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

Biomedical Research on Health and Performance of Military Women: Accomplishments of the Defense Women's Health Research Program (DWHRP)

In 1994, Congress provided dollar 40 M for biomedical research on issues of importance for military women. This supported 104 intramural and 30 extramural studies and launched an era of research to narrow the knowledge gap on protection and enhancement of health and performance of military women. Projects addressed issues specific to female physiology (e.g., gynecological health in the field, maternal malaria), problems with higher prevalence for women (e.g., marginal iron deficiency, stress fracture), and issues of drug and materiel safety that had only been extrapolated from studies of men (e.g., chemical agent prophylaxis, fatigue countermeasures). Several important assumptions about female physiology and occupational risks were found to be astoundingly wrong. Hormonal changes through the menstrual cycle were less important to acute health risks and performance than predicted, exercise did not increase risk for amenorrhea and consequent bone mineral loss, and women tolerated G-forces and could be as safe as men in the cockpit if their equipment was designed for normal size and strength ranges. Data on personal readiness issues, such as body fat, physical fitness, nutrition, and postpartum return to duty, allowed reconsideration of standards that were gender appropriate and not simply disconnected adjustments to existing male standards. Other discoveries directly benefited men as well as women, including development of medical surveillance databases, identification of task strength demands jeopardizing safety and performance, and greater understanding of the effects of psychosocial stress on health and performance. This surge of research has translated into advances for the welfare of service women and the readiness of the entire force; relevant gender issues are now routine considerations for researchers and equipment developers, and some key remaining research gaps of special importance to military women continue to be investigated.

Sex differences in muscle fatigability and activation patterns of the human quadriceps femoris

The purposes of this study were to determine if the fatigability of the quadriceps femoris varies by biological sex under conditions of normal muscle blood flow and ischemia, and if differences in neuromuscular activation patterns exist. Young men and women (n = 11/group; age 20-39 years) performed a sustained knee extension contraction at 25% of maximal force under conditions of occluded (OCC) and normal muscle blood flow (NON-OCC). Electromyographic (EMG) activity was recorded from the vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM) and biceps femoris (BF) muscles, and analyzed for fatigue-induced changes in the amplitude and burst rate and duration (transient changes in motor unit recruitment) of the signal. Additionally, force fluctuations during the sustained contraction were quantified. Women had a longer time to task failure during the NON-OCC task [214.9 +/- 20.5 vs. 169.1 +/- 20.5 (SE) s] (P = 0.02), but not during the OCC task (179.6 + 19.6 vs. 165.2 +/- 19.6 s). EMG data demonstrated sex differences in the neuromuscular activation pattern of the RF muscle and the collectively averaged QF muscles. During the NON-OCC and OCC tasks women achieved a higher relative activation of the RF at task failure than men (NON-OCC: 40.68 +/- 4.57 vs. 24.49 +/- 4.19%; OCC: 36.80 +/- 5.45 vs. 24.41 +/- 2.12%) (P = 0.02 and 0.05, respectively). Also, during both tasks, they demonstrated a greater relative activation at task failure than men when an average of the VL, VM and RF was considered. Additionally, women exhibited a greater coefficient of variation in force fluctuations during the last-third of the fatiguing NON-OCC task (6.21 +/- 0.567 vs. 4.56 +/- 0.56%) (P = 0.001). No sex differences in EMG burst rate or duration were observed, although there was a trend towards greater EMG burst rate of the RF in association with muscle fatigue in the women (P = 0.09). Interestingly, the only neuromuscular activation variable that displayed a significant relationship with the time to task failure was the average relative EMG of the QF at task failure, and this relationship was observed under both experimental blood flow conditions (NON-OCC: r = 0.47, P = 0.03; OCC: r = 0.44, P = 0.04). These results indicate that sex differences in muscle blood flow and/or muscle metabolism are in part responsible for the female advantage in fatigue-resistance. Additionally, these findings suggest that men synergistically recruit the RF compartment to a lesser extent than women in association with muscle fatigue, and that women achieve an overall greater relative activation of the QF at task failure than men. However, the implications of these sex differences in neuromuscular activation patterns during fatiguing muscular contractions on the ability to withstand muscle fatigue (prolonged time to task failure) does not appear to be causally related.

Skeletal muscle fatigue, strength, and quality in the elderly: the Health ABC Study

