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1 Department of Exercise Science, University of Massachusetts, Amherst, Massachusetts 01003; 2 Exercise Science Program, Marquette University, Milwaukee, Wisconsin 53201; and 3 Magnetic Resonance Unit, Department of Radiology, University of California, San Francisco, California 94121
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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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 Pi and
H2PO
physical activity; magnetic resonance spectroscopy; central fatigue; activation; excitation-contraction coupling; metabolism
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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MUSCLE FATIGUE CAN BE DEFINED as the fall in maximum force-generating capacity of the muscle. During exercise, the magnitude and mechanisms of human skeletal muscle fatigue vary widely and depend to a large extent on the individual, the type of muscle, and the exercise stimulus or task. In general, fatigue may arise during muscular contractions due to failure at one or more sites along the pathway of force production from the central nervous system to the contractile apparatus (16).
There is reason to believe that both age and gender can affect the fatigue process, although our understanding of these effects is hampered by a lack of consensus in the literature. Although it has been reported that older adults fatigue relatively more than young adults (12, 33) and that men fatigue more than women (18, 20, 36), some investigators have found no effect of age (31, 35, 44) or gender (14) on fatigue. Still others have found that older subjects fatigue relatively less than younger subjects (3, 14).
Along with the lack of clarity regarding the effects of age and gender on the magnitude of muscle fatigue, the mechanisms of these differences have not been established. Differences in fatigability across age or gender could occur as a result of differences in neural drive, fiber-type composition, contractile function, muscle membrane excitability, metabolic capacity, or muscle mass and blood flow. For example, it was recently suggested that central activation failure may play a relatively larger role in the fatigue of older compared with younger adults (2, 44). Other investigators have reported impairments in excitation-contraction coupling in the muscle of older adults (13), although the possible role of this impairment in fatigue has not been established. The results of some (37), but not all (9, 28), studies suggest that oxidative capacity may be impaired with aging, despite a general shift toward a more oxidative fiber-type profile in older compared with younger muscle (30, 34). An impaired oxidative capacity in the muscle of older adults might contribute to fatigue in this group. Finally, it is unclear how a gender-based difference in fatigue might interact with the aging process.
In addition to the effects of activation, contractile function, and metabolism on muscle performance, the degree of fatigue that develops during exercise may be affected by muscle size and, consequently, vascular constriction during contraction. The impact of larger muscle mass, greater strength, and higher target tensions during exercise in men compared with women has been addressed in several studies. In the adductor pollicis, a gender-based difference in endurance time during a submaximal contraction persisted despite matching subjects to similar strengths (18). More recently, Hunter and Enoka (21) showed a gender difference in endurance (time to failure to maintain target tension) of the elbow flexor muscles during a contraction sustained at 20% maximal voluntary contraction (MVC) force but similar fatigue (fall in MVC) in men and women at the end of this exercise. Notably, the gender difference in endurance was negated by accounting for preexercise differences in muscle strength. These and other (14) results suggest that the relationship between muscle strength and fatigue should be examined in studies of the effects of age or gender on fatigue.
The purpose of this study was to investigate the magnitude and mechanisms of fatigue (i.e., fall in MVC of the ankle dorsiflexor muscles) in healthy young and older men and women during a progressive, intermittent isometric dorsiflexion exercise protocol that proceeds from a steady-state oxidative phase to a more glycolytic, fatiguing phase (26). The dorsiflexor muscles are functionally important for locomotion, posture, balance (49), and the prevention of falls in older adults (7). Furthermore, habitual use of the dorsiflexor muscles may make men and women less susceptible to disuse deconditioning than use of muscles more typically involved in high-power activities (e.g., quadriceps femoris), which are not often used by older adults.
