iii) contractions of the left leg to failure, followed 1 min later by contractions of the right leg to failure (CONT); and (iv) the same protocol but with cuff occlusion applied to the left leg throughout both the recovery period and right leg contractions (CONT-OCC). Supramaximal electrical stimulation during maximal voluntary contractions was used to determine the degree of central and peripheral fatigue, whilst complexity was determined using approximate entropy (ApEn) and detrended fluctuation analysis α exponent (DFA α). Neuromuscular fatigue was consistently associated with a loss of torque complexity in all conditions [e.g. IPS bout 1, ApEn from (mean ± SD) 0.46 ± 0.14 to 0.12 ± 0.06 (P < 0.001)]. In IPS-OCC, occlusion abolished the recovery from fatigue, and torque complexity remained at the values observed at task failure in the preceding bout (IPS-OCC bout 2, first minute 0.14 ± 0.03, P < 0.001). Prior contralateral contractions, with or without blood flow occlusion, had no effect on torque complexity.

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.

Bioenergetics and ATP Synthesis during Exercise: Role of Group III/IV Muscle Afferents

PURPOSE

The purpose of this study was to investigate the role of the group III/IV muscle afferents in the bioenergetics of exercising skeletal muscle beyond constraining the magnitude of metabolic perturbation.

METHODS

Eight healthy men performed intermittent isometric knee-extensor exercise to task failure at ~58% maximal voluntary contraction under control conditions (CTRL) and with lumbar intrathecal fentanyl to attenuate group III/IV leg muscle afferents (FENT). Intramuscular concentrations of phosphocreatine (PCr), inorganic phosphate (Pi), diprotonated phosphate (H2PO4), adenosine triphosphate (ATP), and pH were determined using phosphorous magnetic resonance spectroscopy (P-MRS).

RESULTS

The magnitude of metabolic perturbation was significantly greater in FENT compared with CTRL for [Pi] (37.8 ± 16.8 vs 28.6 ± 8.6 mM), [H2PO4] (24.3 ± 12.2 vs 17.9 ± 7.1 mM), and [ATP] (75.8% ± 17.5% vs 81.9% ± 15.8% of baseline), whereas there was no significant difference in [PCr] (4.5 ± 2.4 vs 4.4 ± 2.3 mM) or pH (6.51 ± 0.10 vs 6.54 ± 0.14). The rate of perturbation in [PCr], [Pi], [H2PO4], and pH was significantly faster in FENT compared with CTRL. Oxidative ATP synthesis was not significantly different between conditions. However, anaerobic ATP synthesis, through augmented creatine kinase and glycolysis reactions, was significantly greater in FENT than in CTRL, resulting in a significantly greater ATP cost of contraction (0.049 ± 0.016 vs 0.038 ± 0.010 mM·min·N).

CONCLUSION

Group III/IV muscle afferents not only constrain the magnitude of perturbation in intramuscular Pi, H2PO4, and ATP during small muscle mass exercise but also seem to play a role in maintaining efficient skeletal muscle contractile function in men.

Do naked mole rats accumulate a metabolic acidosis or an oxygen debt in severe hypoxia?

In severe hypoxia, most vertebrates increase anaerobic energy production, which results in the development of a metabolic acidosis and an O2 debt that must be repaid during reoxygenation. Naked mole rats (NMRs) are among the most hypoxia-tolerant mammals, capable of drastically reducing their metabolic rate in acute hypoxia; while staying active and alert. We hypothesized that a key component of remaining active is an increased reliance on anaerobic metabolism during severe hypoxia. To test this hypothesis, we exposed NMRs to progressive reductions in inspired O2 (9 to 3% O2) followed by reoxygenation (21% O2) and measured breathing frequency, heart rate, behavioural activity, body temperature, metabolic rate, and also metabolic substrates and pH in blood and tissues. We found that NMRs exhibit robust metabolic rate depression in acute hypoxia, accompanied by declines in all physiological and behavioural variables examined. However, blood and tissue pH were unchanged and tissue [ATP] and [phosphocreatine] were maintained. Naked mole rats increased their reliance on carbohydrates in hypoxia, and glucose was mobilized from the liver to the blood. Upon reoxygenation NMRs entered into a coma-like state for∼15-20 mins during which metabolic rate was negligible and body temperature remained suppressed. However, an imbalance in the rates at which V̇O2 and V̇CO2 returned to normoxic levels during reoxygenation hint at the possibility that NMRs do utilize anaerobic metabolism during hypoxia but have a tissue and/or blood buffering capacity that mask typical markers of metabolic acidosis, and prioritize the synthesis of glucose from lactate during recovery.

Muscle fatigue: general understanding and treatment

Muscle fatigue is a common complaint in clinical practice. In humans, muscle fatigue can be defined as exercise-induced decrease in the ability to produce force. Here, to provide a general understanding and describe potential therapies for muscle fatigue, we summarize studies on muscle fatigue, including topics such as the sequence of events observed during force production, in vivo fatigue-site evaluation techniques, diagnostic markers and non-specific but effective treatments.

