J Appl Physiol 99: 389-396, 2005;
doi:10.1152/japplphysiol.00207.2005
8750-7587/05 $8.00
INVITED REVIEW
Task failure during fatiguing contractions performed by humans
Katrina S. Maluf and
Roger M. Enoka
Department of Integrative Physiology, University of Colorado, Boulder, Colorado
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ABSTRACT
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By comparing the physiological adjustments that occur when two similar fatiguing contractions are performed to failure, it is possible to identify mechanisms that limit the duration of the more difficult task. This approach has been used to study two fatiguing contractions, referred to as the force and position tasks, which differed in the type of feedback given to the subject and the amount of support provided by the surroundings. Even though the two tasks required a similar net muscle torque during submaximal isometric contractions, the duration that the position task could be sustained was consistently much briefer than that for the force task. The position task involved a greater rate of increase in EMG activity and more marked changes in motor unit recruitment and rate coding compared with the force task. These observations are consistent with the hypothesis that the motor unit pool was recruited more rapidly during the position task. The difference in motor unit behavior appeared to be caused by variation in synaptic input, likely involving heightened sensitivity of the stretch reflex during the position task. Upon repeat performances of the two fatiguing contractions, some subjects were able to increase the time to failure for the force task but not the position task. Furthermore, the time to failure for the position task could be influenced by the postural demands associated with maintaining the position of the limb, and the difference in the two durations was enhanced when the postural activity evoked a pressor response. These observations indicate that the difference in the duration of the two fatiguing contractions was attributable to differences in the control strategy used to sustain the tasks and the magnitude of the associated postural activity.
muscle fatigue; maximal force capacity; motor unit; reflexes; posture
DESPITE A CENTURY OF OBSERVATIONS since the seminal work of Mosso (53) and Reid (57), the cause of muscle fatigue remains elusive. There is a consensus that muscle fatigue can be quantified as the decline in the maximal force capacity of the muscle due to an impairment of either the activation signal or the function of the contractile proteins (20). The mechanisms responsible for the reduction in maximal force, however, depend on the details of the task that is being performed (6, 17, 20). Critical task variables include the type and intensity of exercise, the type of load supported during the contraction, the muscle groups involved, and the physical environment in which the task is performed. Because of the task-dependent variation in the prevailing mechanisms, it has not been possible to identify a single factor that is responsible for muscle fatigue (20). An alternative strategy to study muscle fatigue, therefore, is to identify the mechanisms responsible for the failure of specific tasks. This report illustrates the task-failure approach by describing studies that attempt to determine the effects of load type on the mechanisms responsible for failure of sustained submaximal contractions.
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THE FORCE AND POSITION TASKS
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Our approach has been to compare the performance of two tasks that have similar mechanical requirements, yet that differ substantially in the duration that they can be sustained. By comparing the rate of change in various parameters during the two fatiguing contractions, it is possible to determine the relative significance of different physiological adjustments and hence to identify the mechanisms that limit the duration of the more difficult task. Although this approach does not identify the rate-limiting impairments that are common to both tasks, it can reveal the differential effects of task variables, such as load type, on the multiple processes that limit the duration of a sustained contraction.
The two tasks selected for comparison both involved sustained isometric contractions that required the same submaximal net muscle torque and joint angle yet differed in the type of feedback provided to the subject and the type of load supported by the muscles (30, 32, 33, 49, 62). In one task, referred to as the force task, the limb was attached to a restraint and the subject was required to sustain a constant force with the test muscles (15–20% of maximum) for as long as possible while viewing force feedback on a monitor. In the other task, referred to as the position task, the subject supported an inertial load that was equivalent to the force exerted during the force task and was required to maintain a constant joint angle for as long as possible while viewing position feedback on a monitor. The criteria for terminating the two fatiguing contractions were based on a biomechanical analysis to ensure that the decrease in force during the force task and the reduction in the moment arm during the position task produced similar declines in the net muscle torque (force x moment arm) about the target joint at task failure.
