Slower fatigue and faster recovery of the adductor pollicis muscle in women matched for strength with men

Lower limb peripheral NIRS parameters during a vascular occlusion test: an experimental study in healthy volunteers

OBJECTIVES

The aim of the study was to compare NIRS parameters in combination with a vascular occlusion test (VOT) at a proximal (leg) and a distal (foot) site in male and female.

STUDY DESIGN

A prospective experimental study in healthy subjects.

PATIENTS AND METHODS

Twenty volunteers (10 male, 10 female, 28 ± 4 years) were investigated during 4 experimental steps: baseline, ischemia, reperfusion, and baseline. For each volunteer, 3 NIRS optodes were placed on right and left calves and the left arch of the foot. Blood pressure, heart rate and peripheral pulse oxymetry were monitored.

RESULTS

Significant differences were observed at baseline between regional oxygen saturation (rSO₂) values according to the site of measurement (proximal rSO₂ 81 ± 9% vs distal rSO₂ 60 ± 5%, P<0.001) but not according to gender. Both decreases in proximal and distal rSO₂ during ischemia and increases over baseline values during reperfusion depended on group membership (male or female). NIRS parameters during the VOT were significantly higher in male when compared with female at the proximal site: desaturation rate 5.6% (IQR: 5.5) vs 2.5% (IQR: 0.8), P=0.001; resaturation rate 40.7% (IQR: 6.6) vs 21.7% (IQR: 5.4), P=0.003; and ΔrSO₂ 10.0% (IQR: 7.0) vs 5.5% (IQR: 6.0), P=0.041.

CONCLUSIONS

Values of rSO₂ at the lower limb varied according to the anatomical site of measurement. A VOT induced major changes in rSO₂ that differed between male and female. These results should be taken into account in further clinical studies.

Skeletal muscle fatigue

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

Inspiratory muscle warm-up attenuates muscle deoxygenation during cycling exercise in women athletes

This study examines the effects of inspiratory muscle warm-up (IMW) on performance and muscle oxygenation during cycling exercise. In a randomized crossover study of 10 female soccer players, the IMW, placebo (IMWP) and control (CON) trials were conducted before two 6-min submaximal cycling exercises (100 and 150W) followed by intermittent high-intensity sprint (IHIS, 6×10s with 60s recovery). The reduction in tissue saturation index (TSI) in legs in the IMW were significantly less than those in IMWP and CON (P<0.01) during submaximal cycling exercises. The average reduction in TSI during the IHIS test with IMW was significantly less than those in the IMWP and CON (P=0.023). Nevertheless, the IHIS performance with IMW did not differ from that in other trials. In conclusion, the leg TSI during continuous submaximal cycling exercise followed by intermittent sprinting was likely improved by specific IMW (40% maximal inspiratory mouth pressure), which did not enhance IHIS performance.

The effects of rest interval length on acute bench press performance: the influence of gender and muscle strength

The purpose of this study was to investigate the effects of rest interval (RI) length on bench press performance in subjects with disparity in maximum strength. Two cohorts of subjects performed 3 bench press protocols in random order consisting of 3 sets of up to 10 repetitions with 75% of 1-repetition maximum (1RM) using either 1-, 2-, or 3-minute RIs between sets. In the first cohort, 22 men and women were studied to investigate gender influence. In the second cohort, 23 men were tested for 1RM bench press strength and placed into a low 1RM (mean = 80.7 ± 7.5 kg) or high 1RM (mean = 140.6 ± 11.9 kg) experimental group. The number of successful repetitions completed, average power, and velocity for each set were recorded. Women performed significantly more repetitions than men with 1-minute (26.9 ± 4.4 vs. 21.1 ± 3.5), 2-minute (29.0 ± 2.0 vs. 24.0 ± 4.5), and 3-minute (29.7 ± 1.8 vs. 25.8 ± 5.1) RIs. The magnitude of decline in average velocity and power was significantly higher in men than in women. Total number of repetitions performed was significantly greater in the low 1RM group than in the high 1RM group at 1-minute (21.6 ± 5.0 vs. 18.1 ± 2.0) and 2-minute RIs (24.2 ± 5.4 vs. 21.3 ± 2.8). Significant negative correlations were observed between 1RM bench press and total number of repetitions completed for 1- and 2-minute RIs (r = -0.558 and -0.490, respectively). These data indicate that maximal strength plays a role in bench press performance with varying RIs and suggest that shorter RIs may suffice in women to attain a specific volume.

Habitual physical activity as a determinant of the effect of moderate physical exercise on postural control in older men

The aim of the study was to compare the effect of exercise on postural control (PC) among the elderly with lower or higher level of habitual physical activity (HPA). The study involved 17 elderly men (mean age 72.9 ± 4.79 years). Mean velocity of the center of pressure (COP) displacements was measured using a force plate both before and after cycle ergometer exercise. A significantly higher increase in mean velocity of COP displacements and its component in the sagittal plane were observed in the group with lower level of HPA in comparison with the group with higher HPA level. Simultaneously, a relatively similar reaction to the exercise in the frontal plane was observed in both groups, possibly connected to the specific type of used exercise, which mainly activated the sagittal muscles.

Susceptibility of estrogen receptor rapid responses to xenoestrogens: Physiological outcomes

17β-Estradiol (E2) binding induces rapid modification in the conformation of its cognate receptors (i.e., ERα and ERβ). These allosteric changes allow the association of ERs with cell specific transcriptional cofactors, thus determining cellular contexts specific variations in gene expression. In addition, E2-ER complexes could also interact with membrane and cytosolic signal molecules triggering extra-nuclear signalling pathways. The synergy between these mechanisms is necessary for E2-induced pleiotropic actions in target tissues. Besides E2, the ER ligand binding domains can accommodate many other natural and synthetic ligands. Several of these compounds act as agonist or antagonist of ER transcriptional activity due to their ability to modify the interactions between ERs and transcriptional co-regulators. However, the ability of natural or manmade ER ligands to affect the extra-nuclear interactions of the ERs has been rarely evaluated. Here, the ability of two diet-derived flavonoids (i.e., naringenin and quercetin) and of the synthetic food-contaminant bisphenol A to modulate specifically ER extra-nuclear signalling pathways will be reported. All the tested compounds bind to both ER subtypes even if lesser than E2 activating divergent signal transduction pathways. In fact, in the presence of ERα, both naringenin and quercetin decouple ERα activities by specifically interfering with ERα membrane initiating signals. On the other hand, bisphenol A, but not flavonoids, maintains ERβ at the membrane thus impairing the activation of the downstream kinases. As a whole, extra-nuclear ER signals are highly susceptible to different ligands that, by unbalancing E2-induced cell functions drive cells to different functional endpoints.