Modulators of actin-myosin dissociation: basis for muscle type functional differences during fatigue

The muscle types present with variable fatigue tolerance, in part due to the myosin isoform expressed. However, the critical steps that define “fatigability” in vivo of fast vs. slow myosin isoforms, at the molecular level, are not yet fully understood. We examined the modulation of the ATP-induced myosin subfragment 1 (S1) dissociation from pyrene-actin by inorganic phosphate (P_i), pH, and temperature using a specially modified stopped-flow system that allowed fast kinetics measurements at physiological temperature. We contrasted the properties of rabbit psoas (fast) and bovine masseter (slow) myosins (obtained from samples collected from New Zealand rabbits and from a licensed abattoir, respectively, according to institutional and national ethics permits). To identify ATP cycling biochemical intermediates, we assessed ATP binding to a preequilibrated mixture of actomyosin and variable [ADP], pH (pH 7 vs. pH 6.2), and P_i (zero, 15, or 30 added mM P_i) in a range of temperatures (5 to 45°C). Temperature and pH variations had little, if any, effect on the ADP dissociation constant (K_ADP) for fast S1, but for slow S1, K_ADP was weakened with increasing temperature or low pH. In the absence of ADP, the dissociation constant for phosphate (K_Pi) was weakened with increasing temperature for fast S1. In the presence of ADP, myosin type differences were revealed at the apparent phosphate affinity, depending on pH and temperature. Overall, the newly revealed kinetic differences between myosin types could help explain the in vivo observed muscle type functional differences at rest and during fatigue.

Muscle and Limb Mechanics

Understanding of the musculoskeletal system has evolved from the collection of individual phenomena in highly selected experimental preparations under highly controlled and often unphysiological conditions. At the systems level, it is now possible to construct complete and reasonably accurate models of the kinetics and energetics of realistic muscles and to combine them to understand the dynamics of complete musculoskeletal systems performing natural behaviors. At the reductionist level, it is possible to relate most of the individual phenomena to the anatomical structures and biochemical processes that account for them. Two large challenges remain. At a systems level, neuroscience must now account for how the nervous system learns to exploit the many complex features that evolution has incorporated into muscle and limb mechanics. At a reductionist level, medicine must now account for the many forms of pathology and disability that arise from the many diseases and injuries to which this highly evolved system is inevitably prone. © 2017 American Physiological Society. Compr Physiol 7:429-462, 2017.

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.

Mechanisms of in vivo muscle fatigue in humans: investigating age-related fatigue resistance with a computational model

KEY POINTS

Muscle fatigue can be defined as the transient decrease in maximal force that occurs in response to muscle use. Fatigue develops because of a complex set of changes within the neuromuscular system that are difficult to evaluate simultaneously in humans. The skeletal muscle of older adults fatigues less than that of young adults during static contractions. The potential sources of this difference are multiple and intertwined. To evaluate the individual mechanisms of fatigue, we developed an integrative computational model based on neural, biochemical, morphological and physiological properties of human skeletal muscle. Our results indicate first that the model provides accurate predictions of fatigue and second that the age-related resistance to fatigue is due largely to a lower reliance on glycolytic metabolism during contraction. This model should prove useful for generating hypotheses for future experimental studies into the mechanisms of muscle fatigue.

ABSTRACT

During repeated or sustained muscle activation, force-generating capacity becomes limited in a process referred to as fatigue. Multiple factors, including motor unit activation patterns, muscle fibre contractile properties and bioenergetic function, can impact force-generating capacity and thus the potential to resist fatigue. Given that neuromuscular fatigue depends on interrelated factors, quantifying their independent effects on force-generating capacity is not possible in vivo. Computational models can provide insight into complex systems in which multiple inputs determine discrete outputs. However, few computational models to date have investigated neuromuscular fatigue by incorporating the multiple levels of neuromuscular function known to impact human in vivo function. To address this limitation, we present a computational model that predicts neural activation, biomechanical forces, intracellular metabolic perturbations and, ultimately, fatigue during repeated isometric contractions. This model was compared with metabolic and contractile responses to repeated activation using values reported in the literature. Once validated in this way, the model was modified to reflect age-related changes in neuromuscular function. Comparisons between initial and age-modified simulations indicated that the age-modified model predicted less fatigue during repeated isometric contractions, consistent with reports in the literature. Together, our simulations suggest that reduced glycolytic flux is the greatest contributor to the phenomenon of age-related fatigue resistance. In contrast, oxidative resynthesis of phosphocreatine between intermittent contractions and inherent buffering capacity had minimal impact on predicted fatigue during isometric contractions. The insights gained from these simulations cannot be achieved through traditional in vivo or in vitro experimentation alone.

Influence of blood flow occlusion on the development of peripheral and central fatigue during small muscle mass handgrip exercise

