How many motor units is enough? An assessment of the influence of the number of motor units on firing rate calculations.

The number of motor units included in calculations of mean firing rates varies widely in the literature. It is unknown how the number of decomposed motor units included in the calculation of firing rate per participant compares to the total number of active motor units in the muscle, and if this is different for males and females. Bootstrapped distributions and confidence intervals (CI) of mean motor unit firing rates decomposed from the tibialis anterior were used to represent the total number of active motor units for individual participants in trials from 20 to 100 % of maximal voluntary contraction. Bootstrapped distributions of mean firing rates were constructed using different numbers of motor units, from one to the maximum number for each participant, and compared to the CIs. A probability measure for each number of motor units involved in firing rate was calculated and then averaged across all individuals. Motor unit numbers required for similar levels of probability increased as contraction intensity increased (p < 0.001). Increased levels of probability also required higher numbers of motor units (p < 0.001). There was no effect of sex (p ≥ 0.97) for any comparison. This methodology should be repeated in other muscles, and aged populations.

Neuromuscular behaviour in the first dorsal interosseus following mental fatigue.

We examined sex-specific changes to neuromuscular function in response to mental fatigue. Twenty-five young, healthy adults (13 F, 12 M) performed a mentally fatiguing task and control condition for 30 min on two separate days. Neuromuscular function was assessed in the first dorsal interosseous before and after each condition. Reaction time decreased after the mentally fatiguing task (P < 0.001, η2  = 0.47). Males and females reported higher levels of subjective fatigue after the mentally fatiguing task (P < 0.02, η2  = 0.07). Motor unit firing rate increased over time at 10% maximal voluntary contraction (MVC; P < 0.04, η2  = 0.16), and decreased over time at 50% MVC (P < 0.01, η2  = 0.14); however, this was not unique to either sex. During a variable force contraction, error decreased in females over time and increased in males (P < 0.05, η2  = 0.13), although changes were not unique to mental fatigue. Physiological function of the neuromuscular system was not specifically affected by mental fatigue in males or females.

Sex-Related Differences in Functional Fitness Outcomes in Older Adults.

Sex-related differences in changes in functional fitness over time were longitudinally assessed in older adults participating in a group-based multimodal exercise program. From a database, functional fitness scores were obtained for 89 older adults (71.6 ± 6.5 years old) who had completed two assessments, 5-8 years apart. Lower body strength, upper body strength, aerobic endurance, flexibility, and change of direction performances were compared over time and with normative values. Females (p = .02), but not males, had an improvement in upper body strength over time. Females were also more flexible than males at both assessments (p ≤ .02). Of those who had five consecutive assessments, females were more flexible than males (p ≤ .05) and had a faster change of direction ability (p < .001). When compared with normative values, our results indicate that typical time-related functional fitness loss can be attenuated with group exercise. Our results further support the need to tailor exercise prescription according to the individual.

Mental fatigue does not substantially alter neuromuscular function in young, healthy males and females.

The neuromuscular mechanisms leading to impaired motor performance after mental fatigue (MF) are not well-understood and little is known of sex-specific differences in the neuromuscular response to MF. The purpose of this study was to investigate sex-related differences in the impact of MF on neuromuscular function. Thirty young, healthy adults (15F, 15M) performed the Psychomotor Vigilance Task (PVT) to induce MF and watched the Earth documentary (control) for 30 min in a random and counterbalanced order. Before and after each task, measurements of neuromuscular function during submaximal dorsiflexion contractions were obtained. At the end of the PVT, females and males had a slower reaction time (p<0.001, η2p=0.41) and reported higher fatigue (p<0.001, η2p=0.50), suggesting the PVT induced MF. After the PVT, females and males demonstrated a decline in force during 10% maximum voluntary contractions (MVC) (p=0.006, η2p=0.24), slower motor unit firing rate during 20% MVC (p=0.04, η2p=0.15) and a longer cortical silent period (p=0.01, η2p=0.22). However, similar changes were observed in the control condition suggesting MF is unlikely to substantially alter neuromuscular function during submaximal isometric contractions in young, healthy adults. Results also suggest neuromuscular function after a MF task is similar between young, healthy females and males. Further research is required to investigate populations with higher fatigue, such as multiple sclerosis or chronic fatigue syndrome.

Sex differences in the impact of state and trait fatigue on gait variability.