Although the feedback signal and the amount of support provided by the surroundings differed for the two tasks, both the maximal load that a subject could support and the relation between EMG amplitude and the magnitude of the load were similar for brief contractions of the force and position tasks (49). Furthermore, the amplitude of the EMG recorded for each test muscle was identical at the beginning of the two tasks (33, 49, 62).
The initial experiments were performed with the elbow flexor muscles and the subject seated in the position shown in Fig. 1A (29, 30, 32). The criterion for task failure was an inability to sustain the target force or position for at least 5 s, despite strong verbal encouragement to correct the deviation. The key markers for displacement of the limb from the prescribed position were a 10° increase in elbow angle, lifting the elbow off the underlying force transducer, or internal rotation of the arm about the shoulder joint. In practice, the force task was typically terminated because of a gradual inability to sustain the target force for 5 s, whereas the position task usually ended abruptly because of an inability to maintain the required elbow angle or prevent internal rotation of the arm.
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Fig. 1. Limb position and test muscle for the task failure studies. Experimental arrangement for the force (A) and position (B, C) tasks as performed by the elbow flexor (A, B) and first dorsal interosseus (C) muscles.
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The time to task failure for 16 subjects (8 men and 8 women; 27 ± 4 yr) was consistently less for the position task (mean ± SD; 702 ± 582 s) compared with the force task (1,402 ± 728 s) (33). Nonetheless, the amount of fatigue experienced by the subjects during the two tasks was similar, as indicated by comparable ratings of perceived exertion at failure and reductions of 28–35% in maximal voluntary contraction (MVC) force immediately after each fatiguing contraction (31, 32). Furthermore, a second force task could be sustained for a similar duration whether it was performed 30 s after failure of a preceding force or position task, which suggests that the muscle experienced a similar amount of fatigue at failure of the two tasks (49). Subsequent studies also demonstrated a briefer time to failure for the position task when the two tasks were performed by the elbow flexor muscles (10 men and 10 women; 25 ± 6 yr) with the arm in the position shown in Fig. 1B (62) and by the first dorsal interosseus muscle (20 men; 23 ± 5 yr) with the hand in the position indicated in Fig. 1C (49). The magnitude of the relative difference in the durations of the two tasks, however, differed across the three conditions (Table 1).
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MOTOR OUTPUT FROM THE SPINAL CORD
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Despite each subject exerting the same net muscle torque for the two tasks, the rate of change in motor output from the spinal cord increased more rapidly during the position task. This difference was manifested as a greater rate of increase in the surface EMG signal for the position task when performed by the elbow flexor muscles (short and long heads of biceps brachii, brachialis, and brachioradialis) with the upper arm horizontal (62) and by an intrinsic hand muscle (Fig. 2, 49). A similar difference in the rate of change in EMG activity between the two tasks was observed when the elbow flexor muscles performed intermittent contractions for 30 min with a load of 30% MVC force (48, 66). Furthermore, coactivation of the antagonist muscle increased only slightly and by a similar amount (<10% of maximum) during the two fatiguing contractions (Fig. 2C; 49; see also 62).
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Fig. 2. EMG activity during the force and position tasks performed with the first dorsal interosseus muscle. A: rectified EMG for the agonist (first dorsal interosseus, FDI) and the antagonist (second palmar interosseus, SPI) muscles during the performance of the force task to failure. The load was 20% maximal voluntary contraction (MVC) force. Bottom trace indicates the abduction force exerted at the level of the proximal interphalangeal joint of the index finger. B: rectified EMG signals for the agonist and antagonist muscles during the performance of the position task to failure. Bottom trace denotes the angle of the metacarpophalangeal joint of the index finger, with a downward deflection indicating adduction of the finger. C: group data (means ± SE) for 10 men who performed the force (circles) and position (triangles) tasks to failure. Solid symbols represent the root mean square data for first dorsal interosseus, and open symbols indicate the data for second palmar interosseus. The data in this figure are from Maluf et al. (49).