To examine the mechanisms of fatigue, we obtained simultaneous measures of central and peripheral muscle activation, muscle contractile properties, and intramuscular energy metabolism by using a unique combination of voluntary and electrically stimulated muscle contractions, electromyography (EMG), and phosphorus-31-magnetic resonance spectroscopy. The relationship between strength and fatigue was also examined. To control for the effects of varying levels of physical activity on these measures, we studied individuals with similar, relatively sedentary habitual activity levels. By simultaneously measuring many of the factors that have been suggested to explain age- and gender-based differences in fatigue, we hoped to resolve some of the current discrepancies in this area.
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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We used a cross-sectional design to make comparisons across age and gender. Measures of MVC force were made before, every 2 min during, and 0, 2, 5, and 10 min after the exercise protocol. Central activation was measured before and immediately after exercise. Measures of peripheral activation and contractile function were made before and 0, 2, 5, and 10 min after exercise. The metabolic measures were made before and continuously throughout exercise.
Subjects. Forty-one healthy, nonsmoking men and women aged 25-45 (10 men, 10 women) or 65-85 yr (11 men, 10 women) were recruited from the community. Individuals with chronic disease or those taking medications that might affect muscle function were excluded. All subjects were relatively sedentary in that they participated in two or less periods of continuous (>20 min) activity per week. To minimize the chances of including individuals with latent peripheral vascular disease, any subject with resting supine ankle and/or brachial systolic blood pressure of <1.0 was eliminated from the study. All subjects provided written, informed consent, as was approved by the Committee on Human Research at the University of California, San Francisco. All studies were conducted in San Francisco.
To establish that our study groups were similarly active, physical activity was quantified by using a three-dimensional accelerometer (Tritrac R3D, Professional Products, Madison, WI). Each subject wore an accelerometer at the waist during waking hours for a period of 1 wk, as previously described (27). All subjects maintained a brief written log of activities that was examined and discussed on returning the monitor to the laboratory. The vector magnitude (arbitrary units) for all three dimensions was averaged over the 7 days and divided by 1,000 (for ease of expression) and used as the measure of physical activity.Experimental arrangement. All measures of force, contractile properties, activation, and metabolism were acquired with the subject seated with one leg extended (knee fixed at ~170° extension) into the 30-cm-bore superconducting magnet, as described in detail elsewhere (24, 25, 28). The foot was secured to a platform (ankle angle 120°) under which was mounted a nonmagnetic force transducer, which in turn was coupled to a personal computer.
The stimulating electrode (pair of 10-mm nonmagnetic disks, Grass Instruments, West Warwick, RI; mounted on plastic) was placed over the peroneal nerve, ~1 cm distal to the fibular head. A copper ground plate was placed distally between the stimulating electrode and the EMG recording electrodes (see below). For each subject, supramaximal intensity [15% greater than that necessary to elicit a maximal compound muscle action potential (CMAP)] was determined and then used for all subsequent stimuli. Twitch (0.1-ms pulse) and tetanic (50-Hz, 500-ms train) forces were obtained at sampling rates of 2,500 and 500 Hz, respectively. CMAP was recorded at 2,500 Hz with nonmagnetic surface electrodes (10-mm disks) taped over the belly and distal tendon of the tibialis anterior muscle, as previously described and used (24, 25, 29, 39). Force and EMG data were acquired and transferred to spreadsheet for analysis. Phosphorus magnetic resonance spectroscopy was used to acquire information regarding intramuscular energy metabolism, as performed previously (24). Data were collected in the 1.9-T superconducting magnet by using a 3 × 5-cm elliptical surface coil taped over the belly of the tibialis anterior muscle, just proximal to the EMG recording electrode. After collection, data were transferred to personal computer for analysis, described in Force and contractile measurements.Force and contractile measurements. Before the fatigue protocol, the following measures were made, in order: CMAP and accompanying twitch, MVC force, central activation ratio (CAR), potentiated CMAP + twitch, and stimulated tetanus. Each measure was separated by 1 min of rest. The CMAP + twitch measure and the MVC measure were each repeated three times at 1-min intervals. Peak MVC force was determined from the best of the three 3- to 4-s trials. To ensure optimal performance by the subject, any MVC trial that resulted in a force of <90% of the other trials was repeated. Twitches were acquired before and 0, 2, 5, and 10 min after exercise, and tetani were acquired before and 0, 5, and 10 min after exercise. The postexercise and recovery twitch forces were scaled to the potentiated twitch, which was obtained immediately after the third baseline MVC.