Conditions requiring greater attention or cognitive control, such as fatigue, lead to changes in the motor performance of a task. Perceived fatigue refers to subjective feelings of fatigue, can be expressed as a state variable or trait characteristic and is influenced by demographic factors, such as sex. The purpose of this study was to determine how sex interacts with state and trait physical fatigue (PF) and mental fatigue (MF) to influence gait variability. METHODS: 123 healthy adults (77 female, 46 male), aged 18-36 years, completed the Mental and Physical State and Trait Energy and Fatigue Scale. Using a median split for each fatigue variable, participants were placed into "low" or "high" fatigue categories. Gait variability was defined as the asymmetry of lateral step variability (ALSV) and coefficient of variation (CV) of gait speed, stride length and double limb (DL) support during overground walking. RESULTS: Males with low state PF had greater ALSV than females with low state PF (p = 0.05, η2p = 0.07) and males with high state PF (p = 0.007, η2p = 0.15). Females with high trait MF had greater CV of gait speed than females with low trait mental fatigue (p = 0.02, η2p = 0.08). Males with low trait MF had greater CV of gait speed (p = 0.01, η2p = 0.10) and stride length (p = 0.002, η2p = 0.17) than females with low trait MF. CV of DL support did not vary based on fatigue level or sex (p ≥ 0.11). CONCLUSIONS: There are sex-specific differences in the impact of state PF on asymmetry of lateral step variability and trait MF on the variability of gait speed and stride length.

State and Trait Fatigue and Energy Predictors of Postural Control and Gait.

Compromised attentional resources during perceived fatigue has been suggested to alter motor control. The authors determined if measures of postural control and gait are predicted by state and trait physical and mental fatigue and energy, and how these relationships are modified by sex, sleep quality, and physical activity. Young adults (n = 119) completed the Modified Clinical Test of Sensory Integration, overground walking, and questionnaires to quantify fatigue and energy, sleep quality, and physical activity. Regression models indicated that trait fatigue, trait energy, and sleep quality were predictors of postural control (p ≤ .02, R2 ≥ .04). State fatigue, state energy, and sex were predictors of gait (p ≤ .05, R2 ≥ .03). While the variance explained was low (3-13%), the results demonstrate perceptions of fatigue and energy may influence posture and gait.

Motor unit firing rates during constant isometric contraction: establishing and comparing an age-related pattern among muscles.

Motor unit (MU) firing rates (FRs) are lower in aged adults, compared with young, at relative voluntary contraction intensities. However, from a variety of independent studies of disparate muscles, the age-related degree of difference in FR among muscles is unclear. Using a standardized statistical approach with data derived from primary studies, we quantified differences in FRs across several muscles between younger and older adults. The data set included 12 different muscles in young (18-35 yr) and older adults (62-93 yr) from 18 published and one unpublished study. Experiments recorded single MU activity from intramuscular electromyography during constant isometric contraction at different (step-like) voluntary intensities. For each muscle, FR ranges and FR variance explained by voluntary contraction intensity were determined using bootstrapping. Dissimilarity of FR variance among muscles was calculated by Euclidean distances. There were threefold differences in the absolute frequency of FR ranges across muscles in the young (soleus 8-16 and superior trapezius 20-49 Hz), but in the old, FR ranges were more similar and lower for nine out of 12 muscles. In contrast, the explained FR variance from voluntary contraction intensity in the older group had 1.6-fold greater dissimilarity among muscles than the young (P < 0.001), with FR variance differences being muscle dependent. Therefore, differences between muscle FR ranges were not explained by how FRs scale to changes in voluntary contraction intensity within each muscle. Instead, FRs were muscle dependent but were more dissimilar among muscles in the older group in their responsiveness to voluntary contraction intensity.NEW & NOTEWORTHY The mean frequency of motor unit firing rates were compared systematically among several muscles and between young and older adults from new and published data sets. Firing rates among muscles were lower and more similar during voluntary isometric contraction in older than younger adults. Firing rate responses from voluntary contraction intensity were muscle dependent and more dissimilar among muscles in the older than young adults.

The Role of Active Rehabilitation in Concussion Management: A Systematic Review and Meta-analysis.

