A cross-validation of near-infrared spectroscopy measurements of skeletal muscle oxidative capacity with phosphorus magnetic resonance spectroscopy

Impact of sprint interval training on post-fatigue mitochondrial rate in professional boxers

PURPOSE

Professional boxing is a sport that requires a high aerobic capacity to prevent fatigue and allow athletes to perform over 4-12 rounds. Typically, athletes will go into a heavy training period in a pre-bout camp lasting 6 to 9 weeks. This study investigates the impact of 3 weeks of repeated Wingate sprint interval training, performed on standard gym ergometer bikes, on skeletal muscle endurance and mitochondrial function.

METHODS

Ten male professional boxers (age: 26 ± 4 years, height: 175 ± 5 cm, weight: 70 ± 5 kg) participated in the study. Baseline testing involved a NIRS monitor attached to the rectus femoris muscle prior to an incremental time to exhaustion test on a treadmill. After the treadmill test participants underwent a series of arterial occlusions to determine mitochondrial function post-volitional exhaustion. Participants then continued their own training for 3 weeks and then repeated baseline testing. After the second testing session, participants undertook three weekly sprint sessions consisting of 3 × 30 s maximal sprints with 60 s recovery. Testing was repeated 3 weeks later.

RESULTS

The time to exhaustion increased by > 6% after 3 weeks of sprint interval training as compared to baseline and control (p < 0.05). Skeletal muscle oxygen saturation (SmO2) at exhaustion was increased by 5.5% after 3 weeks of sprint interval training as compared to baseline and control (p = 0.008). Skeletal muscle mitochondrial rate post exhaustion was increased by 160% after 3 weeks of sprint interval training as compared to baseline and control (p < 0.001).

CONCLUSION

The study demonstrated that SIT led to increased incremental time to exhaustion, higher SmO2 levels at volitional exhaustion and increased mitochondrial rates in professional boxers. These findings suggest that SIT should be an integral part of a boxe's conditioning regimen to improve performance and safety within the ring.

Daily Physical Activity Does Not Contribute to Differences in Muscle Oxidative Capacity Between Overweight and Obesity

BACKGROUND

The interaction between physical activity, skeletal muscle health, and adiposity has been explored in normal weight and overweight/obesity grouped together; however, the overall risks associated with being overweight are less than those observed with obesity and can be confounded by disparities in both sex and race. Thus, the present study sought to investigate the intricate interplay of daily physical activity and skeletal muscle oxidative capacity (SMOC) in overweight and obesity, while exploring how sex and race impact this dynamic relationship.

METHODS

One hundred and forty participants were grouped by body mass index (BMI) as overweight (n = 73; BMI >25-<30 kg/m2) or obese (n = 67; BMI ≥30 kg/m2). SMOC was assessed using near-infrared spectroscopy and daily physical activity was assessed for 7 days using accelerometry.

RESULTS

Overweight individuals exhibited a higher (p = 0.004) SMOC and engaged in more (p = 0.007) vigorous physical activity compared to obese individuals. In addition, SMOC was lower (p = 0.005) in obese non-Hispanic Black (NHB) men compared to overweight NHB men. No relationships between physical activity and SMOC were observed.

CONCLUSION

Physical activity is not associated with differences in SMOC in overweight and obesity. Obese individuals engage in less vigorous physical activity and exhibit lower SMOC compared to overweight individuals and these differences are emphasised in NHB men.

Muscle reoxygenation is slower after higher cycling intensity, and is faster and more reliable in locomotor than in accessory muscle sites

Introduction

Wearable near-infrared spectroscopy (NIRS) can be used during dynamic exercise to reflect the balance of muscle oxygen delivery and uptake. This study describes the behaviour and reliability of postexercise reoxygenation with NIRS as a function of exercise intensity at four muscle sites during an incremental cycling test. We discuss physiological components of faster and slower reoxygenation kinetics in the context of sport science and clinical applications. We hypothesised that reoxygenation would be slower at higher intensity, and that locomotor muscles would be faster than accessory muscles. We quantified test-retest reliability and agreement for each site.

Methods

Twenty-one trained cyclists performed two trials of an incremental cycling protocol with 5-min work stages and 1-min rest between stages. NIRS was recorded from the locomotor vastus lateralis and rectus femoris muscles, and accessory lumbar paraspinal and lateral deltoid muscles. Reoxygenation time course was analysed as the half-recovery time (HRT) from the end of work to half of the peak reoxygenation amplitude during rest. Coefficient of variability (CV) between participants, standard error of the measurement (SEM) within participants, and intraclass correlation coefficient (ICC) for test-retest reliability were evaluated at 50%, 75%, and 100% peak workloads. A linear mixed-effects model was used to compare differences between workloads and muscle sites.

Results

HRT was slower with increasing workload in the VL, RF, and PS, but not DL. VL had the fastest reoxygenation (lowest HRT) across muscle sites at all workloads (HRT = 8, 12, 17 s at 50%, 75%, 100% workload, respectively). VL also had the greatest reliability and agreement. HRT was sequentially slower between muscle sites in the order of VL < RF < PS < DL, and reliability was lower than for the VL.

Discussion

This study highlights the potential for using wearable NIRS on multiple muscle sites during exercise. Reoxygenation kinetics differ between local muscle sites with increasing intensity. Moderate-to-good reliability in the VL support its increasing use in sport science and clinical applications. Lower reliability in other muscle sites suggest they are not appropriate to be used alone, but may add information when combined to better reflect systemic intensity and fatigue during exercise at different intensities.

A polyphenol-rich cranberry supplement improves muscle oxidative capacity in healthy adults

Cranberries are rich in polyphenols, have a high antioxidant capacity, and may protect against exercise-induced free radical production. Mitochondria are known producers of free radical in skeletal muscle, and preventing overproduction of radicals may be a viable approach to improve muscle health. This study aimed to investigate the effect of a polyphenol-rich cranberry extract (CE) on muscle oxidative capacity and oxygenation metrics in healthy active adults. 17 participants (9 males and 8 females) were tested at: (i) baseline, (ii) 2 h following an acute CE dose (0.7 g/kg of body mass), and (iii) after 4 weeks of daily supplement consumption (0.3 g/kg of body mass). At each time point, muscle oxidative capacity was determined using near-infrared spectroscopy to measure the recovery kinetics of muscle oxygen consumption following a 15-20 s contraction of the vastus lateralis. Cranberry supplementation over 28 days significantly improved muscle oxidative capacity (k-constant, 2.8 ± 1.8 vs. 3.9 ± 2.2; p = 0.02). This was supported by a greater rate of oxygen depletion during a sustained cuff occlusion (-0.04 ± 0.02 vs. -0.07 ± 0.03; p = 0.02). Resting muscle oxygen consumption was not affected by cranberry consumption. Our results suggest that cranberry supplementation may play a role in improving mitochondrial health, which could lead to better muscle oxidative capacity in healthy active adult populations.

Methodological considerations on near-infrared spectroscopy derived muscle oxidative capacity

PURPOSE

Different strategies for near-infrared spectroscopy (NIRS)-derived muscle oxidative capacity assessment have been reported. This study compared and evaluated (I) approaches for averaging trials; (II) NIRS signals and blood volume correction equations; (III) the assessment of vastus lateralis (VL) and tibialis anterior (TA) muscles in two fitness levels groups.

METHODS

Thirty-six participants [18 chronically trained (CT: 14 males, 4 females) and 18 untrained (UT: 10 males, 8 females)] participated in this study. Two trials of twenty transient arterial occlusions were performed for NIRS-derived muscle oxidative capacity assessment. Muscle oxygen consumption ( V ˙ O2m) was estimated from deoxygenated hemoglobin (HHb), corrected for blood volume changes following Ryan (HHbR) and Beever (HHbB) equations, and from oxygen saturation (StO2) in VL and TA.

RESULTS

Superimposing or averaging V ˙ O2m or averaging the rate constants (k) from the two trials resulted in equivalent k values [two one-sided tests (TOST) procedure with 5% equivalence margin-P < 0.001]. Whereas HHbR (2.35 ± 0.61 min-1) and HHbB (2.34 ± 0.58 min-1) derived k were equivalent (P < 0.001), StO2 derived k (2.81 ± 0.92 min-1) was greater (P < 0.001) than both. k values were greater in CT vs UT in both muscles (VL: + 0.68 min-1, P = 0.002; TA: + 0.43 min-1, P = 0.01).

CONCLUSION

Different approaches for averaging trials lead to similar k. HHb and StO2 signals provided different k, although different blood volume corrections did not impact k. Group differences in k were detected in both muscles.

Impact of low-load resistance exercise with and without blood flow restriction on muscle strength, endurance, and oxidative capacity: A pilot study

Low-load resistance exercise (LLRE) to failure can increase muscle mass, strength, endurance, and mitochondrial oxidative capacity (OXPHOS). However, the impact of adding blood flow restriction to low-load resistance exercise (LLBFR) when matched for volume on these outcomes is incompletely understood. This pilot study examined the impact of 6 weeks of single-legged LLBFR and volume-matched LLRE on thigh bone-free lean mass, strength, endurance, and mitochondrial OXPHOS. Twenty (12 males and 8 females) untrained young adults (mean ± SD; 21 ± 2 years, 168 ± 11 cm, 68 ± 12 kg) completed 6 weeks of either single-legged LLBFR or volume-matched LLRE. Participants performed four sets of 30, 15, 15, and 15 repetitions at 25% 1-RM of leg press and knee extension with or without BFR three times per week. LLBFR increased knee extension 1-RM, knee extension endurance, and thigh bone-free lean mass relative to control (all p < 0.05). LLRE increased leg press and knee extension 1-RM relative to control (p = 0.012 and p = 0.054, respectively). LLRE also increased mitochondrial OXPHOS (p = 0.047 (nonparametric)). Our study showed that LLBFR increased muscle strength, muscle endurance, and thigh bone-free lean mass in the absence of improvements in mitochondrial OXPHOS. LLRE improved muscle strength and mitochondrial OXPHOS in the absence of improvements in thigh bone-free lean mass or muscle endurance.