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A more rapid augmentation of the activation signal during the position task was evident as progressive increases in both the amplitude of the interference EMG and the rate of bursts in the EMG signal (30, 32, 33). When a task was sustained at a relatively modest force, the gradual augmentation of the interference EMG comprised occasional bursts of activity that corresponded to the transient recruitment of motor units (9, 18, 21, 51). The EMG bursts were minimal at the onset of the contraction and began to appear more frequently as the task progressed. In one study, burst rate increased to 0.64 ± 0.75 bursts/min at the end of the force task compared with an increase to 0.72 ± 1.23 bursts/min at the end of the position task (33). Because the position task had a briefer duration, the rate of increase in EMG bursts was more rapid during the position task, suggesting earlier recruitment of the motor unit pool.
Consistent with this interpretation, Mottram et al. (55) observed greater levels of motor unit recruitment during the position task compared with the force task (15 men; 26 ± 6 yr). On the basis of recordings obtained with a fine-wire electrode placed between the subcutaneous tissue and the fascia of biceps brachii as the two tasks were performed for the same duration (161 ± 96 s) in a single session, Mottram et al. observed the recruitment of 46 new motor units during the position task compared with 32 during the force task. As detected by the electrode, 26 of the newly recruited motor units were recruited in both tasks, which meant that 6 different motor units were recruited during the force task compared with 20 during the position task. The mean discharge rate [10.5 ± 2.1 pulses per second (pps)] and the coefficient of variation for discharge rate (21.5 ± 8.5%) were similar for the newly recruited motor units during the two tasks.
Taken together, these observations suggested that the briefer time to failure for the position task is due to the earlier recruitment of the motor unit pool. To test this hypothesis, Maluf et al. (49) compared the time to failure when the first dorsal interosseus muscle performed the force and position tasks at a contraction intensity of 60% MVC, which exceeds the upper limit of motor unit recruitment for this muscle (12, 52). Consistent with the hypothesis, the time to failure for the position task (86 ± 31 s) was not statistically different from that for the force task (93 ± 41 s) when the load was 60% MVC force (Table 1). This result indicates that when the nervous system is unable to recruit additional motor units to compensate for the decline in force capacity as the muscle fatigues, there is no difference in the duration that the two tasks can be sustained.
These data also indicate that the surface EMG signal is insensitive to modest differences in motor unit activity. For example, overlapping positive and negative phases of motor unit potentials cancel one another and substantially reduce the summed value, which results in the amplitude of the surface EMG underestimating the actual motor output from the spinal cord (11, 19, 41). Although the surface-recorded interference EMG for biceps brachii increased at a similar rate during the two fatiguing contractions when the upper arm was vertical (33, 55), there were significant differences in motor unit recruitment and rate coding during the two tasks (55). As suggested by Adrian (1), these observations caution against the overinterpretation of the interference EMG signal, especially during fatiguing contractions.
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CONTROL STRATEGIES FOR THE TWO FATIGUING CONTRACTIONS
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Why are motor units recruited more rapidly during low-intensity contractions in the position task? Evidence suggests that the control strategy differs for the two tasks, perhaps requiring heightened sensitivity of the stretch reflex during the position task. For example, muscle spindle sensitivity is augmented during precision tasks, reinforcement maneuvers, and voluntary contractions (27, 37, 38, 58), the response of the spindle afferents can increase during position holding compared with shortening contractions (4, 28), and the amplitude of the stretch reflex is enhanced when the limb acts against a compliant load compared with a rigid restraint (2, 13, 14). These investigations suggest that heightened input from muscle spindle afferents may assist in the accurate control of limb position against externally imposed loads.
