Enzyme activities in the tibialis anterior muscle of young moderately active men and women: relationship with body composition, muscle cross-sectional area and fibre type composition

The paradox of fatty-acid β-oxidation in muscle insulin resistance: Metabolic control and muscle heterogeneity

The skeletal muscle is a metabolically heterogeneous tissue that plays a key role in maintaining whole-body glucose homeostasis. It is well known that muscle insulin resistance (IR) precedes the development of type 2 diabetes. There is a consensus that the accumulation of specific lipid species in the tissue can drive IR. However, the role of the mitochondrial fatty-acid β-oxidation in IR and, consequently, in the control of glucose uptake remains paradoxical: interventions that either inhibit or activate fatty-acid β-oxidation have been shown to prevent IR. We here discuss the current theories and evidence for the interplay between β-oxidation and glucose uptake in IR. To address the underlying intricacies, we (1) dive into the control of glucose uptake fluxes into muscle tissues using the framework of Metabolic Control Analysis, and (2) disentangle concepts of flux and catalytic capacities taking into account skeletal muscle heterogeneity. Finally, we speculate about hitherto unexplored mechanisms that could bring contrasting evidence together. Elucidating how β-oxidation is connected to muscle IR and the underlying role of muscle heterogeneity enhances disease understanding and paves the way for new treatments for type 2 diabetes.

V ˙ Lamax: determining the optimal test duration for maximal lactate formation rate during all-out sprint cycle ergometry

PURPOSE

This study aimed to ascertain the optimal test duration to elicit the highest maximal lactate formation rate ( V ˙ Lamax), whilst exploring the underpinning energetics, and identifying the optimal blood lactate sampling period.

METHODS

Fifteen trained to well-trained males (age 27 ± 6 years; peak power: 1134 ± 174 W) participated in a randomised cross-over design completing three all-out sprint cycling tests of differing test durations (10, 15, and 30 s). Peak and mean power output (W and W.kg-1), oxygen uptake, and blood lactate concentrations were measured. V ˙ Lamax and energetic contributions (phosphagen, glycolytic, and oxidative) were determined using these parameters.

RESULTS

The shortest test duration of 10 s elicited a significantly (p = 0.003; p < 0.001) higher V ˙ Lamax (0.86 ± 0.17 mmol.L-1.s-1; 95% CI 0.802-0.974) compared with both 15 s (0.68 ± 0.18 mmol.L-1.s-1; 95% CI 0.596-0.794) and 30 s (0.45 ± 0.07 mmol.L-1.s-1; 95% CI 0.410-0.487). Differences in V ˙ Lamax were associated with large effect sizes (d = 1.07, d = 3.15). We observed 81% of the PCr and 53% of the glycolytic work completed over the 30 s sprint duration was attained after 10 s. BLamaxpost were achieved at 5 ± 2 min (ttest 10 s), 6 ± 2 min (ttest 15 s), and 7 ± 2 min (ttest 30 s), respectively.

CONCLUSION

Our findings demonstrated a 10 s test duration elicited the highest V ˙ Lamax. Furthermore, the 10 s test duration mitigated the influence of the oxidative metabolism during all-out cycling. The optimal sample time to determine peak blood lactate concentration following 10 s was 5 ± 2 min.

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 ([Formula: see text]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 [Formula: see text]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.

Sex differences in skeletal muscle fiber types: A meta-analysis

Biopsies have been acquired from living men and women to determine proportions of Type I (slow-twitch) and II (fast-twitch) skeletal muscle fibers since the 1970s. Sex differences have been assumed but the literature has not been submitted to meta-analysis. Here, the aim was to generate effect sizes of sex differences in muscle fiber cross-sectional areas, distribution percentages, and area percentages. Data from 2875 men and 2452 women, who participated in 110 studies, were analyzed. Myofibrillar adenosine triphosphatase histochemistry was used in 71.8% of studies to classify fibers as Type I, II, IIA, and/or IIX; immunohistochemistry, immunofluorescence, or sodium dodecyl sulfate-polyacrylamide gel electrophoresis were used in 35.4% of studies to similarly classify myosin heavy chain (MHC) isoform content. Most studies involved biopsies from vastus lateralis (79.1%) in healthy individuals (92.7%) between 18 and 59 years old (80.9%). Men exhibited greater cross-sectional areas for all fiber types (g = 0.40-1.68); greater distribution percentages for Type II, MHC II, IIA, IIX fibers (g = 0.26-0.34); greater area percentages for Type II, IIA, MHC IIA, IIX fibers (g = 0.39-0.93); greater Type II/I and Type IIA/I fiber area ratios (g = 0.63, 0.94). Women exhibited greater Type I and MHC I distribution percentages (g = -0.13, -0.44); greater Type I and MHC I area percentages (g = -0.53, -0.69); greater Type I/II fiber area ratios (g = -1.24). These data, which represent the largest repository of comparative muscle fiber type data from living men and women, can inform discussions about biological sex and its impact on pathologies and sports performance (e.g., explaining sex differences in muscle strength and muscle endurance).

The Biological Basis of Sex Differences in Athletic Performance: Consensus Statement for the American College of Sports Medicine

ABSTRACT

Biological sex is a primary determinant of athletic performance because of fundamental sex differences in anatomy and physiology dictated by sex chromosomes and sex hormones. Adult men are typically stronger, more powerful, and faster than women of similar age and training status. Thus, for athletic events and sports relying on endurance, muscle strength, speed, and power, males typically outperform females by 10%-30% depending on the requirements of the event. These sex differences in performance emerge with the onset of puberty and coincide with the increase in endogenous sex steroid hormones, in particular testosterone in males, which increases 30-fold by adulthood, but remains low in females. The primary goal of this consensus statement is to provide the latest scientific knowledge and mechanisms for the sex differences in athletic performance. This review highlights the differences in anatomy and physiology between males and females that are primary determinants of the sex differences in athletic performance and in response to exercise training, and the role of sex steroid hormones (particularly testosterone and estradiol). We also identify historical and nonphysiological factors that influence the sex differences in performance. Finally, we identify gaps in the knowledge of sex differences in athletic performance and the underlying mechanisms, providing substantial opportunities for high-impact studies. A major step toward closing the knowledge gap is to include more and equitable numbers of women to that of men in mechanistic studies that determine any of the sex differences in response to an acute bout of exercise, exercise training, and athletic performance.

Force-velocity profile in sprinting: sex effect

The ability to produce muscle power during sprint acceleration is a major determinant of physical performance. The comparison of the force-velocity (F-v: theoretical maximal force, F₀; velocity, v₀ and maximal power output, P_max) profile between men and women has attracted little attention. Most studies of sex differences have failed to apply a scaling ratio when reporting data. The present study investigated the sex effect on the F-v profile using an allometric model applied with body mass (BM), fat-free mass (FFM), fat-free mass of the lower limb (FFMLL), cross-sectional area (CSA) and leg length (LL) to mechanical parameters. Thirty students (15 men, 15 women) participated. Raw velocity-time data for three maximal 35 m sprints were measured with a radar. Mechanical parameters of the F-v relationship were calculated from the modelling of the velocity-time curve. When F₀ and P_max were allometrically scaled with BM (p = 0.538; ES = 0.23) and FFM (p = 0.176; ES = 0.51), there were no significant differences between men and women. However, when the allometric model was applied to P_max with FFMLL (p = 0.015; ES = 0.52), F₀ with CSA (p = 0.016; ES = 0.93) and v₀ with LL (p ≤ 0.001; ES = 1.98) differences between men and women persisted. FFM explained 83% of the sex differences in the F-v profile (p ≤ 0.001). After applying an allometric model, sex differences in the F-v profile are explained by other factors than body dimensions (i.e., physiological qualitative differences).

Narrative Review of Sex Differences in Muscle Strength, Endurance, Activation, Size, Fiber Type, and Strength Training Participation Rates, Preferences, Motivations, Injuries, and Neuromuscular Adaptations

ABSTRACT

Nuzzo, JL. Narrative review of sex differences in muscle strength, endurance, activation, size, fiber type, and strength training participation rates, preferences, motivations, injuries, and neuromuscular adaptations. J Strength Cond Res 37(2): 494-536, 2023-Biological sex and its relation with exercise participation and sports performance continue to be discussed. Here, the purpose was to inform such discussions by summarizing the literature on sex differences in numerous strength training-related variables and outcomes-muscle strength and endurance, muscle mass and size, muscle fiber type, muscle twitch forces, and voluntary activation; strength training participation rates, motivations, preferences, and practices; and injuries and changes in muscle size and strength with strength training. Male subjects become notably stronger than female subjects around age 15 years. In adults, sex differences in strength are more pronounced in upper-body than lower-body muscles and in concentric than eccentric contractions. Greater male than female strength is not because of higher voluntary activation but to greater muscle mass and type II fiber areas. Men participate in strength training more frequently than women. Men are motivated more by challenge, competition, social recognition, and a desire to increase muscle size and strength. Men also have greater preference for competitive, high-intensity, and upper-body exercise. Women are motivated more by improved attractiveness, muscle "toning," and body mass management. Women have greater preference for supervised and lower-body exercise. Intrasexual competition, mate selection, and the drive for muscularity are likely fundamental causes of exercise behaviors in men and women. Men and women increase muscle size and strength after weeks of strength training, but women experience greater relative strength improvements depending on age and muscle group. Men exhibit higher strength training injury rates. No sex difference exists in strength loss and muscle soreness after muscle-damaging exercise.