Analysis of fatigue and tremor during sustained maximal grip contractions using Hilbert-Huang Transformation

The objective of this study was to evaluate muscle fatigue and tremor during a Sustained Maximal Grip Contraction (SMGC) using the Hilbert-Huang Transformation (HHT). Thirty-nine healthy subjects volunteered for the study and performed a 25-s SMGC. Fatigue parameters such as the relative force output (RFO) were calculated from the residual of SMGC after applying Empirical Mode Decomposition (EMD). Using the energy spectrum of the Intrinsic Mode Functions (IMF) obtained using HHT, isometric force tremor was identified from the 4 to 12 Hz region in IMF3 and IMF4. Data were analysed for five consecutive 5-s epochs to identify changes in fatigue and tremor over time. The HHT method was able to identify a greater resistance to fatigue in women compared to men (p≤0.05) and in non-dominant hands compared to dominant hands (p≤0.05). Consistent with the results for fatigue, women had less tremor than men (p≤0.05), while non-dominant hands trembled less than did dominant hands (p≤0.05). Higher levels of tremor were observed for non-fatigue-resistant subjects for both 10-15 s and 15-20 s epochs (p≤0.05). The HHT is an appropriate method to identify both fatigue and tremor during SMGC. It would be of interest to apply this method to the study the elderly or patients with neuromuscular disorders.

Neuroendocrine-immune interactions and responses to exercise

This article reviews the interaction between the neuroendocrine and immune systems in response to exercise stress, considering gender differences. The body's response to exercise stress is a system-wide effort coordinated by the integration between the immune and the neuroendocrine systems. Although considered distinct systems, increasing evidence supports the close communication between them. Like any stressor, the body's response to exercise triggers a systematic series of neuroendocrine and immune events directed at bringing the system back to a state of homeostasis. Physical exercise presents a unique physiological stress where the neuroendocrine and immune systems contribute to accommodating the increase in physiological demands. These systems of the body also adapt to chronic overload, or exercise training. Such adaptations alleviate the magnitude of subsequent stress or minimize the exercise challenge to within homeostatic limits. This adaptive capacity of collaborating systems resembles the acquired, or adaptive, branch of the immune system, characterized by the memory capacity of the cells involved. Specific to the adaptive immune response, once a specific antigen is encountered, memory cells, or lymphocytes, mount a response that reduces the magnitude of the immune response to subsequent encounters of the same stress. In each case, the endocrine response to physical exercise and the adaptive branch of the immune system share the ability to adapt to a stressful encounter. Moreover, each of these systemic responses to stress is influenced by gender. In both the neuroendocrine responses to exercise and the adaptive (B lymphocyte) immune response, gender differences have been attributed to the 'protective' effects of estrogens. Thus, this review will create a paradigm to explain the neuroendocrine communication with leukocytes during exercise by reviewing (i) endocrine and immune interactions; (ii) endocrine and immune systems response to physiological stress; and (iii) gender differences (and the role of estrogen) in both endocrine response to physiological stress and adaptive immune response.

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.

Nutrition and human health from a sex-gender perspective

Nutrition exerts a life-long impact on human health, and the interaction between nutrition and health has been known for centuries. The recent literature has suggested that nutrition could differently influence the health of male and female individuals. Until the last decade of the 20th century, research on women has been neglected, and the results obtained in men have been directly translated to women in both the medicine and nutrition fields. Consequently, most modern guidelines are based on studies predominantly conducted on men. However, there are many sex-gender differences that are the result of multifactorial inputs, including gene repertoires, sex steroid hormones, and environmental factors (e.g., food components). The effects of these different inputs in male and female physiology will be different in different periods of ontogenetic development as well as during pregnancy and the ovarian cycle in females, which are also age dependent. As a result, different strategies have evolved to maintain male and female body homeostasis, which, in turn, implies that there are important differences in the bioavailability, metabolism, distribution, and elimination of foods and beverages in males and females. This article will review some of these differences underlying the impact of food components on the risk of developing diseases from a sex-gender perspective.

Influência do nível de força máxima na produção e manutenção da potência muscular

Indivíduos mais fortes (com nível mais elevado de força máxima, Fmax) demonstram menor resistência de força que indivíduos mais fracos (com nível mais baixo de Fmax) em uma mesma intensidade relativa. Como o nível de Fmax influencia a produção de potência, espera-se que sujeitos mais fortes também apresentem uma menor resistência de potência. O objetivo deste estudo foi avaliar a influência do nível de Fmax na produção e na resistência de potência durante repetições e séries múltiplas do exercício meio-agachamento. Quarenta e dois sujeitos foram classificados de acordo com o resultado no teste de força dinâmica máxima (1RM) e destes os 10 mais fortes e os 10 mais fracos foram selecionados para participar no estudo. Para avaliar a resistência de potência os dois grupos realizaram 10 séries de seis repetições a 40% e a 60% 1RM na maior velocidade possível. A potência absoluta (PA) e a potência relativa ao peso corporal (PR) desenvolvidas na fase concêntrica do exercício foram medidas. A análise de variância (ANOVA two-way) revelou que os sujeitos mais fortes diminuíram a PA a 60% 1RM a partir da quarta repetição e a PR a partir da quinta repetição. Já os sujeitos mais fracos diminuíram a PA apenas na sexta repetição e mantiveram o rendimento na PR ao longo das 10 séries. Não houve efeito significante na intensidade de 40% 1RM. Isso sugere que sujeitos mais fortes fadigam antes em maiores intensidades de carga. Essa fadiga precoce nos sujeitos mais fortes poderia estar ligada a diferentes fatores associados ao controle da homeostase orgânica como o comportamento da pressão arterial, da atividade eletromiográfica e a proporção de fibras musculares dos tipos I e II.

Sex differences in exercise-induced diaphragmatic fatigue in endurance-trained athletes

There is evidence that female athletes may be more susceptible to exercise-induced arterial hypoxemia and expiratory flow limitation and have greater increases in operational lung volumes during exercise relative to men. These pulmonary limitations may ultimately lead to greater levels of diaphragmatic fatigue in women. Accordingly, the purpose of this study was to determine whether there are sex differences in the prevalence and severity of exercise-induced diaphragmatic fatigue in 38 healthy endurance-trained men ( n = 19; maximal aerobic capacity = 64.0 ± 1.9 ml·kg−1·min−1) and women ( n = 19; maximal aerobic capacity = 57.1 ± 1.5 ml·kg−1·min−1). Transdiaphragmatic pressure (Pdi) was calculated as the difference between gastric and esophageal pressures. Inspiratory pressure-time products of the diaphragm and esophagus were calculated as the product of breathing frequency and the Pdi and esophageal pressure time integrals, respectively. Cervical magnetic stimulation was used to measure potentiated Pdi twitches (Pdi,tw) before and 10, 30, and 60 min after a constant-load cycling test performed at 90% of peak work rate until exhaustion. Diaphragm fatigue was considered present if there was a ≥15% reduction in Pdi,tw after exercise. Diaphragm fatigue occurred in 11 of 19 men (58%) and 8 of 19 women (42%). The percent drop in Pdi,tw at 10, 30, and 60 min after exercise in men ( n = 11) was 30.6 ± 2.3, 20.7 ± 3.2, and 13.3 ± 4.5%, respectively, whereas results in women ( n = 8) were 21.0 ± 2.1, 11.6 ± 2.9, and 9.7 ± 4.2%, respectively, with sex differences occurring at 10 and 30 min ( P < 0.05). Men continued to have a reduced contribution of the diaphragm to total inspiratory force output (pressure-time product of the diaphragm/pressure-time product of the esophagus) during exercise, whereas diaphragmatic contribution in women changed very little over time. The findings from this study point to a female diaphragm that is more resistant to fatigue relative to their male counterparts.

