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Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Submitted 1 June 2004 ; accepted in final form 11 July 2004
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ABSTRACT |
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91% peak oxygen consumption for 6 min once per hour for 16 h. Muscle isometric properties assessed before and after selected repetitions (R1, R2, R4, R7, R12, and R15) were used to quantitate fatigue (before vs. after repetitions) and weakness (before vs. before repetitions). Muscle fatigue at R1 was indicated by reductions (P < 0.05) in peak twitch force (135 ± 13 vs. 106 ± 11 N) and by a reduction (P < 0.05) in the force-frequency response, which ranged between 53% at 10 Hz (113 ± 12 vs. 52.6 ± 7.4 N) and 17% at 50 Hz (324 ± 27 vs. 270 ± 30 N). No recovery of force, regardless of stimulation frequency, was observed during the 54 min between R1 and R2. At R2 and for all subsequent repetitions, no reduction in force, regardless of stimulation frequency, was generally found after the exercise. The only exception was for R2, where, at 20 Hz, force was reduced (P < 0.05) by 18%. At R15, force before repetitions for high frequencies (i.e., 100 Hz) returned to R1 (333 ± 29 vs. 324 ± 27 N), whereas force at low frequency (i.e., 10 Hz) was only partially (P < 0.05) recovered (113 ± 12 vs. 70 ± 6.6 N). It is concluded that multiple sessions of heavy exercise can reverse the fatigue noted early and reduce or eliminate weakness depending on the frequency of stimulation. repetitive dynamic activity; isometric force; weakness
As might be expected, fatigue and weakness can display a variety of manifestations depending on the specifics of the task (6). The different mechanical manifestations suggest the differential involvement of selective sites in the motor command to the muscle and/or in the muscle itself (6). A major challenge in exercise physiology is to identify the specific sites responsible for the altered mechanical behavior observed during fatigue and weakness and the mechanisms responsible for the dysfunction observed in the particular sites involved.
One of the most controversial issues is the relative roles of central command vs. the muscle cell itself in the disturbances observed with repetitive voluntary activity (19). There is evidence, particularly in heavy exercise, that inhibition in neural drive can be implicated in fatigue. It has also been proposed that the reduction in central command occurs as a consequence of the accumulation of selective metabolic by-products in the muscle that triggers central inhibition via reflexes generated from the muscle (20).
It is also possible that, under some conditions, a failure in one or more excitation-contraction (E-C) process in muscle could be responsible for exercise-induced fatigue and weakness. High-intensity exercise has been shown to induce disturbances in the properties of the muscle compound action potential (M wave), which suggests that membrane excitability and the ability of the sarcolemma and T tubule to conduct action potentials may be compromised (25). There is also abundant evidence implicating a failure in sarcoplasmic reticulum (SR) Ca2+ handling as a site of fatigue (1). Repetitive exercise in known to depress Ca2+ uptake and Ca2+ release in humans (21). The alterations in these properties appear to disturb the cytosolic free Ca2+ ([Ca2+f]) integral, resulting in a reduced activation of the myofibrillar complex (1).
The alteration in specific E-C coupling processes could be responsible for different muscle mechanical responses under different activating conditions. Decreases in force during high-frequency stimulation, as an example, are most frequently attributed to the loss of membrane excitability (25). In contrast, the reduction in force at low frequency has been attributed to lower [Ca2+f] levels (1). Interestingly, the persistence of this type of weakness after exercise suggests that SR impairment is not easily reversed (1). It has also been observed that, if the [Ca2+f] is reduced sufficiently by exercise, depressions in high-frequency force can also be observed (1).
Unclear from the studies performed to date are the characteristics of fatigue and weakness induced by repeated bouts of heavy exercise. A single bout of heavy exercise would be expected to induce fatigue, as indicated by a depression in both the maximal voluntary contraction (MVC) force and a shift in the force-frequency response to the left (3). If sufficient time is provided after the exercise to allow for full recovery of metabolites, weakness should persist (14) at least at low frequencies of stimulation. As the number of repetitions is increased, both fatigue and weakness should be exacerbated, resulting in a progressive failure in the mechanical response in response to both low and high frequencies of electrical stimulation (ES).