Differences in motor unit behavior during isometric contractions in patients with diabetic peripheral neuropathy at various disease severities

The aim of this study was to determine whether HD-sEMG is sensitive to detecting changes in motor unit behavior amongst healthy adults and type 2 diabetes mellitus (T2DM) patients presenting diabetic peripheral neuropathy (DPN) at different levels. Healthy control subjects (CON, n = 8) and T2DM patients presenting no DPN symptoms (ABS, n = 8), moderate DPN (MOD, n = 18), and severe DPN (SEV, n = 12) performed isometric ankle dorsiflexion at 30 % maximum voluntary contraction while high-density surface EMG (HD-sEMG) was recorded from the tibialis anterior muscle. HD-sEMG signals were decomposed, providing estimates of discharge rate, motor unit conduction velocity (MUCV), and motor unit territory area (MUTA). As a result, the ABS group presented reduced MUCV compared to CON. The groups with diabetes presented significantly larger MUTA compared to the CON group (p < 0.01), and the SEV group presented a significantly lower discharge rate compared to CON and ABS (p < 0.01). In addition, the SEV group presented significantly higher CoV_force compared to CON and MOD. These results support the use of HD-SEMG as a method to detect peripheral and central changes related to DPN.

Do men and women have different musculoskeletal symptoms at the same musculoskeletal discomfort level?

Owing to biological and social factors, illness-related musculoskeletal symptoms tend to vary between men and women. However, in the past, conceptualised discomfort metrics were applied uniformly to both genders. This study aimed to develop a scale to measure musculoskeletal discomfort that compares the symptoms between men and women. The scale aimed to determine the gender-based response patterns related to symptoms. A total of 707 men and 1302 women reported their symptoms on a body map. Factor analysis and item response theory were used to differentiate the identified symptoms in the construction of a musculoskeletal discomfort scale. Differences in work exposure appeared to explain the symptom patterns between men and women. The scale had eight levels, and it was found that at the same level of discomfort, men and women reported symptoms in different body regions.Practitioner summary: On this discomfort scale, the response patterns of men and women were categorised into eight levels. Symptoms differed by gender at the same musculoskeletal discomfort level. This is in contrast to previous studies in which scales were devised without considering differences between the genders.Abbreviations: WMSDs: work-related musculoskeletal disorders; BMI: body mass index; FA: factor analysis; IRT: item response theory; KMO: Kaiser-Meyer-Olkin; BST: Bartlett's test of sphericity; F: factor loading; h2: communality; α: Cronbach's alpha; ωt: McDonald's omega; ai: parameters of discrimination of the items; bik: parameters of difficulty of response categories; θj: latent trait; RMSEA: root mean square error of approximation; CFI: comparative fit index; TLI: Tucker-Lewis index; odu: musculoskeletal discomfort units; RA: rarely; OF: often; AL: always.

Metabolic and morphological profile in skeletal muscle of healthy boys and girls

It was hypothesized that the typical adult pattern of higher glycolytic capacity in skeletal muscle of males compared to females is not observed in children and that fiber cross-sectional area (CSA) is a determinant of glycolytic capacity in children. Biopsies were performed in vastus lateralis in 9-12 years-old healthy boys and girls (N = 27). Fiber types were classified by myofibrillar ATPase staining and CSA was measured using planimetry. Citrate synthase (CS) and lactate dehydrogenase (LD) were analyzed using fluorometric and spectrophotometric methods. There was no significant difference between boys and girls in CS activity (0.45 ± 0.1 μkat g^-1 dry muscle in boys and 0.42 ± 0.1 in girls) or LD activity (24 ± 6 μkat g^-1 dry muscle in boys and 25 ± 7 in girls). CSA did not differ between boys and girls. CS was inversely related to type I CSA (r = -0.62, p < 0.001) and LD was directly related to type IIA (r = 0.63, p < 0.001) and type IIB CSA (r = 0.72, p < 0.001). CSA was a significant determinant of CS and LD, even after adjusting for sex and relative fiber type area in multiple regression analysis. This suggests that the typical adult pattern of higher muscle glycolytic capacity in males than in females, as estimated by LD activity, was not observed in children. Sex-specific patterns in glycolytic capacity thus appear to develop during the transition from childhood to adulthood. In addition, fiber CSA was a strong determinant of both muscle glycolytic and oxidative capacity in children, regardless of sex.

Expanding the Gap: An Updated Look Into Sex Differences in Running Performance

Males consistently outperform females in athletic endeavors, including running events of standard Olympic distances (100 m to Marathon). The magnitude of this percentage sex difference, i.e., the sex gap, has evolved over time. Two clear trends in sex gap evolution are evident; a narrowing of the gap during the 20th century, followed by a period of stability thereafter. However, an updated perspective on the average sex gap from top 20 athlete performances over the past two decades reveals nuanced trends over time, indicating the sex gap is not fixed. Additionally, the sex gap varies with performance level; the difference in absolute running performance between males and females is lowest for world record/world lead performances and increases in lower-ranked elite athletes. This observation of an increased sex gap with world rank is evident in events 400 m and longer and indicates a lower depth in female competitive standards. Explanations for the sex difference in absolute performance and competition depth include physical (physiological, anatomical, neuromuscular, biomechanical), sociocultural, psychological, and sport-specific factors. It is apparent that females are the disadvantaged sex in sport; therefore, measures should be taken to reduce this discrepancy and enable both sexes to reach their biological performance potential. There is scope to narrow the sex performance gap by addressing inequalities between the sexes in opportunities, provisions, incentives, attitudes/perceptions, research, and media representation.

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

Functional reserve and sex differences during exercise to exhaustion revealed by post-exercise ischaemia and repeated supramaximal exercise

KEY POINTS

Females have lower fatigability than males during single limb isometric and dynamic contractions, but whether sex-differences exist during high-intensity whole-body exercise remains unknown. This study shows that males and females respond similarly to repeated supramaximal whole-body exercise, and that at task failure a large functional reserve remains in both sexes. Using post-exercise ischaemia with repeated exercise, we have shown that this functional reserve depends on the glycolytic component of substrate-level phosphorylation and is almost identical in both sexes. Metaboreflex activation during post-exercise ischaemia and the O₂ debt per kg of active lean mass are also similar in males and females after supramaximal exercise. Females have a greater capacity to extract oxygen during repeated supramaximal exercise and reach lower P ETC O 2 , experiencing a larger drop in brain oxygenation than males, without apparent negative repercussion on performance. Females had no faster recovery of performance after accounting for sex differences in lean mass.

ABSTRACT

The purpose of this study was to ascertain what mechanisms explain sex differences at task failure and to determine whether males and females have a functional reserve at exhaustion. Exercise performance, cardiorespiratory variables, oxygen deficit, and brain and muscle oxygenation were determined in 18 males and 18 females (21-36 years old) in two sessions consisting of three bouts of constant-power exercise at 120% of V ̇ O 2 max until exhaustion interspaced by 20 s recovery periods. In one of the two sessions, the circulation of both legs was occluded instantaneously (300 mmHg) during the recovery periods. Females had a higher muscle O₂ extraction during fatiguing supramaximal exercise than males. Metaboreflex activation, and lean mass-adjusted O₂ deficit and debt were similar in males and females. Compared to males, females reached lower P ETC O 2 and brain oxygenation during supramaximal exercise, without apparent negative consequences on performance. After the occlusions, males and females were able to restart exercising at 120% of V ̇ O 2 max , revealing a similar functional reserve, which depends on glycolytic component of substrate-level phosphorylation and its rate of utilization. After ischaemia, muscle O₂ extraction was increased, and muscle V ̇ O 2 was similarly reduced in males and females. The physiological response to repeated supramaximal exercise to exhaustion is remarkably similar in males and females when differences in lean mass are considered. Both sexes fatigue with a large functional reserve, which depends on the glycolytic energy supply, yet females have higher oxygen extraction capacity, but reduced P ETC O 2 and brain oxygenation.