PURPOSE: This study aimed to conduct a systematic review and meta-analysis of active rehabilitation on concussion management. We also examined moderator variables that may contribute to differences across studies: symptom scale, physical activity type, time of injury to recruitment, and mechanism of injury. METHODS: The standardized effect size of physical activity on concussion management was computed for 23 studies (29 effect sizes). Effect sizes were coded as positive when studies reported an improvement in symptom scores, which was represented by a decrease in postconcussive symptom scores. RESULTS: The overall effect size of physical activity on concussion recovery was large and positive (g = 1.03). Subthreshold aerobic activity provided the largest effect size (g = 1.71), whereas multimodal interventions had a moderate effect size (g = 0.70). All other moderator variables produced positive effect sizes ranging from g = 0.59 to g = 1.46. CONCLUSIONS: This systematic review and meta-analysis demonstrates that current evidence supports the notion that physical activity is beneficial in decreasing postconcussive symptoms in both the acute and chronic phases after concussion. The results indicate that unimodal subthreshold aerobic activity may be the best course of action compared with multimodal interventions. Despite this growing body of evidence, additional research is needed to determine the optimal intensity, duration, and time to initiation of aerobic exercise after concussion.

Motor Cortex Function in APOE4 Carriers and Noncarriers.

PURPOSE: The ε4 allele of the apolipoprotein-E gene has been associated with disease activity including Alzheimer disease, multiple sclerosis, and cardiovascular disease. Individuals who possess the ε4 variant of this gene (ε4 carriers) also demonstrate higher levels of cognitive impairment and lower motor scores compared with noncarriers. The purpose of this study was to establish whether there is a difference in motor cortex function between apoε4 carriers and noncarriers. We hypothesized that carriers would have lower levels of excitability and excitatory transmitter (glutamate) and similar levels of intracortical inhibition and inhibitory neurotransmitter (gamma-aminobutyric acid) than noncarriers. METHODS: Fifty-two participants provided saliva samples to determine apoε4 carrier status. Measures of motor cortex excitability and inhibition were obtained using transcranial magnetic stimulation, and measures of glutamate and gamma-aminobutyric acid concentrations were obtained using proton magnetic resonance spectroscopy. RESULTS: No significant differences in transcranial magnetic stimulation (P ≥ 0.19) or proton magnetic resonance spectroscopy measures (P ≥ 0.90) were found between carriers and noncarriers. CONCLUSIONS: The results from this study suggest that motor cortex function, as assessed by transcranial magnetic stimulation measures of excitability and inhibition, and MRS measures of excitatory and inhibitory neurotransmitter are similar in those who possess an apoε4 allele and those who do not.

Excitability, Inhibition, and Neurotransmitter Levels in the Motor Cortex of Symptomatic and Asymptomatic Individuals Following Mild Traumatic Brain Injury.

Purpose: The purpose of this study was to determine the level of excitability and inhibition, as well as the concentrations of excitatory and inhibitory neurotransmitters, in the motor cortex of individuals with acute and chronic symptoms from mTBI. Methods: Fifty-three individuals were assigned to one of four groups: (i) without history of mTBI (Control), (ii) within 72-h of diagnosis of mTBI (Acute), (iii) with history of mTBI and no remaining symptoms (Chronic Asymptomatic), and (iv) with chronic symptoms from mTBI, lasting at least 3 months post-injury (Chronic Symptomatic). Measures of corticospinal excitability and inhibition were obtained using transcranial magnetic stimulation (TMS). On the same day, measures of glutamate and GABA concentrations were obtained from the primary motor cortex (M1) using proton magnetic resonance spectroscopy. Results: MEP amplitude and area were both significantly lower in the Chronic Symptomatic group compared to the Control and Chronic Asymptomatic groups (p ≤ 0.05). Intracortical inhibition was not significantly different among groups (p = 0.14). The concentration of glutamate in M1 was similar between groups (p = 0.93) while there was a trend for a lower concentration of GABA in the Chronic Symptomatic group compared to the Acute group (p = 0.06). Conclusions: Individuals with chronic mTBI symptoms appear to have lower corticospinal excitability compared with acutely-injured individuals and asymptomatic controls, but the absence of differences in intracortical inhibition, and concentrations of excitatory and inhibitory neurotransmitters in M1 suggests that neurotransmitter changes in the human brain post-mTBI do not follow the pattern typically seen in the animal literature.

The Effect of Mental Fatigue on Neuromuscular Function is Similar in Young and Older Women.