The purpose of this study was to investigate the effects of repetitive intermittent exercise performed over an extended period of time on the mechanical manifestations of fatigue and weakness. We have hypothesized that, after exercise, fatigue would increase with the number of repetitions, resulting in progressive depressions in MVC force and in the force elicited at different frequencies of stimulation. We have also postulated that, during the recovery period, the low-frequency fatigue observed early in the repetitive exercise protocol would become more emphasized and ultimately result in weakness both during maximal voluntary effort and during different frequencies of stimulation.
In this study, we have defined fatigue as a depression in mechanical responses observed after exercise and weakness as the persistent disturbance in the mechanical response observed at the end of the recovery period between exercise repetitions.
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METHODS |
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O2 peak), as determined by a progressive cycle exercise to fatigue, was 3.09 ± 0.17 l/min. This study was approved by the Office of Research Ethics at the University of Waterloo, and all participants were fully informed of all experimental procedures and associated risks before written consent was obtained.
Experimental design.
Changes in fatigue and weakness in the quadriceps muscle were investigated in response to 16 repetitions of heavy cycling. Each repetition was performed for 6 min at 91% O2 peak, as determined by the gas-exchange values determined during the final minute of exercise at the initial repetition. During the 54-min recovery period between repetitions, participants remained in the test area and were not permitted to exercise except as provided by the testing protocols. The basic testing protocol included the assessment of muscle mechanical behavior both before (B) and after exercise (A) at repetitions 1 (R1), 2 (R2), 4 (R4), 7 (R7), 12 (R12), and 15 (R15). Fatigue and weakness were determined by the disturbances in isometric force recorded both during MVC and ES at different frequencies. For this study, fatigue is defined as the loss of force occurring in response to each repetition (B vs. A), whereas weakness is defined as the persistent impairment in force observed between exercise repetitions (B vs. B).
On the experimental day, each participant reported to the laboratory at 7 AM for preliminary preparation. The preliminary preparation involved insertion of catheters in the back of the hand for extraction of venous blood samples and preparation of sites on the vastus lateralis for harvesting muscle tissue (to be reported elsewhere). Before reporting to the laboratory, volunteers were required to ingest an Ensure (250 kcal) meal replacement consisting of 9.4 g of protein, 6.7 g of fat, and 38 g of carbohydrate (CHO) (Ross Products Division, Saint-Laurent, Canada). This was done to control for the preexercise nutritional intake between participants. Approximately 2 h elapsed between ingestion of the Ensure and the first exercise bout. During the 16-h experimental period, water and dietary supplements were not allowed during the first 2 h. Thereafter, water ingestion was ad libitum. After 2 h, the volunteers were allowed to consume selected vegetables, fruits, and Gatorade bars on a regulated basis. The macronutrient content of the foods ingested and the time of ingestion were recorded for each individual. Dietary composition as well as energy intake was calculated using nutritional analyses software (ESHA-Diet Analysis Plus, version 5.0, Salem, OR). The analyses addressed different segments of the intermittent exercise repetitions as well as the combined 16-h characteristics. The environmental conditions remained constant throughout the experimental day. The temperature and relative humidity were 20–22°C and 39–48%, respectively.
At least 2 wk before the intermittent exercise protocol, subjects performed progressive cycle exercise to fatigue for measurement of O2 peak and related properties. The specifics of the protocol and the gas-collection system were as previously reported (23, 34). The oxygen consumption responses were plotted against power output and the relationship used to establish individual power outputs for the intermittent exercise sessions. The power output selected was designed to elicit 90% O2 peak. The absolute power outputs selected remained constant for each of the work repetitions. For determinations of O2 peak and repetitive exercise sessions, an electrically braked cycle ergometer (Siemens Elma 380 B) was used. The cycle was calibrated on a daily basis (O2 peak tests) or at regular intervals (intermittent exercise). Seat height was individually adjusted and remained standardized for each repetition for each individual. All protocols were performed at a pedaling rate of 60 rpm. The intensity and duration of each of the intermittent work bouts were based on previous research from our laboratory (22, 36). We have found that, in untrained subjects, the protocol produces a pronounced strain on the metabolic systems. It was particularly important that subjects were able to perform complete multiple sessions of this protocol. Moreover, extensive adaptations also occurred in muscle metabolism and SR function when measured in the days after the intermittent exercise.