JAK/STAT inhibition augments soleus muscle function in a rat model of critical illness myopathy via regulation of complement C3/3R

KEY POINTS

Critical illness myopathy (CIM) is a frequently observed negative consequence of modern critical care. Chronic Janus kinase (JAK)/signal transducer and activator of transcription activation impairs muscle size and function and is prominent following mechanical ventilation. We identify pSTAT-3 activation in tibialis anterior of CIM patients, before examining the potential benefits of JAK1/2 inhibition in an experimental model of CIM, where muscle mass and function are impaired. CIM activates complement cascade and increased monocyte infiltration in the soleus muscle, which was ameliorated by JAK1/2 inhibition, leading to reduced muscle degeneration and improved muscle force. Here, we demonstrate that JAK1/2 inhibition augments CIM muscle function through regulation of the complement cascade.

ABSTRACT

Critical illness myopathy (CIM) is frequently observed in response to modern critical care with negative consequences for patient quality of life, morbidity, mortality and healthcare costs. Janus kinase (JAK)/signal transducer and activator of transcription (STAT) activation is observed in limb muscles following controlled mechanical ventilation. Chronic JAK/STAT activation promotes loss of muscle mass and function. Thus, we hypothesized that JAK1/2 inhibition would improve muscle outcomes for CIM. Following 12 days of intensive care unit conditions, pSTAT-3 levels increased in tibialis anterior muscle of CIM patients (P = 0.0489). The potential of JAK1/2 inhibition was assessed in an experimental model of CIM, where soleus muscle size and force are impaired. JAK1/2 inhibition restores soleus force (P < 0.0001). CIM activated muscle complement cascade, which was ameliorated by JAK1/2 inhibition (P < 0.05, respectively). Soleus macrophage number corresponded with complement activity, leading to reduced muscle degeneration and augmented muscle function (P < 0.05). Thus, JAK/STAT inhibition improves soleus function by modulating the complement cascade and muscle monocyte infiltration. Collectively, we demonstrate that JAK/STAT inhibition augments muscle function in CIM.

Flexibility of equine bioenergetics and muscle plasticity in response to different types of training: An integrative approach, questioning existing paradigms

Equine bioenergetics have predominantly been studied focusing on glycogen and fatty acids. Combining omics with conventional techniques allows for an integrative approach to broadly explore and identify important biomolecules. Friesian horses were aquatrained (n = 5) or dry treadmill trained (n = 7) (8 weeks) and monitored for: evolution of muscle diameter in response to aquatraining and dry treadmill training, fiber type composition and fiber cross-sectional area of the M. pectoralis, M. vastus lateralis and M. semitendinosus and untargeted metabolomics of the M. pectoralis and M. vastus lateralis in response to dry treadmill training. Aquatraining was superior to dry treadmill training to increase muscle diameter in the hindquarters, with maximum effect after 4 weeks. After dry treadmill training, the M. pectoralis showed increased muscle diameter, more type I fibers, decreased fiber mean cross sectional area, and an upregulated oxidative metabolic profile: increased β-oxidation (key metabolites: decreased long chain fatty acids and increased long chain acylcarnitines), TCA activity (intermediates including succinyl-carnitine and 2-methylcitrate), amino acid metabolism (glutamine, aromatic amino acids, serine, urea cycle metabolites such as proline, arginine and ornithine) and xenobiotic metabolism (especially p-cresol glucuronide). The M. vastus lateralis expanded its fast twitch profile, with decreased muscle diameter, type I fibers and an upregulation of glycolytic and pentose phosphate pathway activity, and increased branched-chain and aromatic amino acid metabolism (cis-urocanate, carnosine, homocarnosine, tyrosine, tryptophan, p-cresol-glucuronide, serine, methionine, cysteine, proline and ornithine). Trained Friesians showed increased collagen and elastin turn-over. Results show that branched-chain amino acids, aromatic amino acids and microbiome-derived xenobiotics need further study in horses. They feed the TCA cycle at steps further downstream from acetyl CoA and most likely, they are oxidized in type IIA fibers, the predominant fiber type of the horse. These study results underline the importance of reviewing existing paradigms on equine bioenergetics.

Recovery from Different High-Intensity Interval Training Protocols: Comparing Well-Trained Women and Men

Due to physiological and anatomical sex differences, there are variations in the training response, and the recovery periods following exercise may be different. High-intensity interval training (HIIT) protocols are well-suited to differentially investigate the course of recovery. This study was conducted to determine sex-specific differences in the recovery following HIIT intervals interspersed with recovery phases of different lengths. Methods: Well-trained cyclists and triathletes (n = 11 females, n = 11 males) participated in this study. There were no significant sex differences in maximal heart rate (HR), relative peak power to body mass and fat-free mass, training volume, and VO_2max-percentiles (females: 91.8 ± 5.5 %, males: 94.6 ± 5.4 %). A 30 s Wingate test was performed four times, separated by different active recovery periods (1, 3, or 10 min). Lactate, HR, oxygen uptake, and subjective rating of exertion and recovery were determined. Results: For the recovery time of three and ten minutes, men showed significantly higher lactate concentrations (p = 0.04, p = 0.004). Contrary, HR recovery and subjective recovery were significant slower in women than in men. Conclusion: During HIIT, women may be more resistant to fatigue and have a greater ability to recover metabolically, but have a slower HR and subjective recovery.

A Comparison between Male and Female Athletes in Relative Strength and Power Performances

The aim of this study was to compare male vs. female athletes in strength and power performance relative to body mass (BM) and lean body mass (LBM) and to investigate the relationships between muscle architecture and strength in both genders. Sixteen men (age = 26.4 ± 5.0 years; body mass = 88.9 ± 16.6 kg; height = 177.6 ± 9.3 cm) and fourteen women (age = 25.1 ± 3.2 years; body mass = 58.1 ± 9.1 kg; height = 161.7 ± 4.8 cm) were tested for body composition and muscle thickness (MT) of vastus lateralis muscle (VT), pectoralis major (PEC), and trapezius (TRAP). In addition, participants were tested for lower body power at countermovement jump (CMJP) and upper-body power at bench press throw (BPT). Participants were also assessed for one repetition maximum (1RM) at bench press (1RMBP), deadlift (1RMDE), and squat (1RMSQ). Significantly greater (p < 0.01) MT of the VL, PEC and TRAP muscles and LBM were detected in men compared to women. Significantly greater (p < 0.05) 1RMBP and BPT adjusted for LBM were detected in men than in women. No significant gender differences after adjusting for LBM were detected for 1RMSQ (p = 0.945); 1RMDE (p = 0.472) and CMJP (p = 0.656). Significantly greater (p < 0.05) results in all performance assessments adjusted for MT of the specific muscles, were detected in males compared to females. Superior performances adjusted for MT and LBM in men compared to women, may be related to gender differences in muscle morphology and LBM distribution, respectively.

Network Physiology of Cortico-Muscular Interactions

Skeletal muscle activity is continuously modulated across physiologic states to provide coordination, flexibility and responsiveness to body tasks and external inputs. Despite the central role the muscular system plays in facilitating vital body functions, the network of brain-muscle interactions required to control hundreds of muscles and synchronize their activation in relation to distinct physiologic states has not been investigated. Recent approaches have focused on general associations between individual brain rhythms and muscle activation during movement tasks. However, the specific forms of coupling, the functional network of cortico-muscular coordination, and how network structure and dynamics are modulated by autonomic regulation across physiologic states remains unknown. To identify and quantify the cortico-muscular interaction network and uncover basic features of neuro-autonomic control of muscle function, we investigate the coupling between synchronous bursts in cortical rhythms and peripheral muscle activation during sleep and wake. Utilizing the concept of time delay stability and a novel network physiology approach, we find that the brain-muscle network exhibits complex dynamic patterns of communication involving multiple brain rhythms across cortical locations and different electromyographic frequency bands. Moreover, our results show that during each physiologic state the cortico-muscular network is characterized by a specific profile of network links strength, where particular brain rhythms play role of main mediators of interaction and control. Further, we discover a hierarchical reorganization in network structure across physiologic states, with high connectivity and network link strength during wake, intermediate during REM and light sleep, and low during deep sleep, a sleep-stage stratification that demonstrates a unique association between physiologic states and cortico-muscular network structure. The reported empirical observations are consistent across individual subjects, indicating universal behavior in network structure and dynamics, and high sensitivity of cortico-muscular control to changes in autonomic regulation, even at low levels of physical activity and muscle tone during sleep. Our findings demonstrate previously unrecognized basic principles of brain-muscle network communication and control, and provide new perspectives on the regulatory mechanisms of brain dynamics and locomotor activation, with potential clinical implications for neurodegenerative, movement and sleep disorders, and for developing efficient treatment strategies.

Sex differences in diaphragmatic fatigue: Effects of hypoxia during inspiratory loading

KEY POINTS

Under normoxic conditions, both healthy female and male diaphragms fatigue at a similar degree when matched for absolute diaphragmatic work during inspiratory loading. We investigated whether similarities in diaphragm fatigability persist under acute hypoxic conditions. We found that, in acute hypoxia, fatigue of the diaphragm is greater in women compared to men, whereas the magnitude of fatigue in normoxia did not differ between sexes. When matched for maximal diaphragm strength, women and men had a similar pressor response to work-matched inspiratory loading, independent of oxygen availability.