The addition of blood flow restriction during resistance exercise does not increase prolonged low-frequency force depression

At a given exercise intensity, blood flow restriction (BFR) reduces the volume of exercise required to impair post-exercise neuromuscular function. Compared to traditional exercise, the time course of recovery is less clear. After strenuous exercise, force output assessed with electrical muscle stimulation is impaired to a greater extent at low versus high stimulation frequencies, a condition known as prolonged low-frequency force depression (PLFFD). It is unclear if BFR increases PLFFD after exercise. This study tested if BFR during exercise increases PLFFD and slows recovery of neuromuscular function compared to regular exercise. Fifteen physically active participants performed six low-load sets of knee-extensions across four conditions: resistance exercise to task failure (RETF), resistance exercise to task failure with BFR applied continuously (BFRCONT) or intermittently (BFRINT), and resistance exercise matched to the lowest exercise volume condition (REVM). Maximal voluntary contraction (MVC) force output, voluntary activation and a force-frequency (1-100 Hz) curve were measured before and 0, 1, 2, 3, 4 and 24 h after exercise. Exercise to task failure caused similar reductions at 0 h for voluntary activation (RETF = 81.0 ± 14.2%, BFRINT = 80.9 ± 12.4% and BFRCONT = 78.6 ± 10.7%) and MVC force output (RETF = 482 ± 168 N, BFRINT = 432 ± 174 N, and BFRCONT = 443 ± 196 N), which recovered to baseline values between 4 and 24 h. PLFFD occurred only after RETF at 1 h supported by a higher frequency to evoke 50% of the force production at 100 Hz (1 h: 17.5 ± 4.4 vs. baseline: 15 ± 4.1 Hz, P = 0.0023), BFRINT (15.5 ± 4.0 Hz; P = 0.03), and REVM (14.9 ± 3.1 Hz; P = 0.002), with a trend versus BFRCONT (15.7 ± 3.5 Hz; P = 0.063). These findings indicate that, in physically active individuals, using BFR during exercise does not impair the recovery of neuromuscular function by 24 h post-exercise.

Short-term cutaneous vasodilatory and thermosensory effects of topical methyl salicylate

Introduction

Methyl salicylate, the main compound of wintergreen oil, is widely used in topical applications. However, its vascular and thermosensory effects are not fully understood. The primary aim was to investigate the effects of topical methyl salicylate on skin temperature (Tskin), skin microcirculation (MCskin) and muscle oxygen saturation (SmO2) compared to a placebo gel. The secondary aim was to assess thermosensory responses (thermal sensation, thermal comfort) and to explore to which extent these sensations correspond to the physiological responses over time.

Methods

21 healthy women (22.2 ± 2.9 years) participated in this single-blind, randomized controlled trial. Custom-made natural wintergreen oil (12.9%), containing methyl salicylate (>99%) and a placebo gel, 1 g each, were applied simultaneously to two paravertebral skin areas (5 cm × 10 cm, Th4-Th7). Tskin (infrared thermal imaging), MCskin (laser speckle contrast imaging) and SmO2 (deep tissue oxygenation monitoring) and thermosensation (Likert scales) were assessed at baseline (BL) and at 5-min intervals during a 45 min post-application period (T0-T45).

Results

Both gels caused an initial decrease in Tskin, with Tskin(min) at T5 for both methyl salicylate (BL-T5: Δ-3.36°C) and placebo (BL-T5: Δ-3.90°C), followed by a gradual increase (p < .001). Methyl salicylate gel resulted in significantly higher Tskin than placebo between T5 and T40 (p < .05). For methyl salicylate, MCskin increased, with MCskin(max) at T5 (BL-T5: Δ88.7%). For placebo, MCskin decreased (BL-T5: Δ-17.5%), with significantly lower values compared to methyl salicylate between T0 and T45 (p < .05). Both gels had minimal effects on SmO2, with no significant differences between methyl salicylate and placebo (p > .05). Thermal sensation responses to topical methyl salicylate ranged from "cool" to "hot", with more intense sensations reported at T5.

Discussion

The findings indicate that topical methyl salicylate induces short-term cutaneous vasodilation, but it may not enhance skeletal muscle blood flow. This study highlights the complex sensory responses to its application, which may be based on the short-term modulation of thermosensitive transient receptor potential channels.

Assessment of Microvascular Hemodynamic Adaptations in Finger Flexors of Climbers

Climbing performance is greatly dependent on the endurance of the finger flexors which, in turn, depends on the ability to deliver and use oxygen within the muscle. Near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS) have provided new possibilities to explore these phenomena in the microvascular environment. The aim of the present study was to explore climbing-related microvascular adaptations through the comparison of the oxygen concentration and hemodynamics of the forearm between climbers and non-climber active individuals during a vascular occlusion test (VOT). Seventeen climbers and fifteen non-climbers joined the study. Through NIRS and DCS, the oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb) concentrations, tissue saturation index (TSI), and blood flow index (BFI) were obtained from the flexor digitorum profundus during the VOT. During the reactive hyperemia, climbers presented greater blood flow slopes (p = 0.043, d = 0.573), as well as greater O2Hb maximum values (p = 0.001, d = 1.263) and HHb minimum values (p = 0.009, d = 0.998), than non-climbers. The superior hemodynamics presented by climbers could indicate potential training-induced structural and functional adaptations that could enhance oxygen transportation to the muscle, and thus enhance muscle endurance and climbing performance.

Near-InfraRed Spectroscopy Provides a Reproducible Estimate of Muscle Aerobic Capacity, but Not Whole-Body Aerobic Power

Near-infrared spectroscopy (NIRS) during repeated limb occlusions is a noninvasive tool for assessing muscle oxidative capacity. However, the method's reliability and validity remain under investigation. This study aimed to determine the reliability of the NIRS-derived mitochondrial power of the musculus vastus lateralis and its correlation with whole-body (cycling) aerobic power (V̇O2 peak). Eleven healthy active men (28 ± 10 y) twice (2 days apart) underwent repeated arterial occlusions to induce changes in muscle oxygen delivery after 15 s of electrical muscle stimulation. The muscle oxygen consumption (mV̇O2) recovery time and rate (k) constants were calculated from the NIRS O2Hb signal. We assessed the reliability (coefficient of variation and intraclass coefficient of correlation [ICC]) and equivalency (t-test) between visits. The results showed high reproducibility for the mV̇O2 recovery time constant (ICC = 0.859) and moderate reproducibility for the k value (ICC = 0.674), with no significant differences between visits (p > 0.05). NIRS-derived k did not correlate with the V̇O2 peak relative to body mass (r = 0.441, p = 0.17) or the absolute V̇O2 peak (r = 0.366, p = 0.26). In conclusion, NIRS provides a reproducible estimate of muscle mitochondrial power, which, however, was not correlated with whole-body aerobic capacity in the current study, suggesting that even if somewhat overlapping, not the same set of factors underpin these distinct indices of aerobic capacity at the different (peripheral and whole-body systemic) levels.

Association of skeletal muscle oxidative capacity with muscle function, sarcopenia-related exercise performance, and intramuscular adipose tissue in older adults

Muscle function and exercise performance measures, such as muscle endurance capacity, maximal strength, chair stand score, gait speed, and Timed Up and Go score, are evaluated to diagnose sarcopenia and frailty in older individuals. Furthermore, intramuscular adipose tissue (IntraMAT) content increases with age. Skeletal muscle oxidative capacity determines muscle metabolism and maintains muscle performance. This study aimed to investigate the association of skeletal muscle oxidative capacity with muscle function, exercise performance, and IntraMAT content in older individuals. Thirteen older men and women participated in this study. Skeletal muscle oxidative capacity was assessed by the recovery speed of muscle oxygen saturation after exercise using near-infrared spectroscopy from the medial gastrocnemius. We assessed two muscle functions, peak torque and time to task failure, and four sarcopenia-related exercise performances: handgrip strength, gait speed, 30-s chair stand, and Timed Up and Go. The IntraMAT content was measured using axial magnetic resonance imaging. The results showed a relationship between skeletal muscle oxidative capacity and gait speed but not with muscle functions and other exercise performance measures. Skeletal muscle oxidative capacity was not related to IntraMAT content. Skeletal muscle oxidative capacity, which may be indicative of the capacity of muscle energy production in the mitochondria, is related to locomotive functions but not to other functional parameters or skeletal fat infiltration.

HDL-C and apolipoprotein A-I are independently associated with skeletal muscle mitochondrial function in healthy humans

Prior animal and cell studies have demonstrated a direct role of high-density lipoprotein (HDL) and apolipoprotein A-I (ApoA-I) in enhancing skeletal muscle mitochondrial function and exercise capacity. However, the relevance of these animal and cell investigations in humans remains unknown. Therefore, a cross-sectional study was conducted in 48 adults (67% female, 8% Black participants, age 39 ± 15.4 yr old) to characterize the associations between HDL measures, ApoA-I, and muscle mitochondrial function. Forearm muscle oxygen recovery time (tau) from postexercise recovery kinetics was used to assess skeletal muscle mitochondrial function. Lipoprotein measures were assessed by nuclear magnetic resonance. HDL efflux capacity was assessed using J774 macrophages, radiolabeled cholesterol, and apolipoprotein B-depleted plasma both with and without added cyclic adenosine monophosphate. In univariate analyses, faster skeletal muscle oxygen recovery time (lower tau) was significantly associated with higher levels of HDL cholesterol (HDL-C), ApoA-I, and larger mean HDL size, but not HDL cholesterol efflux capacity. Slower recovery time (higher tau) was positively associated with body mass index (BMI) and fasting plasma glucose (FPG). In multivariable linear regression analyses, higher levels of HDL-C and ApoA-I, as well as larger HDL size, were independently associated with faster skeletal muscle oxygen recovery times that persisted after adjusting for BMI and FPG (all P < 0.05). In conclusion, higher levels of HDL-C, ApoA-I, and larger mean HDL size were independently associated with enhanced skeletal muscle mitochondrial function in healthy humans.NEW & NOTEWORTHY Our study provides the first direct evidence supporting the beneficial role of HDL-C and ApoA-I on enhanced skeletal muscle mitochondrial function in healthy young to middle-aged humans without cardiometabolic disease.

Muscle Oximetry in Sports Science: An Updated Systematic Review

BACKGROUND

In the last 5 years since our last systematic review, a significant number of articles have been published on the technical aspects of muscle near-infrared spectroscopy (NIRS), the interpretation of the signals and the benefits of using the NIRS technique to measure the physiological status of muscles and to determine the workload of working muscles.

OBJECTIVES

Considering the consistent number of studies on the application of muscle oximetry in sports science published over the last 5 years, the objectives of this updated systematic review were to highlight the applications of muscle oximetry in the assessment of skeletal muscle oxidative performance in sports activities and to emphasize how this technology has been applied to exercise and training over the last 5 years. In addition, some recent instrumental developments will be briefly summarized.

METHODS

Preferred Reporting Items for Systematic Reviews guidelines were followed in a systematic fashion to search, appraise and synthesize existing literature on this topic. Electronic databases such as Scopus, MEDLINE/PubMed and SPORTDiscus were searched from March 2017 up to March 2023. Potential inclusions were screened against eligibility criteria relating to recreationally trained to elite athletes, with or without training programmes, who must have assessed physiological variables monitored by commercial oximeters or NIRS instrumentation.