Several observations indicate that the synaptic input received by motor neurons differs during performances of the force and position tasks. For example, Tax et al. (67) found that the recruitment thresholds of a small sample of motor units in biceps brachii were lower when subjects performed a position-hold task (3.4 ± 2.3 N·m) compared with a restrained isometric contraction (6.9 ± 4.2 N·m). Furthermore, Mottram et al. (55) reported that the discharge characteristics of the same motor units changed differently during the force and position tasks. Although the discharge rate characteristics of the 32 motor units were similar at the start of the two tasks (mean: 13.3 ± 2.9 pps; coefficient of variation: 22.8 ± 9.1%), the mean discharge rate declined more during the position task (final values: 10.6 ± 2.6 pps and 12.0 ± 2.6 pps, respectively) and the coefficient of variation for discharge rate increased during the position task but not the force task (final values: 26.7 ± 9.2% and 22.4 ± 8.2%, respectively). Because the discharge times were recorded in the same motor unit during the two tasks, the different adjustments must have been attributable to differences in the synaptic input received by the motor neurons.
The significance of the synaptic input was underscored by a study (14 men and 15 women; 24 ± 6 yr) that compared the performance of the position task when the gain of the position-feedback signal was varied by changing the sensitivity of the display on the monitor (46), which is known to increase activation of the motor neuron pool (2, 64). The time to task failure for the fatiguing contractions was 7.3 ± 2.6 min when the gain was high (0.13°/cm), whereas it was 9.0 ± 4.8 min when the gain was low (30°/cm). The amount of fatigue, as indicated by the decline in MVC force at task failure, was similar for the two conditions (–23 ± 12%). Furthermore, the high-gain condition involved greater rates of increase in perceived exertion and a greater reduction in the discharge rate of motor units in the biceps brachii (P 
0.02). Discharge rate was similar at the start (12.1 ± 2.5 pps) and middle (9.0 ± 1.5 pps) of the task for both gain conditions, yet it was lower at the end of the contraction for the high-gain condition (7.9 ± 1.5 pps) compared with the low-gain condition (8.5 ± 1.0 pps, P = 0.04) (54). Also, the amplitude of the elbow flexor average rectified EMG (%MVC) was lower at task termination for the high-gain condition (13.5 ± 8.9%) than for the low-gain condition (14.8 ± 9.7%; P = 0.01) (54). These data suggest that an increase in the sensitivity of the feedback signal, which required greater attention by the subject for successful performance of the position task (64) and likely increased reflex sensitivity (35, 38), was associated with a reduction in the time to failure for the position task.
Figure 3 illustrates five of the potential mechanisms that might be responsible for heightened stretch reflex sensitivity during the position task: 1) reduced presynaptic inhibition of feedback from group Ia afferents due to withdrawal of excitation from cortical centers onto the interneurons that mediate presynaptic inhibition (15, 16, 44, 59); 2) enhanced neuromodulatory drive from supraspinal centers that may depend on the task being performed (22, 26, 34); 3) an augmentation of 
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coactivation, perhaps involving an imbalance between the two efferent pathways (5, 24, 68); 4) decreased recurrent inhibition of -motor neurons due to the inhibitory effect of chemosensitive group III–IV afferents on Renshaw cells (39, 45); and 5) excitation of -motor neurons by chemosensitive group III–IV afferents (36, 63).
In experimental conditions when there was no apparent difference between the two tasks in the feedback transmitted by group III–IV afferents, such as occurred when there was no difference in the rate of change in mean arterial pressure (49, 62), then the differential influence on stretch reflex sensitivity was most likely mediated by one or more of the pathways not involving the chemosensitive afferents (pathways 1-3 in Fig. 3). In contrast, a more rapid increase in mean arterial pressure during the position task (32, 55) suggests a greater rate of increase in feedback delivered by group III–IV afferents (3, 40, 60) during the position task compared with the force task. Accordingly, the greater changes in motor unit discharge rate during the position task (55) may have been caused by enhanced activation of the inhibitory interneurons that receive group III–IV afferent input and impinge on -motor neurons (42, 63). Presumably, the greater decline in mean discharge rate during the position task necessitated the recruitment of more motor units to achieve the target force. Greater feedback by group III–IV afferents during the position task could also heighten stretch reflex sensitivity by two additional mechanisms (pathways 4 and 5 in Fig. 3).