Because contractile failure is often a source of fatigue (17) and because it has been suggested that excitation-contraction coupling may be impaired with age (13), we measured several indexes of contractile function by using stimulated twitch and tetanic contractions of the dorsiflexor muscles before, immediately after, and during recovery from exercise, as performed previously (27, 39). Contractile function may be quantified by the peak forces elicited during stimulated contractions as well as by the speeds of contraction and relaxation. Together, these provide indirect information related to changes in the periphery before (e.g., due to differences in fiber type) and during fatiguing exercise, in particular excitation-contraction coupling and calcium resequestration (5, 16, 17, 47). To fully represent the contraction and relaxation characteristics of the muscle, both the maximum rates of force development (dF/dt) and relaxation (
dF/dt), and the more
global measures of twitch contraction time and tetanic half-relaxation
time, were determined. The dF/dt and dF/dt were
calculated for both the twitch and tetanic contractions. Because the
rate of force development is faster with higher force production
(38), dF was expressed as a percentage of the peak force
achieved during each contraction. Thus these rates are expressed as
percent peak force per millisecond. This approach allows comparisons of
rates across individuals with differing torque-producing capacities.
For the tetanus, the half time of force relaxation was calculated as
the time (in ms) from the last CMAP in the train to the point at which
force fell to 50%. For the twitch, the time to peak force (in ms) was
calculated, with the use of the differential of the force trace, from
the time of force onset to the time at which dF/dt = 0. These calculations were performed in an Excel spreadsheet (Microsoft,
Redmond, WA), as previously reported (24, 27).
Activation measurements. Central activation, measured here as that portion of neuromuscular activation located proximal to the stimulating electrode, was quantified by using the CAR [CAR = MVC/(MVC + superimposed stimulated force); Ref. 25].
The stimulated force was elicited with a supramaximal train (50 Hz, 500 ms) that was superimposed on the voluntary contraction when force had reached maximal and plateaued. CAR was determined before and at the end of exercise. Peripheral activation was assessed from CMAP, which reflects the excitability of the neuromuscular junction and muscle membrane (1). CMAP peak-to-peak amplitude (in mV) and duration of the negative peak (in ms) were determined.Metabolic measurements.
After acquisition of the baseline force and contractile and activation
measures, the subject sat quietly while the magnet was shimmed and
phosphorus data were acquired from the resting muscle. The repetition
time for all acquisitions was 1.25 s. The data were averaged over
1 min for the rest spectrum (48 acquisitions) and every 30 s (24 acquisitions) during exercise. To ensure accurate quantification of
overlapping peaks, all peaks in the spectra [bone broad component,
phosphomonoesters, Pi, phosphodiesters, phosphocreatine
(PCr), 3 peaks of adenosine triphosphate] were fit by using NMR1
software (New Methods Research, White Plains, NY). The data were then
imported into a spreadsheet, corrected for partial saturation, and used
to calculate Pi/PCr, Pi (in mM), diprotonated Pi (H2PO
Exercise and recovery protocol. After acquisition of baseline measures of force, contractile properties, activation, and metabolism, each subject practiced several contractions at 10% MVC to become familiar with the target intensity and duty cycle of the exercise protocol. The subject then performed 16 min of isometric contractions (4-s contraction, 6-s relaxation). Exercise began at 10% of MVC and was incremented by 10% every 2 min. To determine the time course of fatigue, an MVC was performed at the beginning of each 2-min stage. Immediately postexercise, MVC with superimposed train (CAR), tetanic force, and CMAP with accompanying twitch force were measured. The primary fatigue variable was the fall of MVC at end of exercise, i.e., postexercise MVC/preexercise MVC. This protocol typically causes MVC to fall to ~70-75% of initial levels (26, 29).