We examined the muscle fatigue characteristics in older men and women and determined whether these were related to the size, strength, or quality of muscle. A total of 1,512 men and women aged 70-79 yr from the Health, Aging, and Body Composition Study participated in this study. Muscle cross-sectional area and attenuation were determined with computed tomography. Skeletal muscle fatigue and strength (peak torque) of the knee extensors and flexors were measured using isokinetic dynamometry. Men were more fatigue resistant than women for both knee extension (fatigue index: 70.4 +/- 15.3 vs. 66.9 +/- 14.3%; P < 0.05) and knee flexion (67.9 +/- 16.4 vs. 64.9 +/- 17.6%; P < 0.05). Peak torque and muscle quality (specific torque) were higher in men than women for knee extension (99.6 +/- 28.2 vs. 63.0 +/- 16.8 N x m and 1.62 +/- 0.43 vs. 1.51 +/- 0.39 N x m/cm2; both P < 0.05) and for knee flexion (74.0 +/- 26.4 vs. 49.6 +/- 15.9 N x m and 2.47 +/- 1.29 vs. 2.22 +/- 0.78 N x m/cm2; both P < 0.05). Total work and power output was greater in men compared with women for both the quadriceps (1,353 +/- 451 vs. 832 +/- 264 J and 87.7 +/- 33.5 vs. 53.3 +/- 19.2 W; both P < 0.05) and the hamstrings (741 +/- 244 vs. 510 +/- 141 J and 35.4 +/- 16.0 vs. 23.7 +/- 10.2 W; both P < 0.05). In both genders, the quadriceps was able to perform more work with greater power compared with the hamstrings. Those who were stronger actually had greater fatigue after adjusting for age, race, physical activity, and total body fat. In conclusion, older men were more fatigue resistant than women, although in both men and women greater fatigue was not related to muscle weakness.

Sex differences in respiratory exercise physiology

Respiratory exercise physiology research has historically focused on male subjects. In the last 20 years, important physiological and functional differences have been noted between the male and female response to dynamic exercise where sex differences have been reported for most of the major determinants of exercise capacity. Female participation in competitive and recreational sport is growing worldwide and it is universally accepted that participation in regular physical activity is of health benefit for both sexes. Understanding sex differences is of potential importance to both the clinician-scientist and the exercise physiologist since differences could impact upon exercise rehabilitation programmes for patient populations, exercise prescription for disease prevention in healthy individuals and training strategies for competitive athletes. Sex differences have been shown in resting pulmonary function, which may impact on the respiratory response to exercise. Women typically have smaller lung volumes and maximal expiratory flow rates even when corrected for height relative to men. Differences in resting and exercising ventilation across the menstrual cycle and relative to men have also been reported, although the functional significance remains unclear. Expiratory flow limitation and a high work of breathing are seen in women. Pulmonary system limitations, in particular exercise-induced arterial hypoxia, have been reported in both men and women; however, the prevalence in women is not yet known. From the available literature, it appears that there are sex differences in some areas of respiratory exercise physiology. However, detailed sex comparisons are difficult because the number of subjects studied to date has been woefully small.

Difference in skeletal muscle function in males vs. females: role of estrogen receptor-beta

Male skeletal muscles are generally faster and have higher maximum power output than female muscles. Conversely, during repeated contractions, female muscles are generally more fatigue resistant and recover faster. We studied the role of estrogen receptor-beta (ERbeta) in this gender difference by comparing contractile function of soleus (mainly slow-twitch) and extensor digitorum longus (fast-twitch) muscles isolated from ERbeta-deficient (ERbeta(-/-)) and wild-type mice of both sexes. Results showed generally shorter contraction and relaxation times in male compared with female muscles, and ERbeta deficiency had no effect on this. Fatigue (induced by repeated tetanic contractions) and recovery of female muscles were not affected by ERbeta deficiency. However, male ERbeta(-/-) muscles were slightly more fatigue resistant and produced higher forces during the recovery period than wild-type male muscles. In fact, female muscles and male ERbeta(-/-) muscles displayed markedly better recovery than male wild-type muscles. Gene screening of male soleus muscles showed 25 genes that were differently expressed in ERbeta(-/-) and wild-type mice. Five of these genes were selected for further analysis: muscle ankyrin repeat protein-2, muscle LIM protein, calsequestrin, parvalbumin, and aquaporin-1. Expression of these genes showed a similar general pattern: increased expression in male and decreased expression in female ERbeta(-/-) muscles. In conclusion, ERbeta deficiency results in increased performance during fatigue and recovery of male muscles, whereas female muscles are not affected. Improved contractile performance of male ERbeta(-/-) mouse muscles was associated with increased expression of mRNAs encoding important muscle proteins.

Gender differences in perceived exertion during fatiguing knee extensions

PURPOSE

To examine gender differences in knee extensor strength, fatigue, and perceived exertion during a single set of continuous dynamic knee extensor contractions.

METHODS

Fifteen men and 15 women were evaluated for their one-repetition maximum (1RM) during a single-leg, inertial knee extension with their right leg. All subjects then completed a single set of repeated knee extensions with a load equivalent to 50% of their 1RM to failure. Subjects lifted the weight by performing a knee extension, held the weight with the knee extended for 1-2 s, and then lowered the weight in a slow and controlled manner. Perceived exertion was measured after completion of each repetition, by viewing a modified Borg category-ratio (CR-10) scale. Perceived exertion responses were standardized across subjects via linear interpolation and power function modeling. The linear interpolated perceived exertion estimates were then examined for linear, quadratic, and cubic trends across the repetitions.

RESULTS

Men lifted a significantly greater amount of mass than women, when corrected for body mass. Men and women did not differ significantly in the number of repetitions performed to failure. Women displayed significantly higher power function exponents for the perceived exertion response than men (0.72 +/- 0.16 and 0.57 +/- 0.16, respectively) and demonstrated statistically nonsignificant greater increases in perceived exertion than men across the repetitions.