Critical power represents an important threshold for neuromuscular fatigue development and may, therefore, dictate intensities for which exercise tolerance is determined by the magnitude of fatigue accrued. Peripheral fatigue appears to be constant across O2 delivery conditions for large muscle mass exercise, but this consistency is equivocal for smaller muscle mass exercise. We sought to determine the influence of blood flow occlusion during handgrip exercise on neuromuscular fatigue development and to examine the relationship between neuromuscular fatigue development and W '. Blood flow occlusion influenced the development of both peripheral and central fatigue, thus providing further evidence that the magnitude of peripheral fatigue is not constant across O2 delivery conditions for small muscle mass exercise. W ' appears to be related to the magnitude of fatigue accrued during exercise, which may explain the reported consistency of intramuscular metabolic perturbations and work performed for severe-intensity exercise. The influence of the muscle metabolic milieu on peripheral and central fatigue is currently unclear. Moreover, the relationships between peripheral and central fatigue and the curvature constant (W ') have not been investigated. Six men (age: 25 ± 4 years, body mass: 82 ± 10 kg, height: 179 ± 4 cm) completed four constant power handgrip tests to exhaustion under conditions of control exercise (Con), blood flow occlusion exercise (Occ), Con with 5 min post-exercise blood flow occlusion (Con + Occ), and Occ with 5 min post-exercise blood flow occlusion (Occ + Occ). Neuromuscular fatigue measurements and W ' were obtained for each subject. Each trial resulted in significant peripheral and central fatigue. Significantly greater peripheral (79.7 ± 5.1% vs. 22.7 ± 6.0%) and central (42.6 ± 3.9% vs. 4.9 ± 2.0%) fatigue occurred for Occ than for Con. In addition, significantly greater peripheral (83.0 ± 4.2% vs. 69.0 ± 6.2%) and central (65.5 ± 14.6% vs. 18.6 ± 4.1%) fatigue occurred for Occ + Occ than for Con + Occ. W ' was significantly related to the magnitude of global (r = 0.91) and peripheral (r = 0.83) fatigue. The current findings demonstrate that blood flow occlusion exacerbated the development of both peripheral and central fatigue and that post-exercise blood flow occlusion prevented the recovery of both peripheral and central fatigue. Moreover, the current findings suggest that W ' may be determined by the magnitude of fatigue accrued during exercise.

The effect of resting blood flow occlusion on exercise tolerance and W'

It has previously been postulated that the anaerobic work capacity (W') may be utilized during resting blood flow occlusion in the absence of mechanical work. We tested the hypothesis that W' would not be utilized during an initial range of time following the onset of resting blood flow occlusion, after which W' would be utilized progressively more. Seven men completed blood flow occlusion constant power severe intensity handgrip exercise to task failure following 0, 300, 600, 900, and 1,200 s of resting blood flow occlusion. The work performed above critical power (CP) was not significantly different between the 0-, 300-, and 600-s conditions and was not significantly different from the total W' available. Significantly less work was performed above CP during the 1,200-s condition than the 900-s condition (P < 0.05), while both conditions were significantly less than the 0-, 300-, and 600-s conditions (P < 0.05). The work performed above CP during these conditions was significantly less than the total W' available (P < 0.05). The utilization of W' during resting blood flow occlusion did not begin until 751 ± 118 s, after which time W' was progressively utilized. The current findings demonstrate that W' is not utilized during the initial ∼751 s of resting blood flow occlusion, but is progressively utilized thereafter, despite no mechanical work being performed. Thus, the utilization of W' is not exclusive to exercise, and a constant amount of work that can be performed above CP is not the determining mechanism of W'.

Influence of blood flow occlusion on muscle oxygenation characteristics and the parameters of the power-duration relationship

It was previously (Monod H, Scherrer J. Ergonomics 8: 329-338, 1965) postulated that blood flow occlusion during exercise would reduce critical power (CP) to 0 Watts (W), while not altering the curvature constant (W'). We empirically assessed the influence of blood flow occlusion on CP, W', and muscle oxygenation characteristics. Ten healthy men (age: 24.8 ± 2.6 yr; height: 180 ± 5 cm; weight: 84.6 ± 10.1 kg) completed four constant-power handgrip exercise tests during both control blood flow (control) and blood flow occlusion (occlusion) for the determination of the power-duration relationship. Occlusion CP (-0.7 ± 0.4 W) was significantly (P < 0.001) lower than control CP (4.1 ± 0.7 W) and significantly (P < 0.001) lower than 0 W. Occlusion W' (808 ± 155 J) was significantly (P < 0.001) different from control W' (558 ± 129 J), and all 10 subjects demonstrated an increased occlusion W' with a mean increase of ∼49%. The present findings support the aerobic nature of CP. The findings also demonstrate that the amount of work that can be performed above CP is constant for a given condition, but can vary across conditions. Moreover, this amount of work that can be performed above CP does not appear to be the determinant of W', but rather a consequence of the depletion of intramuscular energy stores and/or the accumulation of fatigue-inducing metabolites, which limit exercise tolerance and determine W'.

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Fatigue-related firing of muscle nociceptors reduces voluntary activation of ipsilateral but not contralateral lower limb muscles

During fatiguing upper limb exercise, maintained firing of group III/IV muscle afferents can limit voluntary drive to muscles within the same limb. It is not known if this effect occurs in the lower limb. We investigated the effects of group III/IV muscle afferent firing from fatigued ipsilateral and contralateral extensor muscles and ipsilateral flexor muscles of the knee on voluntary activation of the knee extensors. In three experiments, we examined voluntary activation of the knee extensors by measuring changes in superimposed twitches evoked by femoral nerve stimulation. Subjects attended on 2 days for each experiment. On one day a sphygmomanometer cuff occluded blood flow of the fatigued muscles to maintain firing of group III/IV muscle afferents. After a 2-min extensor contraction (experiment 1; n = 9), mean voluntary activation was lower with than without maintained ischemia (47 ± 19% vs. 87 ± 8%, respectively; P < 0.001). After a 2-min knee flexor maximal voluntary contraction (MVC) (experiment 2; n = 8), mean voluntary activation was also lower with than without ischemia (59 ± 21% vs. 79 ± 9%; P < 0.01). After the contralateral (left) MVC (experiment 3; n = 8), mean voluntary activation of the right leg was similar with or without ischemia (92 ± 6% vs. 93 ± 4%; P = 0.65). After fatiguing exercise, activity in group III/IV muscle afferents reduces voluntary activation of the fatigued muscle and nonfatigued antagonist muscles in the same leg. However, group III/IV muscle afferents from the fatigued left leg had no effect on the unfatigued right leg. This suggests that any "crossover" of central fatigue in the lower limbs is not mediated by group III/IV muscle afferents.