Assessment of muscle function.
Assessment of mechanical function was performed 10–15 min before the cycle exercise and 4–5 min after the exercise. The 4- to 5-min period, which was standardized for each volunteer, was needed to position and prepare for the measurements. The mechanical properties were based on isometric knee extension. For these measurements, the participant sat upright in a specially designed, straight-backed chair with hips and legs firmly secured, the knee at 90° to the thigh, and the arms folded across the chest. A 5-cm plastic cuff was positioned around the lower leg just proximal to the ankle malleoli and attached to a linear variable differential transducer. The linear variable transducer was amplified by a Daytronic carrier amplifier at 1 kHz, converted to a digital signal, and fed into an IBM computer for analyses. Two aluminum chloride electrodes (8 x 13 cm) coated with warm electrode gel were used to deliver the electrical impulse to the quadriceps muscle. The ground electrode was placed centrally on the interior aspect of the thigh just above the patella while the active electrode was toward the hip on the proximal portion of the belly of the vastus lateralis.
ES, applied to the right quadriceps muscle using a Grass model S48 stimulator with an isolation unit, was used to assess muscle mechanical behavior. The properties assessed were based on different frequencies of stimulation. To characterize the twitch properties, a single supramaximal (150 V) impulse with a 50-µs duration was employed. The twitch properties assessed included the peak twitch force (Pt), contraction time (CT), one-half relaxation time, the maximal rate of force development (+dF/dtmax), and the maximal rate of force decline (–dF/dtmax). Tetanic properties were based on stimulating the muscle at different frequencies (10, 20, 30, 50, and 100 Hz) using a pulse duration of 50 µs and train durations of 1 s. The stimulation voltage employed for each participant was established during the days before the experimental day and was set to elicit 50% MVC at 100 Hz. The same absolute voltage was employed for each individual both B and A for each of the repetitions used for measurement. For the tetanic stimulations, peak force was measured as well as +dF/dtmax and –dF/dtmax for each frequency.
A standardized sequence of measurements were performed both before and after selected repetitions of the exercise. The protocol consisted of two consecutive twitches (separated by 5 s) and stimulation at increasing frequencies, namely 10, 20, 30, 50, and 100 Hz. This was followed by two repetitions of MVC. A 30-s period separated each of the ES and MVC measurements. For the MVC, the subject was instructed to produce a maximal effort and to sustain the effort for 4–5 s. Verbal encouragement was provided. The time to administer the total protocol was 5 min.
Additional details describing the measurement procedures and analyses of raw data (Labview 5.1 software routine) are as published earlier from our laboratory (18). We have previously determined the reliability of the forces produced by transcutaneous stimulation at different frequencies (26). Under the conditions of this experiment, reliabilities as calculated by the correlation coefficient (r), ranged between 0.81 and 0.96 for 10 and 100 Hz, respectively.
We have also assessed membrane excitability at the time of the muscle function measurements using the properties of the muscle compound action potential (M wave). The properties assessed included the peak-to-peak amplitude (mV), duration (ms), and area (µV/s). The M wave was produced using a supramaximal twitch delivered to the vastus lateralis muscle as detailed previously by our group (18). No verification occurred to determine whether the M wave was maximized for each individual. However, in pilot work, we have determined that the voltage used for the twitch was well in excess of that needed to induce a maximal level for each of the M-wave properties of interest.
Electromyogram recordings were made with 10-mm-diameter Ag-AgCl (Meditrace 60) surface electrodes. Electrodes were placed over the belly of the vastus medialis (interelectrode distance was 2 cm) with one ground electrode positioned on the lateral epicondycle of the fibia. The skin was shaved, abraded, and cleaned with alcohol before electrode placement. Electrode positioning was kept constant for each individual for each of the M-wave determinations. The electromyogram signal (bandwidth of 20–500 Hz) was passed through an alternating-current amplifier (National Instruments, AT-M10–16H multifunction board). The gain was calibrated to optimize signal amplitude for analog-to-digital conversion and collected at 2,048 Hz. Custom-modified National Institute of Allergy and Infectious Diseases software (National Instruments) was used to acquire electromyogram and analyze raw data (Labview 5.1 software routine). For assessment of the properties of the M wave, amplitude was defined as the sum of the absolute values for maximum and minimum points of the biphasic (one positive and one negative deflection); duration was defined as the time from baseline to baseline from the beginning to the end of the biphasic M wave, where the beginning is established as a positive deflection two standard deviations above baseline harmonic mean and the end as a return to baseline; area was calculated as the integral of the absolute value of the entire M wave.