Hypohydration and muscular fatigue of the thumb alter median nerve somatosensory evoked potentials

The mechanisms by which dehydration impairs endurance performance remain unresolved but may involve alterations in afferent neural processing. The purpose of this study was to determine the effect of hypohydration on somatosensory evoked potentials (SEPs) at rest and during recovery from fatiguing exercise. Fourteen volunteers (12 men, 2 women) performed repetitive isometric thumb contractions (50% maximal voluntary contractions (MVC) and 100% MVC in a 5:1 ratio, each contraction separated by 5 s of rest) until exhaustion when euhydrated (EU) and when hypohydrated by 4% body mass (HY). SEPs were obtained from the median nerve. The results indicated that HY did not produce statistical differences in time to exhaustion (EU = 754 (SD 255); HY = 714 (SD 318) s; p = 0.66) or rate of muscle fatigue. However, HY was associated with greater subjective feelings of fatigue and loss of vigor after exhaustive exercise (p < 0.01). HY affected N20 latency with an interaction effect of hydration by fatigue state (EU-Rest: 18.5 (SD 1.6) ms; EU-Fatigue: 19.0 (SD 1.6) ms; HY-Rest: 18.3 (SD 1.3) ms; HY-Fatigue: 18.4 (SD 1.5) ms; p = 0.034), but N20 and N20-P22 amplitude responses were similar between HY and EU trials. We concluded that moderate water deficits appear to alter afferent signal processing within the cerebral cortex.

The influence of estrogen on skeletal muscle: sex matters

As women enter menopause, the concentration of estrogen and other female hormones declines. This hormonal decrease has been associated with a number of negative outcomes, including a greater incidence of injury as well as a delay in recovery from these injuries. Over the past two decades, our understanding of the protective effects of estrogen against various types of injury and disease states has grown immensely. In skeletal muscle, studies with animals have demonstrated that sex and estrogen may potentially influence muscle contractile properties and attenuate indices of post-exercise muscle damage, including the release of creatine kinase into the bloodstream and activity of the intramuscular lysosomal acid hydrolase, beta-glucuronidase. Furthermore, numerous studies have revealed an estrogen-mediated attenuation of infiltration of inflammatory cells such as neutrophils and macrophages into the skeletal muscles of rats following exercise or injury. Estrogen has also been shown to play a significant role in stimulating muscle repair and regenerative processes, including the activation and proliferation of satellite cells. Although the mechanisms by which estrogen exerts its influence upon indices of skeletal muscle damage, inflammation and repair have not been fully elucidated, it is thought that estrogen may potentially exert its protective effects by: (i) acting as an antioxidant, thus limiting oxidative damage; (ii) acting as a membrane stabilizer by intercalating within membrane phospholipids; and (iii) binding to estrogen receptors, thus governing the regulation of a number of downstream genes and molecular targets. In contrast to animal studies, studies with humans have not as clearly delineated an effect of estrogen on muscle contractile function or on indices of post-exercise muscle damage and inflammation. These inconsistencies have been attributed to a number of factors, including age and fitness level of subjects, the type and intensity of exercise protocols, and a focus on sex differences that typically involve factors and hormones in addition to estrogen. In recent years, hormone replacement therapy (HRT) or estrogen combined with exercise have been proposed as potentially therapeutic agents for postmenopausal women, as these agents may potentially limit muscle damage and inflammation and stimulate repair in this population. While the benefits and potential health risks of long-term HRT use have been widely debated, controlled studies using short-term HRT or other estrogen agonists may provide future new and valuable insights into understanding the effects of estrogen on skeletal muscle, and greatly benefit the aging female population. Recent studies with older females have begun to demonstrate their benefits.

Men and women exhibit a similar time to task failure for a sustained, submaximal elbow extensor contraction

Sex differences in muscle fatigue-resistance have been observed in a variety of muscles and under several conditions. This study compared the time to task failure (TTF) of a sustained isometric elbow extensor (intensity 15% of maximal strength) contraction in young men (n = 12) and women (n = 11), and examined if their neurophysiologic adjustments to fatigue differed. Motor-evoked potential amplitude (MEP), silent period duration, interference electromyogram (EMG) amplitude, maximal muscle action potential (M (max)), heart rate, and mean arterial pressure were measured at baseline, during the task, and during a 2-min ischemia period. Men and women did not differ in TTF (478.2 +/- 31.9 vs. 500.4 +/- 41.3 s; P = 0.67). We also performed an exploratory post hoc cluster analysis, and classified subjects as low (n = 15) or high endurance (n = 8) based on TTF (415.3 +/- 16.0 vs. 626.7 +/- 25.8 s, respectively). The high-endurance group exhibited a lower MEP and EMG at baseline (MEP 16.3 +/- 4.1 vs. 37.2 +/- 3.0% M (max), P < 0.01; EMG 0.98 +/- 0.18 vs. 1.85 +/- 0.26% M (max), P = 0.03). These findings suggest no sex differences in elbow extensor fatigability, in contrast to observations from other muscle groups. The cluster analyses results indicated that high- and low-endurance groups displayed neurophysiologic differences at baseline (before performing the fatigue task), but that they did not differ in fatigue-induced changes in their neurophysiologic adjustments to the task.

17β-Estradiol regulates the first steps of skeletal muscle cell differentiation via ER-α-mediated signals

17β-Estradiol (E2) mediates a wide variety of complex biological processes determining the growth and development of reproductive tract as well as nonreproductive tissues of male and female individuals. While E2 effects on the reproductive system, bone, and cardiovascular system are quite well established, less is known about how it affects the physiology of other tissues. Skeletal muscle is a tissue that is expected to be E2 responsive since both isoforms of estrogen receptor (ER-α and ER-β) are expressed. Significant sex-related differences have been described in skeletal muscle, although the role played by E2 and the mechanisms underlying it remain to be determined. Here, we demonstrate that E2 increases the glucose transporter type 4 translocation at membranes as well as the expression of well-known differentiation markers of myogenesis (i.e., myogenin and myosin heavy chain) in rat myoblast cells (L6). These E2-induced effects require rapid extranuclear signals and the presence of ER-α, whereas no contribution of IGF-I receptor has been observed. In particular, ER-α-dependent Akt activation participates in regulating the first step of myogenic differentiation. Moreover, both receptors mediate the E2-induced activation of p38, which, in turn, affects the expression of myogenin and myosin heavy chain. All together, these data indicate that E2 should be included in the list of skeletal muscle trophic factors.