Effect of age on in vivo oxidative capacity in two locomotory muscles of the leg

To determine the effects of age and sex on in vivo mitochondrial function of distinct locomotory muscles, the tibialis anterior (TA) and medial gastrocnemius (MG), of young (Y; 24 ± 3 years) and older (O; 69 ± 4) men (M) and women (W) of similar overall physical activity (PA) was compared. In vivo mitochondrial function was measured using phosphorus magnetic resonance spectroscopy, and PA and physical function were measured in all subjects. Overall PA was similar among the groups, although O (n = 17) had fewer daily minutes of moderate-to-vigorous PA (p = 0.001), and slowed physical function (p < 0.05 for all variables), compared with Y (n = 17). In TA, oxidative capacity (V max; mM s(-1)) was higher in O than Y (p < 0.001; Y = 0.90 ± 0.12; O = 1.12 ± 0.18). There was no effect of age in MG (p = 0.5; Y = 0.91 ± 0.17; O = 0.96 ± 0.24), but women had higher oxidative capacity than men (p = 0.007; M = 0.84 ± 0.18; W = 1.03 ± 0.18). In vivo mitochondrial function was preserved in healthy O men and women, despite lower intensity PA and physical function in this group. The extent to which compensatory changes in gait may be responsible for this preservation warrants further investigation. Furthermore, women had higher oxidative capacity in the MG, but not the TA.

Human skeletal muscle metabolic economy in vivo: effects of contraction intensity, age, and mobility impairment

We tested the hypothesis that older muscle has greater metabolic economy (ME) in vivo than young, in a manner dependent, in part, on contraction intensity. Twenty young (Y; 24±1 yr, 10 women), 18 older healthy (O; 73±2, 9 women) and 9 older individuals with mild-to-moderate mobility impairment (OI; 74±1, 7 women) received stimulated twitches (2 Hz, 3 min) and performed nonfatiguing voluntary (20, 50, and 100% maximal; 12 s each) isometric dorsiflexion contractions. Torque-time integrals (TTI; Nm·s) were calculated and expressed relative to maximal fat-free muscle cross-sectional area (cm2), and torque variability during voluntary contractions was calculated as the coefficient of variation. Total ATP cost of contraction (mM) was determined from flux through the creatine kinase reaction, nonoxidative glycolysis and oxidative phosphorylation, and used to calculate ME (Nm·s·cm(-2)·mM ATP(-1)). While twitch torque relaxation was slower in O and OI compared with Y (P≤0.001), twitch TTI, ATP cost, and economy were similar across groups (P≥0.15), indicating comparable intrinsic muscle economy during electrically induced isometric contractions in vivo. During voluntary contractions, normalized TTI and total ATP cost did not differ significantly across groups (P≥0.20). However, ME was lower in OI than Y or O at 20% and 50% MVC (P≤0.02), and torque variability was greater in OI than Y or O at 20% MVC (P≤0.05). These results refute the hypothesis of greater muscle ME in old age, and provide support for lower ME in impaired older adults as a potential mechanism or consequence of age-related reductions in functional mobility.

Mathematical simulation of energy expenditure and recovery during sprint cross-country skiing

Purpose

A cross-country sprint competition relies on maximal effort durations of 3–4 minutes. Significant anaerobic energy contribution is expected. Anaerobic energy contribution has been estimated in different sports to date from the accumulated O₂ deficit. However, the O₂-deficit model can be questioned. We investigate anaerobic energy contribution by applying other methods than the O₂ deficit.

Methods

Theoretical model development.

Results

For sprint cross-country competitions, the anaerobic energy contribution was 20%–25% independent of the employed mathematical model. Recovery times of a minimum 20 minutes were found to be required after sprint races to be sure that the performance in subsequent heats was not influenced.

Conclusion

The O₂-deficit model gave anaerobic energy results in agreement with other models from the literature. Recovery times of a minimum 20 minutes were found to be required after sprint races to be sure that the performance in subsequent heats was not influenced.

High-intensity interval training alters ATP pathway flux during maximal muscle contractions in humans

AIM

High-intensity interval training (HIT) results in potent metabolic adaptations in skeletal muscle; however, little is known about the influence of these adaptations on energetics in vivo. We used magnetic resonance spectroscopy to examine the effects of HIT on ATP synthesis from net PCr breakdown (ATPCK ), oxidative phosphorylation (ATPOX ) and non-oxidative glycolysis (ATPGLY ) in vivo in vastus lateralis during a 24-s maximal voluntary contraction (MVC).

METHODS

Eight young men performed 6 sessions of repeated, 30-s 'all-out' sprints on a cycle ergometer; measures of muscle energetics were obtained at baseline and after the first and sixth sessions.

RESULTS

Training increased peak oxygen consumption (35.8 ± 1.4 to 39.3 ± 1.6 mL min(-1) kg(-1) , P = 0.01) and exercise capacity (217.0 ± 11.0 to 230.5 ± 11.7 W, P = 0.04) on the ergometer, with no effects on total ATP production or force-time integral during the MVC. While ATP production by each pathway was unchanged after the first session, 6 sessions increased the relative contribution of ATPOX (from 31 ± 2 to 39 ± 2% of total ATP turnover, P < 0.001) and lowered the relative contribution from both ATPCK (49 ± 2 to 44 ± 1%, P = 0.004) and ATPGLY (20 ± 2 to 17 ± 1%, P = 0.03).

