Download THE ASSESSMENT OF NEURAL FACTORS IN MUSCLE FATIGUE

THE ASSESSMENT OF NEURAL FACTORS IN MUSCLE FATIGUE
Roger M. Enoka
Department of Integrative Physiology
University of Colorado, Boulder, USA
[email protected]
INTRODUCTION
Muscle fatigue is defined as an exercise-induced decrease in
the maximum force capacity of muscle (Gandevia, 2001).
This transient reduction in strength differs from the
weakness and tiredness experienced by patients.
THE CLASSIC APPROACH
The typical strategy to identify the cause of muscle fatigue
has been to distinguish between neural and muscular
mechanisms. This can be done, for example, by comparing
the decline in muscle force during a voluntary contraction
with that evoked by electrical stimulation (Bigland-Ritchie
et al 1986). When the reduction in voluntary force exceeds
that for the evoked force, the muscle fatigue includes a contribution from neural mechanisms. Such interventions have
demonstrated that neural mechanisms are impaired in many
fatiguing contractions (Gandevia, 2001).
To identify the adjustments that occur in the nervous system
during fatiguing contractions, studies have examined the
roles of afferent feedback, descending inputs, and spinal
circuitry in modifying the output of the motor neurons. The
central effects of sustained contractions are diverse. For
example, the metabolites that accumulate in muscle during a
prolonged contraction excite afferent fibers that both enhance the central drive to maintain muscle perfusion and
reduce the discharge of the motor neurons.
ceived exertion, and the transient recruitment of motor units
all increased more rapidly during the position task (Hunter et
al 2002). These results were interpreted as indicating that
the motor neurons in the spinal cord received greater
amounts of excitatory and inhibitory input during the position task.
To examine this possibility, we have addressed two questions: (1) Do interventions that influence motor unit activity
also alter the time to task failure? (2) Does the behaviour of
single motor units differ for the two tasks? For the first
question, we have examined the effect of practice on performance of the two tasks (Hunter & Enoka, 2001, 2003).
With practice of the force task, some subjects increased the
time to task failure (responders), whereas other subjects did
not (non-responders). The improvement in performance for
the responders was accompanied by a decrease in the rate of
rise in the average EMG, a delay in the transient recruitment
of motor units, and a reduced rate of increase in the force
fluctuations. Practice, however, did not alter the time to task
failure for the position task (Hunter et al 2003).
One consequence of these studies has been the failure to
identify a single fatigue factor. Rather, the decline in force
capacity during a fatiguing contraction usually involves multiple mechanisms, which depend on the type and intensity of
exercise, the muscle groups involved, and the physical environment in which the task is performed.
For the second question, we measured the discharge rate of
the same motor units in biceps brachii as subjects performed
the two tasks for brief durations (MacGillis et al 2003): 59 ±
4 s for high-threshold motor units and 222 ± 66 s for lowthreshold motor units. The initial discharge rate was ~ 14
Hz in both tasks and declined to 12.5 ± 2.7 Hz for the force
task and 10.9 ± 2.6 Hz for the position task. Furthermore,
the coefficient of variation for discharge rate began at about
19% for both tasks and increased to 20.6 ± 9.1% at the end
of the force task and 24.7 ± 13.7% at the end of the position
task. Thus, motor unit discharge differed for the two tasks,
as did the fluctuations in motor output, mean arterial pressure, heart rate, and ratings of perceived exertion.
A DIFFERENT QUESTION
SUMMARY
Instead of attempting to determine the cause of muscle
fatigue, we have focused on identifying the factors that
cause task failure. The task was to support a load (15-20%
of maximum) with the elbow flexor muscles for as long as
possible. In a randomized order on separate days, subjects
performed two isometric contractions: (1) Force task –
exerted a force against a resistance with an infinite spring
stiffness; (2) Position task – maintained a constant elbow
angle while supporting an inertial load that was equal to the
force exerted during the force task. The net muscle torque
for each subject was identical for the two tasks.
The mechanisms that contribute to muscle fatigue depend on
the task that is performed. Relatively subtle changes in the
task, such as the type of load supported, have a major effect
on the involved mechanisms during a fatiguing contraction.
The time to task failure was 1402 ± 728 s (mean ± SD) for
the force task and 702 ± 582 s for the position task (Hunter
& Enoka, 2002). Despite the difference in duration for the
two tasks, the average EMG for the elbow flexor muscles
increased at a similar rate for the force and position tasks.
Nonetheless, the increase in the fluctuations in force and
acceleration, the mean arterial pressure, the rating of per-
REFERENCES
Bigland-Ritchie B, et al. (1986) J Appl Physiol 61, 421-429.
Gandevia SC. (2001) Physiol Reviews 81, 1725-1772.
Hunter SK, Enoka RM. (2001) J Appl Physiol 91, 26862694.
Hunter SK, Enoka RM. (2003) J Appl Physiol 94, 108-118.
Hunter SK, et al. (2002) J Neurophysiol 88, 3087-3096.
Hunter SK, et al. (2003) J Appl Physiol 94, 2438-2447.
MacGillis CJ, et al. (2003) Med Sci Sports Exerc 35, S000.
ACKNOWLEDGEMENTS
Supported by the National Institute of Neurological Disorders and Stroke (NS 43275).