Statistical analyses. Two-factor (age, gender) ANOVAs were used to examine differences between groups in preexercise force (MVC, tetanic force, twitch force), contractile properties (maximum rates of twitch and tetanic force development and relaxation, twitch contraction time, and tetanic half-relaxation time), peripheral activation (CMAP amplitude and duration), and metabolic variables at rest (Pi/PCr, pH). Due to the ceiling effect of the CAR measure, Mann-Whitney and Wilcoxon nonparametric procedures were used to detect differences across groups in pre- and postexercise CAR values. Two-factor (age, gender) repeated-measures (pre-, postexercise) ANOVAs were used to compare changes in force, contractile properties, and activation before vs. immediately after exercise. Changes in metabolites throughout exercise and the recoveries of MVC, tetanic force, twitch force, CMAP, and contractile properties were also compared across groups by using two-factor, repeated-measures ANOVA.
To investigate the role of muscle mass in fatigue, the association between preexercise MVC (a surrogate for mass, given complete activation and a consistent ankle angle) and fatigue was determined by using univariate linear regression analysis for all subjects combined. Likewise, to determine whether initial strength (mass) was related to the metabolic response to exercise, the relationship between preexercise MVC and end-exercise H2PO
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RESULTS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Subjects. Descriptive data are provided in Table 1. To aid interpretation, data are grouped by age and gender throughout the paper. The men were taller and heavier than the women, with no age effect. Seven of the older women were on estrogen replacement therapy. There were no age or gender main effects (P > 0.05) for physical activity level. Group sizes for the activity measurement were 10 young women, 8 older women, 9 young men, and 11 older men.
For each category of variables in the following sections, the data from measurements taken before the fatigue protocol are presented first, followed by comparisons of the exercise and recovery data.Force and fatigue.
Preexercise values for MVC, tetanic force, and twitch force are
provided in Table 2. Overall, men were
stronger than women, with no significant effect of age.
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Contractile properties. Before the fatigue protocol, older subjects showed the expected age-related slowing of contractile properties in response to twitch and tetanic stimuli (Table 2). Twitch contraction time was longer, and the maximum rates of twitch force development and relaxation were lower, in older compared with young subjects (P < 0.01, all). Likewise, the maximum rates of tetanic force development and relaxation were lower (P < 0.01, both), and the half-time of force relaxation tended to be longer (P = 0.07) in older compared with young subjects. There was no effect of gender on these variables.
Before the exercise protocol, the potentiation of twitch force after baseline MVCs was greater in men (161 ± 12%) than in women (133 ± 7%, P = 0.02), with no effect of age (P = 0.34). The ratio of potentiated twitch to tetanic force was higher in older (0.17 ± 0.01) compared with young subjects (0.13 ± 0.02, P < 0.01). There was no effect of gender on this variable. Exercise had no effect on twitch contraction time or the maximum rates of twitch force development and relaxation in any group. However, there was an increase in the maximum rate of tetanic force production after exercise (P < 0.01), with a significant gender effect indicating that women had a greater increase in the speed of tetanic force production compared with men (P < 0.01; Fig. 2). In contrast, there was a significant slowing of both the maximum rate and the half-time of tetanic force relaxation in response to exercise (P < 0.01, all; Fig. 2). The recovery of all twitch and tetanic contractile variables was similar across groups.
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Muscle activation. As shown by the CAR data in Table 2, there was no difference between groups in the ability to maximally activate the dorsiflexor muscles before the exercise protocol. Likewise, CAR was unchanged in all groups at the end of the exercise protocol (Table 3).