CONCLUSION

These alterations to muscle ATP production in vivo indicate that brief, maximal contractions are performed with increased support of oxidative ATP synthesis and relatively less contribution from anaerobic ATP production following training. These results extend previous reports of molecular and cellular adaptations to HIT and show that 6 training sessions are sufficient to alter in vivo muscle energetics, which likely contributes to increased exercise capacity after short-term HIT.

In vivo evidence of an age-related increase in ATP cost of contraction in the plantar flexor muscles

Impaired skeletal muscle efficiency potentially contributes to the age-related decline in exercise capacity and may explain the altered haemodynamic response to exercise in the elderly. Thus we examined whether (i) the ATP cost of contraction increases with age, and (ii) this results in altered convective O(2) delivery to maintain microvascular oxygenation in the calf muscle. To this aim, we used an integrative experimental approach combining (31)P-MRS (magnetic resonance spectroscopy), Doppler ultrasound imaging and NIRS (near-IR spectroscopy) during dynamic plantar flexion exercise at 40% of WR(max) (maximal power output) in 20 healthy young and 20 older subjects matched for physical activity. The ATP cost of contraction was significantly higher in the old (7.2±4.1 mM/min per W) compared with the young (2.4±1.9 mM/min per W; P<0.05) and this was only significantly correlated with the plantar flexion WR(max) value in the old subjects (r=-0.52; P<0.05). Even when differences in power output were taken into account, end-exercise blood flow (old, 259±168 ml/min per W and young, 134±40 ml/min per W; P<0.05) and convective O(2) delivery (old, 0.048±0.031 l/min per W and young, 0.026±0.008 l/min per W; P<0.05) were greater in the old in comparison with the young subjects. In contrast, the NIRS oxyhaemoglobin, deoxyhaemoglobin and microvascular oxygenation indices were not significantly different between the groups (P>0.05). Therefore the present study reveals that, although the peripheral haemodynamic responses to plantar flexion exercise appear to be appropriate, the elevated energy cost of contraction and associated reduction in the WR(max) value in this muscle group may play a role in limiting exercise capacity with age.

Fatigue-related firing of distal muscle nociceptors reduces voluntary activation of proximal muscles of the same limb

With fatiguing exercise, firing of group III/IV muscle afferents reduces voluntary activation and force of the exercised muscles. These afferents can also act across agonist/antagonist pairs, reducing voluntary activation and force in nonfatigued muscles. We hypothesized that maintained firing of group III/IV muscle afferents after a fatiguing adductor pollicis (AP) contraction would decrease voluntary activation and force of AP and ipsilateral elbow flexors. In two experiments (n = 10) we examined voluntary activation of AP and elbow flexors by measuring changes in superimposed twitches evoked by ulnar nerve stimulation and transcranial magnetic stimulation of the motor cortex, respectively. Inflation of a sphygmomanometer cuff after a 2-min AP maximal voluntary contraction (MVC) blocked circulation of the hand for 2 min and maintained firing of group III/IV muscle afferents. After a 2-min AP MVC, maximal AP voluntary activation was lower with than without ischemia (56.2 ± 17.7% vs. 76.3 ± 14.6%; mean ± SD; P < 0.05) as was force (40.3 ± 12.8% vs. 57.1 ± 13.8% peak MVC; P < 0.05). Likewise, after a 2-min AP MVC, elbow flexion voluntary activation was lower with than without ischemia (88.3 ± 7.5% vs. 93.6 ± 3.9%; P < 0.05) as was torque (80.2 ± 4.6% vs. 86.6 ± 1.0% peak MVC; P < 0.05). Pain during ischemia was reported as Moderate to Very Strong. Postfatigue firing of group III/IV muscle afferents from the hand decreased voluntary drive and force of AP. Moreover, this effect decreased voluntary drive and torque of proximal unfatigued muscles, the elbow flexors. Fatigue-sensitive group III/IV muscle nociceptors act to limit voluntary drive not only to fatigued muscles but also to unfatigued muscles within the same limb.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Firing of antagonist small-diameter muscle afferents reduces voluntary activation and torque of elbow flexors

During muscle fatigue, firing of small-diameter muscle afferents can decrease voluntary activation of the fatigued muscle. However, these afferents may have a more widespread effect on other muscles in the exercising limb. We examined if the firing of fatigue-sensitive afferents from elbow extensor muscles in the same arm reduces torque production and voluntary activation of elbow flexors. In nine subjects we examined voluntary activation of elbow flexors by measuring changes in superimposed twitches evoked by transcranial magnetic stimulation of the motor cortex during brief (2-3 s) maximal voluntary contractions (MVC). Inflation of a blood pressure cuff following a 2-min sustained MVC blocked blood flow to the fatigued muscle and maintained firing of small-diameter afferents. After a fatiguing elbow flexion contraction, maximal flexion torque was lower (26.0 ± 4.4% versus 67.9 ± 5.2% of initial maximal torque; means ± s.d.; P < 0.001) and superimposed twitches were larger (4.1 ± 1.1% versus 1.8 ± 0.2% ongoing MVC, P = 0.01) with than without ischaemia. After a fatiguing elbow extensor contraction, maximal flexion torque was also reduced (82.2 ± 4.9% versus 91.4 ± 2.3% of initial maximal torque; P = 0.007), superimposed twitches were larger (2.7 ± 0.7% versus 1.3 ± 0.2% ongoing MVC; P = 0.02) and voluntary activation lower (81.6 ± 8.2% versus 95.5 ± 6.9%; P = 0.04) with than without ischaemia. After a fatiguing contraction, voluntary drive to the fatigued muscles is reduced with continued input from small-diameter muscle afferents. Furthermore, fatigue of the elbow extensor muscles decreases voluntary drive to unfatigued elbow flexors of the same arm. Therefore, firing of small-diameter muscle afferents from one muscle can affect voluntary activation and hence torque generation of another muscle in the same limb.