RESULTS

Of the identified records, 191 studies regrouping 3435 participants, met the eligibility criteria. This systematic review highlighted a number of key findings in 37 domains of sport activities. Overall, NIRS information can be used as a meaningful marker of skeletal muscle oxidative capacity and can become one of the primary monitoring tools in practice in conjunction with, or in comparison with, heart rate or mechanical power indices in diverse exercise contexts and across different types of training and interventions.

CONCLUSIONS

Although the feasibility and success of the use of muscle oximetry in sports science is well documented, there is still a need for further instrumental development to overcome current instrumental limitations. Longitudinal studies are urgently needed to strengthen the benefits of using muscle oximetry in sports science.

Acute physiological responses and muscle recovery in females: a randomised controlled trial of muscle damaging exercise in hypoxia

BACKGROUND

Studies have investigated the effects of training under hypoxia (HYP) after several weeks in a male population. However, there is still a lack of knowledge on the acute hypoxic effects on physiology and muscle recovery in a female population.

METHODS

This randomized-controlled trial aimed to investigate the acute effects of muscle damaging exercise, performed in HYP and normoxia (CON), on physiological responses and recovery characteristics in healthy females. Key inclusion criteria were recreationally active female participants between the age of 18 to 35 years without any previous surgeries and injuries, whilst key exclusion criteria were acute pain situations, pregnancy, and medication intake. The females conducted a muscle-damaging protocol, comprising 5 × 20 drop-jumps, in either HYP (FiO2: 12%) or CON (FiO2: 21%). Physiological responses, including capillary oxygenation (SpO2), muscle oxygenation (SmO2), heart rate (HR), core- (Tcore) and skin- (Tskin) temperature were assessed at the end of each exercise set. Recovery characteristics were quantified by taking venous blood samples (serum creatine-kinase [CK], C-reactive protein [CRP] and blood sedimentation rate [BSR]), assessing muscle swelling of the quadriceps femoris muscle, maximum voluntary isometric contraction (MVIC) of the knee extensor muscles, countermovement jump (CMJ) performance and muscle soreness ratings (DOMS) at 24-, 48- and 72-hrs post-exercise.

RESULTS

SpO2 (HYP: 76.7 ± 3.8%, CON: 95.5 ± 1.7%, p < 0.001) and SmO2 (HYP: 60.0 ± 9.3, CON: 73.4 ± 5.8%, p = 0.03) values were lower (p < 0.05) in HYP compared to CON at the end of the exercise-protocol. No physiological differences between HYP and CON were observed for HR, Tcore, and Tskin (all p > 0.05). There were also no differences detected for any recovery variable (CK, CRP, BSR, MVIC, CMJ, and DOMS) during the 72-hrs follow-up period between HYP and CON (all p > 0.05).

CONCLUSION

In conclusion, our results showed that muscle damaging exercise under HYP leads to reduced capillary and muscle oxygenation levels compared to normoxia with no difference in inflammatory response and muscle recovery during 72 h post-exercise.

TRIAL REGISTRATION

NCT04902924, May 26th 2021.

Limb, sex, but not acute dietary capsaicin, modulate the near-infrared spectroscopy-vascular occlusion test estimate of muscle metabolism

The downward slope during the near-infrared spectroscopy (NIRS)-vascular occlusion test (NIRS-VOT) is purported as a simplified estimate of metabolism. Whether or not the NIRS-VOT exhibits sex- or limb-specificity or may be acutely altered remains to be elucidated. Thus, we investigated if there is limb- or sex specificity in tissue desaturation rates (DeO2) during a NIRS-VOT, and if acute dietary capsaicin may alter this estimate of muscle metabolism. Young healthy men (n = 25, 21 ± 4 years) and women (n = 20, 20 ± 1 years) ingested either placebo or capsaicin, in a counterbalanced, single-blind, crossover design after which a simplified NIRS-VOT was conducted to determine the DeO2 (%/s), as an estimate of oxidative muscle metabolism, in both the forearm (flexors) and thigh (vastus lateralis). There was a significant limb effect with the quadriceps having a greater DeO2 than the forearm (-2.31 ± 1.34 vs. -1.78 ± 1.22%/s, p = 0.007, ηp 2 = 0.19). There was a significant effect of sex on DeO2 (p = 0.005, ηp 2 = 0.203) with men exhibiting a lesser DeO2 than women (-1.73 ± 1.03 vs. -2.36 ± 1.32%/s, respectively). This manifested in significant interactions of limb*capsaicin (p = 0.001, ηp 2 = 0.26) as well as limb*capsaicin*sex on DeO2 (p = 0.013, ηp 2 = 0.16) being observed. Capsaicin does not clearly alter O2-dependent muscle metabolism, but there was apparent limb and sex specificity, interacting with capsaicin in this NIRS-derived assessment.

NIRS-Derived Muscle-Deoxygenation and Microvascular Reactivity During Occlusion-Reperfusion at Rest Are Associated With Whole-Body Aerobic Fitness

Purpose: Near-infrared spectroscopy (NIRS) indices during arterial occlusion-reperfusion maneuver have been used to examine the muscle's oxidative metabolism and microvascular function-important determinants of whole-body aerobic-fitness. The association of NIRS-derived parameters with whole-body VO2max was previously examined using a method requiring exercise (or electrical stimulation) followed by multiple arterial occlusions. We examined whether NIRS-derived indices of muscle deoxygenation and microvascular reactivity assessed during a single occlusion-reperfusion at rest are (a) associated with maximal/submaximal indices of whole-body aerobic-fitness and (b) could discriminate individuals with different VO2max. We, also, investigated which NIRS-parameter during occlusion-reperfusion correlates best with whole-body aerobic-fitness. Methods: Twenty-five young individuals performed an arterial occlusion-reperfusion at rest. Changes in oxygenated- and deoxygenated-hemoglobin (O2Hb and HHb, respectively) in vastus-lateralis were monitored; adipose tissue thickness (ATT) at NIRS-application was assessed. Participants also underwent a maximal incremental exercise test for VO2max, maximal aerobic velocity (MAV), and ventilatory-thresholds (VTs) assessments. Results: The HHbslope and HHbmagnitude of increase (occlusion-phase) and O2Hbmagnitude of increase (reperfusion-phase) were strongly correlated with VO2max (r = .695-.763, p < .001) and moderately with MAV (r = .468-.530; p < .05). O2Hbmagnitude was moderately correlated with VTs (r = .399-.414; p < .05). After controlling for ATT, the correlations remained significant for VO2max (r = .672-.704; p < .001) and MAV (r = .407; p < .05). Individuals in the high percentiles after median and tritile splits for HHbslope and O2Hbmagnitude had significantly greater VO2max vs. those in low percentiles (p < .01-.05). The HHbslope during occlusion was the best predictor of VO2max. Conclusion: NIRS-derived muscle deoxygenation/reoxygenation indices during a single arterial occlusion-reperfusion maneuver are strongly associated with whole-body maximal indices of aerobic-fitness (VO2max, MAV) and may discriminate individuals with different VO2max.

High-dose atorvastatin therapy progressively decreases skeletal muscle mitochondrial respiratory capacity in humans

BACKGROUNDWhile the benefits of statin therapy on atherosclerotic cardiovascular disease are clear, patients often experience mild to moderate skeletal myopathic symptoms, the mechanism for which is unknown. This study investigated the potential effect of high-dose atorvastatin therapy on skeletal muscle mitochondrial function and whole-body aerobic capacity in humans.METHODSEight overweight (BMI, 31.9 ± 2.0) but otherwise healthy sedentary adults (4 females, 4 males) were studied before (day 0) and 14, 28, and 56 days after initiating atorvastatin (80 mg/d) therapy.RESULTSMaximal ADP-stimulated respiration, measured in permeabilized fiber bundles from muscle biopsies taken at each time point, declined gradually over the course of atorvastatin treatment, resulting in > 30% loss of skeletal muscle mitochondrial oxidative phosphorylation capacity by day 56. Indices of in vivo muscle oxidative capacity (via near-infrared spectroscopy) decreased by 23% to 45%. In whole muscle homogenates from day 0 biopsies, atorvastatin inhibited complex III activity at midmicromolar concentrations, whereas complex IV activity was inhibited at low nanomolar concentrations.CONCLUSIONThese findings demonstrate that high-dose atorvastatin treatment elicits a striking progressive decline in skeletal muscle mitochondrial respiratory capacity, highlighting the need for longer-term dose-response studies in different patient populations to thoroughly define the effect of statin therapy on skeletal muscle health.FUNDINGNIH R01 AR071263.

Use of NIRS to explore skeletal muscle oxygenation during different training sessions in professional boxing

PURPOSE

The physiological examination of boxing has been limited to systemic response in amateur athletes. The demands of professional boxing have been overlooked, despite the different competition format. We sought to determine the physiological demands placed on skeletal muscle in professional boxing.

METHODS

Ten male professional boxers (age 26 ± 5 years, height 177 ± 4 cm, weight 71 ± 6 kg) were recruited for this observational study. On different days, the athletes completed 6 × 3 min rounds of pad, bag or spar-based training with 1 min recovery between each round. Prior to each session, participants put on a heart rate monitor and near-infrared spectroscopy attached to the belly of the rectus femoris muscle to record heart rate and muscle oxygenation.

RESULTS

There were significantly less punches thrown in sparring compared to other training modalities (p < 0.001). Skeletal muscle oxygenation across training modalities consisted of a delay, fast desaturation and steady state. Across rounds there was a significant increase in time delay for desaturation (p = 0.016), rate of fast desaturation (p < 0.001) and duration of fast desaturation (p = 0.019). There was a significant difference in sparring for the heart rate where skeletal muscle oxygenation changes occurred compared to pads or bag sessions (p < 0.001).

CONCLUSION

The findings highlight differences in the skeletal muscle response to the different training modalities. Practitioners need to be aware of the muscular demands of each session to allow optimal adaptation across a training camp. Training needs to allow the skeletal muscle to achieve a new oxygenation steady state rapidly to promote efficient performance across rounds.