The purpose of this study was to examine the effect of a mentally fatiguing task on neuromuscular function in young and older women. Neuromuscular measures were obtained prior to and following 20 minutes of a mentally fatiguing task. Maximal force output significantly decreased after the mental fatigue task (p = 0.02) and this was not different between age groups (p = 0.32). Increases in cortical silent period duration approached significance in both young and older groups (p = 0.06), suggesting that mental fatigue may cause increased cortical inhibition. Measures of peripheral neuromuscular function (contractile properties of the muscle, M-wave) did not change (p ≥ 0.09), suggesting that changes in force production with mental fatigue are more likely due to supraspinal than peripheral mechanisms. These findings provide further evidence of an interaction between mental fatigue and physical function.

The effect of a mentally fatiguing task on postural balance control in young and older women.

The purpose of this study was to examine the effect of a mentally fatiguing task on postural responses to unexpected backward perturbations in sixteen young and sixteen older women. Postural responses were characterized by center of pressure (COP) displacement, corrective COP peak velocity, and electromyography (EMG) of the medial gastrocnemius. Older women had slower reaction time (p = .002), longer EMG onset times (p = .03), larger COP displacement (p = .001), and faster COP velocity (p = .02) than younger women overall. However, only young women experienced mental fatigue (slower reaction times; p = .001) and this was accompanied by significantly faster COP velocity during the mental fatigue condition (p = .02) than the control condition. Performance of mental fatigue task, not necessarily the development of mental fatigue, affects neuromuscular activation in young women only, but does not affect the magnitude of postural response to perturbation.

The Impact of Physical Activity and Sleep on Physiology Following a mTBI.

A mild traumatic brain injury (mTBI) is a common injury that happens to people across a range of lifestyles. Rest has been regarded as the most effective remedy to restore impaired physiological functions in those who have experienced this kind of injury. However, recent data suggests that a gradual increase in physical activity post-injury can lead to quicker recovery from negative symptoms. The impact of physical activity on physiologic recovery is not completely understood and the purpose of this study was to better understand how physical activity and sleep influence physiological outcomes following a mTBI. The data collected in this study came from two groups of young adults: a healthy control (n=5) and those that had a mTBI (n = 5). The daily activity was measured using an activity monitor for a week immediately after injury. Physiological measurements of excitability and inhibition within the motor cortex and neurotransmitter concentrations were collected within 72 hours of injury and again at two months to determine if physical activity or sleep within one week after injury had an influence on physiologic recovery. Control participants had higher levels of total (d = 1.2), light (d = 1.3) and moderate physical activity (d = 0.7), and lower total sleep time (d = -1.9), compared with the mTBI group. Across the two months, relative to the control group the mTBI group showed similar changes in excitability (d = 0.13), but greater changes in inhibition (d = -0.66). Changes in the excitatory (d = 0.06) and inhibitory (d = -0.16) neurotransmitters were similar between groups. Neither physical activity, nor sleep quality were significantly associated with the change in motor cortex excitability (R 2 ≤ 0.18, p ≥ 0.22), inhibition (R 2 ≤ 0.3, p ≥ 0.08) or neurotransmitter concentrations (R 2 ≤ 0.15, p ≥ 0.20) between 72 hours and two months post-injury. These preliminary results suggest there is no direct effect of physical activity on physiological outcomes.

Essential Amino Acid Supplementation Mitigates Muscle Atrophy After Total Knee Arthroplasty: A Randomized, Double-Blind, Placebo-Controlled Trial.