The amplitude of the unpotentiated CMAP was significantly lower in older compared with young subjects (P < 0.01; Table 2). Before the start of the exercise protocol, CMAP amplitude increased in all groups after potentiation by the three baseline MVCs (P < 0.01). The duration of the unpotentiated CMAP was similar in all groups (Table 2), and there was a similar increase of CMAP duration in all groups after the MVCs (P < 0.01). All postexercise CMAP values were compared with the potentiated CMAP. At the end of exercise, there was no further change in CMAP amplitude from the potentiated level in any group. However, there was a significant shortening of CMAP duration (P < 0.01) immediately after exercise, which was similar in all groups. There were no gender main effects for CMAP amplitude or duration, either before or after exercise. During the recovery period, CMAP amplitude increased in young relative to older adults (P = 0.04).Muscle metabolism.
At rest, Pi/PCr was higher in young compared with
older (P < 0.01) subjects, with no effect of gender.
Resting pH was similar in all groups (Table 2). There was no age effect
on the change in Pi/PCr during exercise (P = 0.76; Fig. 3), but there was a significant gender effect in that
women had a smaller increase in Pi/PCr compared with
men (P < 0.01; Fig. 3). As shown in Fig. 3, the rate of change in
Pi/PCr increased in the young and older men after
~8 min of exercise, indicating the end of the steady-state, oxidative
phase of this exercise protocol (8, 26). In contrast, women showed little change in the rate of increase of
Pi/PCr during exercise, suggesting that oxidative
metabolism was able to keep pace with energy needs in the women
throughout exercise.
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during the final minute of exercise was
linearly related to preexercise MVC or strength (r = 0.53, P < 0.001).
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DISCUSSION |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The primary results of this study were that, during incremental isometric exercise, 1) older subjects exhibited less fatigue compared with young subjects, 2) there was no effect of gender on fatigue, and 3) the metabolic response to exercise varied with age and gender in a manner that suggests a greater reliance on nonoxidative sources of ATP in young compared with older subjects and in men compared with women. Of note is the fact that these results were obtained in groups with similar habitual physical activity levels, which minimizes the effect of activity on our measures.
Contractile function. There were three main findings related to contractile function. First, contraction and relaxation rates were slowed in the unfatigued muscles of the older compared with young subjects, as expected. Second, there were no age-based differences in the degree of twitch potentiation before exercise or in the change in twitch-to-tetanus ratio, the increase in the rate of tetanic force development, and the slowing of force relaxation of the tetanus after exercise. Third, there was earlier potentiation of twitch force in men compared with women, with no effect of age. Overall, alterations in contractile function did not explain the age-related difference in fatigue that we observed.
Our finding of an age-related slowing of electrically evoked twitch and tetanic contractile properties in the unfatigued muscle is similar to the results from previous studies of the dorsiflexor (10, 39) and other (15) muscles. This slowing is consistent with the age-related shift toward a higher percentage of type I fiber content reported by others (22, 34). Exercise caused a similar slowing of tetanic force relaxation in all groups, which is often a consequence of fatigue (5). Slowed force relaxation is likely due to the slowing of calcium resequestration by the sarcoplasmic reticulum in fatigued muscle (47). The fact that the older subjects showed no excessive slowing in either force development or relaxation during fatigue suggests that neither excitation-contraction coupling nor calcium kinetics were altered in this group compared with the young. The lack of an age-related effect on excitation-contraction coupling is further supported by the lack of an effect of age on the change in the twitch-to-tetanus ratio after exercise and by the similar recoveries of all force and contractile variables in all groups (17). Before the exercise protocol, the potentiation of twitch force after three MVCs was greater in men compared with women, with no effect of age. This result suggests that there was a greater increase in actin-myosin Ca2+ sensitivity in response to three 3- to 4-s MVCs in the men and no impact of age on this system (41). At the end of exercise, tetanic force fell similarly in all groups, whereas twitch force fell more in men compared with women (Table 3). As a result, the twitch-to-tetanic force ratio increased in women but decreased in men during fatigue. Furthermore, although the maximum rate of tetanic force development increased in all groups in response to the exercise protocol, this increase was significantly higher in women compared with men (Table 3). Taken together, these observations suggest that the majority of force potentiation occurred very rapidly (i.e., after baseline MVCs) in men, whereas potentiation reached its maximum later, during the exercise protocol, in women. These results indicate a gender-based difference in the magnitude and timing of force potentiation that may reflect differences between men and women in actin-myosin Ca2+ sensitivity (41).Activation. Neural activation may be separated broadly into central and peripheral components. In the present study, these are delineated by the location of the stimulating electrode, with all elements proximal to the electrode representing central activation and all elements distal to the electrode comprising peripheral activation. The main findings related to activation in this study were that neither central nor peripheral activation failure contributed to fatigue in any group in response to this incremental isometric protocol.