Contraction intensity affects NIRS-derived skeletal muscle oxidative capacity but not its relationships to mitochondrial protein content or aerobic fitness

To further refine the near-infrared spectroscopy (NIRS)-derived measure of skeletal muscle oxidative capacity in humans, we sought to determine whether the exercise stimulus intensity affected the τ value and/or influenced the magnitude of correlations with in vitro measures of mitochondrial content and in vivo indices of exercise performance. Males (n = 12) and females (n = 12), matched for maximal aerobic fitness per fat-free mass, completed NIRS-derived skeletal muscle oxidative capacity tests for the vastus lateralis following repeated contractions at 40% (τ40) and 100% (τ100) of maximum voluntary contraction, underwent a skeletal muscle biopsy of the same muscle, and performed multiple intermittent isometric knee extension tests to task failure to establish critical torque (CT). The value of τ100 (34.4 ± 7.0 s) was greater than τ40 (24.2 ± 6.9 s, P < 0.001), but the values were correlated (r = 0.688; P < 0.001). The values of τ40 (r = -0.692, P < 0.001) and τ100 (r = -0.488, P = 0.016) correlated with myosin heavy chain I percentage and several markers of mitochondrial content, including COX II protein content in whole muscle (τ40: r = -0.547, P = 0.006; τ100: r = -0.466, P = 0.022), type I pooled fibers (τ40: r = -0.547, P = 0.006; τ100: r = -0.547, P = 0.006), and type II pooled fibers (τ40: r = -0.516, P = 0.009; τ100: r = -0.635, P = 0.001). The value of τ40 (r = -0.702, P < 0.001), but not τ100 (r = -0.378, P = 0.083) correlated with critical torque (CT); however, neither value correlated with W' (τ40: r = 0.071, P = 0.753; τ100: r = 0.054, P = 0.812). Overall, the NIRS method of assessing skeletal muscle oxidative capacity is sensitive to the intensity of skeletal muscle contraction but maintains relationships to whole body fitness, isolated limb critical intensity, and mitochondrial content regardless of intensity.NEW & NOTEWORTHY Skeletal muscle oxidative capacity measured using near-infrared spectroscopy (NIRS) was lower following high-intensity compared with low-intensity isometric knee extension contractions. At both intensities, skeletal muscle oxidative capacity was correlated with protein markers of mitochondrial content (in whole muscle and pooled type I and type II muscle fibers) and critical torque. These findings highlight the importance of standardizing contraction intensity while using the NIRS method with isometric contractions and further demonstrate its validity.

A systematic review of the relationship between muscle oxygen dynamics and energy rich phosphates. Can NIRS help?

BACKGROUND

Phosphocreatine dynamics provide the gold standard evaluation of in-vivo mitochondrial function and is tightly coupled with oxygen availability. Low mitochondrial oxidative capacity has been associated with health issues and low exercise performance.

METHODS

To evaluate the relationship between near-infrared spectroscopy-based muscle oxygen dynamics and magnetic resonance spectroscopy-based energy-rich phosphates, a systematic review of the literature related to muscle oxygen dynamics and energy-rich phosphates was conducted. PRISMA guidelines were followed to perform a comprehensive and systematic search of four databases on 02-11-2021 (PubMed, MEDLINE, Scopus and Web of Science). Beforehand pre-registration with the Open Science Framework was performed. Studies had to include healthy humans aged 18-55, measures related to NIRS-based muscle oxygen measures in combination with energy-rich phosphates. Exclusion criteria were clinical populations, laboratory animals, acutely injured subjects, data that only assessed oxygen dynamics or energy-rich phosphates, or grey literature. The Effective Public Health Practice Project Quality Assessment Tool was used to assess methodological quality, and data extraction was presented in a table.

RESULTS

Out of 1483 records, 28 were eligible. All included studies were rated moderate. The studies suggest muscle oxygen dynamics could indicate energy-rich phosphates under appropriate protocol settings.

CONCLUSION

Arterial occlusion and exercise intensity might be important factors to control if NIRS application should be used to examine energetics. However, more research needs to be conducted without arterial occlusion and with high-intensity exercises to support the applicability of NIRS and provide an agreement level in the concurrent course of muscle oxygen kinetics and muscle energetics.

TRIAL REGISTRATION

https://osf.io/py32n/ .

KEY POINTS

1. NIRS derived measures of muscle oxygenation agree with gold-standard measures of high energy phosphates when assessed in an appropriate protocol setting. 2. At rest when applying the AO protocol, in the absence of muscle activity, an initial disjunction between the NIRS signal and high energy phosphates can been seen, suggesting a cascading relationship. 3. During exercise and recovery a disruption of oxygen delivery is required to provide the appropriate setting for evaluation through either an AO protocol or high intensity contractions.

Muscle Oxidative Capacity in Vivo Is Associated With Physiological Parameters in Trained Rowers

Purpose: The muscle oxygen uptake (m V ˙ O 2 ) kinetics following exercise, measured by near-infrared spectroscopy, has been used as a functional evaluation of muscle oxidative metabolism. This study aimed to determine the m V ˙ O 2 off-kinetics and verify the relationship of the recovery rate of m V ˙ O 2 (k) with time-trial performance and different aerobic parameters in trained rowers. Methods: Eleven male rowers (age: 20 ± 3 years; V ˙ O 2 m a x : 4.28 ± 0.35 L·min-1) used a rowing ergometer to perform (I) an incremental test to determine the maximal oxygen uptake (V ˙ O 2 m a x ) and peak power output (Ppeak); (II) several visits to determine maximal lactate steady state (MLSS); and (III) a 2000-m rowing ergometer performance test. Also, one test to determine m V ˙ O 2 off-kinetics of the vastus lateralis muscle using a repeated arterial occlusions protocol. Results: The m V ˙ O 2 generated a good monoexponential fit (R2 = 0.960 ± 0.030; SEE = 0.041 ± 0.018%.s-1). The k of m V ˙ O 2 (2.06 ± 0.58 min-1) was associated with relative V ˙ O 2 m a x (r = 0.79), power output at MLSS (r = 0.76), and Ppeak (r = 0.83); however, it was not related with 2000-m rowing performance (r = -0.38 to 0.52; p > .152). Conclusion: These findings suggest that although not associated with rowing performance, the m V ˙ O 2 off-kinetics determined after a submaximal isometric knee extension may be a practical and less-exhaustive approach than invasive responses and incremental tests to assess the muscle oxidative metabolism during a training program.

Muscle oxygen saturation rates coincide with lactate-based exercise thresholds

INTRODUCTION

Monitoring muscle metabolic activity via blood lactate is a useful tool for understanding the physiological response to a given exercise intensity. Recent indications suggest that skeletal muscle oxygen saturation (SmO2), an index of the balance between local O2 supply and demand, may describe and predict endurance performance outcomes.

PURPOSE

We tested the hypothesis that SmO2 rate is tightly related to blood lactate concentration across exercise intensities, and that deflections in SmO2 rate would coincide with established blood lactate thresholds (i.e., lactate thresholds 1 and 2).

METHODS

Ten elite male soccer players completed an incremental running protocol to exhaustion using 3-min work to 30 s rest intervals. Blood lactate samples were collected during rest and SmO2 was collected continuously via near-infrared spectroscopy from the right and left vastus lateralis, left biceps femoris and the left gastrocnemius.

RESULTS

Muscle O2 saturation rate (%/min) was quantified after the initial 60 s of each 3-min segment. The SmO2 rate was significantly correlated with blood lactate concentrations for all muscle sites; RVL, r = - 0.974; LVL, r = - 0.969; LG, r = - 0.942; LHAM, r = - 0.907. Breakpoints in SmO2 rate were not significantly different from LT1 or LT2 at any muscle sites (P > 0.05). Bland-Altman analysis showed speed threshold estimates via SmO2 rate and lactate are similar at LT2, but slightly greater for SmO2 rate at LT1.

CONCLUSIONS

Muscle O2 saturation rate appears to provide actionable information about maximal metabolic steady state and is consistent with bioenergetic reliance on oxygen and its involvement in the attainment of metabolic steady state.

Applications of near-infrared spectroscopy in "anaerobic" diagnostics - SmO2 kinetics reflect PCr dephosphorylation and correlate with maximal lactate accumulation and maximal pedalling rate

We investigated the relationship of the time-dependent behaviour of muscle oxygen saturation SmO2(t), phosphagen energy supply WPCr(t) and blood lactate accumulation ΔBLC(t) during a 60-s all-out cycling sprint and tested SmO2(t) for correlations with the end of the fatigue-free state tFf, maximal pedalling rate PRmax and maximal blood lactate accumulation rate v̇Lamax. Nine male elite track cyclists performed four maximal sprints (3, 8, 12, 60 s) on a cycle ergometer. Crank force and cadence were monitored continuously to determine PRmax and tFf based on force-velocity profiles. SmO2 of the vastus lateralis muscle and respiratory gases were measured until the 30th minute after exercise. WPCr was calculated based on the fast component of the post-exercise oxygen uptake for each sprint. Before and for 30 minutes after each sprint, capillary blood samples were taken to determine the associated ΔBLC. Temporal changes of SmO2, WPCr and ΔBLC were analysed via non-linear regression analysis. v̇Lamax was calculated based on ΔBLC(t) as the highest blood lactate accumulation rate. All models showed excellent quality (R2 > 0.95). The time constant of SmO2(t) τSmO2 = 2.93 ± 0.65 s was correlated with the time constant of WPCr(t) τPCr = 3.23 ± 0.67 s (r = 0.790, p < 0.012), v̇Lamax = 0.95 ± 0.18 mmol · l-1 · s-1 (r = 0.768, p < 0.017) and PRmax = 299.51 ± 14.70 rpm (r = -0.670, p < 0.049). tFf was correlated with τSmO2 (r = 0.885, p < 0.001). Our results show a time-dependent reflection of SmO2 kinetics and phosphagen energy contribution during a 60-s maximal cycling sprint. A high v̇Lamax results in a reduction, a high PRmax in an increase of the desaturation rate. The half-life of SmO2 desaturation indicates the end of the fatigue-free state.

Analysis of electrical stimulation and voluntary muscle contraction on skeletal muscle oxygen uptake and mitochondrial recovery using near-infrared spectroscopy

PURPOSE

This investigation was to compare differences in skeletal muscle oxygen consumption ([Formula: see text]) and mitochondrial recovery between voluntary (VOL) and electrically stimulated (ES) plantarflexion contractions.

METHODS

Twelve men and women (26 ± 4.0 years; 171.8 ± 5.1 cm; 74.0 ± 13.7 kg) were seated in a chair with their right knee fully extended and right foot secured to a force transducer. ES electrodes and a near-infrared spectroscopy device were placed on the gastrocnemius. Participants performed ES plantarflexion contractions across a range of stimulation intensities at frequencies of 1 and 2 Hz and similar VOL contractions. Cuff occlusion occurred immediately following each series of contractions to measure [Formula: see text]. A standardized mitochondrial function assessment protocol was also performed to calculate K-constants between work-matched ES and VOL contractions.