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.

Low mean level sustained and intermittent grip exertions: influence of age on fatigue and recovery

The goal of this study was to quantify localised muscle fatigue resulting from low mean levels of exertion in younger (< 40 years) and older (> 50 years) adults. Fatigue, elicited in the finger flexor muscles by intermittent (10% mean maximum voluntary contraction (MVC)) and sustained (8% MVC) handgrip exercises, was quantified by a muscle twitch force response before, immediately after and during 3 h following exercise. Despite greater mean loads, recovery time was shorter following intermittent than sustained contractions, which suggests that recovery from fatigue is more sensitive to rest within the work cycle than mean work. The more pronounced effects for younger than older individuals following the sustained exertion indicate that changes in muscle fibre type composition might predispose older individuals to be more resistant to fatigue resulting from sustained contractions of low level. Performing hand exertion tasks requiring low mean force levels contributes to similar long-lasting fatigue effects regardless of gender and age. Intermittent periods of complete rest reduce muscle fatigue. Since fatigue was not perceived during recovery from the tested sustained and intermittent contractions, subjective evaluations may not be a reliable indicator of localised muscle fatigue.

Measurement of muscle strength in the intensive care unit

Traditional (indirect) techniques, such as electromyography and nerve conduction velocity measurement, do not reliably predict intensive care unit-acquired muscle weakness and its clinical consequences. Therefore, quantitative assessment of skeletal muscle force is important for diagnosis of intensive care unit-acquired motor dysfunction. There are a number of ways for assessing objectively muscle strength, which can be categorized as techniques that quantify maximum voluntary contraction force and those that assess evoked (stimulated) muscle force. Important factors that limit the repetitive evaluation of maximum voluntary contraction force in intensive care unit patients are learning effects, pain during muscular contraction, and alteration of consciousness.The selection of the appropriate muscle is crucial for making adequate predictions of a patient's outcome. The upper airway dilators are much more susceptible to a decrease in muscle strength than the diaphragm, and impairment of upper airway patency is a key mechanism of extubation failure in intensive care unit patients. Data suggest that the adductor pollicis muscle is an appropriate reference muscle to predict weakness of muscles that are typically affected by intensive care unit-acquired weakness, i.e., upper airway as well as extremity muscles. Stimulated (evoked) force of skeletal muscles, such as the adductor pollicis, can be assessed repetitively, independent of brain function, even in heavily sedated patients during high acuity of their disease.

Scapula kinematic alterations following a modified push-up plus task

Scapular kinematic and muscle activity alterations have been identified in individuals with subacromial impingement syndrome of the shoulder, including workers and athletes who regularly perform overhead activities. Serratus anterior fatigue is a proposed mechanism for these kinematic alterations, although no direct evidence supports such a relationship. The purpose of this study was to examine three-dimensional scapula kinematics in asymptomatic participants before and after a task intended to preferentially fatigue serratus anterior. Twenty-eight participants completed the study. The experimental task consisted of five repetitions of scapular plane arm elevation and the fatigue task was the isometric hold of scapular protraction in the push-up position. Electromyography of four shoulder muscles was collected during the task to determine level of muscle fatigue, and the Borg CR10 scale was used to assess subjective fatigue. The fatigue task resulted in decreased median power frequency in all four muscles and significantly increased Borg scores. Scapula posterior tilting and internal rotation were most impacted by muscle fatigue, with decreased posterior tilting and increased internal rotation after the task. There was no effect on scapular upward rotation. Effect sizes were low to moderate (.13-.51) where these kinematic alterations reached statistical significance. Shoulder muscle fatigue contributes to scapular kinematic alterations and is a plausible risk factor for subacromial impingement syndrome.

Sex differences and mechanisms of task-specific muscle fatigue

Women can be less fatigable than men because of sex-related differences within the neuromuscular system that impact physiological adjustments during a fatiguing task. However, the involved mechanism(s) for the sex difference is task specific. This review explores the novel hypothesis that variation of the task will alter the magnitude of the sex-difference in muscle fatigue and the contribution of involved mechanisms.

Gender differences in muscle fatigue of the biceps brachii and influences of female menstrual cycle in electromyography variables

Several studies report gender differences in response to fatigue. Most results suggest that females have higher muscle endurance than males. Possible explanations lie on differences in muscle mass, substrate utilization, muscle morphology and neuromuscular activation. One relevant aspect not always considered is the hormonal fluctuations during the female menstrual cycle. The present work observed eighteen healthy and untrained adults (eight males, 26.9 +/- 4.0 yr and ten females, 24.0 +/- 2.8 yr) performing fatiguing isometric contractions to evaluate both the influence of menstrual cycle and gender differences in fatigability. Surface electromyographic signals were recorded from the biceps brachii using a linear electrode array of eight electrodes during 90 seconds at 40% of maximal voluntary contraction. Root mean square (RMS), mean frequency (MNF) the conduction velocity (CV) values were estimated using windows of 0.5 seconds. Female subjects showed overall lower fatigability, demonstrated by the lower mean CV decrease (1.494) compared to males (1.787). However, in periods of high decreases in hormones concentrations in females (the end of both the follicular and luteal phases), higher CV decreases were observed (1.921 and 2.183). These results indicate the need of considering the effects of hormonal fluctuations in females when observing gender effects on muscle fatigue.

Sex differences in time to task failure and blood flow for an intermittent isometric fatiguing contraction

The purpose of this study was to compare the time to task failure, postcontraction hyperemia, and vascular conductance of young men and women for a submaximal intermittent fatiguing contraction performed with the handgrip muscles. Twenty men and 20 women (mean +/- SD: 22 +/- 4 years) performed an isometric contraction at 50% of maximal voluntary contraction (MVC) (6-s contraction, 4-s rest) until task failure. Forearm venous occlusion plethysmography was used to estimate the peak blood flow (after 10-min occlusion) and blood flow at rest after 6-s submaximal contractions of varying intensities, and during an intermittent fatiguing contraction at 1-min intervals and task failure. The time to task failure was longer for the women compared with the men (408 +/- 205 s vs. 297 +/- 57 s, P < 0.05). Postcontraction hyperemia and vascular conductance were greater for men than for women after nonfatiguing 6-s submaximal contractions performed at 20%, 40%, 50%, 60%, and 80% of MVC force (P < 0.05). In contrast, hyperemia and vascular conductance were similar for both genders when measured at 50 s into the fatiguing contraction, at each minute thereafter, and at task failure. Regression analysis indicated that the rate of electromyographic activity and perceived exertion were the significant predictors of the time to task failure. The longer time to task failure for women compared with men for an intermittent fatiguing contraction with handgrip muscles was not explained by postcontraction hyperemia or vascular conductance with fatigue.