CONCLUSIONS

The major findings of this study indicated that: 1) men inherently possessed greater knee extensor strength than women; 2) submaximal fatiguing knee extensor performance did not differ between genders; 3) model selection had a significant impact on standardizing perceived exertion estimates; and 4) subtle gender differences in the perceived exertion response may have existed during submaximal, fatiguing resistance exercise.

A comparison of adductor pollicis fatigue in older men and women

Sex differences in fatigue resistance of the adductor pollicis (AP) muscle were studied in 24 older adults who were divided into three groups: 12 older men (69.8 +/- 4.60 years), 6 older women not on hormone replacement therapy (HRT) (70.2 +/- 4.02 years), and 6 older women on HRT (68.7 +/- 6.47 years). Fatigue in the AP muscle was induced using an intermittent (5 s contraction, 5 s rest) submaximal voluntary contraction (50% of maximal voluntary contraction (MVC)) protocol, which was continued until exhaustion (i.e., when subjects could either no longer maintain a 5-s contraction at 50% MVC or when the MVC was deemed to be lower than the target force). There was no effect of HRT on MVC or time to fatigue (TTF); therefore, the older women were pooled as one subject group. At baseline, men were stronger than women for MVC (75.9 +/- 18.8 N in men vs. 56.8 +/- 10.0 N in women; P < 0.05) and evoked twitch force (7.3 +/- 1.7 N in men vs. 5.2 +/- 0.8 N in women; P < 0.05). There was no difference in TTF between men and women (14.77 +/- 7.06 min in men vs. 11.53 +/- 4.91 min in women; P > 0.20), nor was there a significant relationship between baseline muscle force and TTF (r = 0.14). There was also no difference in the pattern of fatigue and recovery between the men and women. These results suggest that there is no difference in endurance or fatigue characteristics of the AP muscle in men and women over the age of 65 years, and that baseline muscle force does not predict fatigue resistance in this muscle.

MRI measures of perfusion-related changes in human skeletal muscle during progressive contractions

Although skeletal muscle perfusion is fundamental to proper muscle function, in vivo measurements are typically limited to those of limb or arterial blood flow, rather than flow within the muscle bed itself. We present a noninvasive functional MRI (fMRI) technique for measuring perfusion-related signal intensity (SI) changes in human skeletal muscle during and after contractions and demonstrate its application to the question of occlusion during a range of contraction intensities. Eight healthy men (aged 20-31 yr) performed a series of isometric ankle dorsiflexor contractions from 10 to 100% maximal voluntary contraction. Axial gradient-echo echo-planar images (repetition time = 500 ms, echo time = 18.6 ms) were acquired continuously before, during, and following each 10-s contraction, with 4.5-min rest between contractions. Average SI in the dorsiflexor muscles was calculated for all 240 images in each contraction series. Postcontraction hyperemia for each force level was determined as peak change in SI after contraction, which was then scaled to that obtained following a 5-min cuff occlusion of the thigh (i.e., maximal hyperemia). A subset of subjects (n = 4) performed parallel studies using venous occlusion plethysmography to measure limb blood flow. Hyperemia measured by fMRI and plethysmography demonstrated good agreement. Postcontraction hyperemia measured by fMRI scaled with contraction intensity up to approximately 60% maximal voluntary contraction. fMRI provides a noninvasive means of quantifying perfusion-related changes during and following skeletal muscle contractions in humans. Temporal changes in perfusion can be observed, as can the heterogeneity of perfusion across the muscle bed.

Influence of aging on sex differences in muscle fatigability

The purpose of this study was to compare time to task failure for a sustained isometric contraction performed at a submaximal intensity with elbow flexor muscles by young and old men and women. Twenty-seven young (14 men and 13 women, 18-35 yr) and 18 old (10 men and 8 women, 65-80 yr) adults sustained an isometric contraction at 20% of maximal voluntary contraction torque until target torque could no longer be achieved for > or = 5 s. Young adults were stronger than old adults (66.8 +/- 17.9 vs. 47.7 +/- 18.1 N x m, P < 0.05), and men were stronger than women (69.8 +/- 17.9 vs. 47.1 +/- 15.3 N x m, P < 0.05), with no interaction between age and sex (P > 0.05). Time to task failure was longer for old than for young adults (22.8 +/- 9.1 vs. 14.4 +/- 7.6 min, P < 0.05) and for young women than for young men (18.3 +/- 8.0 vs. 10.8 +/- 5.2, P < 0.05), but there was no difference between old women and men (21.3 +/- 10.7 and 24.1 +/- 8.0 min, respectively, P > 0.05) or between young women and old adults (P > 0.05). Mean arterial pressure, heart rate, average electromyographic (EMG) activity, and torque fluctuations of elbow flexor muscles increased during the fatiguing contraction (P < 0.05) for all subjects. Rates of increase in mean arterial pressure, heart rate, and torque fluctuations were greater for young men and old adults, with no differences between old men and women (P > 0.05). Similarly, the rate of increase in EMG activity was greater for young men than for the other three groups. EMG bursts were less frequent for old adults (P < 0.05) at the end of the fatiguing contraction, and this was accompanied by reduced fluctuations in torque. Consequently, time to task failure was related to target torque for young, but not old, adults, and differences in task duration were accompanied by parallel changes in the pressor response.