RESULTS

For mitochondrial assessments, there were no significant differences between ES and VOL rate constants (2.03 ± 0.98 vs. 1.25 ± 1.35 min-1, p = 0.266). ES resulted in a significantly greater workrate-[Formula: see text] slope at 1 Hz (0.007 ± 0.007 vs. 0.001 ± 0.002% [Formula: see text]/s/N, p = 0.014) and 2 Hz (0.010 ± 0.010 vs. 0.001 ± 0.001% [Formula: see text]/s/N, p = 0.012), as well as a significantly greater workrate-[Formula: see text] Y-intercept at 2 Hz (1.603 ± 1.513 vs. 0.556 ± 0.564% [Formula: see text]/s, p = 0.035) but not 1 Hz (0.579 ± 0.448 vs. 0.442 ± 0.357% mV̇O2/s, p = 0.535) when compared to VOL.

CONCLUSION

ES results in a significantly greater [Formula: see text] at similar work rates compared to VOL, however, the mitochondrial recovery rate constants were similar. The greater mVO2 with ES may partially contribute to the increased rate of fatigue during ES exercise in individuals with muscle paralysis.

Treatment side affects exercising microvascular oxygenation response in active breast cancer survivors: A pilot study

Cancer treatment is associated with cardiovascular toxicity, skeletal muscle dysfunction and interruptions in mitochondrial respiration. Microvascular oxygenation responses, measured via near-infrared spectroscopy (NIRS), at peak exercise intensity has previously been associated with aerobic capacity. Specifically, the greater magnitude of microvascular deoxygenation observed at peak exercise intensity has been associated with higher aerobic capacity. Therefore, a pilot study investigated if diagnosis side (uninvolved side, treatment side) and/or exercise side (paddle side, non-paddle side) affected microvascular oxygenation responses at peak intensity during paddle exercise. Thirty-three breast cancer survivors (age = 57 ± 9 years, height = 1.64 ± 0.05 m, weight = 76.5 ± 15.6 kg, 7 ± 7 years since treatment) who also competed as dragon boat racers performed a unilateral (paddle), discontinuous graded exercise test (2-min exercise, 1-min rest) on a rowing ergometer to volitional fatigue. Tissue oxygenation saturation (StO_2DIFF ) and total haemoglobin concentration (total[heme]_DIFF ) responses at peak exercise intensity were measured bilaterally from the posterior deltoids using NIRS. Two-way analysis of variance determined if diagnosis side and/or exercise side effected StO_2DIFF or total[heme]_DIFF . Diagnosis side elicited a moderate effect (effect size = 0.66) on StO_2DIFF , as the treatment side deoxygenated less (-6.0 ± 14.7 ∆BSL) compared to the uninvolved side (-16.9 ± 16.9 ∆BSL) at peak exercise intensity. No other significant main effects or interactions were observed for StO_2DIFF or total[heme]_DIFF . The pilot findings suggest that the ability of the exercising muscle to use oxygen for the purpose of mitochondrial oxidative respiration may be impaired on the treatment side.

Epinephrine iontophoresis attenuates changes in skin blood flow and abolishes cutaneous contamination of near-infrared diffuse correlation spectroscopy estimations of muscle perfusion

Near-infrared diffuse correlation spectroscopy (NIR-DCS) is an optical imaging technique for measuring relative changes in skeletal muscle microvascular perfusion (i.e., fold change above baseline) during reactive hyperemia testing and exercise and is reported as a blood flow index (BFI). Although it is generally accepted that changes in BFI are primarily driven by changes in muscle perfusion, it is well known that large, hyperthermia-induced changes in cutaneous blood flow can uncouple this relationship. What remains unknown, is how much of an impact that changes in cutaneous perfusion have on NIR-DCS BFI and estimates of skeletal muscle perfusion under thermoneutral conditions, where changes in cutaneous blood flow are assumed to be relatively low. We therefore used epinephrine iontophoresis to pharmacologically block changes in cutaneous perfusion throughout a battery of experimental procedures. The data show that 1) epinephrine iontophoresis attenuates changes in cutaneous perfusion for up to 4-h posttreatment, even in the face of significant neural and local stimuli, 2) under thermoneutral conditions, cutaneous perfusion does not significantly impact NIR-DCS BFI during reactive hyperemia testing or moderate-intensity exercise, and 3) during passive whole body heat stress, when cutaneous vasodilation is pronounced, epinephrine iontophoresis preserves NIR-DCS measures of skeletal muscle BFI during moderate-intensity exercise. Collectively, these data suggest that cutaneous perfusion is unlikely to have a major impact on NIR-DCS estimates of skeletal muscle BFI under thermoneutral conditions, but that epinephrine iontophoresis can be used to abolish cutaneous contamination of the NIR-DCS BFI signal during studies where skin blood flow may be elevated but skeletal muscle perfusion is of specific interest.

Comparing the effects of dynamic and holding isometric contractions on cardiovascular, perceptual, and near-infrared spectroscopy parameters: A pilot study

The aim of this pilot study was to assess the effect of muscle contraction type on SmO2 during a dynamic contraction protocol (DYN) and a holding isometric contraction protocol (ISO) in the back squat exercise. Ten voluntary participants (age: 26.6 ± 5.0 years, height: 176.8 ± 8.0 cm, body mass: 76.7 ± 8.1 kg, and one-repetition maximum (1RM): 112.0 ± 33.1 kg) with back squat experience were recruited. The DYN consisted of 3 sets of 16 repetitions at 50% of 1RM (56.0 ± 17.4 kg), with a 120-second rest interval between sets and 2 seconds per movement cycle. The ISO consisted of 3 sets of 1 isometric contraction with the same weight and duration as the DYN (32 seconds). Through near-infrared spectroscopy (NIRS) in the vastus lateralis (VL), soleus (SL), longissimus (LG), and semitendinosus (ST) muscles, the minimum SmO2 (SmO2 min), mean SmO2 (SmO2 avg), percent change from baseline (SmO2 Δdeoxy) and time to recovery 50% of baseline value (t SmO2 50%reoxy) were determined. No changes in SmO2 avg were found in the VL, LG, and ST muscles, however the SL muscle had lower values in DYN, in the 1st set (p = 0.002) and in the 2nd set (p = 0.044). In terms of SmO2 min and ΔSmO2 deoxy, only the SL muscle showed differences (p≤0.05) and lower values in the DYN compared to ISO regardless of the set. The t SmO2 50%reoxy was higher in the VL muscle after ISO, only in the 3rd set. These preliminary data suggested that varying the type of muscle contraction in back squat with the same load and exercise time resulted in a lower SmO2 min in the SL muscle in DYN, most likely because of a higher demand for specialized muscle activation, indicating a larger oxygen supply-consumption gap.

Changes in Muscle Deoxygenation During Squat Exercise After 6-Week Resistance Training With Different Percentages of Velocity Loss

ABSTRACT

Costilla, M, Casals, C, Marín-Galindo, A, Sánchez-Sixto, A, Muñoz-López, A, Sañudo, B, Corral-Pérez, J, and Ponce-González, JG. Changes in muscle deoxygenation during squat exercise after 6-week resistance training with different percentages of velocity loss. J Strength Cond Res XX(X): 000-000, 2022-The present study compared, for the first time, the effects of 6 weeks of 20% (20VL) vs. 40% (40VL) velocity loss (VL) resistance training (RT) programs on muscle oxygen dynamics during the squat exercise. Twenty-three young men (21.4 ± 2.4 years) were randomly allocated into the 20VL group (n = 8), 40VL group (n = 7), or control group (CG; n = 8). The RT program consisted of 3 sets of Smith machine back squat exercise at 20VL or 40VL with a 3-minute rest between sets, twice per week for 6 weeks. Tissue oxygenation index (TOI) was measured using near-infrared spectroscopy in the vastus medialis and vastus lateralis during a squat test (8-repetition 1 m·s-1 load test), and the maximum (maxTOI) and minimum (minTOI) TOIs were measured during a 3-min recovery period. After the 6-week RT program, TOI increased significantly at the beginning of the test in both muscles (during the first 4 repetitions in the vastus lateralis and 5 repetitions in the vastus medialis) in the 20VL group (p < 0.05), with nonsignificant changes in the 40VL group and CG. The maxTOI was significantly increased in the vastus medialis (+3.76%) and vastus lateralis (+3.97%) after the training only in the 20VL group (p < 0.05). The minTOI in the vastus medialis reached during the test remained unchanged postintervention for both training groups, with the CG showing significantly higher values compared with the 20VL group (+14.1%; p < 0.05). In conclusion, depending on the VL reached during a squat RT program, different changes in muscle oxygen dynamics can be expected. Training at 20% of VL improves metabolic efficiency and the reoxygenation peak after the set.

Haemodynamics and oxygenation in the lower-limb muscles of young ambulatory adults with cerebral palsy

AIM

To evaluate muscle haemodynamics and oxygen metabolism in adults with cerebral palsy (CP) at rest and during exercise.

METHOD

This cross-sectional study included 12 adults with spastic CP (four females, eight males; mean age [SD] 29 years 6 months [7 years 10.8 months]) and 13 typically developing individuals (seven females, six males; mean age [SD] 26 years 6 months [1 year 1.9 months]). Near-infrared spectroscopy was used to assess changes in muscle blood flow (mBF), muscle oxygen consumption (mVO₂ ), and muscle oxygen saturation in the vastus lateralis and rectus femoris muscles during three conditions: rest, low load at 20% maximum voluntary contraction (MVC), and high load at 80% MVC.

RESULTS

MBF was lower in participants with CP than in typically developing participants at rest (p < 0.001) and at 20% MVC (p = 0.007) in both muscles. Increased load caused a reduction in mBF in typically developing participants and an increase in CP. MVO₂ in typically developing participants increased from rest to 20% MVC and was reduced at 80% MVC compared with 20% MVC. In participants with CP, there was no change with load in the rectus femoris muscle; however, there was an increase in the vastus lateralis muscle from rest to 20% MVC, and 80% MVC had a similar value. Muscle saturation was higher in participants with CP across all conditions (vastus lateralis, p < 0.001; rectus femoris, p = 0.0518).

INTERPRETATION

Oxidative metabolism in CP is not limited by oxygen delivery (mBF), because high muscle saturation suggests oxygen availability. Adults with CP demonstrate muscular responses to exercise that are inconsistent with typical high-workload activation, probably because of inefficient fibre recruitment and secondary anomalies.

Metabolic and microvascular function assessed using near-infrared spectroscopy with vascular occlusion in women: age differences and reliability

NEW FINDINGS

What is the central question of this study? Can the near-infrared spectroscopy with vascular occlusion test (NIRS-VOT) reliably measure skeletal muscle metabolic and microvascular function in women? What is the main finding and its importance? The NIRS-VOT can be used as a reliable technique for the assessment of skeletal muscle metabolism and microvascular function in women, with reliability being generally greater in younger women. These findings have important implications for the planning and development of future studies employing the NIRS-VOT in women, and provide insights into the effects of age on these parameters in women specifically.