Several observations suggest that performance during sustained submaximal contractions is more likely influenced by presynaptic rather than postsynaptic mechanisms, at least in the absence of a pressor response. First, the decline in the amplitude of the Hoffmann reflex (short-latency response) during a fatiguing contraction was accompanied by an increase in the amplitude of the long-latency reflex, which suggests differential adjustments in the pathways that mediate these responses. Because the long-latency reflex involves cortical structures and the short-latency response does not, the different changes in the two reflexes suggests that a sustained submaximal contraction involves an augmentation of the excitatory drive onto the - and -motor neurons from supraspinal structures that receive input from group Ia afferents (16, 29). Second, the application of muscle vibration during a fatiguing contraction can transiently restore the discharge rate of motor units (7, 23), which indicates that the decrease in discharge rate was attributable to a reduction in excitatory input rather than the postsynaptic depression (recurrent inhibition) of the motor neuron output. Furthermore, the reduction in the discharge rate of spindle afferents was accompanied by a progressive increase in EMG amplitude to sustain the submaximal force (47), which meant that the decline in excitation from muscle spindles during the sustained contraction was compensated by an increase in excitation of the motor neuron pool from another source. Third, the amplitude of the Hoffmann reflex declined in a hand muscle that performed a sustained submaximal contraction, but not in an inactive adjacent muscle despite the hand being ischemic during the postcontraction recordings (16). Thus feedback transmitted by group III–IV afferents from the active muscle did not exert an effect on heteronymous motor neurons (see also Ref. 8). When the task involves a significant pressor response, however, the group III–IV afferents may assist in mediating any task-dependent difference in the sensitivity of the stretch reflex pathway during the force and position tasks.
An alternative hypothesis to explain the difference in the time to failure for the two tasks is that the muscle performs more external work during the position task and hence fatigues more rapidly. Because the position of the limb was less constrained during the position task compared with the force task, changes in potential and kinetic energy performed by the limb are greater during the position task. Accordingly, the metabolic demands are presumed to be greater during the position task, which might contribute to the briefer duration for the position task. According to this scheme, the energy-demanding corrective adjustments during the position task should be greater when the limb supports a larger mass because of greater changes in potential and kinetic energy, and the difference in the time to failure between the two fatiguing contractions should increase. On the contrary, Maluf and colleagues (49) found no difference in the time to failure for the two tasks with a load equal to 60% MVC force, whereas there was a significant difference with a lesser load (Table 1). Furthermore, the calculated force fluctuations during the position task, which were derived from the acceleration of the limb, were less than those measured during the force task: the coefficient of variation for force was 2.5 ± 1.6% during the position task and 10.2 ± 4.4% during the force task (49). Although these observations suggest that greater external work during the position task is not the primary mechanism responsible for its briefer duration, future studies should quantify the contribution of external work and its associated metabolic costs to the differences in the duration of the two fatiguing contractions.
The mechanisms that are responsible for the difference in the time to failure for the force and position tasks must also be able to explain the difference in the adaptability of the two tasks. When subjects (7 men and 7 women; 27 ± 4 yr) performed the force task (Fig. 1A) on three occasions separated by 1 wk, some subjects (responders; n = 9) were able to increase the time to task failure, whereas other subjects (nonresponders; n = 5) did not experience a change in the duration of the task (30). The responders increased the time to failure by 60 ± 28% from session 1 (21.1 ± 12.6 min) to session 3 (34.1 ± 22.3 min), whereas the duration did not change from session 1 (19.2 ± 10.1 min) to session 3 (18.7 ± 9.5 min) for the nonresponders. The MVC force, the target force, the feedback signal, and the amount of fatigue (decline in MVC force) did not differ either between the two groups of subjects or across sessions. In contrast, none of the 24 subjects (11 men and 13 women; 26 ± 6 yr) who performed the position task on three different occasions separated by 1 wk were able to alter the time to task failure (5.3 ± 2.5 min), and there were no changes in any of the variables that had changed with repeat performances of the force task (32).