Data analyses. Two-way ANOVA procedures for repeated measurements were used to examine differences between exercise (B and A) and time (number of repetitions). Where significance was found, the Newman-Keuls technique was applied to determine which means were significantly different. Pearson's product-moment correlation coefficients were also calculated (using standardized techniques) between the change in MVC and the change in force elicited at each frequency of stimulation for each repetition and for the pooled sample. The probability for statistical significance was accepted at P < 0.05. Data are represented as means ± SE.
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14 and 18%, respectively. Weakness, defined as the persistent loss of force and assessed before performance of the heavy exercise bouts, was evident by the fourth repetition where reductions of 14 and 10% were observed compared with B at R1 and R2, respectively. The force exerted during MVC at B did not recover throughout the remaining repetitions.
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21% reduction in Pt (Table 2). A reduction in Pt was also observed after the second repetition of the exercise. Thereafter, no reductions in Pt were observed with exercise regardless of the number of repetitions performed. The failure of the heavy exercise protocol to induce fatigue after the second exercise session coincided with a reduction in the force elicited before exercise. At B, force failed to recover during the first four repetitions. By R7, force was higher compared with the second repetition but still depressed compared with the value measured before R1. By R12, weakness was not detected since no differences were found with the force measured before the first bout of exercise.
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Both properties used to assess relaxation characteristics of the twitch were altered by the exercise protocol (Table 2). In the case of one-half relaxation time, only a main effect occurred when R1 was greater than R2. This effect persisted throughout the remaining sessions. Changes in –dF/dtmax were also observed with exercise that depended on the number of repetitions. At R1 and R2, decreases were observed in –dF/dtmax. At each of the remaining repetitions, no differences were observed between B and A. By R12 and extending throughout R15, the fatigue observed at R1 and R2 was reversed such that no differences were noted with the values obtained before the first exercise bout.
Force-frequency responses.
The heavy intermittent exercise protocol induced significant changes in isometric force that were dependent both on the frequency of stimulation and the number of repetitions of the exercise that were performed (Fig. 2). Low-frequency fatigue was induced by the first session of exercise as evidenced by the 53% reduction in force that occurred at a stimulation frequency of 10 Hz. The depression in force persisted throughout recovery and was evident at B for all of the remaining exercise sessions. However, some recovery in weakness occurred as the number of exercise bouts increased. Beginning at R7 and extending through R12 and R15, the force at B was higher than that observed at B for R2 and R4. The same general fatigue and weakness pattern was observed at 20-Hz stimulation. At the higher frequencies of stimulation, namely 50 and 100 Hz, some differences in the response pattern to the repetitions of heavy exercise were observed compared with the 10- and 20-Hz frequencies. Although a pronounced fatigue occurred in response to the first exercise bout, which resulted in a persistent weakness through bout R7, at bouts R12 and R15 the force exerted at B was not different from that observed at B before R1. For R12 and R15, force was not depressed with exercise despite the recovery that was found compared with the start of the repetitive exercise protocol. A typical response pattern illustrating the reduction in force at low and high frequencies with the 6 min of cycling exercise is presented in Fig. 3.