Manual wheelchair pushrim dynamics in people with multiple sclerosis11No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors(s) or upon any organization with which the author(s) is/are associated

OBJECTIVES

To define differences in pushrim dynamics during manual wheelchair propulsion by users with multiple sclerosis (MS) relative to 2 control groups of subjects with spinal cord injury (SCI) and no disability (ND) and to investigate changes in propulsion biomechanics at different speeds and with fatigue.

DESIGN

Case-control.

SETTING

Biomechanics laboratory.

PARTICIPANTS

A convenience sample of 42 people, 8 women and 6 men per group.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

The propulsion pattern, mean maximum speed, percentage of time in push and recovery phases, push angle, push frequency, mean maximum resultant pushrim force, mean work and push cycle, and hand-to-pushrim coupling and decoupling effects.

RESULTS

The MS groups tended to use an arcing propulsion pattern more than did the control groups (Pmax=.003). The MS group pushed at a lower mean maximum velocity (v) when allowed to choose the speed of propulsion (v(MS),.67+/-.20m/s; v(SCI), 1.10+/-0.23m/s; P=.001), when asked to push at 1m/s (v(MS),.91+/-.26m/s; v(SCI), 1.12+/-0.11m/s; v(ND), 1.05+/-0.13m/s; P=.010), and were unable to maintain a self-selected speed during a 5-minute trial (Deltav(MS),.15+/-.02m/s; Deltav(ND),.03+/-.06m/s; P<.001). The MS group spent a higher proportion of time in the push phase of propulsion (Pmax=.001). In general, the MS group had smaller push angles, but push frequencies similar to the controls. Statistically adjusted MS group pushrim forces and work per push measures varied depending on context, but in all trials the MS group displayed a braking effect when grasping and releasing the pushrim. The MS group had declines in most measures when pushing at a self-selected speed for 5 minutes. In all trials, the MS group was more likely to display asymmetry between right and left sides in biomechanic parameters.

CONCLUSIONS

Manual wheelchair users with MS have difficulty grasping and releasing the pushrim and maintaining speed during a fatigue trial. This likely leads to a slow self-selected speed of propulsion that may not be functional. Clinicians should remember these results when prescribing manual wheelchairs for people with MS.

Physiologic considerations for exercise performance in women

Women exhibit several anatomic and physiologic characteristics that distinguish their responses to exercise from those of men. Women are smaller than men, have less muscle mass, and more fat mass for a given body size. Blood volume, stroke volume, and cardiac output are all lower in women than in men. These and other factors contribute to lower maximal aerobic power (even for similar training status) in women. The reproductive hormones, estrogen and progesterone, can influence ventilation, substrate metabolism, and thermoregulation during exercise. Women have a greater tendency for EIAH, which can limit VO2max as well as submaximal exercise performance at higher intensities. Women tend to use a greater percentage of fats during exercise, but also rely on CHOs. Thermoregulatory control is altered significantly over the course of the menstrual cycle by fluctuations in circulating levels of progesterone and estrogen. It is important for women to include regular exercise in their daily routines, particularly because regular physical activity has been implicated in the prevention of osteoporosis, breast cancer, heart disease, and depression.

The effects of estradiol and progesterone on plantarflexor muscle fatigue in ovariectomized mice

The aim of this study was to examine specific and interactional effects of estradiol and progesterone on the time-to-fatigue of eccentrically contracted plantarflexor muscles and on the percent of plantarflexor isometric torque remaining immediately after an eccentric contraction (EC) protocol. Ovariectomized 6- to 8-week-old C57BL/6 mice were implanted with 21-day 0.05 mg-placebo, 0.05 mg-17-beta estradiol (OE), 15 mg-progesterone (OP), or estradiol and progesterone pellets (OEP). On the 16th day of hormone treatment, the isometric torque of the left plantarflexor muscles was measured. The left plantarflexor muscles then underwent 1 set of 150 ECs followed by 2 immediate post-EC isometric torque measurements. A group of ovarian-intact female mice of a similar age underwent the same isometric torque measurements and EC protocol. Plantarflexor muscle fatigue during ECs took 30%-41% longer to occur in the OP group (n = 9) than it did in the intact (n = 8, P = 0.02), OC (n = 11, P = 0.003), and OEP (n = 9, P = 0.007) groups. Peak active isometric torque had decreased immediately after ECs at 2 time points (M1 and M2). The OP group exhibited the greatest percent of isometric torque remaining immediately after ECs (M1, P = 0.03; M2, P = 0.04). These findings suggest that progesterone reduces muscle fatigue in response to ECs and that this progesterone effect is blunted when estradiol also is present. Therefore, ovarian hormone status may need to be considered when evaluating a response to physical activities, especially those activities involving ECs.