ABSTRACT

In normoxia, women and men display a comparable magnitude of diaphragmatic fatigue (DF) after work-matched inspiratory loading. Whether these sex similarities are maintained under acute hypoxic conditions is unknown. We investigated the influence of acute hypoxia during work-matched inspiratory pressure-threshold loading (PTL) on DF in healthy women (n = 8) and men (n = 8). Two 5 min isocapnic PTL tasks targeting a transdiaphragmatic pressure (P_di ) of 92 cmH₂ O in normoxia and hypoxia (8% O₂ ) were performed on separate days (≥48 h). DF was quantified by twitch P_di (P_di,tw ) via cervical magnetic stimulation post-PTL. Women and men had similar maximal P_di (P_di,max ; women: 171 ± 16, men: 178 ± 20 cmH₂ O) and relative target workload (women: 54 ± 5%, men: 53 ± 6% P_di,max ). The absolute cumulative diaphragmatic work did not differ between sexes in normoxia (women: 12,653 ± 1796 cmH₂ O s^-1 , men: 13,717 ± 1231 cmH₂ O s^-1 ; P = 0.202) or hypoxia (women: 11,624 ± 1860 cmH₂ O s^-1 , men: 12 722 ± 1502 cmH₂ O s^-1 ; P = 0.189). In normoxia, the magnitude of reduction in P_di,tw post-PTL was similar between sexes (women: -21.1 ± 8.4%, men: -22.5 ± 4.9 %; P = 0.193); however, a higher degree of DF was observed in women compared to men following PTL in acute hypoxia (women: -27.6 ± 7.7%, men: -23.4 ± 9.6%, P = 0.019). We conclude that the female diaphragm is more susceptible to fatigue after inspiratory loading under acute hypoxic conditions. This finding may be related to sex differences in diaphragm muscle metabolism, such as fibre type composition, contractile properties, substrate utilisation and blood perfusion.

Is sex a proxy for mechanical variables during an upper limb repetitive movement task? An investigation of the effects of sex and of anthropometric load on muscle fatigue

BACKGROUND

Women report more work-related pain and neck/shoulder musculoskeletal disorders than men. For the same absolute workload, due to lower strength, females generally work at a higher relative intensity, which could induce more fatigue. However, the arm's anthropometric load (AL) of men is higher. Therefore, simply lifting their arm could be more fatiguing. Sex as a variable is formed of many constructs, and analyses can become muddied by their differing responses to fatigue. No studies have considered AL, when comparing how fatigue affects men and women. The purpose was to determine if including the arm's AL in the statistical analysis would impact findings of sex-specific effects of shoulder fatigue on muscle EMG.

METHODS

Fifty-five (29m/26f) participants completed a repetitive pointing task (RPT) at shoulder height until they reported fatigue of 8+ on the BORG CR-10 scale. Muscle activities were measured using surface electrodes placed over the anterior deltoid (AD) and upper trapezius (UT) muscles. Muscle activity amplitude was quantified using root mean square (RMS). First- and last-minute data were used to assess change from no-fatigue (NF) to fatigue-terminal (FT) conditions. AL was calculated using sex-specific body parameter equations. General estimating equations (GEE) were used to determine the effects of sex and fatigue on RMS values, while including AL in the GEE.

RESULTS

There was no sex difference in time to reach fatigue. A significant main effect of sex on RMS was observed (χ²(1) = 4.17, p = 0.04) when including AL as a covariate. Females displayed a significantly higher percentage change in AD RMS from NF to FT, compared to males (p = 0.03), when AL was included in the GEE. No sex differences in UT were observed.

CONCLUSIONS

This sex difference emerged when AL was included as a covariate, suggesting that sex-associated anthropometric differences may contribute to sex differences in the fatigue response. Differences in the impact of AL on AD compared to UT could be explained by differences in their respective mechanical roles or muscle fiber content. Anthropometrics may be useful to include as covariates in future research to separate individual anthropometric differences from sex differences.

The Acute Physiological Responses to Traditional vs. Practical Blood Flow Restriction Resistance Exercise in Untrained Men and Women

This study compared the acute physiological responses of traditional and practical blood flow restriction resistance exercise (tBFR and pBFR, respectively) and high- and low-load resistance exercise without BFR (HL and LL, respectively), as well as the potential sex differences within the aforementioned exercise methods. Fourteen men and fifteen women randomly completed the following experimental conditions: (1) tBFR, consisting of four sets of 30-15-15-15 repetitions of the bilateral horizontal leg press and knee extension exercises, at 30% of one-repetition maximum (1-RM), with a 13.5 cm wide pneumatic cuff placed at the most proximal portion of each thigh and inflated to a pressure equivalent to 50% of the participant’s total occlusion pressure; (2) pBFR, which was the same as the tBFR condition, except that an elastic band wrapped around the proximal portion of each thigh at a tightness of 7 on a 0 to 10 perceived pressure scale was used to reduce blood flow; (3) LL, same as the tBFR and pBFR protocols, except that no BFR was applied; and (4) HL, consisting of 3 sets of 10 repetitions at 80% of 1-RM, with the same 1-min rest interval between sets and a 3-min rest period between exercises. At baseline, immediately post-, 5 min post-, and 15 min post-exercise, whole-blood lactate (WBL), indices of muscle swelling (muscle thickness and thigh circumference), hematocrit and plasma volume changes, were measured as well as superficial electromyography (sEMG) amplitude during exercise. There were no significant (p > 0.05) differences between the tBFR and pBFR exercise protocols for any of the physiological parameters assessed. However, significantly greater (p < 0.05) WBL and sEMG values were observed for HL compared to the remaining exercise conditions. Finally, males displayed greater WBL levels than females at 15 min post-exercise. Interestingly, males also displayed significantly (p < 0.05) greater sEMG amplitude than females within the low-load trials during leg press, but no significant (p < 0.05) sex differences were observed during knee extension. In conclusion, tBFR and pBFR seemed to be capable of inducing the same acute physiological responses. Furthermore, males displayed greater responses than females for some of the physiological parameters measured.

Universal spectral profile and dynamic evolution of muscle activation: a hallmark of muscle type and physiological state

The skeletal muscle is an integrated multicomponent system with complex dynamics of continuous myoelectrical activation of various muscle types across time scales to facilitate muscle coordination among units and adaptation to physiological states. To understand the multiscale dynamics of neuromuscular activity, we investigated spectral characteristics of different muscle types across time scales and their evolution with physiological states. We hypothesized that each muscle type is characterized by a specific spectral profile, reflecting muscle composition and function, that remains invariant over time scales and is universal across subjects. Furthermore, we hypothesized that the myoelectrical activation and corresponding spectral profile during certain movements exhibit an evolution path in time that is unique for each muscle type and reflects responses in muscle dynamics to exercise, fatigue, and aging. To probe the multiscale mechanism of neuromuscular regulation, we developed a novel protocol of repeated squat exercise segments, each performed until exhaustion, and we analyzed differentiated spectral power responses over a range of frequency bands for leg and back muscle activation in young and old subjects. We found that leg and back muscle activation is characterized by muscle-specific spectral profiles, with differentiated frequency band contribution, and a muscle-specific evolution path in response to fatigue and aging that is universal across subjects in each age group. The uncovered universality among subjects in the spectral profile of each muscle at a given physiological state, as well as the robustness in the evolution of these profiles over a range of time scales and states, reveals a previously unrecognized multiscale mechanism underlying the differentiated response of distinct muscle types to exercise-induced fatigue and aging.NEW & NOTEWORTHY To understand coordinated function of distinct fibers in a muscle, we investigated spectral dynamics of muscle activation during maximal exercise across a range of frequency bands and time scales of observation. We discovered a spectral profile that is specific for each muscle type, robust at short, intermediate, and large time scales, universal across subjects, and characterized by a muscle-specific evolution path with accumulation of fatigue and aging, indicating a previously unrecognized multiscale mechanism of muscle tone regulation.