Methods for assessing mitochondrial function in diabetes

A growing body of research is investigating the potential contribution of mitochondrial function to the etiology of type 2 diabetes. Numerous in vitro, in situ, and in vivo methodologies are available to examine various aspects of mitochondrial function, each requiring an understanding of their principles, advantages, and limitations. This review provides investigators with a critical overview of the strengths, limitations and critical experimental parameters to consider when selecting and conducting studies on mitochondrial function. In vitro (isolated mitochondria) and in situ (permeabilized cells/tissue) approaches provide direct access to the mitochondria, allowing for study of mitochondrial bioenergetics and redox function under defined substrate conditions. Several experimental parameters must be tightly controlled, including assay media, temperature, oxygen concentration, and in the case of permeabilized skeletal muscle, the contractile state of the fibers. Recently developed technology now offers the opportunity to measure oxygen consumption in intact cultured cells. Magnetic resonance spectroscopy provides the most direct way of assessing mitochondrial function in vivo with interpretations based on specific modeling approaches. The continuing rapid evolution of these technologies offers new and exciting opportunities for deciphering the potential role of mitochondrial function in the etiology and treatment of diabetes.

Influence of intermittent hypoxic training on muscle energetics and exercise tolerance

Intermittent hypoxic training (IHT) is sometimes used by athletes to enhance nonhematological physiological adaptations to simulated altitude. We investigated whether IHT would result in greater improvements in muscle energetics and exercise tolerance compared with work-matched intermittent normoxic training (INT). Nine physically active men completed 3 wk of intensive, single-leg knee-extensor exercise training. Each training session consisted of 25 min of IHT (FiO2 14.5 ± 0.1%) with the experimental leg and 25 min of INT with the alternate leg, which served as a control. Before and after the training intervention, subjects completed a test protocol consisting of a bout of submaximal constant-work-rate exercise, a 24-s high-intensity exercise bout to quantify the phosphocreatine recovery time constant ([PCr]-τ), and an incremental test to the limit of tolerance. The tests were completed in normoxia and hypoxia in both INT and IHT legs. Muscle metabolism was assessed noninvasively using 31P-magnetic resonance spectroscopy. Improvements in the time-to-exhaustion during incremental exercise were not significantly different between training conditions either in normoxia (INT, 28 ± 20% vs. IHT, 25 ± 9%; P = 0.86) or hypoxia (INT, 21 ± 10% vs. IHT, 15 ± 11%; P = 0.29). In hypoxia, [PCr]-τ was speeded slightly but significantly more post-IHT compared with post-INT (−7.3 ± 2.9 s vs. −3.7 ± 1.7 s; P < 0.01), but changes in muscle metabolite concentrations during exercise were essentially not different between IHT and INT. Under the conditions of this investigation, IHT does not appreciably alter muscle metabolic responses or incremental exercise performance compared with INT.

A computational model of torque generation: neural, contractile, metabolic and musculoskeletal components

The pathway of voluntary joint torque production includes motor neuron recruitment and rate-coding, sarcolemmal depolarization and calcium release by the sarcoplasmic reticulum, force generation by motor proteins within skeletal muscle, and force transmission by tendon across the joint. The direct source of energetic support for this process is ATP hydrolysis. It is possible to examine portions of this physiologic pathway using various in vivo and in vitro techniques, but an integrated view of the multiple processes that ultimately impact joint torque remains elusive. To address this gap, we present a comprehensive computational model of the combined neuromuscular and musculoskeletal systems that includes novel components related to intracellular bioenergetics function. Components representing excitatory drive, muscle activation, force generation, metabolic perturbations, and torque production during voluntary human ankle dorsiflexion were constructed, using a combination of experimentally-derived data and literature values. Simulation results were validated by comparison with torque and metabolic data obtained in vivo. The model successfully predicted peak and submaximal voluntary and electrically-elicited torque output, and accurately simulated the metabolic perturbations associated with voluntary contractions. This novel, comprehensive model could be used to better understand impact of global effectors such as age and disease on various components of the neuromuscular system, and ultimately, voluntary torque output.

The multiple roles of phosphate in muscle fatigue

Muscle fatigue is the decline in performance of muscles observed during periods of intense activity. ATP consumption exceeds production during intense activity and there are multiple changes in intracellular metabolites which may contribute to the changes in crossbridge activity. It is also well-established that a reduction in activation, either through action potential changes or reduction in Ca(2+) release from the sarcoplasmic reticulum (SR), makes an additional contribution to fatigue. In this review we focus on the role of intracellular inorganic phosphate (P(i)) whose concentration can increase rapidly from around 5-30 mM during intense fatigue. Studies from skinned muscle fibers show that these changes substantially impair myofibrillar performance although the effects are strongly temperature dependent. Increased P(i) can also cause reduced Ca(2+) release from the SR and may therefore contribute to the reduced activation. In a recent study, we have measured both P(i) and Ca(2+) release in a blood-perfused mammalian preparation and the results from this preparation allows us to test the extent to which the combined effects of P(i) and Ca(2+) changes may contribute to fatigue.