ABSTRACT

We investigated the test-retest reliability of, and age-related differences in, markers of skeletal muscle metabolism and microvascular function derived from the near-infrared spectroscopy with vascular occlusion test (NIRS-VOT) in younger women (YW) and middle-aged and older women (MAOW). Seventeen YW (age 23 ± 4 years) and 17 MAOW (age 59 ± 8 years) completed this study. Participants completed identical experimental visits separated by ∼4 weeks during which the NIRS-VOT was used to quantify the occlusion slope, minimum and maximum tissue saturation, ischaemic index, reperfusion magnitude, the reperfusion and 10-s reperfusion slopes (slope 2 and slope 210-s ), time to max tissue saturation, and area under the reperfusion curve using the local tissue oxygen saturation signal. Except for slope 210-s (intraclass correlation coefficient (ICC) = 0.37; coefficient of variation (CV) = 31%), time to max tissue saturation (ICC = 0.21), and ischaemic index (ICC = 0.37) for MAOW, all of the NIRS variables demonstrated good to excellent relative reliability for the YW (ICCs = 0.74-0.86) and the MAOW (ICCs = 0.51-0.87), with CVs of 2-21% and 2-22%, respectively. The occlusion slope was significantly lower, indicating accelerated deoxygenation, while maximum tissue saturation, reperfusion magnitude, and ischaemic index were significantly higher in YW versus MAOW. No other group differences were found. In conclusion, our data support the use of the NIRS-VOT as a simple, reliable, non-invasive technique for the assessment of peripheral skeletal muscle metabolism and microvascular function in women, with the reliability being generally greater in YW versus MAOW. Further, our data suggest that ageing is associated with lower skeletal muscle metabolism and microvascular hyperaemic responsiveness in women.

The effects of normobaric and hypobaric hypoxia on cognitive performance and physiological responses: A crossover study

This partially randomised controlled, crossover study sought to investigate the effects of normobaric hypoxia (NH) and hypobaric hypoxia (HH) on cognitive performance, the physiological response at rest and after a 3-min step-test. Twenty healthy participants (10 females and 10 males, 27.6±6.2yrs, 73.6±13.7kg, 175.3±8.9cm) completed a cognitive performance test, followed by the modified Harvard-step protocol, in four environments: normobaric normoxia (NN; PiO2: 146.0±1.5mmHg), NH (PiO2: 100.9±1.3mmHg), HH at the first day of ascent (HH1: PiO2 = 105.6±0.4mmHg) and HH after an overnight stay (HH2: PiO2 = 106.0±0.5mmHg). At rest and/or exercise, SpO2, NIRS, and cardiovascular and perceptual data were collected. The cerebral tissue oxygenation index and the cognitive performance (throughput, accuracy, and reaction time) were not different between the hypoxic conditions (all p>0.05). In NH, SpO2 was higher compared to HH1 (ΔSpO2 NH vs HH1: 1.7±0.5%, p = 0.003) whilst heart rate (ΔHR NH vs HH2: 5.8±2.6 bpm, p = 0.03) and sympathetic activation (ΔSNSi NH vs HH2: 0.8±0.4, p = 0.03) were lower in NH compared to HH2. Heart rate (ΔHR HH1 vs HH2: 6.9±2.6 bpm, p = 0.01) and sympathetic action (ΔSNSi HH1 vs HH2: 0.9±0.4, p = 0.02) were both lower in HH1 compared to HH2. In conclusion, cognitive performance and cerebral oxygenation didn’t differ between the hypoxic conditions. SpO2 was only higher in NH compared to HH1. In HH2, heart rate and sympathetic activation were higher compared to both NH and HH1. These conclusions account for a PiO2 between 100–106 mmHg.

Age-Related Changes in Skeletal Muscle Oxygen Utilization

The cardiovascular and skeletal muscle systems are intrinsically interconnected, sharing the goal of delivering oxygen to metabolically active tissue. Deficiencies within those systems that affect oxygen delivery to working tissues are a hallmark of advancing age. Oxygen delivery and utilization are reflected as muscle oxygen saturation (SmO2) and are assessed using near-infrared resonance spectroscopy (NIRS). SmO2 has been observed to be reduced by ~38% at rest, ~24% during submaximal exercise, and ~59% during maximal exercise with aging (>65 y). Furthermore, aging prolongs restoration of SmO2 back to baseline by >50% after intense exercise. Regulatory factors that contribute to reduced SmO2 with age include blood flow, capillarization, endothelial cells, nitric oxide, and mitochondrial function. These mechanisms are governed by reactive oxygen species (ROS) at the cellular level. However, mishandling of ROS with age ultimately leads to alterations in structure and function of the regulatory factors tasked with maintaining SmO2. The purpose of this review is to provide an update on the current state of the literature regarding age-related effects in SmO2. Furthermore, we attempt to bridge the gap between SmO2 and associated underlying mechanisms affected by aging.

Near-infrared spectroscopy estimation of combined skeletal muscle oxidative capacity and O2 diffusion capacity in humans

The final steps of the O2 cascade during exercise depend on the product of the microvascular-to-intramyocyteP O 2 ${P}_{{{\rm{O}}}_{\rm{2}}}$ difference and muscle O2 diffusing capacity (D m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ ). Non-invasive methods to determineD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ in humans are currently unavailable. Muscle oxygen uptake (mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) recovery rate constant (k), measured by near-infrared spectroscopy (NIRS) using intermittent arterial occlusions, is associated with muscle oxidative capacity in vivo. We reasoned that k would be limited byD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ when muscle oxygenation is low (kLOW ), and hypothesized that: (i) k in well oxygenated muscle (kHIGH ) is associated with maximal O2 flux in fibre bundles; and (ii) ∆k (kHIGH  - kLOW ) is associated with capillary density (CD). Vastus lateralis k was measured in 12 participants using NIRS after moderate exercise. The timing and duration of arterial occlusions were manipulated to maintain tissue saturation index within a 10% range either below (LOW) or above (HIGH) half-maximal desaturation, assessed during sustained arterial occlusion. Maximal O2 flux in phosphorylating state was 37.7 ± 10.6 pmol s-1  mg-1 (∼5.8 ml min-1  100 g-1 ). CD ranged 348 to 586 mm-2 . kHIGH was greater than kLOW (3.15 ± 0.45 vs. 1.56 ± 0.79 min-1 , P < 0.001). Maximal O2 flux was correlated with kHIGH (r = 0.80, P = 0.002) but not kLOW (r = -0.10, P = 0.755). Δk ranged -0.26 to -2.55 min-1 , and correlated with CD (r = -0.68, P = 0.015). mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ k reflects muscle oxidative capacity only in well oxygenated muscle. ∆k, the difference in k between well and poorly oxygenated muscle, was associated with CD, a mediator ofD m O 2 $D{{\rm{m}}}_{{{\rm{O}}}_2}$ . Assessment of muscle k and ∆k using NIRS provides a non-invasive window on muscle oxidative and O2 diffusing capacity. KEY POINTS: We determined post-exercise recovery kinetics of quadriceps muscle oxygen uptake (mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ) measured by near-infrared spectroscopy (NIRS) in humans under conditions of both non-limiting (HIGH) and limiting (LOW) O2 availability, for comparison with biopsy variables. The mV ̇ O 2 ${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ recovery rate constant in HIGH O2 availability was hypothesized to reflect muscle oxidative capacity (kHIGH ) and the difference in k between HIGH and LOW O2 availability (∆k) was hypothesized to reflect muscle O2 diffusing capacity. kHIGH was correlated with phosphorylating oxidative capacity of permeabilized muscle fibre bundles (r = 0.80). ∆k was negatively correlated with capillary density (r = -0.68) of biopsy samples. NIRS provides non-invasive means of assessing both muscle oxidative and oxygen diffusing capacity in vivo.

Wearables in Sports Cardiology

The expanding array and adoption of consumer health wearables is creating a new dynamic to the patient-health-care provider relationship. Providers are increasingly tasked with integrating the biometric data collected from their patients into clinical care. Further, a growing body of evidence is supporting the provider-driven utility of wearables in the screening, diagnosis, and monitoring of cardiovascular disease. Here we highlight existing and emerging wearable health technologies and the potential applications for use within sports cardiology. We additionally highlight how wearables can advance the remote cardiovascular care of patients within the context of the COVID-19 pandemic. Finally, despite these promising advances, we acknowledge some of the significant challenges that remain before wearables can be routinely incorporated into clinical care.

Assessment of mitochondrial respiratory capacity using minimally invasive and noninvasive techniques in persons with spinal cord injury

Purpose

Muscle biopsies are the gold standard to assess mitochondrial respiration; however, biopsies are not always a feasible approach in persons with spinal cord injury (SCI). Peripheral blood mononuclear cells (PBMCs) and near-infrared spectroscopy (NIRS) may alternatively be predictive of mitochondrial respiration. The purpose of the study was to evaluate whether mitochondrial respiration of PBMCs and NIRS are predictive of respiration of permeabilized muscle fibers after SCI.

Methods

Twenty-two individuals with chronic complete and incomplete motor SCI between 18–65 years old were recruited to participate in the current trial. Using high-resolution respirometry, mitochondrial respiratory capacity was measured for PBMCs and muscle fibers of the vastus lateralis oxidizing complex I, II, and IV substrates. NIRS was used to assess mitochondrial capacity of the vastus lateralis with serial cuff occlusions and electrical stimulation.

Results

Positive relationships were observed between PBMC and permeabilized muscle fibers for mitochondrial complex IV (r = 0.86, P < 0.0001). Bland-Altman displayed agreement for complex IV (MD = 0.18, LOA = -0.86 to 1.21), between PBMCs and permeabilized muscles fibers. No significant relationships were observed between NIRS mitochondrial capacity and respiration in permeabilized muscle fibers.

Conclusions

This is the first study to explore and support the agreement of less invasive clinical techniques for assessing mitochondrial respiratory capacity in individuals with SCI. The findings will assist in the application of PBMCs as a viable alternative for assessing mitochondrial health in persons with SCI in future clinical studies.

NIRS-derived muscle V̇O2 kinetics after moderate running exercise in healthy males: reliability and associations with parameters of aerobic fitness

NEW FINDINGS

What is the central question of this study? In vivo muscle oxidative capacity has been evaluated through the mV̇O₂ kinetics following single joint exercise using NIRS system. Here, we demonstrated its utility following running exercise. What is the main finding and its importance? We demonstrated that time constant of mV̇O₂ kinetics in gastrocnemius following moderate running exercise presents good to excellent reliability. In addition, it was well correlated with parameters of aerobic fitness, such as maximal speed of the incremental test, ventilatory threshold and pulmonary V̇O₂ on-kinetics. Therefore, NIRS-derived muscle oxidative capacity together with other physiological measurements may allow a concomitant local and systemic analysis of the components of the oxidative system.