Strength and Electromyographic Responses of Upper and Lower Limbs During Maximal Intermittent Contractions in Males and Females

Carr, JC and Ye, X. Strength and electromyographic responses of upper and lower limbs during maximal intermittent contractions in males and females. J Strength Cond Res XX(X): 000-000, 2020-This study examined the strength and electromyographic (EMG) responses of upper vs. lower limb muscles during intermittent maximal contractions in both sexes. Twenty subjects (n = 7 women) performed a fatiguing protocol (6, 30-second intermittent maximal isometric contractions with a 50% duty cycle) with either the elbow flexors or the knee extensors on separate visits. Bipolar surface EMG signals were detected from the biceps brachii and vastus lateralis muscles (n = 5 women retained). Women maintained more of their maximal force than men (Δforce: men vs. women = -55.0 ± 12.8% vs. -43.3 ± 9.9%, p = 0.042). Although force loss was similar between the elbow flexors and knee extensors, the EMG responses showed greater reductions for the biceps brachii than those for the vastus lateralis (Δamplitude: biceps brachii vs. vastus lateralis: -32.0 ± 22.3% vs. -18.9 ± 28.9%; Δmedian frequency: biceps brachii vs. vastus lateralis: -31.1 ± 14.5% vs. -10.3 ± 17.0%). During a series of maximal intermittent isometric contractions with 30 seconds of recovery between work bouts, women are more fatigue resistant than men. In addition, the greater electrophysiological fatigue exhibited by the biceps brachii than that by the vastus lateralis suggests that high-intensity contractions involving elbow flexion will have a greater rate of fatigue progression than those involving knee extension.

Repeatability and reliability of muscle relaxation properties induced by motor cortical stimulation

Impaired muscle relaxation is a feature of many neuromuscular disorders. However, few tests are available to quantify muscle relaxation. Transcranial magnetic stimulation (TMS) of the motor cortex can induce muscle relaxation by abruptly inhibiting corticospinal drive. The aim of our study was to investigate whether repeatability and reliability of TMS-induced relaxation are greater than voluntary relaxation. Furthermore, effects of sex, cooling, and fatigue on muscle relaxation properties were studied. Muscle relaxation of deep finger flexors was assessed in 25 healthy subjects (14 men and 11 women, age 39.1 ± 12.7 and 45.3 ± 8.7 yr, respectively) with handgrip dynamometry. All outcome measures showed greater repeatability and reliability in TMS-induced relaxation compared with voluntary relaxation. The within-subject coefficient of variability of normalized peak relaxation rate was lower in TMS-induced relaxation than in voluntary relaxation (3.0% vs. 19.7% in men and 6.1% vs. 14.3% in women). The repeatability coefficient was lower (1.3 vs. 6.1 s^-1 in men and 2.3 vs. 3.1 s^-1 in women) and the intraclass correlation coefficient was higher (0.95 vs. 0.53 in men and 0.78 vs. 0.69 in women) for TMS-induced relaxation compared with voluntary relaxation. TMS enabled demonstration of slowing effects of sex, muscle cooling, and muscle fatigue on relaxation properties that voluntary relaxation could not. In conclusion, repeatability and reliability of TMS-induced muscle relaxation were greater compared with voluntary muscle relaxation. TMS-induced muscle relaxation has the potential to be used in clinical practice for diagnostic purposes and therapy effect monitoring in patients with impaired muscle relaxation. NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS)-induced muscle relaxation demonstrates greater repeatability and reliability compared with voluntary relaxation, represented by the ability to demonstrate typical effects of sex, cooling, and fatigue on muscle relaxation properties that were not seen in voluntary relaxation. In clinical practice, TMS-induced muscle relaxation could be used for diagnostic purposes and therapy effect monitoring. Furthermore, fewer subjects will be needed for future studies when using TMS to demonstrate differences in muscle relaxation properties.

High Incomplete Skeletal Muscle Fatty Acid Oxidation Explains Low Muscle Insulin Sensitivity in Poorly Controlled T2D

CONTEXT

Almost 50% of type 2 diabetic (T2D) patients are poorly controlled [glycated hemoglobin (HbA1c) ≥ 7%]; however, the mechanisms responsible for progressively worsening glycemic control are poorly understood. Lower skeletal muscle mitochondrial respiratory capacity is associated with low insulin sensitivity and the development of T2D.

OBJECTIVE

We investigated if skeletal muscle insulin sensitivity (SI) was different between well-controlled T2D (WCD) and poorly controlled T2D (PCD) and if the difference was associated with differences resulting from mitochondrial respiratory function.

DESIGN

Vastus lateralis muscle mitochondrial respiration, mitochondrial content, mitochondrial enzyme activity, and fatty acid oxidation (FAO) were measured. SI and the acute response to glucose (AIRg) were calculated by MINMOD analysis from glucose and insulin obtained during a modified, frequently sampled, intravenous glucose tolerance test.

RESULTS

SI and AIRg were lower in PCD than WCD. Muscle incomplete FAO was greater in PCD than WCD and greater incomplete FAO was associated with lower SI and higher HbA1c. Hydroxyacyl-coenzyme A dehydrogenase expression and activity were greater in PCD than WCD. There was no difference in maximal mitochondrial respiration or content between WCD and PCD.

CONCLUSION

The current results suggest that greater skeletal muscle incomplete FAO in poorly controlled T2D is due to elevated β oxidation and is associated with worsening muscle SI.

Distribution of muscle fibre conduction velocity for representative samples of motor units in the full recruitment range of the tibialis anterior muscle

AIM

Motor units are recruited in an orderly manner according to the size of motor neurones. Moreover, because larger motor neurones innervate fibres with larger diameters than smaller motor neurones, motor units should be recruited orderly according to their conduction velocity (MUCV). Because of technical limitations, these relations have been previously tested either indirectly or in small motor unit samples that revealed weak associations between motor unit recruitment threshold (RT) and MUCV. Here, we analyse the relation between MUCV and RT for large samples of motor units.

METHODS

Ten healthy volunteers completed a series of isometric ankle dorsiflexions at forces up to 70% of the maximum. Multi-channel surface electromyographic signals recorded from the tibialis anterior muscle were decomposed into single motor unit action potentials, from which the corresponding motor unit RT, MUCV and action potential amplitude were estimated. Established relations between muscle fibre diameter and CV were used to estimate the fibre size.

RESULTS

Within individual subjects, the distributions of MUCV and fibre diameters were unimodal and did not show distinct populations. MUCV was strongly correlated with RT (mean (SD) R²  = 0.7 (0.09), P < 0.001; 406 motor units), which supported the hypothesis that fibre diameter is associated with RT.

CONCLUSION

The results provide further evidence for the relations between motor neurone and muscle fibre properties for large samples of motor units. The proposed methodology for motor unit analysis has also the potential to open new perspectives in the study of chronic and acute neuromuscular adaptations to ageing, training and pathology.

Fine-mapping of genes determining extrafusal fiber properties in murine soleus muscle

Muscle fiber cross-sectional area (CSA) and proportion of different fiber types are important determinants of muscle function and overall metabolism. Genetic variation plays a substantial role in phenotypic variation of these traits; however, the underlying genes remain poorly understood. This study aimed to map quantitative trait loci (QTL) affecting differences in soleus muscle fiber traits between the LG/J and SM/J mouse strains. Fiber number, CSA, and proportion of oxidative type I fibers were assessed in the soleus of 334 genotyped female and male mice of the F₃₄ generation of advanced intercross lines (AIL) derived from the LG/J and SM/J strains. To increase the QTL detection power, these data were combined with 94 soleus samples from the F₂ intercross of the same strains. Transcriptome of the soleus muscle of LG/J and SM/J females was analyzed by microarray. Genome-wide association analysis mapped four QTL (genome-wide P < 0.05) affecting the properties of muscle fibers to chromosome 2, 3, 4, and 11. A 1.5-LOD QTL support interval ranged between 2.36 and 4.67 Mb. On the basis of the genomic sequence information and functional and transcriptome data, we identified candidate genes for each of these QTL. The combination of analyses in F₂ and F₃₄ AIL populations with transcriptome and genomic sequence data in the parental strains is an effective strategy for refining QTL and nomination of the candidate genes.

Neural, biomechanical, and physiological factors involved in sex-related differences in the maximal rate of isometric torque development

Objective

Recent research has reported that lower maximal rate of torque development (dτ/dt_max) exhibited by females, relative to males, during knee extension can be accounted for by normalization to a maximal voluntary contraction (MVC); however, this was not seen in the upper limb.

Purpose

The aim of the current work was to examine the contribution of maximum strength (τ_max), twitch contraction time (CT), muscle fiber condition velocity (MFCV), and rate of muscle activation (Q₃₀) to sex-differences in the dτ/dt_max during maximal isometric dorsiflexion.

Methods

Thirty-eight participants (20 males; 18 females) performed both maximal voluntary and evoked isometric contractions of the tibialis anterior across 3 days. Ten maximal compound muscle action potentials were elicited and subsequently followed by three, 5-s contractions. From the recordings, MFCV, dτ/dt_max, τ_max, CT, electromechanical delay (EMD), root-mean squared (RMS) amplitude, peak-to-peak voltage (Vpp), and Q₃₀ were calculated.

Results

An ANCOVA showed that τ_max accounted for all the sex-differences in dτ/dt_max (p = 0.96). There were no significant differences between groups with respect to MFCV, RMS amplitude, Vpp amplitude, or CT. However, there was a significant sex-difference in dτ/dt_max, τ_max, and Q₃₀. Females had longer evoked EMD times compared with males (15.69 ± 10.57 ms versus 9.95 ± 3.46 ms; p = 0.01), but the voluntary EMD times were not different.