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DISCUSSION |
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91% O2 peak resulted in adaptations that reduce or eliminate the fatigue and weakness induced as an early response. The potent effect of the exercise protocol on muscle fatigue and weakness in the unconditioned state can be appreciated by examining the mechanical response to ES before and after the first repetition of the exercise. After exercise, force was reduced across a range of frequencies, with the greatest relative reduction observed at 10 Hz (53%) and the lowest reduction observed at 100 Hz (17%). At R1, neuromuscular fatigue was also found as indicated by the 14% reduction in MVC. Weakness, defined as the persistent decrease in force during the 54-min period of recovery between exercise repetitions, was also pronounced after R1. Compared with R1, no recovery of force occurred at R2, regardless of the frequency of stimulation. In contrast, some recovery of MVC occurred since no differences were found between R1 and R2 before exercise. During the initial repetitions, the persistent weakness, resulting in low preexercise force levels, were in general associated with a reversal of fatigue since further depressions in force were not observed after exercise. As the number of repetitions of the heavy exercise increased, recovery of force was observed, with the magnitude depending on the frequency of stimulation. At the higher frequencies of 50 and 100 Hz, no differences in force were observed between R1 and R15 before exercise. At R15, fatigue induced by exercise was not observed at any frequency. The repetition-dependent effect on the mechanical behavior of the quadriceps is dramatic testimony to the rapidity with which adaptations can occur to protect the functional integrity of a muscle, which was initially perturbed by a heavy bout of exercise.
To appreciate the nature of the adaptations that occur in the mechanical responses over the 16-h period of intermittent exercise, it is necessary to gain insight into the mechanisms responsible for fatigue with the first repetition. Although reductions in MVC occurred with R1 and R2, suggesting a possible inability of central neural processes to maximally activate the quadriceps muscle, our results clearly indicate that much of the functional loss is peripheral in nature. Because the fatigue, which appears to activate the muscle cell by stimulating nerve branches (5, 24), was characterized by using surface electrodes, failure at the neuromuscular junction must be added to the potential list of processes that may be involved in mechanical failure. Among the excitation sites and processes that may be rate limiting are the sarcolemma and T tubules (which could result in an inability to conduct repetitive action potentials), the coupling between the T tubules and the Ca2+ release channel (CRC) of the SR (resulting in a disturbance in signal transmission to the CRC), and direct alterations to the SR itself (which could modify the recruitment of the contractile apparatus via alterations in the [Ca2+]f transient). Changes in [Ca2+f] transient could occur via disruptions in Ca2+ release from the CRC and/or by disruptions in Ca2+ uptake. Direct changes in contractile function could occur via modifications in the regulatory proteins troponin and tropomyosin, or via changes in the contractile proteins themselves, namely actin and myosin (28). Collectively, these changes, regardless of the site involved, could compromise weak to strong actomyosin binding with consequent loss of force-generating capabilities (16). All of the sites and processes cited have been differentially implicated previously in the loss of muscle to generate a desired and expected force depending on the specifics of the task and the type of preparation employed (16).
In an attempt to gain insight into how the changes in MVC force induced by the repetitive exercise correlated with the changes observed at the different frequencies of stimulation, correlation coefficients were calculated. In general, correlations were found only with the first repetition of the exercise. These results indicated that the loss of force assessed with an MVC is positively related to the force loss observed with each frequency of stimulation. This might be expected since MVC includes both central and peripheral components. After each of the remaining repetitions, correlations between MVC and ES, regardless of frequency, were generally not significant. This would suggest no relationship between the voluntary and involuntary induced effects of exercise on force loss. The general lack of a significant relationship after the first repetition was expected given the relatively small fatigue that occurred with each of the subsequent repetitions. It must also be emphasized that the correlations for each repetition are based on small sample sizes.