Obesity does not reduce maximum acceptable weights of lift

The maximum acceptable weights of lift (MAWL) of obese and non-obese participants were empirically investigated. Three obesity levels were considered: non-obese (18.5 kg/m(2)< or= body mass index (BMI)<or=24.9 kg/m(2)), moderately obese (35 kg/m(2)<or=BMI<or=39.9 kg/m(2)) and extremely obese (BMI>or= 40 kg/m(2)). Ten male and 10 female participants were recruited for each obesity level. The participants determined their MAWL for 18 different lifting task conditions (six lifting frequencies x three lifting heights). An analysis of variance (ANOVA) was conducted to determine the effects of obesity level, gender, lifting height, lifting frequency and their interactions on MAWL. Overall, the ANOVA results indicated that obesity does not reduce MAWL, and thus, suggested that the existing MAWL data can be used to accommodate both general and obese workers. However, further studies based on the biomechanical and physiological approaches are required to provide more complete understanding of obesity effects on lifting tolerance limits.

Muscle function in men and women during maximal eccentric exercise

This study assessed muscle fatigue patterns of the elbow flexors in untrained men and women to determine if sex differences exist during acute maximal eccentric exercise. High-intensity eccentric exercise is often used by athletes to elicit gains in muscle strength and size gains. Development of fatigue during this type of exercise can increase risk of injury; therefore, it is important to understand fatigue patterns during eccentric exercise to minimize injury risk exposure while still promoting training effects. While many isometric exercise studies have demonstrated that women show less fatigue, the patterns of fatigue during purely eccentric exercise have not been assessed in men and women. Based on the lack of sex differences in overall strength loss immediately post-eccentric exercise, it was hypothesized that women and men would have similar relative fatigue pattern responses (i.e., change from baseline) during a single bout of maximal eccentric exercise. Forty-six subjects (24 women and 22 men) completed 5 sets of 10 maximal eccentric contractions on an isokinetic dynamometer. Maximal voluntary isometric contraction strength was assessed at baseline and immediately following each exercise set. Maximal eccentric torque and contractile properties (i.e., contraction time, work, half relaxation time, and maximal rate of torque development) were calculated for each contraction. Men and women demonstrated similar relative isometric (32% for men and 39% for women) and eccentric (32% for men and 39% for women) fatigue as well as similar deficits in work done and rates of torque development and relaxation during exercise (p > 0.05). Untrained men and women displayed similar relative responses in all measures of muscle function during a single bout of maximal eccentric exercise of the elbow flexors. Thus, there is no reason to suspect that women may be more vulnerable to fatigue-related injury.

Contrasting influences of age and sex on muscle fatigue

PURPOSE

Greater resistance to muscle fatigue has been observed in women versus men and in older versus young individuals. As suggested mechanisms for these differences include task intensity and duty cycle, the purpose of this study was to evaluate fatigue in healthy young and older men and women during maximum-effort isometric contractions with a 70% duty cycle (7 s of contraction, 3 s of rest). We hypothesized that no differences in fatigue would be observed across age or sex, in contrast to studies incorporating lower duty cycles.

METHODS

The protocol was carried out on ankle dorsiflexors of older (73 +/- 1 yr) and younger (25 +/- 1 yr) men and women. Volitional and stimulated force, compound muscle action potential, and muscle contractile responses were collected before, during, and immediately after the fatigue protocol. These measurements allowed for assessment of fatigue as well as central and peripheral activation.

RESULTS

At baseline, older subjects had longer force half-relaxation times and less twitch potentiation than younger subjects, consistent with a slower muscle phenotype. During contractions, younger subjects fatigued more than older subjects did, with no differences between men and women. Central activation decreased similarly in all groups with fatigue. There were no fatigue-related differences in peripheral excitation or contractile properties attributable to age or sex.

CONCLUSIONS

These data indicate that age-related differences in fatigue are observed even during intermittent MVC with a high duty cycle, and that these differences are independent of central and peripheral activation. Further, it seems that sex-based differences in both fatigue and central activation failure were abolished with this duty cycle. Overall, these results suggest that age- and sex-based differences in fatigue arise from distinct mechanisms.

Enhanced muscle fatigue occurs in male but not female ASIC3-/- mice

Muscle fatigue is associated with a number of clinical diseases, including chronic pain conditions. Decreases in extracellular pH activates acid-sensing ion channel 3 (ASIC3), depolarizes muscle, protects against fatigue, and produces pain. We examined whether ASIC3-/- mice were more fatigable than ASIC3+/+ mice in a task-dependent manner. We developed two exercise protocols to measure exercise-induced muscle fatigue: (fatigue task 1, three 1-h runs; fatigue task 2, three 30-min runs). In fatigue task 1, male ASIC3+/+ mice muscle showed less fatigue than male ASIC3-/- mice and female ASIC3+/+ mice. No differences in fatigue were observed in fatigue task 2. We then tested whether the development of muscle fatigue was dependent on sex and modulated by testosterone. Female ASIC3+/+ mice that were ovariectomized and administered testosterone developed less muscle fatigue than female ASIC3+/+ mice and behaved similarly to male ASIC3+/+ mice. However, testosterone was unable to rescue the muscle fatigue responses in ovariectomized ASIC3-/- mice. Plasma levels of testosterone from male ASIC3-/- mice were significantly lower than in male ASIC3+/+ mice and were similar to female ASIC3+/+ mice. Muscle fiber types, measured by counting ATPase-stained whole muscle sections, were similar in calf muscles from male and female ASIC3+/+ mice. These data suggest that both ASIC3 and testosterone are necessary to protect against muscle fatigue in a task-dependent manner. Also, differences in expression of ASIC3 and the development of exercise-induced fatigue could explain the female predominance in clinical syndromes of pain that include muscle fatigue.

Sex differences in contractile properties and fatigue resistance of human skeletal muscle

To explore the cause of higher skeletal muscle fatigue resistance in women than men, we used electrically evoked contractions (1 s on, 1 s off, 30 Hz, 2 min), which circumvent motivational bias and allow examination of contractile properties. We compared 29 men [26.5 (7.0) years old; mean (s.d.)] with 35 women [25.4 (7.6) years old]. Strength of the quadriceps muscle was higher in men than women (P < 0.001). The lower maximal rate of relaxation in women (P = 0.002) indicates that their muscles were slower than those of men. The torque declined less in women than in men [37.7 (10.7) versus 29.9 (10.0)%; P = 0.002], and was not related to muscle strength or size, as determined with magnetic resonance imaging. The sex difference in fatigability was also seen when the circulation to the leg was occluded [torque declined 76.9 (10.8) versus 59.5 (16.9)% in men versus women, respectively; P = 0.008]. The maximal rate of relaxation correlated with the fatigability of the muscle under all conditions (correlations ranging from 0.34 to 0.51, P < 0.02). We conclude that the sex-related difference in skeletal muscle fatigue resistance is not explicable by differences in motivation, muscle size, oxidative capacity and/or blood flow between sexes, but might be related to differences in fibre type composition.