The thickness of the adductor pollicis muscle reflects the muscle compartment and may be used as a new anthropometric parameter for nutritional assessment

PURPOSE OF REVIEW

Contraction of the adductor pollicis muscle after electrical stimulation (electromyogram) or dynamometry (hand-grip tests) has been evaluated in a variety of clinical conditions as a parameter to assess nutritional status. However, adductor pollicis muscle thickness has not been investigated as an anthropometric parameter.

RECENT FINDINGS

Prolonged immobilization and non-use of lower and upper limb muscles causes atrophy. Adductor pollicis muscle function is normal in patients with stable chronic obstructive pulmonary disease and multiple sclerosis, whereas the musculature of lower limbs suffers more pronounced functional alterations. Structure and function are relatively preserved in upper limb muscles, probably because of the maintenance of some daily activities involving the arms. Inactivity as a result of a reduction in daily activities is probably the driving factor for these changes. Forearm immobilization for 21 days caused no significant change in muscle morphology, but caused a deterioration in muscle function. Virtually all routinely developed activities requiring opposition of the thumb muscle and repetitive exercise of one muscle group for a given period of time maintain muscle size and function. Apathy is often observed as malnutrition progresses, reducing daily working activity and aggravating adductor pollicis muscle loss besides the muscular catabolism caused by disease.

SUMMARY

This study provides the first estimates of adductor pollicis muscle thickness in normal healthy individuals. The adductor pollicis muscle has a positive correlation with anthropometric variables that estimate muscle mass, but fails to correlate with parameters that estimate fat mass. This measurement is now being evaluated as an anthropometric parameter in clinical studies.

Men are more fatigable than strength-matched women when performing intermittent submaximal contractions

The purpose of this study was to compare the time to task failure for a series of intermittent submaximal contractions performed with the elbow flexor muscles by men and women who were matched for strength (n = 20, 18-34 yr). The fatigue task comprised isometric contractions at 50% of maximal voluntary contraction (MVC) torque (6-s contraction, 4-s rest). The MVC torque was similar for the men and women [64.8 +/- 9.2 (SD) vs. 62.2 +/- 7.9 N.m; P > 0.05]. However, the time to task failure was longer for the women (1,408 +/- 1,133 vs. 513 +/- 194 s; P < 0.05), despite the similar torque levels. The mean arterial pressure, heart rate, and rating of perceived exertion started and ended at similar values for the men and women, but the rate of increase was less for the women. The rate of increase in the average of the rectified electromyogram (AEMG; % peak MVC) for the elbow flexor muscles was less for the women: the AEMG was greater for the men compared with the women at task failure (72 +/- 28 vs. 50 +/- 21%; P < 0.05), despite similar AEMG values at the start of the fatiguing contraction (32 +/- 9 vs. 36 +/- 13%). These results indicate that for intermittent contractions performed with the elbow flexor muscles 1) the sex difference in time to task failure was not explained by the absolute strength of the men and women, but involved another mechanism that is present during perfused conditions, and 2) men required a more rapid increase in descending drive to maintain a similar torque.

Fatigability of the elbow flexor muscles for a sustained submaximal contraction is similar in men and women matched for strength

The purpose of this study was to compare the time to task failure for a submaximal fatiguing contraction sustained with the elbow flexor muscles by men and women who were matched for strength (n = 20, 18-35 yr). The maximal torque exerted at the wrist was similar for the men and women [64.5 +/- 8.7 (SD) vs. 64.5 +/- 8.3 N.m; P > 0.05], which meant that the average torque exerted during the fatiguing contraction [20% of maximum voluntary contraction (MVC)] was similar for the two sexes. The time to task failure was similar for these strength-matched men and women (819 +/- 306 vs. 864 +/- 391 s; P > 0.05). The mean arterial pressure was similar at the beginning of the contraction for men (97 +/- 12 mmHg) and women (96 +/- 15 mmHg; P > 0.05) and at task failure (134 +/- 18 vs. 126 +/- 26 mmHg; P > 0.05, respectively). Furthermore, the increases in heart rate, torque fluctuations, and rating of perceived exertion during the fatiguing contraction were similar for the two sexes. However, the electromyogram (EMG) activity differed for the men and women: the rate of increase in the average of the rectified EMG (% peak MVC) for all the elbow flexor muscles was less for the women compared with the men (P < 0.05). Furthermore, the bursts of EMG activity for the elbow flexor muscles increased toward exhaustion for all subjects but at a greater rate for the women compared with the men (P < 0.05). The results indicate that strength-matched men and women experienced similar levels of muscle fatigue and cardiovascular adjustments during a sustained low-force isometric contraction, despite differences in the EMG activity for the two groups of subjects.

Surface electromyography assessment of back muscle intrinsic properties

The purpose of this study was to assess (1) the reliability and (2) the sensitivity to low back pain status and gender of different EMG indices developed for the assessment of back muscle weakness, muscle fiber composition and fatigability. Healthy subjects (men and women) and chronic low back pain patients (men only) performed, in a static dynamometer, maximal and submaximal static trunk extension tasks (short and long duration) to assess weakness, fiber composition and fatigue. Surface EMG signals were recorded from four (bilateral) pairs of back muscles and three pairs of abdominal muscles. To assess reliability of the different EMG parameters, 40 male volunteers (20 controls and 20 chronic low back pain patients) were assessed on three occasions. Reliable EMG indices were achieved for both healthy and chronic low back pain subjects when specific measurement strategies were applied. The EMG parameters used to quantify weakness and fiber composition were insensitive to low back status and gender. The EMG fatigue parameters did not detect differences between genders but unexpectedly, healthy men showed higher fatigability than back pain patients. This result was attributed to the smaller absolute load that was attributed to the patients, a load that was defined relative to their maximal strength, a problematic measure with this population. An attempt was made to predict maximal back strength from anthropometric measurements but this prediction was prone to errors. The main difficulties and some potential solutions related to the assessment of back muscle intrinsic properties were discussed.