In contrast to some reports (2, 44), we observed no age-related impairment of central activation, either before (CAR, Table 2) or at the end (Table 3) of fatiguing exercise. Likewise, there was no difference between men and women in the ability to fully activate the dorsiflexor muscles. These results are consistent with our previous work in this muscle group (27), as well as with the work of others (20). It is likely that the moderate degree of fatigue observed with this protocol precluded the development of central fatigue, as central activation failure is often associated with more severely fatiguing exercise (e.g., Ref. 24). Although there was no failure of central activation, per se, it is possible that lower motor unit discharge rates may have played a role in the greater fatigue resistance of the older subjects. It has been reported that discharge rates are reduced in older compared with young adults during both submaximal and MVCs (10, 23). During a progressive exercise protocol such as that used here, lower discharge rates in older adults could serve to 1) acutely limit the extent to which Pi/PCr increases and pH decreases during exercise as the ability to drive the muscle at higher frequencies is limited and 2) shift the muscle toward a more oxidative profile (i.e., the prolonged exposure of all fibers to lower discharge rates would result in adaptation toward a slower, more oxidative muscle). Precedence for the first possibility exists from a study of muscle fatigue in people with multiple sclerosis (MS). Fatigue in the MS and control groups was similar during the same incremental isometric exercise protocol reported here (29). However, the metabolic response to exercise (i.e., Pi/PCr, pH) was markedly smaller in MS (29). The metabolic difference could not be explained by differences in motor unit recruitment in the MS group. Instead, this difference was likely because of an inability of MS patients to generate high discharge rates during exercise, as shown previously by Rice et al. (42). The second possibility, related to a morphological adaptation to chronically reduced activation rates, is consistent with the age-related increase in type I fiber area reported in the tibialis anterior muscle, from 76% in young adults to 84% in older adults (22). The observation of slower contractile properties in the unfatigued muscle of the older adults in this study is compatible with such a fiber-type shift. This shift might arise both from the loss of type II fibers due to the denervation-reinnervation process that occurs with aging (6) as well as from the lower discharge rates experienced by the muscle of older adults. Regardless of the mechanism, the shift toward a slower muscle is consistent with the greater fatigue resistance observed in the older group in this study. Although CMAP amplitude was smaller in older compared with young subjects before voluntary muscle activations (Table 2), the degree of potentiation of the CMAP in response to baseline MVC contractions was similar in all groups. This result suggests that, before fatigue, the enhancement of sarcolemmal Na+-K+ pump activity in response to contraction (19) is unaffected by age or gender. At the end of exercise, there was no change from baseline in CMAP amplitude in any group (Table 3), which suggests that there was no decrease in peripheral excitability during fatigue. The duration of the CMAP was shorter in all groups after exercise, suggesting that conduction velocity across the neuromuscular junction or along the muscle membrane had increased during this submaximal protocol. An increase in conduction velocity may have occurred due to a "warm-up" effect in the muscle. Overall, peripheral activation failure did not appear to play a role in the development of fatigue in any group during this protocol. Furthermore, there was no evidence to suggest that differences in peripheral excitability across age had an impact on the age-related difference observed in fatiguability in this study.Metabolism.