ABSTRACT

NIRS-derived muscle oxygen uptake (mV̇O₂ ) kinetics following single-joint exercise has been used to assess muscle oxidative capacity. However, little evidence is available on the use of this technique following whole-body exercises. Therefore, this study aimed to assess the reliability of the NIRS-derived mV̇O₂ kinetics following running exercise and to investigate the relationship between the time constant of mV̇O₂ off-kinetics (τmV̇O₂ ) with parameters of aerobic fitness. After an incremental test to determine V̇O₂ max, first (VT₁ ) and second (VT₂ ) ventilatory thresholds, and maximal speed (Smax), thirteen males (age = 21 ± 4 years; V̇O₂ max = 55.9 ± 3.4 mlꞏkg-1ꞏmin-1) performed three sets (two in the first day and one on a subsequent day) of two repetitions of 6-min running exercise at 90%VT₁ . The pulmonary V̇O₂ on-kinetics (pV̇O₂ ) and mV̇O₂ off-kinetics in gastrocnemius were assessed. τmV̇O₂ presented no systematic change and satisfactory reliability (SEM and ICC of 4.21 s and 0.49 for between transitions; and 2.65 s and 0.74 averaging τmV̇O₂ within each time-set), with no difference (p > 0.3) between the within- (SEM = 2.92 s) and between-day variability (SEM = 2.78 s and 2.19 s between first vs. third set, and second vs. third set, respectively). τmV̇O₂ (28.5 ± 4.17 s) correlated significantly (p < 0.05) with Smax (r = -0.66), VT₁ (r = -0.64) and time constant of the pV̇O2 on-kinetics (r = 0.69). These findings indicate that NIRS-derived mV̇O₂ kinetics in the gastrocnemius following moderate running exercise is a useful and reliable method to assess muscle oxidative capacity. This article is protected by copyright. All rights reserved.

Application of near-infrared spectroscopy technology in the complex fermentation system to achieve high-efficiency production

The fermentation process is dynamically changing, and the metabolic status can be grasped through real-time monitoring of environmental parameters. In this study, a real-time and on-line monitoring experiment platform for substrates and products detection was developed based on non-contact type near-infrared (NIR) spectroscopy technology. The prediction models for monitoring the fermentation process of lactic acid, sophorolipids (SLs) and sodium gluconate (SG) were established based on partial least-squares regression and internal cross-validation methods. Through fermentation verification, the accuracy and precision of the NIR model for the complex fermentation environments, different rheological properties (uniform system and multi-phase inhomogeneous system) and different parameter types (substrate, product and nutrients) have good applicability, and R2 was greater than 0.98, exhibiting a good linear relationship. The root mean square error of prediction shows that the model has high credibility. Through the control of appropriate glucose concentration in SG fermentation as well as glucose and oil concentrations SLs fermentation by NIR model, the titers of SG and SLs were increased to 11.8% and 26.8%, respectively. Although high cost of NIR spectrometer is a key issue for its wide application in an industrial scale. This work provides a basis for the application of NIR spectroscopy in complex fermentation systems.

Peripheral impairments of oxidative metabolism after a 10-day bed rest are upstream of mitochondrial respiration

Abstract

In order to identify peripheral biomarkers of impaired oxidative metabolism during exercise following a 10‐day bed rest, 10 males performed an incremental exercise (to determine peak pulmonary V̇O₂ (V̇O₂p)) and moderate‐intensity exercises, before (PRE) and after (POST) bed rest. Blood flow response was evaluated in the common femoral artery by Eco‐Doppler during 1 min of passive leg movements (PLM). The intramuscular matching between O₂ delivery and O₂ utilization was evaluated by near‐infrared spectroscopy (NIRS). Mitochondrial respiration was evaluated ex vivo by high‐resolution respirometry in isolated muscle fibres, and in vivo by NIRS by the evaluation of skeletal muscle V̇O₂ (V̇O₂m) recovery kinetics. Resting V̇O₂m was estimated by NIRS. Peak V̇O₂p was lower in POST vs. PRE. The area under the blood flow vs. time curve during PLM was smaller (P = 0.03) in POST (274 ± 233 mL) vs. PRE (427 ± 291). An increased (P = 0.03) overshoot of muscle deoxygenation during a metabolic transition was identified in POST. Skeletal muscle citrate synthase activity was not different (P = 0.11) in POST (131 ± 16 nmol min^–1 mg^–1) vs. PRE (138 ± 19). Maximal ADP‐stimulated mitochondrial respiration (66 ± 18 pmol s^–1 mg^–1 (POST) vs. 72 ± 14 (PRE), P = 0.41) was not affected by bed rest. Apparent K_m for ADP sensitivity of mitochondrial respiration was reduced in POST vs. PRE (P = 0.04). The V̇O₂m recovery time constant was not different (P = 0.79) in POST (22 ± 6 s) vs. PRE (22 ± 6). Resting V̇O₂m was reduced by 25% in POST vs. PRE (P = 0.006). Microvascular‐endothelial function was impaired following a 10‐day bed rest, whereas mitochondrial mass and function (both in vivo and ex vivo) were unaffected or slightly enhanced.

Key points

Ten days of horizontal bed rest impaired in vivo oxidative function during exercise.

Microvascular impairments were identified by different methods.

Mitochondrial mass and mitochondrial function (evaluated both in vivo and ex vivo) were unchanged or even improved (i.e. enhanced mitochondrial sensitivity to submaximal [ADP]).

Resting muscle oxygen uptake was significantly lower following bed rest, suggesting that muscle catabolic processes induced by bed rest/inactivity are less energy‐consuming than anabolic ones.

Augmented muscle glycogen utilization following a single session of sprint training in hypoxia

PURPOSE

This study determined the effect of a single session of sprint interval training in hypoxia on muscle glycogen content among athletes.

METHODS

Ten male college track and field sprinters (mean ± standard error of the mean: age, 21.1 ± 0.2 years; height, 177 ± 2 cm; body weight, 67 ± 2 kg) performed two exercise trials under either hypoxia [HYPO; fraction of inspired oxygen (F_iO₂), 14.5%] or normoxia (NOR: F_iO₂, 20.9%). The exercise consisted of 3 × 30 s maximal cycle sprints with 8-min rest periods between sets. Before and immediately after the exercise, the muscle glycogen content was measured using carbon magnetic resonance spectroscopy in vastus lateralis and vastus intermedius muscles. Moreover, power output, blood lactate concentrations, metabolic responses (respiratory oxygen uptake and carbon dioxide output), and muscle oxygenation were evaluated.

RESULTS

Exercise significantly decreased muscle glycogen content in both trials (interaction, P = 0.03; main effect for time, P < 0.01). Relative changes in muscle glycogen content following exercise were significantly higher in the HYPO trial (- 43.5 ± 0.4%) than in the NOR trial (- 34.0 ± 0.3%; P < 0.01). The mean power output did not significantly differ between the two trials (P = 0.80). The blood lactate concentration after exercise was not significantly different between trials (P = 0.31).

CONCLUSION

A single session of sprint interval training (3 × 30 s sprints) in hypoxia caused a greater decrease in muscle glycogen content compared with the same exercise under normoxia without interfering with the power output.

Forearm muscle mitochondrial capacity and resting oxygen uptake: Relationship to symptomatic fatigue in persons with multiple sclerosis

Background

Mitochondrial dysfunction has been implicated in the pathogenesis of multiple sclerosis (MS). Whether mitochondrial alterations are a function of ambulatory dysfunction or are of a non-ambulatory systemic nature is unclear.

Objective

To compare oxidative capacity, and rest muscle oxygen consumption (mVO₂) in the upper limb of persons with multiple sclerosis (PwMS) to a control group (CON), whereby an upper limb would be comparatively independent of ambulation or deconditioning.

Methods

Near infra-red spectroscopy was used to measure oxidative capacity of the wrist flexors in PwMS (n = 16) and CON (n = 13). Oxidative capacity was indicated by the time constant (TC) of mVO₂ recovery following brief wrist flexion contractions. Measurements included well-being, depression, symptomatic fatigue, disability, handgrip strength, cognition, and functional endurance. Analysis was by T-tests and Pearson correlations with p ≤ 0.05. Data are mean (SD).

Results

TC of mVO₂ recovery was slower in PwMS (MS = 47(14) sec, CON = 36(11) sec; p = 0.03). No significant correlations were found between oxidative capacity and any other measures. Rest mVO₂ was not different between groups, but correlated with symptomatic fatigue (r = 0.694, p = 0.003) and strength (0.585, p = 0.017) in PwMS.

Conclusion

Oxidative capacity was lower in the wrist flexors of PwMS, possibly indicating a systemic component of the disease. Within PwMS, rest mVO₂ was associated with symptomatic fatigue.

The effect of HIIT vs. SIT on muscle oxygenation in trained sprint kayakers

PURPOSE

To assess the performance change and physiological adaptations following nine sessions of short high-intensity interval training (HIIT) or sprint-interval training (SIT) in sprint kayakers.

METHODS

Twelve trained kayakers performed an incremental test and 3 time trials (200 m, 500 m and 1000 m) on a kayak ergometer. Oxygen consumption (V̇O₂) and muscle oxygenation of the latissimus dorsi, biceps brachii, and vastus lateralis were measured. Athletes were then paired for sex and V̇O₂max and randomized into a HIIT or a SIT training group, and performed nine training sessions before repeating the tests.

RESULTS

Training improved performance in HIIT (200 m: + 3.8 ± 3.1%, p = 0.06; 500 m: + 2.1 ± 4.1%, p = 0.056; 1000 m: + 3.0 ± 4.6%, p = 0.13) but changes in performance remained within the smallest worthwhile change in SIT (200 m: + 0.8 ± 4.1%, p = 0.59; 500 m: + 0.5 ± 4.1%, p = 0.87; 1000 m: + 1.3 ± 4.6%, p = 0.57). In the 1000 m, training led to a greater deoxygenation in the biceps brachii and vastus lateralis in HIIT, and in the latissimus dorsi in SIT. In HIIT, the best predictors of improvements in 1000 m performance were increases in latissimus dorsi and vastus lateralis maximal deoxygenation.

CONCLUSION

In a group of trained sprint kayakers, greater improvements in performance can be obtained with HIIT compared with SIT, for any distance. Training did not change V̇O₂peak, but increased muscle maximal deoxygenation, suggesting both HIIT and SIT elicit peripheral adaptations. Performance improvement in the 1000 m was associated with increased maximal muscle deoxygenation, reinforcing the contribution of peripheral adaptations to performance in sprint kayaking.