Sex differences in human skeletal muscle fatigue are eliminated under ischemic conditions

Several studies have suggested that women may be more resistant to muscle fatigue than men (Fulco CS, Rock PB, Muza SA, Lammi E, Cymerman A, Butterfield G, Moore, LG, Braun B, and Lewis SF. Acta Physiol Scand 167: 233-239, 1999) possibly because of differences in muscle oxidative metabolism. We evaluated muscle fatigue produced by intermittent, maximal volitional isometric contractions of the dorsiflexor muscles of healthy young (21-34 yr) men (n = 8) and women (n = 8) under two conditions: free-flow (FF) circulation and ischemia. Measures of voluntary and stimulated (10- and 50-Hz) force, central activation ratio (CAR), and compound muscle action potential (CMAP) were collected in each session. The ischemic protocol induced greater fatigue than the FF protocol, in both sexes, and was associated with greater reductions in CAR, CMAP, stimulated force, and the ratio of 10- to 50-Hz force compared with the FF condition. Women fatigued less than men in FF but not during ischemia, and this difference was roughly paralleled by a difference in CAR. No sex effects on the CMAP, tetanic force, and measures of excitation-contraction coupling function were found in the FF condition, suggesting that the primary mechanism behind the difference in fatigue was a relatively greater impairment of central activation in men. The observation that ischemia eliminated the sex differences in fatigue is consistent with a number of studies (Kent-Braun JA, Ng AV, Doyle JW, and Towse TF. J Appl Physiol 93: 1813-1823, 2002) relating fatigue to muscle metabolism and might be the result of sex-based differences in metabolic pathway utilization during muscle contraction.

Gender differences in skeletal muscle fatigability are related to contraction type and EMG spectral compression

The purposes of this study were 1) to evaluate gender differences in back extensor endurance capacity during isometric and isotonic muscular contractions, 2) to determine the relation between absolute load and endurance time, and 3) to compare men [n = 10, age 22.4 +/- 0.69 (SE) yr] and women (n = 10, age 21.7 +/- 1.07 yr) in terms of neuromuscular activation patterns and median frequency (MF) shifts in the electromyogram (EMG) power spectrum of the lumbar and hip extensor muscles during fatiguing submaximal isometric trunk extension exercise. Subjects performed isotonic and isometric trunk extension exercise to muscular failure at 50% of maximum voluntary contraction force. Women exhibited a longer endurance time than men during the isometric task (146.0 +/- 10.9 vs. 105.4 +/- 7.9 s), but there was no difference in endurance performance during the isotonic exercise (24.3 +/- 3.4 vs. 24.0 +/- 2.8 repetitions). Absolute load was significantly related to isometric endurance time in the pooled sample (R(2) = 0.34) but not when men and women were analyzed separately (R(2) = 0.05 and 0.04, respectively). EMG data showed no differences in neuromuscular activation patterns; however, gender differences in MF shifts were observed. Women demonstrated a similar fatigability in the biceps femoris and lumbar extensors, whereas in men, the fatigability was more pronounced in the lumbar musculature than in the biceps femoris. Additionally, the MF of the lumbar extensors demonstrated a greater association with endurance time in men than in women (R(2) = 0.45 vs. 0.19). These findings suggest that gender differences in muscle fatigue are influenced by muscle contraction type and frequency shifts in the EMG signal but not by alterations in the synergistic activation patterns.

Human skeletal muscle responses vary with age and gender during fatigue due to incremental isometric exercise

The purpose of this study was to compare the magnitude and mechanisms of ankle dorsiflexor muscle fatigue in 20 young (33 +/- 6 yr, mean +/- SD) and 21 older (75 +/- 6 yr) healthy men and women of similar physical activity status. Noninvasive measures of central and peripheral (neuromuscular junction, sarcolemma) muscle activation, muscle contractile function, and intramuscular energy metabolism were made before, during, and after incremental isometric exercise. Older subjects fatigued less than young (P < 0.01); there was no effect of gender on fatigue (P = 0.24). For all subjects combined, fatigue was modestly related to preexercise strength (r = 0.49, P < 0.01). Neither central (central activation ratio) nor peripheral (compound muscle action potential) activation played a significant role in fatigue in any group. During exercise, intracellular concentrations of P(i) and H(2)PO increased more and pH fell more in young compared with older subjects (P < 0.01) and in men compared with women (P < 0.01). These varied metabolic responses to exercise suggest a greater reliance on nonoxidative sources of ATP in young compared with older subjects and in men compared with women. These results suggest that the mechanisms of fatigue vary with age and gender, regardless of whether differences in the magnitude of fatigue are observed.