Forearm blood flow follows work rate during submaximal dynamic forearm exercise independent of sex

To test the hypothesis that sex influences forearm blood flow (FBF) during exercise, 15 women and 16 men of similar age [women 24.3 +/- 4.0 (SD) vs. men 24.9 +/- 4.5 yr] but different forearm muscle strength (women 290.7 +/- 44.4 vs. men 509.6 +/- 97.8 N; P < 0.05) performed dynamic handgrip exercise as the same absolute workload was increased in a ramp function (0.25 W/min). Task failure was defined as the inability to maintain contraction rate. Blood pressure and FBF were measured on separate arms during exercise by auscultation and Doppler ultrasound, respectively. Muscle strength was positively correlated with endurance time (r = 0.72, P < 0.01) such that women had a shorter time to task failure than men (450.5 +/- 113.0 vs. 831.3 +/- 272.9 s; P < 0.05). However, the percentage of maximal handgrip strength achieved at task failure was similar between sexes (14% maximum voluntary contraction). FBF was similar between women and men throughout exercise and at task failure (women 13.6 +/- 5.3 vs. men 14.5 +/- 4.9 ml.min(-1).100 ml(-1)). Mean arterial pressure was lower in women at rest and during exercise; thus calculated forearm vascular conductance (FVC) was higher in women during exercise but similar between sexes at task failure (women 0.13 +/- 0.05 vs. men 0.11 +/- 0.04 ml.min(-1).100 ml(-1).mmHg(-1)). In conclusion, the similar FBF during exercise was achieved by a higher FVC in the presence of a lower MAP in women than men. Still, FBF remained coupled to work rate (and presumably metabolic demand) during exercise irrespective of sex.

Stereological estimates indicate that aging does not alter the capillary length density in the human posterior cricoarytenoid muscle

Studies of some human skeletal muscles demonstrate an age-related capillarity decrease. An age-related decrease in blood flow to the posterior cricoarytenoid muscle (PCA) in rats has been reported, as well as a decreased ability to abduct the vocal folds. We, therefore, hypothesized that decreased muscle capillarity may contribute to PCA dysfunction in the elderly. Using immunological and stereological techniques, human PCAs (ages 18-98 yr; 28 men, 23 women) were examined for age-related changes in muscle fiber-type-specific and/or total capillary length density. While analysis shows no age-related changes in total muscle or fiber-type-specific capillary length densities (L(V cap)), there are significant age-related increases in L(V cap) within the interstitial tissue (P = 0.001) and in the ratio of the type I L(V cap) to type I surface (P = 0.002), with a strong trend for type II L(V cap) (P = 0.055). There is also an age-related decrease in the muscle fiber surface density for both type I and II fibers (P < 0.001 and 0.04, respectively). Data also show that women have a significantly higher type II L(V cap) (P = 0.039), regardless of age. In addition, with the exception of female type I L(V cap), all measured variables are significantly higher for type I fibers (P < 0.001), independent of age or sex. While data indicate there are age-related changes of capillary-muscle fiber relationships within the PCA, they do not support the hypothesis of an age-related loss of capillarity.

Gender effects on trapezius surface EMG during delayed onset muscle soreness due to eccentric shoulder exercise

The purpose of this study was to investigate gender-specific motor control strategies during eccentric exercise and delayed onset muscle soreness (DOMS) in the shoulder region. Twelve healthy males and females participated in the study. Eccentric shoulder exercises were conducted on the dominant shoulder while the other side served as control. The exerted force, range of shoulder elevation, rating of perceived exertion, pain intensity, and surface electromyography (EMG) from the trapezius muscles were recorded and analyzed. A significant decrease in exerted force during exercise was only found in males despite similar rating of perceived exertion among genders. During eccentric exercise: males showed increasing root mean square (RMS) of the EMG while a decrease occurred for females, no difference between genders in mean power frequency of the EMG were seen. During static and dynamic contractions: no differences between genders in pain intensity or RMS were observed; RMS of the exercised side were lower than that of the control side (P<0.05) at 24 h after exercise. The results indicated a more prominent muscle fatigue resistance in females compared with males and mobilization of different muscle activation strategies during eccentric exercise. A protective adaptation to DOMS, i.e. decrease in RMS values was found with no gender differences.

Mechanomyography and electromyography during and after fatiguing shoulder eccentric contractions in males and females

The aim of this study was to investigate changes in mechanomyographic (MMG) and the surface electromyographic (EMG) signals during and after fatiguing shoulder eccentric contractions in a group consisting of 12 males and 12 females. Exerted force, MMG, EMG, pain and rate of perceived exertion were assessed before, during and after repeated high-intensity eccentric exercises. Bouts of eccentric contractions caused a decrease in the exerted force for males (P<0.05) and an increase in the rate of perceived exertion and pain for both genders (P<0.05). During eccentric exercise, the root mean square (RMS) values of the MMG signal increased (P<0.05). The mean power frequency (MPF) values of the EMG signal decreased at the end of each eccentric bout for both genders (P<0.05); the decrease was higher for females compared with males (P<0.05). Immediately after eccentric exercise in static abduction of the upper limbs, the MMG RMS and MPF values increased (P<0.05). The present study showed that (1) neuromuscular changes associated with pain and changes in muscle stiffness and (2) changes in motor units strategy during fatigue development in shoulder muscle are reflected in the MMG and EMG signals.

Central fatigue explains sex differences in muscle fatigue and contralateral cross-over effects of maximal contractions

A sustained voluntary contraction increases central fatigue and produces a 'cross-over' of fatigue during a subsequent contraction of the contralateral limb. These studies compared the magnitude of these changes for men and women. Force and electromyographic responses from dominant (study 1; n = 8 men, 8 women) or non-dominant (study 2; n = 7 men, 8 women) leg extensors to nerve stimulation were recorded at rest and during brief maximal voluntary contractions (MVCs), before and after 100-s sustained MVCs performed with the dominant leg. For the dominant leg, force was reduced more for men (by approximately 24%) than women (by approximately 16%, P < 0.05) after the sustained contractions. Similarly, voluntary activation during these contractions was reduced more for men (by approximately 22%) than women (by approximately 9%, P < 0.05). Conversely, resting twitches changed similarly for both sexes (P > 0.05). For the non-dominant leg, men experienced a reduction in force (by approximately 13%, P < 0.001) and had greater deficits in activation than women ( approximately 9% vs approximately 3%, P < 0.05), after sustained contractions of the dominant leg. Therefore, sustained MVCs produce greater central fatigue and a more pronounced 'cross-over' of effects to the contralateral limb for men compared to women. These findings demonstrate distinct differences between sexes in the way the nervous system adapts to changes associated with fatigue.