On the kinetics of anaerobic power

BACKGROUND

This study investigated two different mathematical models for the kinetics of anaerobic power. Model 1 assumes that the work power is linear with the work rate, while Model 2 assumes a linear relationship between the alactic anaerobic power and the rate of change of the aerobic power. In order to test these models, a cross country skier ran with poles on a treadmill at different exercise intensities. The aerobic power, based on the measured oxygen uptake, was used as input to the models, whereas the simulated blood lactate concentration was compared with experimental results. Thereafter, the metabolic rate from phosphocreatine break down was calculated theoretically. Finally, the models were used to compare phosphocreatine break down during continuous and interval exercises.

RESULTS

Good similarity was found between experimental and simulated blood lactate concentration during steady state exercise intensities. The measured blood lactate concentrations were lower than simulated for intensities above the lactate threshold, but higher than simulated during recovery after high intensity exercise when the simulated lactate concentration was averaged over the whole lactate space. This fit was improved when the simulated lactate concentration was separated into two compartments; muscles + internal organs and blood. Model 2 gave a better behavior of alactic energy than Model 1 when compared against invasive measurements presented in the literature. During continuous exercise, Model 2 showed that the alactic energy storage decreased with time, whereas Model 1 showed a minimum value when steady state aerobic conditions were achieved. During interval exercise the two models showed similar patterns of alactic energy.

CONCLUSIONS

The current study provides useful insight on the kinetics of anaerobic power. Overall, our data indicate that blood lactate levels can be accurately modeled during steady state, and suggests a linear relationship between the alactic anaerobic power and the rate of change of the aerobic power.

Muscle metabolic status and acid-base balance during 10-s work:5-s recovery intermittent and continuous exercise

Gastrocnemius muscle phosphocreatine ([PCr]) and hydrogen ion ([H(+)]) were measured using (31)P-magnetic resonance spectroscopy during repeated bouts of 10-s heavy-intensity (HI) exercise and 5-s rest compared with continuous (CONT) HI exercise. Recreationally active male subjects (n = 7; 28 yr ± 9 yr) performed on separate occasions 12 min of isotonic plantar flexion (0.75 Hz) CONT and intermittent (INT; 10-s exercise, 5-s rest) exercise. The HI power output in both CONT and INT was set at 50% of the difference between the power output associated with the onset of intracellular acidosis and peak exercise determined from a prior incremental plantar flexion protocol. Intracellular concentrations of [PCr] and [H(+)] were calculated at 4 s and 9 s of the work period and at 4 s of the rest period in INT and during CONT exercise. [PCr] and [H(+)] (mean ± SE) were greater at 4 s of the rest periods vs. 9 s of exercise over the course of the INT exercise bout: [PCr] (20.7 mM ± 0.6 vs. 18.7 mM ± 0.5; P < 0.01); [H(+)] (370 nM ± 13.50 vs. 284 nM ± 13.6; P < 0.05). Average [H(+)] was similar for CONT vs. INT. We therefore suggest that there is a glycolytic contribution to ATP recovery during the very short rest period (<5 s) of INT and that the greater average power output of CONT did not manifest in greater [H(+)] and greater glycolytic contribution compared with INT exercise.

The kinetics of lactate production and removal during whole-body exercise

BACKGROUND

Based on a literature review, the current study aimed to construct mathematical models of lactate production and removal in both muscles and blood during steady state and at varying intensities during whole-body exercise. In order to experimentally test the models in dynamic situations, a cross-country skier performed laboratory tests while treadmill roller skiing, from where work rate, aerobic power and blood lactate concentration were measured. A two-compartment simulation model for blood lactate production and removal was constructed.

RESULTS

The simulated and experimental data differed less than 0.5 mmol/L both during steady state and varying sub-maximal intensities. However, the simulation model for lactate removal after high exercise intensities seems to require further examination.

CONCLUSIONS

Overall, the simulation models of lactate production and removal provide useful insight into the parameters that affect blood lactate response, and specifically how blood lactate concentration during practical training and testing in dynamical situations should be interpreted.

Dietary nitrate reduces muscle metabolic perturbation and improves exercise tolerance in hypoxia

Exercise in hypoxia is associated with reduced muscle oxidative function and impaired exercise tolerance. We hypothesised that dietary nitrate supplementation (which increases plasma [nitrite] and thus NO bioavailability) would ameliorate the adverse effects of hypoxia on muscle metabolism and oxidative function. In a double-blind, randomised crossover study, nine healthy subjects completed knee-extension exercise to the limit of tolerance (T(lim)), once in normoxia (20.9% O(2); CON) and twice in hypoxia (14.5% O(2)). During 24 h prior to the hypoxia trials, subjects consumed 0.75 L of nitrate-rich beetroot juice (9.3 mmol nitrate; H-BR) or 0.75 L of nitrate-depleted beetroot juice as a placebo (0.006 mmol nitrate; H-PL). Muscle metabolism was assessed using calibrated (31)P-MRS. Plasma [nitrite] was elevated (P < 0.01) following BR (194 ± 51 nm) compared to PL (129 ± 23 nm) and CON (142 ± 37 nM). T(lim) was reduced in H-PL compared to CON (393 ± 169 vs. 471 ± 200 s; P < 0.05) but was not different between CON and H-BR (477 ± 200 s). The muscle [PCr], [P(i)] and pH changed at a faster rate in H-PL compared to CON and H-BR. The [PCr] recovery time constant was greater (P < 0.01) in H-PL (29 ± 5 s) compared to CON (23 ± 5 s) and H-BR (24 ± 5 s). Nitrate supplementation reduced muscle metabolic perturbation during exercise in hypoxia and restored exercise tolerance and oxidative function to values observed in normoxia. The results suggest that augmenting the nitrate-nitrite-NO pathway may have important therapeutic applications for improving muscle energetics and functional capacity in hypoxia.