Smaller muscle ATP reduction in women than in men by repeated bouts of sprint exercise

It was hypothesized that the reduction of high-energy phosphates in muscle after repeated sprints is smaller in women than in men. Fifteen healthy and physically active women and men with an average age of 25 yr (range of 19-42 yr) performed three 30-s cycle sprints (Wingate test) with 20 min of rest between sprints. Repeated blood and muscle samples were obtained. Freeze-dried pooled muscle fibers of types I and II were analyzed for high-energy phosphates and their breakdown products and for glycogen. Accumulation of plasma ATP breakdown products, plasma catecholamines, and blood lactate, as well as glycogen reduction in type I fibers, was all lower in women than in men during sprint exercise. Repeated sprints induced smaller reduction of ATP and smaller accumulation of IMP and inosine in women than in men in type II muscle fibers, with no gender differences in changes of ATP and its breakdown products during the bouts of exercise themselves. This indicates that the smaller ATP reduction in women than in men during repeated sprints was created during recovery periods between the sprint exercises and that women possess a faster recovery of ATP via reamination of IMP during these recovery periods.

Sex differences in the fatigability of arm muscles depends on absolute force during isometric contractions

Women are capable of longer endurance times compared with men for contractions performed at low to moderate intensities. The purpose of the study was 1) to determine the relation between the absolute target force and endurance time for a submaximal isometric contraction and 2) to compare the pressor response and muscle activation patterns of men [26.3 +/- 1.1 (SE) yr] and women (27.5 +/- 2.3 yr) during a fatiguing contraction performed with the elbow flexor muscles. Maximal voluntary contraction (MVC) force was greater for men (393 +/- 23 vs. 177 +/- 7 N), which meant that the average target force (20% of MVC) was greater for men (79.7 +/- 6.5 vs. 36.7 +/- 2.0 N). The endurance time for the fatiguing contractions was 118% longer for women (1,806 +/- 239 vs. 829 +/- 94 s). The average of the rectified electromyogram (%MVC) for the elbow flexor muscles at exhaustion was similar for men (31 +/- 2%) and women (30 +/- 2%). In contrast, the heart rate and mean arterial pressure (MAP) were less at exhaustion for women (94 +/- 6 vs. 111 +/- 7 beats/min and 121 +/- 5 vs. 150 +/- 6 mmHg, respectively). The target force and change in MAP during the fatiguing contraction were exponentially related to endurance time (r(2) = 0.68 and r(2) = 0.64, respectively), whereas the change in MAP was linearly related to target force (r(2) = 0.51). The difference in fatigability of men and women when performing a submaximal contraction was related to the absolute contraction intensity and was limited by mechanisms that were distal to the activation of muscle.

Gender alters impact of hypobaric hypoxia on adductor pollicis muscle performance

Recently, we reported that, at similar voluntary force development during static submaximal intermittent contractions of the adductor pollicis muscle, fatigue developed more slowly in women than in men under conditions of normobaric normoxia (NN) (Acta Physiol Scand 167: 233-239, 1999). We postulated that the slower fatigue of women was due, in part, to a greater capacity for muscle oxidative phosphorylation. The present study examined whether a gender difference in adductor pollicis muscle performance also exists during acute exposure to hypobaric hypoxia (HH; 4,300-m altitude). Healthy young men (n = 12) and women (n = 21) performed repeated static contractions at 50% of maximal voluntary contraction (MVC) force of rested muscle for 5 s followed by 5 s of rest until exhaustion. MVC force was measured before and at the end of each minute of exercise and at exhaustion. Exhaustion was defined as an MVC force decline to 50% of that of rested muscle. For each gender, MVC force of rested muscle in HH was not significantly different from that in NN. MVC force tended to decline at a faster rate in HH than in NN for men but not for women. In both environments, MVC force declined faster (P < 0.01) for men than for women. For men, endurance time to exhaustion was shorter (P < 0.01) in HH than in NN [6.08 +/- 0.7 vs. 8.00 +/- 0.7 (SE) min]. However, for women, endurance time to exhaustion was similar (not significant) in HH (12.86 +/- 1.2 min) and NN (13.95 +/- 1.0 min). In both environments, endurance time to exhaustion was longer for women than for men (P < 0.01). Gender differences in the impact of HH on adductor pollicis muscle endurance persisted in a smaller number of men and women matched (n = 4 pairs) for MVC force of rested muscle and thus on submaximal absolute force and, by inference, ATP demand in both environments. In contrast to gender differences in the impact of HH on small-muscle (adductor pollicis) exercise performance, peak O(2) uptake during large-muscle exercise was lower in HH than in NN by a similar (P > 0.05) percentage for men and women (-27.6 +/- 2 and -25.1 +/- 2%, respectively). Our findings are consistent with the postulate of a higher adductor pollicis muscle oxidative capacity in women than in men and imply that isolated performance of muscle with a higher oxidative capacity may be less impaired when the muscle is exposed to HH.