Mechanisms of fatigue differ after low- and high-force fatiguing contractions in men and women

The magnitude of failure in voluntary drive after fatiguing contractions of different intensities in men and women is not known. The purpose of this study was to compare the time to task failure and voluntary activation of men and women for a sustained isometric contraction performed at a low and high intensity with the elbow flexor muscles. Nine men and nine women sustained an isometric contraction at 20% and 80% of maximal voluntary contraction (MVC) force until task failure during separate sessions. The men had a shorter time to failure than women for the 20% but not the 80% MVC task. Voluntary activation was reduced to similar levels for the men and women at the end of the fatiguing contractions but was reduced less after the 80% MVC task than the 20% MVC contraction. Twitch amplitude was reduced similarly at task failure for both sexes and to similar levels at termination of the 20% and 80% MVC tasks. The rate of change in mean arterial pressure was the main predictor of time to failure for the low-force sustained contraction. These results suggest that women experienced greater muscle perfusion, less peripheral fatigue, and a longer time to task failure than men during the low-force fatiguing contraction. However, the low-force task induced greater central fatigue than the high-force contraction for both men and women. Thus, low-force, long-duration fatiguing contractions can be used in rehabilitation to induce significant fatigue within the central nervous system and potentially greater neural adaptations in men and women.

Gender influence on fatigability of back muscles during intermittent isometric contractions: a study of neuromuscular activation patterns

BACKGROUND

Gender difference in the fatigability of muscles can be attributed to muscle mass (or strength) and associated level of vascular occlusion, substrate utilization, muscle composition, and neuromuscular activation patterns. The purpose of this study was to assess the role of neuromuscular activation patterns to explain gender differences in back muscle fatigability during intermittent isometric tasks.

METHODS

Sixteen males and 15 females performed maximal voluntary contractions (Strength) and a fatigue test to exhaustion (fatigue criterion=time to exhaustion), while standing in a static dynamometer measuring L5/S1 extension moment. The fatigue test consisted of repetitions of an 8-s cycle (1.5 s ramp to reach 40% of maximal voluntary contraction +5s plateau at 40% of maximal voluntary contraction +1.5s rest). Surface electromyography signals were collected bilaterally from 4 back muscles (multifidus at the L5 level, iliocostalis lumborum at L3, and longissimus at L1 and T10).

FINDINGS

Males were stronger (P<0.05) than females (316, SD 82>196, SD 25 Nm) but showed significantly shorter time-to-exhaustion values (7.1, SD 5.2<13.0, SD 6.1 min.), the latter result being corroborated by electromyographic indices of fatigue. However, the gender effect on time to exhaustion disappeared when accounting for Strength, thus supporting the muscle mass hypothesis. Among the various electromyographic indices computed to assess neuromuscular activation patterns, the amount of alternating activity between homolateral and between contralateral muscles showed a gender effect (females>males).

INTERPRETATION

These results support the muscle mass hypothesis as well as the neuromuscular activation hypothesis to explain gender differences in back muscle fatigability.

Continuous testosterone administration prevents skeletal muscle atrophy and enhances resistance to fatigue in orchidectomized male mice

Androgens promote anabolism in skeletal muscle; however, effects on subsequent muscle function are less well defined because of a lack of reliable experimental models. We established a rigorous model of androgen withdrawal and administration in male mice and assessed androgen regulation of muscle mass, structure, and function. Adult C57Bl/6J male mice were orchidectomized (Orx) or sham-operated (Sham) and received 10 wk of continuous testosterone (T) or control treatment (C) via intraperitoneal implants. Mass, fiber cross-sectional area (CSA), and in vitro contractile function were assessed for fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles. After 10 wk, Orx+C mice had reduced body weight gain (P < 0.05), seminal vesicle mass (P < 0.01), and levator ani muscle mass (P < 0.001) compared with Sham+C mice, and these effects were prevented with testosterone treatment. Orx+T mice had greater EDL (P < 0.01) and SOL (P < 0.01) muscle mass compared with Orx+C mice; however, median fiber CSA was not significantly altered in these muscles. EDL and SOL muscle force was greater in Sham+T compared with Orx+C mice (P < 0.05) in proportion to muscle mass. Unexpectedly, Orx+T mice had increased fatigue resistance of SOL muscle compared with Orx+C mice (P < 0.001). We used a rigorous model of androgen withdrawal and administration in male mice to demonstrate an essential role of androgens in the maintenance of muscle mass and force. In addition, we showed that testosterone treatment increases resistance to fatigue of slow- but not fast-twitch muscle.

Differences in fatigability between the sexes during a sustained submaximal contraction protocol in prepubertal children

The aim of this study was to determine whether there are differences in the fatigability of plantar flexor muscles during sustained submaximal contractions in prepubertal boys and girls. Fifteen boys (age 10.0 +/- 1.0 years) and 15 girls (age 9.8 +/- 0.9 years) participated in the study. The fatigue protocol consisted of a 10 min isometric plantar flexion at 20% of the maximal voluntary contraction. Immediately after this, five maximal isometric contractions were performed with a 3 min interval between contractions. During the experiment, electromyograms of the agonist muscles soleus and medial gastrocnemius and antagonist tibialis anterior were recorded. We observed no differences between the sexes (P < 0.05) in the decrease in torque or in the recovery rate after the fatigue protocol. Nor were there any differences between the sexes (P < 0.05) in agonist or antagonist muscle activation during the fatigue protocol and recovery period. The results indicate that there are no differences in fatigability between prepubertal boys and girls during submaximal sustained contractions, probably because the agonist and antagonist muscles were activated similarly in both sexes.

Supraspinal fatigue does not explain the sex difference in muscle fatigue of maximal contractions

Young women are less fatigable than young men for maximal and submaximal contractions, but the contribution of supraspinal fatigue to the sex difference is not known. This study used cortical stimulation to compare the magnitude of supraspinal fatigue during sustained isometric maximal voluntary contractions (MVCs) performed with the elbow flexor muscles of young men and women. Eight women (25.6 +/- 3.6 yr, mean +/- SD) and 9 men (25.4 +/- 3.8 yr) performed six sustained MVCs (22-s duration each, separated by 10 s). Before the fatiguing contractions, the men were stronger than the women (75.9 +/- 9.2 vs. 42.7 +/- 8.0 N.m; P < 0.05) in control MVCs. Voluntary activation measured with cortical stimulation before fatigue was similar for the men and women during the final control MVC (95.7 +/- 3.0 vs. 93.3 +/- 3.6%; P > 0.05) and at the start of the fatiguing task (P > 0.05). By the end of the six sustained fatiguing MVCs, the men exhibited greater absolute and relative reductions in torque (65 +/- 3% of initial MVC) than the women (52 +/- 9%; P < 0.05). The increments in torque (superimposed twitch) generated by motor cortex stimulation during each 22-s maximal effort increased with fatigue (P < 0.05). Superimposed twitches were similar for men and women throughout the fatiguing task (5.5 +/- 4.1 vs. 7.3 +/- 4.7%; P > 0.05), as well as in the last sustained contraction (7.8 +/- 5.9 vs. 10.5 +/- 5.5%) and in brief recovery MVCs. Voluntary activation determined using an estimated control twitch was similar for the men and women at the start of the sustained maximal contractions (91.4 +/- 7.4 vs. 90.4 +/- 6.8%, n = 13) and end of the sixth contraction (77.2 +/- 13.3% vs. 73.1 +/- 19.6%, n = 10). The increase in the area of the motor-evoked potential and duration of the silent period did not differ for men and women during the fatiguing task. However, estimated resting twitch amplitude and the peak rates of muscle relaxation showed greater relative reductions at the end of the fatiguing task for the men than the women. These results indicate that the sex difference in fatigue of the elbow flexor muscles is not explained by a difference in supraspinal fatigue in men and women but is largely due to a sex difference of mechanisms located within the elbow flexor muscles.