Conclusion

The current research supports the work by Hannah et al. Exp Physiol 97:618–629, (2012) that normalization to MVC in the quadriceps is able to account for all sex-differences in rate of toque development in the lower limb.

Non-invasive assessment of phosphate metabolism and oxidative capacity in working skeletal muscle in healthy young Chinese volunteers using (31)P Magnetic Resonance Spectroscopy

Background. Generally, males display greater strength and muscle capacity than females while performing a task. Muscle biopsy is regarded as the reference method of evaluating muscle functions; however, it is invasive and has sampling errors, and is not practical for longitudinal studies and dynamic measurement during excise. In this study, we built an in-house force control and gauge system for quantitatively applying force to quadriceps while the subjects underwent ³¹P Magnetic Resonance Spectroscopy (³¹P-MRS); our aim was to investigate if there is a sex difference of phosphate metabolite change in working muscles in young heathy Chinese volunteers.

Methods. Volunteers performed knee-extending excises using a force control and gauge system while lying prone in a Philips 3T Magnetic Resonance (MR) scanner. The ³¹P-MRS coil was firmly placed under the middle of the quadriceps . ³¹P-MRS measurements of inorganic phosphate (Pi), phosphocreatine (PCr) and adenosine triphosphate (ATP) were acquired from quadriceps while subjects were in a state of pre-, during- and post-exercise. The PCr, Pi, PCr/Pi, PCr/ATP, pH, work/energy cost ratio (WE), k_PCr and oxidative capacity were compared between males and females.

Results. A total of 17 volunteers underwent the study. Males: N = 10, age = 23.30 ± 1.25years; females: N = 7, age = 23.57 ± 0.79 years. In this study, males had significantly greater WE (16.33 ± 6.46 vs. 7.82 ± 2.16, p = 0.002) than females. Among PCr, Pi, PCr/Pi, PCr/ATP, pH, kPCr and oxidative capacity at different exercise status, only PCr/Pi (during-exercise, males = 5.630 ± 1.647, females = 4.014 ± 1.298, p = 0.047), PCr/ATP (during-exercise, males =1.273 ± 0.219, females = 1.523 ± 0.167, p = 0.025), and ATP (post-exercise, males = 24.469 ± 3.911 mmol/kg, females = 18.353 ± 4.818 mmol/kg, p = 0.035) had significant sex differences. Males had significantly greater PCr/Pi, but less PCr/ATP than females during exercise, suggesting males had higher energy transfer efficiency than females. At the post-exercise status, the recovery of PCr did not show sex difference.

Conclusions. Our in-house force control and gauge system quantitatively applied force during the exercise for ³¹P-MRS experiments, and a sex difference of higher energy transfer efficiency and WE was detected in males with mild loaded exercising quadriceps. This noninvasive technology allows us to further study and understand the sex difference of high energy phosphate metabolism in the future.

The influence of confounding factors on the relationship between muscle contraction level and MF and MPF values of EMG signal: a review

The purpose of this article is to gather results of studies on the relationship between median frequency (MF) and mean power frequency (MPF) and the level of muscle contraction, and to use those results to discuss the differences in the trends according to factors related to measurement technique and subject. Twenty-one studies with 63 cases for upper limb muscles and nine studies with 31 cases for lower limb muscles were analysed. Most results showed an increase in parameters with an increased level of muscle contraction, only some studies showed a decrease. The influence on parameters of the level of muscle contraction and factors such as subjects, type of contraction, muscle length and electrodes was analysed for each muscle. It was concluded that when analysing the influence of different factors on MF and MPF, because those factors interact they should be considered together, not separately.

FHL3 differentially regulates the expression of MyHC isoforms through interactions with MyoD and pCREB

In skeletal muscle, muscle fiber types are defined by four adult myosin heavy chain (MyHC) isoforms. Four and a half LIM domain protein 3 (FHL3) regulates myoblasts differentiation and gene expression by acting as a transcriptional co-activator or co-repressor. However, how FHL3 regulates MyHC expression is currently not clear. In this study, we found that FHL3 down-regulated the expression of MyHC 1/slow and up-regulated the expression of MyHC 2a and MyHC 2b, whereas no significant effect was found on MyHC 2x expression. MyoD and phosphorylated cAMP response element binding protein (pCREB) played important roles in the regulation of MyHC 1/slow and MyHC 2a expression by FHL3, respectively. FHL3 could interact with MyoD, CREB and pCREB in vivo. pCREB had stronger interaction with the cyclic AMP-responsive elements (CRE) of the MyHC 2a promoter compared with CREB, and FHL3 significantly affected the binding capacity of pCREB to CRE. We established a model in which FHL3 promotes the expression of MyHC 2a through CREB-mediated transcription and inhibits the expression of MyHC 1/slow by inhibiting MyoD transcription activity during myogenesis. Our data support the notion that FHL3 plays important roles in the regulation of muscle fiber type composition.

Physiological and health-related adaptations to low-volume interval training: influences of nutrition and sex

Interval training refers to the basic concept of alternating periods of relatively intense exercise with periods of lower-intensity effort or complete rest for recovery. Low-volume interval training refers to sessions that involve a relatively small total amount of exercise (i.e. ≤10 min of intense exercise), compared with traditional moderate-intensity continuous training (MICT) protocols that are generally reflected in public health guidelines. In an effort to standardize terminology, a classification scheme was recently proposed in which the term 'high-intensity interval training' (HIIT) be used to describe protocols in which the training stimulus is 'near maximal' or the target intensity is between 80 and 100 % of maximal heart rate, and 'sprint interval training' (SIT) be used for protocols that involve 'all out' or 'supramaximal' efforts, in which target intensities correspond to workloads greater than what is required to elicit 100 % of maximal oxygen uptake (VO2max). Both low-volume SIT and HIIT constitute relatively time-efficient training strategies to rapidly enhance the capacity for aerobic energy metabolism and elicit physiological remodeling that resembles changes normally associated with high-volume MICT. Short-term SIT and HIIT protocols have also been shown to improve health-related indices, including cardiorespiratory fitness and markers of glycemic control in both healthy individuals and those at risk for, or afflicted by, cardiometabolic diseases. Recent evidence from a limited number of studies has highlighted potential sex-based differences in the adaptive response to SIT in particular. It has also been suggested that specific nutritional interventions, in particular those that can augment muscle buffering capacity, such as sodium bicarbonate, may enhance the adaptive response to low-volume interval training.

Ergonomic task reduction prevents bone osteopenia in a rat model of upper extremity overuse

We evaluated the effectiveness of ergonomic workload reduction of switching rats from a high repetition high force (HRHF) lever pulling task to a reduced force and reach rate task for preventing task-induced osteopenic changes in distal forelimb bones. Distal radius and ulna trabecular structure was examined in young adult rats performing one of three handle-pulling tasks for 12 wk: (1) HRHF, (2) low repetition low force (LRLF); or (3) HRHF for 4 wk and than LRLF thereafter (HRHF-to-LRLF). Results were compared to age-matched controls rats. Distal forelimb bones of 12-wk HRHF rats showed increased trabecular resorption and decreased volume, as control rats. HRHF-to-LRLF rats had similar trabecular bone quality as control rats; and decreased bone resorption (decreased trabecular bone volume and serum CTX1), increased bone formation (increased mineral apposition, bone formation rate, and serum osteocalcin), and decreased osteoclasts and inflammatory cytokines, than HRHF rats. Thus, an ergonomic intervention of HRHF-to-LRLF prevented loss of trabecular bone volume occurring with prolonged performance of a repetitive upper extremity task. These findings support the idea of reduced workload as an effective approach to management of work-related musculoskeletal disorders, and begin to define reach rate and load level boundaries for such interventions.

Fibre type-specific change in FXYD1 phosphorylation during acute intense exercise in humans

The aim of the present study was to examine fibre type-specific Na(+)-K(+) pump subunit expression and exercise-induced alterations in phospholemman (FXYD1) phosphorylation in humans. Segments of human skeletal muscle fibres were dissected and fibre typed, and protein expression was determined by Western blotting. The protein expression of the Na(+)-K(+) pump α2 isoform was lower in type I than in type II fibres (0.63 ± 0.04 a.u. vs. 1.00 ± 0.07 a.u., P < 0.001), while protein expression of the Na(+)-K(+) pump α1 and β1 isoforms was not different. Protein expression of the ATP-dependent potassium channel Kir6.2 was higher in type I compared with type II fibres. In both type I (P < 0.01) and type II fibres (P < 0.001) the AB_FXYD1 signal was lower after exercise compared with rest, indicating an increase in unspecific FXYD1 phosphorylation. The FXYD1 serine 68 phosphorylation was higher (P < 0.001) after exercise compared with rest in type II fibres (1.90 ± 0.25 vs. 1.00 ± 0.08) and not changed in type I fibres. Total FXYD1 was not expressed in a fibre type-specific manner. Expression of phosphofructokinase was lower (P < 0.001) in type I than in type II fibres, whereas citrate synthase and 3-hydroxyacyl-CoA dehydrogenase were more abundant (P < 0.001) in type I fibres. In conclusion, FXYD1 phosphorylation at serine 68 increased after an acute bout of intense exercise in human type II fibres, while AB_FXYD1 signal intensity was lower in both type I and type II fibres, indicating fibre type-specific differences in FXYD1 phosphorylation on serine 63, serine 68 and threonine 69. This, together with the observation of a higher abundance of the Na(+)-K(+) pump α2 isoform protein in type II fibres, is likely to have importance for the exercise-induced human Na(+)-K(+) pump activity in the different fibre types.