BACKGROUND: Substantial muscle atrophy occurs after total knee arthroplasty (TKA), resulting in decreased strength and impaired mobility. We sought to determine whether perioperative supplementation with essential amino acids (EAA) would attenuate muscle atrophy following TKA and whether the supplements were safe for ingestion in an older surgical population. METHODS: We performed a double-blind, placebo-controlled, randomized trial of 39 adults (age range, 53 to 76 years) undergoing primary unilateral TKA who ingested 20 g of EAA (n = 19) or placebo (n = 20) twice daily for 7 days preoperatively and for 6 weeks postoperatively. At baseline and 6 weeks postoperatively, magnetic resonance imaging (MRI) scans were obtained to measure quadriceps and hamstrings muscle volume. Secondary outcomes included functional mobility and strength. Data on physical activity, diet, and patient-reported outcomes (Veterans RAND 12-Item Health Survey and Knee injury and Osteoarthritis Outcome Score) were collected. Safety was determined through blood tests evaluating blood urea nitrogen, creatinine, creatinine clearance, homocysteine, and renal and liver function. Laboratory values at baseline, on the day of surgery, and at 2 days, 2 weeks, and 6 weeks postoperatively were compared between treatment groups. Analysis of covariance models, with baseline values as covariates, were used to evaluate outcomes between treatment groups. P values were adjusted for multiple tests. RESULTS: Compared with baseline, the EAA group had significantly less decrease in mean quadriceps muscle volume compared with the placebo group in the involved leg (-8.5% ± 2.5% compared with -13.4% ± 1.9%; p = 0.033) and the contralateral leg (-1.5% ± 1.6% compared with -7.2% ± 1.4%; p = 0.014). The hamstrings also demonstrated a greater muscle-volume-sparing effect for the EAA group than for the placebo group in the involved leg (-7.4% ± 2.0% compared with -12.2% ± 1.4%; p = 0.036) and contralateral leg (-2.1% ± 1.3% compared with -7.5% ± 1.5%; p = 0.005). There were no differences between the groups in terms of functional measures or strength. Blood chemistry values varied significantly between assessments periods but did not statistically differ between groups. CONCLUSIONS: The results of the present study suggest that EAA supplementation is safe and reduces the loss of muscle volume in older adults recovering from TKA. LEVEL OF EVIDENCE: Therapeutic Level II. See Instructions for Authors for a complete description of levels of evidence.

Glutamate and GABA concentrations following mild traumatic brain injury: a pilot study.

Animal models of mild traumatic brain injury (mTBI) suggest that metabolic changes in the brain occur immediately after a mechanical injury to the head. Proton magnetic resonance spectroscopy (1H-MRS) can be used to determine relative concentrations of metabolites in vivo in the human brain. The purpose of this study was to determine concentrations of glutamate and GABA in the brain acutely after mTBI and throughout 2 mo of recovery. Concentrations of glutamate and GABA were obtained using 1H-MRS in nine individuals who had suffered an mTBI and nine control individuals in two brain regions of interest: the primary motor cortex (M1), and the dorsolateral prefrontal cortex (DLPFC), and at three different time points postinjury: 72 h, 2 wk, and 2 mo postinjury. There were no differences between groups in concentrations of glutamate or GABA, or the ratio of glutamate to GABA, in M1. In the DLPFC, glutamate concentration was lower in the mTBI group compared with controls at 72 h postinjury (d = 1.02), and GABA concentration was lower in the mTBI group at 72 h and 2 wk postinjury (d = 0.81 and d = 1.21, respectively). The ratio of glutamate to GABA in the DLPFC was higher in the mTBI group at 2 wk postinjury (d = 1.63). These results suggest that changes in glutamate and GABA concentrations in the brain may be region-specific and may depend on the amount of time that has elapsed postinjury. NEW & NOTEWORTHY To our knowledge, this is the first study to examine neurotransmitter concentrations in vivo at multiple time points throughout recovery from mild traumatic brain injury in humans.

Influence of sex on performance fatigability of the plantar flexors following repeated maximal dynamic shortening contractions.

The purpose was to determine sex differences in fatigability during maximal, unconstrained velocity, shortening plantar flexions. The role of time-dependent measures (i.e., rate of torque development, rate of velocity development, and rate of neuromuscular activation) in such sex-related differences was also examined. By task termination, females exhibited smaller reductions in power and similar changes in rate of neuromuscular activation than males, indicating females were less fatigable than males.

Assessment of motor cortex excitability and inhibition during a cognitive task in individuals with concussion.

PRIMARY OBJECTIVE: To examine the function of the motor cortex during executive function tasks in individuals with concussion, relative to healthy controls. METHODS AND PROCEDURES: Transcranial magnetic stimulation (TMS) was used to assess motor cortex excitability and inhibition acutely, within 72 hours, and over two months, post-concussion in 23 participants, nine individuals with concussion and 14 controls. Participants performed a cognitive task during TMS to determine the impact of cognitive task on the motor cortex. MAIN OUTCOMES AND RESULTS: Resting motor threshold (p = 0.02) and motor-evoked potential (MEPRest) amplitude (p = 0.03) were different between groups, both suggesting greater corticospinal excitability in individuals with concussion. Cortical silent period (CSP) duration was greater at 72 hours (p = 0.03), one month (p = 0.003) and two months (p = 0.05) in individuals with concussion, suggesting increased intracortical inhibition. The performance of a cognitive task caused an increase in MEPRest (p = 0.006) and CSP (p = 0.04), compared to baseline in both groups, but no interaction of condition by group (p ≥ 0.91) for either measure. CONCLUSION: Simultaneously performing a cognitive task during motor cortex assessments increased corticospinal excitability and intracortical inhibition; however, the increase was not different between groups.