Effect of ischemic preconditioning in skeletal muscle measured by functional magnetic resonance imaging and spectroscopy: a randomized crossover trial

BACKGROUND

Nuclear magnetic resonance (NMR) imaging and spectroscopy have been applied to assess skeletal muscle oxidative metabolism. Therefore, in-vivo NMR may enable the characterization of ischemia-reperfusion injury. The goal of this study was to evaluate whether NMR could detect the effects of ischemic preconditioning (IPC) in healthy subjects.

METHODS

Twenty-three participants were included in two randomized crossover protocols in which the effects of IPC were measured by NMR and muscle force assessments. Leg ischemia was administered for 20 minutes with or without a subsequent impaired reperfusion for 5 minutes (stenosis model). IPC was administered 4 or 48 hours prior to ischemia. Changes in 31phosphate NMR spectroscopy and blood oxygen level-dependent (BOLD) signals were recorded. 3-Tesla NMR data were compared to those obtained for isometric muscular strength.

RESULTS

The phosphocreatine (PCr) signal decreased robustly during ischemia and recovered rapidly during reperfusion. In contrast to PCr, the recovery of muscular strength was slow. During post-ischemic stenosis, PCr increased only slightly. The BOLD signal intensity decreased during ischemia, ischemic exercise and post-ischemic stenosis but increased during hyperemic reperfusion. IPC 4 hours prior to ischemia significantly increased the maximal PCr reperfusion signal and mitigated the peak BOLD signal during reperfusion.

CONCLUSIONS

Ischemic preconditioning positively influenced muscle metabolism during reperfusion; this resulted in an increase in PCr production and higher oxygen consumption, thereby mitigating the peak BOLD signal. In addition, an impairment of energy replenishment during the low-flow reperfusion was detected in this model. Thus, functional NMR is capable of characterizing changes in reperfusion and in therapeutic interventions in vivo.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT00883467.

Age, muscle fatigue, and walking endurance in pre-menopausal women

Aging is associated with loss of endurance; however, aging is also associated with decreased fatigue during maximal isometric contractions. The aims of this study were to examine the relationship between age and walking endurance (WE) and maximal isometric fatigue (MIF) and to determine which metabolic/fitness components explain the expected age effects on WE and MIF. Subjects were 96 pre-menopausal women. Oxygen uptake (walking economy) was assessed during a 3-mph walk; aerobic capacity and WE by progressive treadmill test; knee extension strength by isometric contractions, MIF during a 90-s isometric plantar flexion (muscle metabolism measured by (31)P MRS). Age was related to increased walking economy (low VO(2), r = -0.19, P < 0.03) and muscle metabolic economy (force/ATP, 0.34, P = 0.01), and reduced MIF (-0.26, P < 0.03). However, age was associated with reduced WE (-0.28, P < 0.01). Multiple regression showed that muscle metabolic economy explained the age-related decrease in MIF (partial r for MIF and age -0.13, P = 0.35) whereas walking economy did not explain the age-related decrease in WE (partial r for WE and age -0.25, P < 0.02). Inclusion of VO(2max) and knee endurance strength accounted for the age-related decreased WE (partial r for WE and age = 0.03, P > 0.80). In premenopausal women, age is related to WE and MIF. In addition, these results support the hypothesis that age-related increases in metabolic economy may decrease MIF. However, decreased muscle strength and oxidative capacity are related to WE.

Intracellular energetics and critical PO2 in resting ischemic human skeletal muscle in vivo

During ischemia and some types of muscular contractions, oxygen tension (Po(2)) declines to the point that mitochondrial ATP synthesis becomes limited by oxygen availability. Although this critical Po(2) has been determined in animal tissue in vitro and in situ, there remains controversy concerning potential disparities between values measured in vivo and ex vivo. To address this issue, we used concurrent heteronuclear magnetic resonance spectroscopy (MRS) to determine the critical intracellular Po(2) in resting human skeletal muscle in vivo. We interleaved measurements of deoxymyoglobin using (1)H-MRS with measures of high-energy phosphates and pH using (31)P-MRS, during 15 min of ischemia in the tibialis anterior muscles of 6 young men. ATP production and intramyocellular Po(2) were quantified throughout ischemia. Critical Po(2), determined as the Po(2) corresponding to the point where PCr begins to decline (PCr(ip)) in resting muscle during ischemia, was 0.35 ± 0.20 Torr, means ± SD. This in vivo value is consistent with reported values ex vivo and does not support the notion that critical Po(2) in resting muscle is higher when measured in vivo. Furthermore, we observed a 4.5-fold range of critical Po(2) values among the individuals studied. Regression analyses revealed that time to PCr(ip) was associated with critical Po(2) and the rate of myoglobin desaturation (r = 0.83, P = 0.04) but not the rate of ATP consumption during ischemia. The apparent dissociation between ATP demand and myoglobin deoxygenation during ischemia suggests that some degree of uncoupling between intracellular energetics and oxygenation is a potentially important factor that influences critical Po(2) in vivo.