Supraspinal fatigue is similar in men and women for a low-force fatiguing contraction

PURPOSE

This study determined the contribution of supraspinal fatigue to the sex difference in neuromuscular fatigue for a low-intensity fatiguing contraction. Because women have greater motor responses to arousal than men, we also examined whether cortical and motor nerve stimulation, techniques used to quantify central fatigue, would alter the sex difference in muscle fatigue.

METHODS

In study 1, cortical stimulation was elicited during maximal voluntary contractions (MVC) before and after a submaximal isometric contraction at 20% MVC with the elbow flexor muscles in 29 young adults (20 ± 2.6 yr, 14 men). In study 2, 10 men and 10 women (19.1 ± 2.9 yr) performed a fatiguing contraction in the presence and absence of cortical and motor nerve stimulation.

RESULTS

Study 1: Men had a briefer time to task failure than women (P = 0.009). Voluntary activation was reduced after the fatiguing contraction (P < 0.001) similarly for men and women. Motor-evoked potential area and the EMG silent period increased similarly with fatigue for both sexes. Peak relaxation rates, however, were greater for men than women and were associated with time to task failure (P < 0.05). Force fluctuations, RPE, HR, and mean arterial pressure increased at a greater rate for men than for women during the fatiguing contraction (P < 0.05). Study 2: Time to task failure, force fluctuations, and all other physiological variables assessed were similar for the control session and stimulation session (P > 0.05) for both men and women.

CONCLUSIONS

Supraspinal fatigue was similar for men and women after the low-force fatiguing contraction, and the sex difference in muscle fatigue was associated with peripheral mechanisms. Furthermore, supraspinal fatigue can be quantified in both men and women without influencing motor performance.

Effects of weight loss and leptin on skeletal muscle in human subjects

Maintenance of a 10% or greater reduced body weight results in decreases in the energy cost of low levels of physical activity beyond those attributable to the altered body weight. These changes in nonresting energy expenditure are due mainly to increased skeletal muscle work efficiency following weight loss and are reversed by the administration of the adipocyte-derived hormone leptin. We have also shown previously that the maintenance of a reduced weight is accompanied by a decrease in ratio of glycolytic (phosphofructokinase) to oxidative (cytochrome c oxidase) activity in vastus lateralis muscle that would suggest an increase in the relative expression of the myosin heavy chain I (MHC I) isoform. We performed analyses of vastus lateralis muscle needle biopsy samples to determine whether maintenance of an altered body weight was associated with changes in skeletal muscle metabolic properties as well as mRNA expression of different isoforms of the MHC and sarcoplasmic endoplasmic reticular Ca(2+)-dependent ATPase (SERCA) in subjects studied before weight loss and then again after losing 10% of their initial weight and receiving twice daily injections of either placebo or replacement leptin in a single blind crossover design. We found that the maintenance of a reduced body weight was associated with significant increases in the relative gene expression of MHC I mRNA that was reversed by the administration of leptin as well as an increase in the expression of SERCA2 that was not significantly affected by leptin. Leptin administration also resulted in a significant increase in the expression of the less MHC IIx isoform compared with subjects receiving placebo. These findings are consistent with the leptin-reversible increase in skeletal muscle chemomechanical work efficiency and decrease in the ratio of glycolytic/oxidative enzyme activities observed in subjects following dietary weight loss.

Skeletal muscle signaling response to sprint exercise in men and women

To determine if there is a sex dimorphism in the skeletal muscle signaling response to sprint exercise, 17 men and ten women performed a 30-s Wingate test. Muscle biopsies were taken before, immediately after the exercise and at 30 and 120 min during the recovery period. Thr(172)-AMPKα, Ser(221)-ACCβ, Thy(705)-STAT3, Thr(202)/Thy(204)-ERK1/2 and Thr(180)/Thy(182)-p38MAPK phosphorylation responses to sprint exercise were not statistically different between men and women. AMPKα phosphorylation was enhanced fourfold 30 min after the sprint exercise in males and females (P < 0.01). ACCβ phosphorylation was enhanced by about threefold just after the sprint test exercise and 30 min into the recovery period in males and females (P < 0.01). STAT3 phosphorylation was increased 2 h after the Wingate test compared to the value observed right after the end of the exercise (P < 0.05), and 30 min after the Wingate test there was a 2.5-fold increase in ERK1/2 phosphorylation, compared to both the pre-exercise and to the value observed right after the Wingate test (both, P < 0.05). In conclusion, the skeletal muscle signaling response to a single bout of sprint exercise mediated by AMPK, ACC, STAT3, ERK and p38MAPK is not statistically different between men and women. Marked increases in AMPKα, ACCβ, STAT3 and ERK phosphorylation were observed after a single 30-s all-out sprint (Wingate test) in the vastus lateralis.

A critical review on physical factors and functional characteristics that may explain a sex/gender difference in work-related neck/shoulder disorders

The objective of this paper is to critically review recent literature on physical and functional sex/gender (s/g) differences, with focus on physical determinants associated with neck/shoulder musculoskeletal injuries. It is well known that there are s/g differences in anthropometrical and functional body characteristics (e.g. size and strength). However, s/g differences may be wrongly attributed if data analysis does not include appropriate corrections (e.g. by strength for endurance). Recent literature on motor control shows that there may indeed be s/g differences in muscle coordination and movement strategies during upper limb tasks that are not currently explained by methodological inadequacies. Moreover, recent studies have shown differences between men and women in sensory hypersensitivity characteristics associated with neck/shoulder injuries. Taken together, the literature points to the importance of accounting for possible s/g differences at all levels of the biopsychosocial system in order to better understand sex- and gender-specific issues relevant to workplace health.

PRACTITIONER SUMMARY

This article critically reviews recent literature and a conceptual model highlighting s/g differences in physical and functional characteristics related to neck/shoulder musculoskeletal disorders (NSMSD). Findings have implications on understanding how personal factors may affect NSMSD risk. With better understanding, practitioners can make more appropriate decisions to prevent work-related NSMSD.

Sex differences in fatigue resistance are muscle group dependent

PURPOSE

Women are often reported to be generally more resistant to fatigue than men for relative-intensity tasks. This has been observed repeatedly for elbow flexors, whereas at the ankle, sex differences appear less robust, suggesting localized rather than systemic influences. Thus, the purpose of this study was to examine sex differences in fatigue resistance at muscle groups in a single cohort and which factors, if any, predict endurance time.

METHODS

Thirty-two young adults (age = 19-44 yr, 16 women) performed sustained isometric contractions at 50% maximum voluntary isometric contraction to failure for elbow flexion and ankle dorsiflexion. Pain, exertion, and muscle EMG were assessed throughout. Self-reported baseline activity was measured using the International Physical Activity Questionnaire.

RESULTS

Women were significantly more resistant to fatigue than men at the elbow (112.3 ± 6.2 vs 80.3 ± 5.8 s, P = 0.001) but not at the ankle (140.6 ± 10.7 vs 129.2 ± 10.5 s, P = 0.45). Peak torque was greater in men than that in women (P < 0.0001) at the ankle (45.0 ± 1.7 vs 30.1 ± 1.0 N·m) and at the elbow (75.7 ± 3.1 vs 34.4 ± 2.2 N·m). Peak torque was significantly related to endurance time at the elbow (R2= 0.30) but not at the ankle (R2 = 0.03). Peak pain, rate of pain increase, peak exertion, EMG, and baseline physical activity did not differ between sexes.

CONCLUSIONS

Sex differences in fatigue resistance are muscle group specific. Women were more fatigue resistant at the elbow but not at the ankle during a sustained isometric contraction. Further, factors that may contribute to fatigue resistance for one muscle group (e.g., sex, peak torque) may not be critical at another.

Cyclic compressive loading facilitates recovery after eccentric exercise

PURPOSE

To assess the biologic basis of massage therapies, we developed an experimental approach to mimic Swedish massage and evaluate this approach on recovery from eccentric exercise-induced muscle damage using a well-controlled animal model.