Cortical and Physical Function after Mild Traumatic Brain Injury.

PURPOSE: The aim of this study was to prospectively examine the association between intracortical inhibition and functional recovery after mild traumatic brain injury (mTBI). METHODS: Twenty individuals with mTBI and 20 matched control participants were assessed using transcranial magnetic stimulation, the Attentional Network Test, and gait analysis. Hierarchical linear modeling was used to longitudinally examine potential differences between groups and relationships in the pattern of recovery in cortical silent period (CSP) duration, cognitive reaction time, and single- and dual-task walking speeds across five testing time points. Individuals with mTBI were assessed within 72 h of injury, and again at 1 wk, 2 wk, 1 month, and 2 months postinjury. After initial testing, control participants followed a similar timeline. RESULTS: At the 72-h time point, the group with mTBI had longer reaction time (b = -91.76, P = 0.01), similar single-task walking speed (b = 0.055, P = 0.10), and slower dual-task walking speed (b = 0.10, P = 0.012) compared with control participants. The CSP duration also tended to be longer in individuals with mTBI than controls at the 72-h time point (b = -16.34, P = 0.062). The change is CSP duration over time was not significantly associated with the change in reaction time (b = -0.19, P = 0.47), single-task walking speed (b = 0.0001, P = 0.53), or dual-task walking speed (b < 0.001, P = 0.68). CONCLUSION: Although cognitive and motor functions were significantly impaired in the mTBI group acutely after injury, levels of intracortical inhibition were not associated with recovery in either functional domain.

Reliability of glutamate and GABA quantification using proton magnetic resonance spectroscopy.

The consistency and reliability of proton magnetic resonance spectroscopy (1H-MRS) assessments of neurotransmitter concentration has not been widely examined over multiple days. The purpose of this study was to determine the reliability of glutamate and GABA measures using a single-voxel 1H-MRS protocol in healthy men and women. Glutamate and GABA quantitations were obtained from the primary motor cortex (M1) and the dorsolateral prefrontal cortex (DLPFC) in 13 healthy individuals across 3 data collection sessions, including a baseline (Visit 1), 2-week (Visit 2), and 2-month time point (Visit 3). Glutamate concentrations were similar across visits in M1 (p=0.72) and the DLPFC (p=0.52). Reliability across days was excellent in M1 (R=0.93), and in the DLPFC (R=0.99). GABA concentrations were similar across visits in M1 (p=0.44) and in the DLPFC (p=0.59). Reliability of GABA concentration across days was excellent in M1 (R=0.93), and in the DLPFC (R=0.97). 1H-MRS is a reliable method for quantifying glutamate and GABA concentration in M1 and the DLPFC in humans.

Heterogeneous effects of old age on human muscle oxidative capacity in vivo: a systematic review and meta-analysis.

Despite intensive efforts to understand the extent to which skeletal muscle mitochondrial capacity changes in older humans, the answer to this important question remains unclear. To determine what the preponderance of evidence from in vivo studies suggests, we conducted a systematic review and meta-analysis of the effects of age on muscle oxidative capacity as measured noninvasively by magnetic resonance spectroscopy. A secondary aim was to examine potential moderators contributing to differences in results across studies, including muscle group, physical activity status, and sex. Candidate papers were identified from PubMed searches (n = 3561 papers) and the reference lists of relevant papers. Standardized effects (Hedges' g) were calculated for age and each moderator using data from the 22 studies that met the inclusion criteria (n = 28 effects). Effects were coded as positive when older (age, ≥55 years) adults had higher muscle oxidative capacity than younger (age, 20-45 years) adults. The overall effect of age on oxidative capacity was positive (g = 0.171, p < 0.001), indicating modestly greater oxidative capacity in old. Notably, there was significant heterogeneity in this result (Q = 245.8, p < 0.001; I2 = ∼70%-90%). Muscle group, physical activity, and sex were all significant moderators of oxidative capacity (p ≤ 0.029). This analysis indicates that the current body of literature does not support a de facto decrease of in vivo muscle oxidative capacity in old age. The heterogeneity of study results and identification of significant moderators provide clarity regarding apparent discrepancies in the literature, and indicate the importance of accounting for these variables when examining purported age-related differences in muscle oxidative capacity.