Effects of muscle fatigue induced by low-level clenching on experimental muscle pain and resting jaw muscle activity: gender differences

The purpose of this study was to investigate the effects of jaw-muscle fatigue evoked by low-level tooth-clenching followed by the induction of experimental muscle pain by injection of glutamate on the perception of fatigue and pain and on the resting electromyographic (EMG) activity. In addition, the role of gender on these interactions was studied. The EMG activities of bilateral masseter (MAL, MAR) and temporalis (TAL, TAR) muscles in 11 healthy young women and 12 men were measured before (Baseline) and after tooth-clenching for 30 min at 10% of maximal force (Post1), after subsequent glutamate (Glu) or isotonic saline (Iso) injection into the MAL following the tooth-clenching (Post2) and 60 min after tooth-clenching (Post3). The intensities of fatigue, fatigue-related muscle pain and headache-like symptoms were scored on 0-10 cm visual analog scales (VAS). The glutamate-evoked pain was continuously scored on an electronic VAS. Sustained low-level tooth-clenching consistently produced fatigue sensation, fatigue-related muscle pain and headache-like symptoms in both genders with significantly higher fatigue VAS scores in men than in women, while the accompanying increase in the resting EMG activity appears higher in women than in men in the masseter muscles. In this study no gender differences were found for the perceived amount of experimental pain induced by glutamate injection. Additional increases of the resting EMG activity after injections occurred only in men in the injected masseter muscle and non-injected temporalis muscles. The present findings provide new information on the complex influence of gender on sensory-motor integration in the trigeminal system which may contribute to differences in susceptibility to develop musculoskeletal pain problems.

Gender Differences in the Fatigability of the Inspiratory Muscles

PURPOSE

The purpose of the present study was to determine whether gender differences are present in the fatigability of the inspiratory muscles of humans. Based on evidence for a gender difference in the oxygen cost of breathing, we hypothesized that females would result in a greater magnitude and/or faster rate of inspiratory muscle fatigue than males.

METHODS

Eleven females and males (N = 22) performed resistive breathing at a target pressure of 70% of maximal inspiratory pressure (PImax). Rate of inspiratory muscle fatigue was calculated from measures of PImax taken every 2 min during resistive breathing, and recovery of inspiratory muscle strength was assessed up to 45 min following task failure.

RESULTS

Resting PImax was found to be lower for females than males (F:137.0 +/- 7.6 cm H2O; M:172.5 +/- 9.8 cm H2O, mean +/- SE, P </= 0.05). During resistive breathing, females exhibited a slower absolute and relative rate of muscle fatigue than males (F: 1.5 +/- 0.4 cm H2O.min(-1); M: -2.9 +/- 0.3 cm H2O.min(-1); P </=0.05). Females and males resulted in a similar decrease in PImax at task failure (15%), and no gender difference was found for recovery of inspiratory muscle strength. Separate analyses were performed in a subgroup of females and males that were matched for resting PImax. Females demonstrated a slower rate of fatigue and less muscle fatigue at task failure than males. No gender difference was found in time to task failure or the recovery of inspiratory muscle strength.

CONCLUSION

The results provide some evidence that the fatigability of the inspiratory muscles is different between genders. Females demonstrated a slower rate of fatigue during resistive breathing than males, a finding independent of muscle strength.

Sex differences in glycolysis during brief, intense isometric contractions

We have previously observed less muscle fatigue in women than men under conditions of intact circulation, but similar fatigue across the sexes during local ischemia. Thus, we hypothesized that women utilize their aerobic metabolic pathways to a greater extent than do men. To test this hypothesis, we examined the extent to which different pathways of intramuscular adenosine triphosphate (ATP) production were utilized by men and women during maximal voluntary isometric contractions. Force production during 15-s and 60-s contractions were recorded in parallel sessions. In one session, central activation was assessed with electrical stimulation. In the other, phosphorus magnetic resonance spectroscopy was used to quantify muscle oxidative capacity, and the contributions of glycolysis and oxidative phosphorylation to ATP synthesis during the 60-s contraction. Fatigue and central activation were similar in men and women during both the 15-s and 60-s contractions. The rate constants of phosphocreatine recovery following the 15-s contraction were similar in men and women, indicating similar oxidative capacities. Men exhibited greater acidosis and peak glycolytic rates compared with women during the 60-s contraction, with no differences observed in creatine kinase flux or the percent of oxidative capacity utilized. We conclude that men exhibit greater in vivo glycolysis during brief, intense isometric contractions. Although this metabolic difference did not contribute to any observable differences in fatigue in the present study, these results highlight a potentially important mechanism to explain sex-related differences in muscle function.

Blood flow does not limit skeletal muscle force production during incremental isometric contractions

It has been suggested that a transient limitation in blood flow during intermittent muscular contractions can contribute to muscle fatigue, and that this limitation is greater as contraction intensity increases. We investigated skeletal muscle blood flow and fatigue in 13 healthy, untrained men (21-27 years) during 16 min of intermittent (4 s contract, 6 s relax) isometric dorsiflexor contractions. Contractions began at 10% of pre-exercise maximal voluntary contraction (MVC) force and increased by 10% every 2 min. Hyperemia (i.e., post-contraction blood flow, measured by venous occlusion plethysmography) and MVC were measured at the end of each stage. Muscle volume measures were obtained using magnetic resonance imaging. After 10 min of exercise, submaximal force and post-contraction hyperemia plateaued. MVC fell from 8 min of exercise onwards (p=0.004), and this onset of fatigue preceded the plateau in submaximal force and hyperemia. Despite a large range in dorsiflexor muscle size (66.3-176.4 cm(3)) and strength (112.5-421.8 N), neither muscle size nor strength were related to fatigue. The temporal dissociation between changes in blood flow and the onset of fatigue (fall of MVC) suggest that limited blood flow was not a factor in the impaired force production observed during intermittent isometric dorsiflexor contractions in healthy young men. Additionally, post-contraction hyperemia increased linearly with increasing contraction intensity, reflecting a match between blood flow and force production throughout the protocol that was independent of fatigue.