In contrast to the activation data, the metabolic data showed
significant age- and gender-related differences in response to
exercise. The exercise protocol used in this study begins with a
low-intensity, metabolically steady-state portion and ends with relatively high-intensity contractions that produce greater changes in
energy metabolites and pH (8, 26). During steady-state exercise, Pi/PCr reflects the ability of the muscle to
respond oxidatively to the need for ATP (8, 26). In the
present study, steady state was maintained similarly in all groups
through the first half of the exercise protocol, which suggests a
similar potential for oxidative metabolism in all groups. Beyond ~8
min, which corresponded to an intensity of 
50% MVC,
Pi/PCr increased at a more rapid rate in men than in women
(Fig. 3). This observation suggests that women continued to keep pace
with the energy demand via oxidative phosphorylation throughout the
exercise protocol, whereas men were less able to do so as the exercise
progressed. There was no effect of age on the change in
Pi/PCr during exercise, suggesting that the gender-based
difference in metabolic pathway "preference" persists with aging.
Interestingly, this difference is consistent with reports indicating a
relatively greater reliance on carbohydrate as a fuel in men compared
with women (45).
in women
compared with men. This observation may, in part, explain the lack of a
gender effect on fatigue despite the greater metabolic changes in men
compared with women. The mechanism of this greater sensitivity in women
is not clear but may be related to a generally lower reliance on
glycolytic metabolism in female muscle. That is, if female muscle is
typically less likely to encounter high concentrations of
H+, Pi, and
H2POMuscle strength.
As noted, it has been suggested that differences in muscle mass might
account for some of the differences in fatigue observed across gender
or age via the impact of intramuscular pressure on muscle perfusion
during the contractions. Higher absolute forces will produce higher
intramuscular pressure and, therefore, relatively less perfusion. This
concept is supported by the recent report from Hunter and Enoka
(21) in which gender differences in elbow flexor endurance
were nullified after adjustment for strength. Similarly, differences in
strength and absolute target tension were likely important factors in
an earlier study of fatigue in which gender-based differences in the
endurance response of the elbow flexors to immobilization were reported
(43). In the present study, preexercise MVC was associated
with ~24% of the fatigue that developed during exercise. This result
provides some support for the possibility that the intramuscular
pressure developed during each contraction may be relatively higher in
the stronger subjects, thus leading to greater occlusion of blood flow
to the working muscle during each contraction. A difference in blood flow would be particularly evident at the higher contraction
intensities in our protocol; interestingly, it is at these intensities
that the metabolic response to exercise diverges in young compared with
older subjects and in men compared with women (Figs. 3 and 4). The
significant relationship between strength (preexercise MVC) and
end-exercise H2PO
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ACKNOWLEDGEMENTS |
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The authors thank the volunteers who participated in this study. We also thank Dr. Milton Hollenberg for medical consultations; Hung Dao, Danielle Bartholomew, and Ian Lanza for assistance with data acquisition and analysis; Drs. John Neuhaus and John Buonoccorsi for statistical advice; and Drs. Kirsten Johansen and David Russ for comments on the manuscript.
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FOOTNOTES |
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This work was supported by National Institute on Aging Grant R29AG-12819.
Address for reprint requests and other correspondence: J. A. Kent-Braun, Dept. of Exercise Science, Totman Bldg. 108, Univ. of Massachusetts, Amherst, MA 01003 (E-mail: janekb{at}excsci.umass.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
August 2, 2002;10.1152/japplphysiol.00091.2002
Received 4 February 2002; accepted in final form 23 July 2002.
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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, Older men; 
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older women; 
, young men; 
, young
women.