The improvement in performance exhibited by the responders during the force task was accompanied by a delay in the development of fluctuations in motor output (Fig. 4A) and the onset of bursts (transient recruitment of motor units) in the EMG signal (Fig. 4B). As is characteristic of fatiguing contractions (43, 44), the motor output during both the force and position tasks became more tremulous as the task progressed. The fluctuations in motor output were quantified as the standard deviation of force during the force task and as the standard deviation of position and acceleration during the position task. Because the increase in tremor during sustained submaximal contractions is attributed to greater peripheral afferent feedback (10, 44), the ability of the responders to delay the progression of the force fluctuations may indicate a downregulation of stretch reflex sensitivity with practice of the task. This might involve, for example, pathway 1 through 3 in Fig. 3. The observation by Macefield et al. (47) that the discharge rate of most muscle spindles declined during a sustained isometric contraction is consistent with this possibility. Furthermore, a reduction in the excitatory drive to the motor neuron pool could also explain the delayed increase in the transient recruitment of motor units (EMG burst rate). Conversely, the inability of subjects to adjust the time to failure for the position task might be due to the need to maintain heightened sensitivity of the stretch reflex to assist with the control of limb position (2, 14).
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Fig. 4. Prolongation of the time to failure for the force task with practice. A: each line indicates 60-s averages (means ± SE) of the coefficient of variation (CV) for force at the beginning, middle, and end of the force task sustained to failure with a load of 20% MVC force. The 4 lines correspond to data for 2 groups of subjects (responders and nonresponders) and 2 experiments (sessions 1 and 3) for each group; the responders are indicated by squares, the nonresponders by circles, session 1 by open symbols, and session 3 by solid symbols. B: similar data for mean burst rate in the EMG recordings of the elbow flexor muscles, averaged across the first, middle, and last third of the time to failure for each session. The bursts were identified with an algorithm that examined the rectified, low-pass filtered, and differentiated EMG recording (see Fig. 1 in Ref. 30). The data in this figure are from Hunter and Enoka (30).
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LIMB POSTURE
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Table 1 indicates that the duration of the position task relative to the force task was less when the two fatiguing contractions were performed with the upper arm in a vertical (Fig. 1A) position compared with a horizontal (Fig. 1B) position. This difference in relative durations was accompanied by marked differences in the rate of change in other dependent measures. For example, Hunter et al. (33) found that the rate of increase in EMG activity for the elbow flexor muscles (biceps brachii, brachialis, brachioradialis) was similar for the two tasks when the upper arm was vertical, whereas Rudroff et al. (62) observed a greater rate of increase in EMG activity during the position task compared with the force task when the upper arm was horizontal. Conversely, mean arterial pressure increased more rapidly during the position task when the upper arm was vertical (33), but the rate of increase was similar for the two fatiguing contractions when the upper arm was horizontal (62).
The difference in the adjustments exhibited in these two limb positions underscores the important role of limb posture in limiting task performance (50, 56). The different rate of increase in mean arterial pressure when the upper arm was vertical suggests that the muscle mass involved differed for the two tasks (66, 69). Indeed, the EMG activity for the anterior deltoid muscle increased more rapidly during the position task when the upper arm was vertical (33), whereas the rate of increase was similar for the two tasks when the upper arm was horizontal (62). When the fatiguing contractions were performed in the vertical (Fig. 1A) position, the arm was abducted from the trunk slightly to accommodate the force transducer; this was not necessary when the upper arm was horizontal. As a consequence, the inertial load suspended from the wrist during the position task when the upper arm was vertical exerted an internal rotation torque about the shoulder joint that had to be counteracted by the rotator cuff muscles. The subjects often attributed the abrupt termination of the position task in this position to an inability to prevent internal rotation at the shoulder joint. Accordingly, preliminary measurements (23 men; 21 ± 3 yr) indicate that the rate of increase in EMG activity of the supraspinatus, infraspinatus, and teres minor muscles is more rapid when the position task is performed in the upper arm in a vertical (Fig. 1A) position compared with a horizontal position (61).