METHODS

Tibialis anterior muscles of six New Zealand White rabbits were subjected to one bout of damaging, eccentric contractions. One muscle was immediately subjected to cyclic compressive loads, and the contralateral served as the exercised control.

RESULTS

We found that commencing 30 min of cyclic compressive loading to the muscle, immediately after a bout of eccentric exercise, facilitated recovery of function and attenuated leukocyte infiltration. In addition, fiber necrosis and wet weight of the tissue were also reduced by compressive loading.

CONCLUSION

We conclude that subjecting muscle to compressive loads immediately after exercise leads to an enhanced recovery of muscle function and attenuation of the damaging effects of inflammation in the rabbit model. Although these observations suggest that skeletal muscle responds to cyclic compressive forces similar to those generated by clinical approaches, such as therapeutic massage, further research is needed to assess the translational efficacy of these findings.

Effects of experimental weight perturbation on skeletal muscle work efficiency, fuel utilization, and biochemistry in human subjects

Maintenance of a body weight 10% above or below that "customary" for lean or obese individuals results in respective increases or decreases in the energy expended in low levels of physical activity (nonresting energy expenditure, NREE). These changes are greater than can be accounted for by the altered body weight or composition and are due mainly to altered skeletal muscle work efficiency at low levels of power generation. We performed biochemical analysis of vastus lateralis muscle needle biopsy samples to determine whether maintenance of an altered body weight was associated with changes in skeletal muscle histomorphology. We found that the maintenance of a 10% reduced body weight was associated with significant declines in glycolytic (phosphofructokinase, PFK) enzyme activity and, in particular, in the ratio of glycolytic to oxidative (cytochrome c oxidase, COX) enzyme activity without significant changes in the activities of enzymes relevant to mitochondrial density, respiratory chain activity, or fuel transport; or in skeletal muscle fiber type or glycogen stores. The fractional change in the ratio of PFK/COX activity in subjects following weight loss was significantly correlated with changes in the systemic respiratory exchange ratio (RER) and measures of mechanical efficiency of skeletal muscle at low workloads (pedaling a bicycle to generate 10 or 25 W of power). Thus, predictable changes in systemic skeletal muscle biochemistry accompany the maintenance of an altered body weight and account for a significant portion of the variance in skeletal muscle work efficiency and fuel utilization at reduced body weight.

Sex alters impact of repeated bouts of sprint exercise on neuromuscular activity in trained athletes

This study characterized the effect of sex on neuromuscular activity during repeated bouts of sprint exercise. Thirty-three healthy male and female athletes performed twenty 5-s cycle sprints separated by 25 s of rest. Mechanical work and integrated electromyograhs (iEMG) of 4 muscles of the dominant lower limb were calculated in every sprint. The iEMG signals from individual muscles were summed to represent overall electrical activity of these muscles (sum-iEMG). Neuromuscular efficiency (NME) was calculated as the ratio of mechanical work and sum-iEMG for every sprint. Arterial oxygen saturation was estimated (SpO2) with pulse oximetry throughout the protocol. The sprint-induced work decrement (18.9% vs. 29.6%; p < 0.05) and sum-iEMG reduction (11.4% vs. 19.4%; p < 0.05) were less for the women than for the men. However, the sprints decreased NME (10.1%; p < 0.05) and SpO2 (3.4%; p < 0.05) without showing sex dimorphism. Changes in SpO2 and sum-iEMG were strongly correlated in both sexes (men, R2 = 0.87; women, R2 = 0.91; all p < 0.05), although the slope of this relationship differed (6.3 +/- 2.9 vs. 3.8 +/- 1.6, respectively; p < 0.05). It is suggested that the sex difference in fatigue during repeated bouts of sprint exercise is not likely to be explained by a difference in muscle contractility impairment in men and women, but may be due to a sex difference in muscle recruitment strategy. We speculate that women would be less sensitive to arterial O2 desaturation than men, which may trigger lower neuromuscular adjustments to exhaustive exercise.

Exercise, sex, menstrual cycle phase, and 17beta-estradiol influence metabolism-related genes in human skeletal muscle

Higher fat and lower carbohydrate and amino acid oxidation are observed in women compared with men during endurance exercise. We hypothesized that the observed sex difference is due to estrogen and that menstrual cycle phase or supplementation of men with 17beta-estradiol (E(2)) would coordinately influence the mRNA content of genes involved in lipid and/or carbohydrate metabolism in skeletal muscle. Twelve men and twelve women had muscle biopsies taken before and immediately after 90 min of cycling at 65% peak oxygen consumption (Vo(2peak)). Women were studied in the midfollicular (Fol) and midluteal (Lut) phases, and men were studied after 8 days of E(2) or placebo supplementation. Targeted RT-PCR was used to compare mRNA content for genes involved in transcriptional regulation and lipid, carbohydrate, and amino acid metabolism. Sex was the greatest predictor of substrate metabolism gene content. Sex affected the mRNA content of FATm, FABPc, SREBP-1c, mtGPAT, PPARdelta, PPARalpha, CPTI, TFP-alpha, GLUT4, HKII, PFK, and BCOADK (P < 0.05). E(2) administration significantly (P < 0.05) affected the mRNA content of PGC-1alpha, PPARalpha, PPARdelta, TFP-alpha, CPTI, SREBP-1c, mtGPAT, GLUT4, GS-1, and AST. Acute exercise increased the mRNA abundance for PGC-1alpha, HSL, FABPc, CPTI, GLUT4, HKII, and AST (P < 0.05). Menstrual cycle had a small effect on PPARdelta, GP, and glycogenin mRNA content. Overall, women have greater mRNA content for several genes involved in lipid metabolism, which is partially due to an effect of E(2).

Expression and localization of augmenter of liver regeneration in human muscle tissue

Mitochondrial DNA (mt-DNA) disorders and abnormal regulation of nuclear-derived proteins devoted to the cross-talk between the two cellular genomes have recently interested researchers in the field of neuromuscular diseases. We have identified, isolated and sequenced a new gene, augmenter of liver regeneration (ALR) that stimulates in vivo hepatocyte proliferation and up-regulates mt-DNA expression and ATP production. ALR protein (Alrp) is mainly located, in rat, in the mitochondrial inter-membrane space and its mRNA is particularly abundant in brain, muscle, testis and liver, tissues whose activity is mostly dependent on mitochondrial metabolism. Studies on rat Alrp sequence revealed the presence of homologous amino-acid sections into proteins derived from mouse, human, Drosophyla, plants and even DNA viruses. In this article, we evaluated ALR expression in normal human muscular tissues, both as protein and as mRNA. The data, obtained by molecular biology, immunohistochemistry and electron microscopy, demonstrated that: (i) Alrp and ALR mRNA are present in human muscular tissue; (ii) Alrp is particularly expressed in muscular fibres rich in mitochondria; (iii) Alrp is localized in the mitochondrial inter-membrane space or associated to mitochondrial cristae; and (iv) in subjects younger then 35 years of age, ALR mRNA expression is different between male and female subjects. In conclusion, the present data set Alrp, as a factor associated with mitochondria also in human tissue, call for future studies aimed at establishing Alrp as an important factor involved in the molecular events that trigger neuromuscular diseases.

In vivo oxidative capacity varies with muscle and training status in young adults

It is well established that exercise training results in increased muscle oxidative capacity. Less is known about how oxidative capacities in distinct muscles, in the same individual, are affected by different levels of physical activity. We hypothesized that 1) trained individuals would have higher oxidative capacity than untrained individuals in both tibialis anterior (TA) and vastus lateralis (VL) and 2) oxidative capacity would be higher in TA than VL in untrained, but not in trained, individuals. Phosphorus magnetic resonance spectroscopy was used to measure the rate of phosphocreatine recovery (k(PCr)), which reflects the rate of oxidative phosphorylation, following a maximal voluntary isometric contraction of the TA and VL in healthy untrained (7 women, 7 men, 25.7 +/- 3.6 yr; mean +/- SD) and trained (5 women, 7 men, 27.5 +/- 3.4 yr) adults. Daily physical activity levels were measured using accelerometry. The trained group spent threefold more time ( approximately 90 vs. approximately 30 min/day; P < 0.001) in moderate to vigorous physical activity (MVPA). Overall, k(PCr) was higher in VL than in TA (P = 0.01) and higher in trained than in untrained participants (P < 0.001). The relationship between k(PCr) and MVPA was more robust in VL (r = 0.64, P = 0.001, n = 25) than in TA (r = 0.38, P = 0.06, n = 25). These results indicate greater oxidative capacity in vivo in trained compared with untrained individuals in two distinct muscles of the lower limb and provide novel evidence of higher oxidative capacity in VL compared with TA in young humans, irrespective of training status. The basis for this difference is not known at this time but likely reflects a difference in usage patterns between the muscles.