Sex differences in human skeletal muscle fatigue

Research on muscle fatigue suggests that greater fatigue resistance may be evident in females compared with males. The possible mechanisms for this sex difference include factors related to muscle mass, substrate utilization, muscle morphology, and neuromuscular activation.

Force deficits after repeated stretches of activated skeletal muscles in female and male rats

Force deficits after stretches of activated plantar flexor muscles were measured in six male Sprague-Dawley rats (285 +/- 10 g, age 62 +/- 4 days, mean +/- SE) and compared with six age-matched (193 +/- 6 g, age 67 +/- 3 days) and six weight-matched female rats (273 +/- 7 g, age 141 +/- 9 days). Twenty stretches, imposed on isometric contractions at 90 degrees (0.2 ms pulse duration, 80 Hz, 5.4 +/- 0.3 V, duty cycle 0.006), were produced by ankle rotation from 90 to 40 degrees. Before the stretch protocol, weight-matched groups had similar isometric forces at an ankle position of 90 degrees at 5, 10, 20, 40, 60 and 80 Hz but forces were lower for age-matched females. For all groups, normalized force-frequency relationships were similar. During the stretch protocol, deficits for isometric force at 90 degrees and peak stretch force at 40 degrees with stretch number were similar for all groups. One hour after the stretches, isometric force deficits at 90 degrees at 40, 60 and 80 Hz were larger for females in weight-matched groups (e.g. 80 Hz, female: 47.8 +/- 1.7%; male: 41.1 +/- 1.7%; P < 0.05), perhaps because of a difference in age (P < 0.05). For age-matched groups, isometric force deficits at 90 degrees were similar at all frequencies. The susceptibility for force deficits by stretches of activated skeletal muscles was not gender-dependent for 2-month-old rats.

The effect of age and gender on the relative fatigability of the human adductor pollicis muscle

The purpose of this study was to examine the relative influence of such factors as age, gender, and absolute force on the fatiguability of the human adductor pollicis muscle. 12 young males (YM, 25.3 ± 2.1 y), 12 young females (YF, 23.5 ± 2.1 y), 12 older males (OM, 71.7 ± 5.6 y) and 12 older females (OF, 69.5 ± 4.6 y) participated. Three minutes of intermittent (5 s contraction, 2 s rest) maximal voluntary contractions (MVC) were used to fatigue the adductor pollicis muscle; the ulnar nerve was also stimulated in each 2 s rest period to evoke a maximal twitch. Males were stronger than females in both voluntary and evoked force (PT) in the young age group (MVC: YM, 10.0 ± 2.7 kg vs. YF, 6.6 ± 1.1 kg, P < 0.05) (PT: YM, 0.99 ± 0.21 kg vs. YF, 0.71 ± 0.12 kg, P < 0.05). In the older adults, however, males were stronger only in the evoked twitch (OM, 0.73 ± 0.24 kg vs. OF, 0.48 ± 0.07 kg, P < 0.05). There was no significant effect of gender or absolute muscle force on relative fatigability; the only variable found to significantly affect fatigability was age. Older adults were significantly less fatigable than young adults as indicated by the voluntary fatigue index (FI) (percentage of force reduction from baseline; FI-young, 40.2 ± 12.6% vs. FI-old, 25.2 ± 12.3%). This age effect, however, was more prominent in males than females (FI-YM, 44.7 ± 10.5% vs. FI-OM, 24.2 ± 10.7%, P < 0.01; FI-YF, 37.8 ± 14.1% vs. FI-OF, 26.3 ± 14.5%, P = 0.13). In conclusion, age was found to be the strongest single predictor of fatigability during short duration, intermittent exercise in human adductor pollicis muscle.Key words: neuromuscular fatigue, gender, age, muscle membrane excitability, absolute force.

Limb immobilization alters muscle activation patterns during a fatiguing isometric contraction

The purpose of the study was to determine the role of excitation-contraction coupling in the increased endurance time for low-force contractions after 4 weeks of elbow joint immobilization. Twelve subjects participated in a protocol that required immobilization of the elbow joint in a fiberglass cast for 4 weeks, and 4 subjects acted as controls. Measurements of muscle strength, contractile properties, and fatigability were performed before and after 4 weeks of limb immobilization, and after 4 weeks of recovery. The immobilization intervention produced significant reductions in the daily activity of the elbow flexor muscles, a 21% decline in the maximum voluntary contraction (MVC) force, and a 31% decrease in the maximum load that could be lifted once. Seven of the immobilized subjects exhibited an unusual pattern of muscle activity during the fatiguing contraction after immobilization, which was associated with an increase in the endurance time of the elbow flexor muscles (mean = 220%) in these subjects. The unusual pattern of muscle activity involved lower relative activity of the brachialis muscle, no increase in the amplitude of the electromyogram (EMG) for the elbow flexor muscles, and intermittent rather than continuous EMG. In contrast, the force-frequency relationship of biceps brachii was not altered by immobilization in these subjects, suggesting that adaptations in excitation-contraction coupling were not the primary cause of the prolonged endurance time after immobilization. Rather, the results suggest that the prolonged endurance time exhibited by some subjects after immobilization was largely due to adaptations within the nervous system.