These observations suggest that the difference in the time to failure for the force and position tasks can depend on two factors. First, the control strategy appears to differ for the two fatiguing contractions; namely, the motor unit pool of the agonist muscle is recruited more rapidly during the position task. Second, the postural activity required to maintain the position of the limb can evoke a significant pressor response that appears to augment changes in motor unit activity to compensate for the decline in discharge rate mediated by group III–IV afferents and thereby hasten the failure of the position task. Because activation of mechanically sensitive afferents during fatiguing contractions has been shown to inhibit both homonymous and heteronymous motor neuron pools (25), greater activation of accessory muscles with the upper arm in a vertical position likely increased differences in the duration of the two fatiguing contractions when performed in this position (Table 1). Thus a greater mechanical stress on the rotator cuff muscle during the position task with the upper arm vertical might increase the heteronymous feedback delivered by group III–IV afferents onto the inhibitory interneurons that could then depress the activity of the -motor neurons innervating the elbow flexor muscles (Fig. 3).
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FUNCTIONAL SIGNIFICANCE
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The extent to which any single process contributes to task failure likely depends on the relative demand placed on each of the processes that contribute to the force exerted by the muscle during a given task (6, 17). The comparison of tasks that place different demands on one or more of these processes helps to identify relations between task variables and the primary sites of impairment that contribute to the failure of the more difficult task. This knowledge can assist clinicians by providing a rationale for exercise prescription to address the specific physiological deficits that impair task performance. For example, findings from this research suggest that unsupported rather than supported exercises (e.g., free weights as opposed to machines) may be optimal to improve the performance of functional tasks that require accurate control of limb position, at least with modest loads. Furthermore, by documenting the benefits of external support during sustained force production, the findings may also be used to optimize the design of ergonomic environments to improve job performance while reducing the risk of overuse injuries in the workplace.
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CONCLUSION
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By comparing the time to failure for two types of fatiguing contractions, it is possible to identify the mechanisms that limit the duration of the more difficult task (e.g., position task) but not the impairments that are common to both tasks. This review synthesizes the results obtained in a series of studies on two tasks that differed in the type of load attached to the limb and the type of feedback provided to the subjects. When the two fatiguing contractions involved moderate loads (15–20% of maximum) that were supported for as long as possible, the earlier termination of the position task was attributable to the more rapid recruitment of the motor unit pool. Furthermore, the difference in the duration of the two fatiguing contractions was exacerbated when the magnitude of the pressor response differed for the two tasks. Some subjects were able, with practice, to manipulate the rate of increase in muscle activation and to prolong the duration of the force task. Expansion of this approach to other tasks will likely generate new knowledge regarding the influence of task variables on the multiple physiological adjustments that occur during fatiguing contractions.
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GRANTS
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The National Institute of Neurological Disorders and Stroke supported this work with an award (R01 NS-43275) to R. M. Enoka, and the National Institute on Aging provided an award (T32 AG-00279) to Robert Schwartz that supported K. S. Maluf.
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ACKNOWLEDGMENTS
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The authors acknowledge the comments made by Prof. Jacques Duchateau on a draft of the manuscript.
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FOOTNOTES
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Address for reprint requests and other correspondence: R. M. Enoka, Dept. of Integrative Physiology, Univ. of Colorado, Boulder, CO 80309-0354 (E-mail: enoka{at}colorado.edu)
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