The balance of muscle oxygen supply and demand reveals critical metabolic rate and predicts time to exhaustion

We tested the hypothesis that during whole body exercise, the balance between muscle O₂ supply and metabolic demand may elucidate intensity domains, reveal a critical metabolic rate, and predict time to exhaustion. Seventeen active, healthy volunteers (12 males, 5 females; 32 ± 2 yr) participated in two distinct protocols. Study 1 (n = 7) consisted of constant work rate cycling in the moderate, heavy, and severe exercise intensity domains with concurrent measures of pulmonary V̇o₂ and local %SmO₂ [via near-infrared spectroscopy (NIRS)] on quadriceps and forearm sites. Average %SmO₂ at both sites displayed a domain-dependent response (P < 0.05). A negative %SmO₂ slope was evident during severe-domain exercise but was positive during exercise below critical power (CP) at both muscle sites. In study 2 (n = 10), quadriceps and forearm site %SmO₂ was measured during three continuous running trials to exhaustion and three intermittent intensity (ratio = 60 s severe: 30 s lower intensity) trials to exhaustion. Intensity-dependent negative %SmO₂ slopes were observed for all trials (P < 0.05) and predicted zero slope at critical velocity. %SmO₂ accurately predicted depletion and repletion of %D' balance on a second-by-second basis (R² = 0.99, P < 0.05; both sites). Time to exhaustion predictions during continuous and intermittent exercise were either not different or better with %SmO₂ [standard error of the estimate (SEE) < 20.52 s for quad, <44.03 s for forearm] versus running velocity (SEE < 65.76 s). Muscle O₂ balance provides a dynamic physiological delineation between sustainable and unsustainable exercise (consistent with a "critical metabolic rate") and predicts real-time depletion and repletion of finite work capacity and time to exhaustion.NEW & NOTEWORTHY Dynamic muscle O₂ saturation discriminates boundaries between exercise intensity domains, exposes a critical metabolic rate as the highest rate of steady state O₂ supply and demand, describes time series depletion and repletion for work above critical power, and predicts time to exhaustion during severe domain whole body exercise. These results highlight the matching of O₂ supply and demand as a primary determinant for sustainable exercise intensities from those that are unsustainable and lead to exhaustion.

Measuring tibial hemodynamics and metabolism at rest and after exercise using near-infrared spectroscopy

The bone vascular system is important, yet evaluation of bone hemodynamics is difficult and expensive. This study evaluated the utility and reliability of near-infrared spectroscopy (NIRS), a portable and relatively inexpensive device, in measuring tibial hemodynamics and metabolic rate. Eleven participants were tested twice using post-occlusive reactive hyperemia technique with the NIRS probes placed on the tibia and the medial gastrocnemius (MG) muscle. Measurements were made at rest and after 2 levels of plantarflexion exercise. The difference between oxygenated and deoxygenated hemoglobin signal could be reliably measured with small coefficients of variation (CV; range 5.7-9.8%) and high intraclass correlation coefficients (ICC; range 0.73-0.91). Deoxygenated hemoglobin rate of change, a potential marker for bone metabolism, also showed good reliability (CV range 7.5-9.8%, ICC range 0.90-0.93). The tibia was characterized with a much slower metabolic rate compared with MG (p < 0.001). While exercise significantly increased MG metabolic rate in a dose-dependent manner (all p < 0.05), no changes were observed for the tibia after exercise compared with rest (all p > 0.05). NIRS is a suitable tool for monitoring hemodynamics and metabolism in the tibia. However, the local muscle exercise protocol utilized in the current study did not influence bone hemodynamics or metabolic rate. Novelty: NIRS can be used to monitor tibial hemodynamics and metabolism with good reliability. Short-duration local muscle exercise increased metabolic rate in muscle but not in bone. High level of loading and exercise volume may be needed to elicit measurable metabolic changes in bone.

Muscle mitochondrial capacity in high- and low-fitness females using near-infrared spectroscopy

The recovery of muscle oxygen consumption (m V ˙ O₂ ) after exercise measured using near-infrared spectroscopy (NIRS) provides a measure of skeletal muscle mitochondrial capacity. Nevertheless, due to sex differences in factors that can influence scattering and thus penetration depth of the NIRS signal in the tissue, e.g., subcutaneous adipose tissue thickness and intramuscular myoglobin and hemoglobin, it is unknown whether results in males can be extrapolated to a female population. Therefore, the aim of this study was to measure skeletal muscle mitochondrial capacity in females at different levels of aerobic fitness to test whether NIRS can measure relevant differences in mitochondrial capacity. Mitochondrial capacity was analyzed in the gastrocnemius muscle and the wrist flexors of 32 young female adults, equally divided in relatively high ( V ˙ O₂ peak ≥ 47 ml/kg/min) and relatively low aerobic fitness group ( V ˙ O₂ peak ≤ 37 ml/kg/min). m V ˙ O₂ recovery was significantly faster in the high- compared to the low-fitness group in the gastrocnemius, but not in the wrist flexors (p = 0.009 and p = 0.0528, respectively). Furthermore, V ˙ O₂ peak was significantly correlated to m V ˙ O₂ recovery in both gastrocnemius (R²  = 0.27, p = 0.0051) and wrist flexors (R²  = 0.13, p = 0.0393). In conclusion, NIRS measurements can be used to assess differences in mitochondrial capacity within a female population and is correlated to V ˙ O₂ peak. This further supports NIRS assessment of muscle mitochondrial capacity providing additional evidence for NIRS as a promising approach to monitor mitochondrial capacity, also in an exclusively female population.

Bilateral NIRS measurements of muscle mitochondrial capacity: Feasibility and repeatability

Background

Non‐invasive determination of mitochondrial capacity via near infrared spectroscopy (NIRS) typically involves voluntary exercise of a single muscle group followed by as many as 26 brief ischemic cuff occlusions to determine a single recovery rate constant (k).

Purpose

To determine the within‐ and between‐visit repeatability of a shortened bilateral NIRS protocol, and to establish the feasibility of hamstring k measurements.

Methods

Sixteen young (eight women, eight men; 22 ± 3 years) active adults underwent a bilateral electrical stimulation protocol in which multiple (n = 4) measurements of k for the vastus lateralis (VL) and medial hamstring (MH) muscles were determined on two visits. Repeatability (CV% and intraclass correlations, ICC) and equivalency across visits were assessed for both muscles.

Results

Mean k values in the VL were consistent with published values and within‐visit ICCs were moderately high for both muscles in both sexes. In men, average k values on visit 2 were within 1% (VL muscle) and 5% (MH muscle) of the values on visit 1 (all p > 0.78). In women, average k values were 10%–15% lower on visit 2 (p = 0.01 and p = 0.15 for MH and VL) with the largest between‐visit differences in a subset of participants with the most days between visits.

Conclusions

This bilateral NIRS protocol is time efficient and provides valid estimates of k in both sexes and muscle groups with acceptable within‐visit repeatability. Lower than expected between‐visit repeatability in some participants reinforces the need for further investigation of this newly developed protocol to identify and control for experimental and behavioral sources of variation.

Critical oxygenation: Can muscle oxygenation inform us about critical power?

The power-duration relationship is well documented for athletic performance and is formulated out mathematically in the critical power (CP) model. The CP model, when applied properly, has great predictive power, e.g. pedaling at a specific power output on an ergometer the model precisely calculates the time over which an athlete can sustain this power. However, CP presents physiological inconsistencies and process-oriented problems. The rapid development of near-infrared spectroscopy (NIRS) to measure muscle oxygenation (SmO₂) dynamics provides a physiological exploration of the CP model on a conceptual and empirical level. Conceptually, the CP model provides two components: first CP is defined as the highest metabolic rate that can be achieved through oxidative means. And second, work capacity above CP named W'. SmO₂ presents a steady-state in oxygen supply and demand and thereby represents CP specifically at a local level of analysis. Empirically, exploratory data quickly illustrates the relationship between performance and SmO₂, as shown during 3-min all-out cycling tests to assess CP. During these tests, performance and SmO₂ essentially mirror each other, and both CP and W' generate solid correlation with what would be deemed their SmO₂ counterparts: first, the steady-state of SmO₂ correlates with CP. And second, the tissue oxygen reserve represented in SmO₂, when calculated as an integral corresponds to W'. While the empirical data presented is preliminary, the proposition of a concurring physiological model to the current CP model is a plausible inference. Here we propose that SmO₂ steady-state representing CP as critical oxygenation or CO. And the tissue oxygen reserve above CO would then be identified as O'. This new CO model could fill in the physiological gap between the highly predictive CP model and at times its inability to track human physiology consistently. For simplicity's sake, this would include acute changes in physiology as a result of changing climate or elevation with travel, which can affect performance. These types of acute fluctuations, but not limited to, would be manageable when applying a CO model in conjunction with the CP model. Further, modeling is needed to investigate the true potential of NIRS to model CP, with a focus on repeatability, recovery, and systemic vs local workloads.

Muscle Oxidative Capacity Is Reduced in Both Upper and Lower Limbs in COPD

INTRODUCTION

Skeletal muscle atrophy, weakness, mitochondrial loss, and dysfunction are characteristics of chronic obstructive pulmonary disease (COPD). It remains unclear whether muscle dysfunction occurs in both upper and lower limbs, because findings are inconsistent in the few studies where upper and lower limb muscle performance properties were compared within an individual. This study determined whether muscle oxidative capacity is low in upper and lower limbs of COPD patients compared with controls.

METHODS

Oxidative capacity of the forearm and medial gastrocnemius was measured using near-infrared spectroscopy to determine the muscle O2 consumption recovery rate constant (k, min) in 20 COPD (Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2/3/4, n = 7/7/6) and 20 smokers with normal spirometry (CON). Muscle k is linearly proportional to oxidative capacity. Steps per day and vector magnitude units per minute (VMU·min) were assessed using triaxial accelerometry. Differences between group and limb were assessed by two-way ANOVA.

RESULTS

There was a significant main effect of group (F = 11.2, ηp = 0.13, P = 0.001): k was lower in both upper and lower limb muscles in COPD (1.01 ± 0.17 and 1.05 ± 0.24 min) compared with CON (1.29 ± 0.49 and 1.54 ± 0.60 min). There was no effect on k of limb (F = 1.8, ηp = 0.02, P = 0.18) or group-limb interaction (P = 0.35). (VMU·min) was significantly lower in COPD (-38%; P = 0.042). Steps per day did not differ between COPD (4738 ± 3194) and CON (6372 ± 2107; P = 0.286), although the difference exceeded a clinically important threshold (>600-1100 steps per day).

CONCLUSIONS

Compared with CON, muscle oxidative capacity was lower in COPD in both upper (-20%) and lower (-30%) limbs. These data suggest that mitochondrial loss in COPD is not isolated to locomotor muscles.