To gain further insight into the potential rate-limiting sites for the fatigue observed in this study, it is necessary to distinguish between the task used to induce the fatigue and the task used to measure the fatigue. The task used to induce the fatigue involved a 6-min bout of heavy dynamic exercise. The measurement of fatigue was accomplished by measuring the isometric characteristics of the quadriceps muscle, which is known to be heavily involved in cycling (30). Moreover, the assessment of fatigue occurred 4–5 min after the end of the cycling exercise. This could have important implications to both the site and mechanism involved in fatigue. The type and duration of the task selected to induce fatigue are known to result in large disturbances in metabolic by-product accumulation (4). Although ATP remains relatively well protected, large decreases in phosphocreatine occur, accompanied by near stoichiometric increases in Pi and creatine (4, 9). Significant recruitment of glycogenolysis and glycolysis also occurs, resulting in substantially increased flux rates and production of lactic acid (4, 33). These by-products have been associated with muscle fatigue, operating potentially at one or more of all of the E-C sites reviewed (16). Inhibition of the catalytic activity of the cellular ATPases appears to be particularly vulnerable (27). As a consequence, disturbances in membrane transport for Na+ and K+ could affect membrane excitability and the generation of action potentials (10). Alternatively, an inhibition of the Ca2+-ATPase activity could reduce Ca2+ sequestration into the SR, compromising Ca2+ storage and Ca2+ release (39). The accumulation of selected by-products could by themselves compromise Ca2+ release. As an example, Pi, which is known to accumulate in the lumen of the SR, may combine with Ca2+ and reduce release rates (35). In addition, the accumulation of a range of by-products such as ADP, glucose-6-phosphate, and lactate, all of which are known to increase with the type of exercise protocol employed in this study, can affect the open state of the Ca2+ release channel and reduce Ca2+ release rates (35). Finally, metabolic by-product accumulation is known to inhibit actomyosin ATPase activity and to depress weak to strong binding transitions and isometric force (11). As a consequence of these changes, Ca2+ sensitivity is reduced, resulting in a reduction in force at a given [Ca2+f] (28).
Because our fatigue assessment was conducted 4–5 min after cycling, the phosphorylation potential of the muscle would be expected to be normalized, given the approximate half-life time for phosphocreatine regeneration, which has been estimated at 20–22 s (32). Lactic acid, on the other hand, which has a half-life time of 15 min (33), would be expected to be substantially elevated at the time of the fatigue assessment. The normalization of the phosphorylation potential during recovery in exercised muscle has also been shown to correlate with a similar fast component of mechanical recovery (3, 38). Despite the fast recovery that occurs early in exercise, a slower, more delayed phase of recovery also persists while the metabolic profile of the muscle is fully normalized (3). Given the role of muscle glycogen reserves in E-C coupling failure (8), we have also provided for intake of nutrients during the 16 h of exercise and recovery. Over the course of the experimental period, a total of 2,339 ± 195 kcal were consumed, with CHO representing the dominant substrate.
The fatigue that we have observed after exercise at R1 does not appear to be explained by disturbances in membrane excitability since it occurred in the absence of reductions in the properties of the M wave. No disturbances were found in the area, amplitude, or duration of the M wave when assessed using the supramaximal twitch. Alterations in the M wave, and specifically reductions in the amplitude, have been noted previously during heavy exercise in untrained subjects (2). These measurements were performed both while the exercise was in progress and for 15–20 min after the exercise (2). It is possible, given our protocol and the time delay between the end of exercise and measurement, that membrane excitability could have recovered. However, as emphasized, decreases in membrane excitability do not appear implicated in the fatigue that was observed at the time of the mechanical assessment.
Alterations in the [Ca2+f] transient, mediated by inhibition of SR Ca2+ release, appear to be at least partly responsible for the loss of the mechanical response. Reductions in Ca2+ release could occur as a result of reduced SR Ca2+ storage secondary to reductions in Ca2+ uptake (39) or to direct alterations in the CRC (15, 35). Previous studies using single-fiber measurements have demonstrated the association between reduced isometric force and depressed SR Ca2+ release during both heavy exercise and recovery (1).
The persistent loss of force-generating capacity, which has been defined as weakness (13), was most conspicuous before R2. At this point, no change in force occurred compared with postexercise measurements despite a period of recovery that exceeded 30 min between the points of measurement. At the time of measurement, the phosphorylation potential and metabolic by-product concentrations would be normalized (9). The delayed recovery in mechanical function is consistent with a sustained depression in the Ca2+ transient mediated primarily by reductions in Ca2+ release (1).
Particularly intriguing about our results was the observation that, when the 6-min session of exercise was performed with the muscle substantially weakened, no fatigue was generally observed. This would suggest that a critical functional reserve in E-C signal transduction remain protected, allowing the heavy exercise session to be performed. What these particular sites and processes are and the mechanisms defending further erosion are uncertain. As emphasized, Ca2+ release remains an inviting site to explain our observations. Before the last repetition of exercise, muscle weakness at the higher stimulation frequencies had been completely eliminated. Remaining, however, was a persistent weakness that was most pronounced at stimulation frequencies of 10 and 20 Hz. The loss of force at low frequencies in the absence of a loss at high frequencies has been attributed to a depression in Ca2+ release (1). The specificity observed is based on the magnitude of the reduction in Ca2+ release and the sigmoid nature of the force-[Ca2+f] curve. At low frequencies, which result in low force levels, a small reduction in [Ca2+f] can have dramatic reductions in force given the steep nature of the curve (1). At higher stimulation frequencies and higher force levels, due to the asymptomatic properties of the curve, small depressions in [Ca2+f] can be accommodated without significant effects on force (1). Our results are entirely consistent with this interpretation.
A surprising and unexpected result was the observation that, by the last repetition, heavy exercise failed to induce fatigue despite either partial or full recovery of the force levels before exercise. In effect, these observations indicate that muscle function is protected during the heavy exercise in the absence or near absence of weakness. This finding is a dramatic illustration of the rapid adaptations that can be invoked in muscle to defend muscle contractility once compromised by the strain provided by the early exercise repetitions. Based on our analyses, although speculative, adaptations at the level of the CRC channel and Ca2+ release may be involved. This could occur either as a result of changes in the internal environment or as a result of modifications to the CRC itself. Our results provide an exciting opportunity to develop specific hypotheses and to probe for the underlying mechanisms that are involved.
Conceivably, the fatigue observed at low frequencies of stimulation could be biased by other factors not resident in the high-frequency protocol. Unlike the stimulations at 50 and 100 Hz, which produce a fused tetanus, stimulations at 10 and 20 Hz produce an unfused tetanus. With an unfused tenanus, changes in the rate of tension development or tension decline could alter the peak force that was observed (17, 37). We have detected that accompanying the reduction in force at varying frequencies of stimulation after R1 were reductions in both +dF/dtmax and –dF/dtmax. The slower +dF/dtmax would be expected to reduce force, whereas the slower –dF/dtmax would be expected to promote increased force (37). Interestingly, these effects were not observed for CT and one-half relaxation time, which are generally considered less sensitive measurements of the kinetics of the force transients. When these properties were examined at the low frequencies of stimulation, similar effects were observed for the twitch, namely a pronounced reduction in both +dF/dtmax and –dF/dtmax after R1. The net effect of the slowing of both of these properties would tend to cancel out, resulting in little effect in peak force. Interestingly, as weakness developed and force was compromised before exercise, a reduced +dF/dtmax and –dF/dtmax persisted. Although a number of processes combine to regulate the kinetics of force development and decay (16), the rates of Ca2+ sequestration and Ca2+ release by the SR remain important factors. Conceivably, changes in these properties are involved in the fatigue-induced responses in the behavior of force transients observed.
By necessity, given the 16 h needed to complete the intermittent work protocol, the assessment of fatigue and weakness was made throughout the experimental day. Recent research investigating circadian rhythms in neuromuscular performance in human adductor muscle indicates increases in force and speed of contraction in the evening compared with the morning (29). If these results apply to larger muscles, such as the quadriceps, it is possible that the differences in weakness could be biased because of the circadian effects. The effects of diurnal variation on fatiguability, however, remain unknown. In this study, we have also included two women as part of the participants who volunteered for the study. It is possible that, in response to the intermittent cycling exercise, men would exhibit a greater decrement in neuromuscular performance than women, given recent research that has shown that the ankle dorsiflexors display a more rapid fatiguability in men (31). However, such a possibility remains only speculative given the differences in task requirements and the muscle group studied.
The mechanical changes that we have observed with the intermittent exercise must also be put in context. The persistent weakness observed during MVC reflects the maximal force-generating capacity of the neuromuscular system. In contrast, due to safety concerns, the ES protocol generated a maximum of only 50% MVC. As a consequence, a large population of motor units are not activated during ES. This could lead to misleading conclusions regarding the magnitude and frequency-dependent nature of fatigue and weakness (12). However, it must be emphasized that, at least for the muscle fibers recruited by ES, the heavy intermittent exercise protocol produced novel findings regarding the nature and time course of adaptation.
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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.
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Significantly different (P < 0.05) from repetition 2. #Significantly different (P < 0.05) from B.
Significantly different (P < 0.05) from repetition 4. #Significantly different (P < 0.05) from B.
