Muscle Fatigue - From Motor Units to Clinical Symptoms
Roger M. Enoka
Motor unit types and fatigue:
They found that two contractile properties could be used to identify three types of motor units; the contractile properties were the profile of a submaximal tetanus and a measure of fatigability.
The protocol began by recording the twitch response of the motor unit to a single electrical stimulus and measuring the time from the onset of the increase in force to the peak force, which is a measure referred to as contraction time. Next, the motor unit was activated with a series of electric stimuli with an inter-stimulus interval of 1.25_ the contraction time of the unit. The resulting tetanus either exhibited a progressive increase in force or there was a slight decline that began soon after an initial peak force. The decline in the titanic force is known as the sag property. Burke and colleagues classified motor units that exhibited sag as fast (type F) units, whereas those that did not display sag were referred to as slow (type S) units. Sag appears to be caused by a transient reduction in the duration of the contractile state.
Those units whose tetanic force did not decline or decreased only slightly were described as fatigue resistant. In contrast, motor units that exhibited a marked decline in the tetanic force were characterized as fatigue sensitive or fatigable.
On the basis of the sag and fatigue tests, Burke et al. identified three types of motor units: (1) type S-very fatigue resistant units with relatively slow twitch contraction; (2) type FR - fatigue resistant units with fast twitch contraction; and (3) type FF - fatigue sensitive units with relatively fast twitch contraction.
The other significant constraint of the motor unit-typing scheme is the measure of fatigability. As indicated by Burke the stimulus frequency was selected to minimize the failure of muscle fiber activation and thereby to stress the physiological processes distal to the muscle fiber action potential. Two of the significant outcomes of this approach are the functional relevance of the measure of fatigability and the interpretation that some motor units are fatigue resistant. One principle to emerge in the literature on muscle fatigue is the concept that the underlying mechanisms vary with the demands of the task being performed. Although the fatigability induced by some tasks can be attributed to mechanisms distal to muscle fiber action potentials, many tasks are limited by impairments in muscle activation. Indeed, it is likely that limitations in activities of daily living are more related to activation issues rather than the capacity of muscle to develop force or power. The second issue involves the fatigability of fatigue-resistant motor units. When human volunteers performed a series of 25 ramp-up and ramp-down contractions that were minimally fatigable (9% decline in maximal voluntary contraction [MVC] force), Farina et al. found that the adjustments were greatest for the first recruited motor units. As the target force for the ramp contractions was only 10% MVC force, the involved motor units were presumably fatigue resistant based on the motor unit-typing scheme. Nonetheless, the motor units that discharged action potentials for a greater proportion of the duration for the initial ramp contractions experienced greater declines in conduction velocity of muscle fiber action potentials and the proportion of the task during which action potentials were discharged. Furthermore, the recruitment thresholds of the least active motor units declined over the course of the 25 ramp contractions, which indicates that the earlier recruited motor units contributed less to the net muscle force as the task progressed. These results indicate that low-threshold motor units, presumably fatigue resistant, are actually fatigable during voluntary contractions. Taken together, these findings indicate that motor units cannot be distinguished on the basis of twitch contraction times and that the characterization of motor unit fatigability with tests of imposed sequences of electrical stimulation does not provide a functionally useful metric of motor unit properties. It is time that we abandoned the concept of motor unit types.
Myoelectric manifestations of fatigue:
Voluntary contractions arise from the activation of a motor neuron pool by synaptic inputs that are provided by descending pathways, spinal inter-neurons, and peripheral afferent feedback.
The simulations by Dideriksen and colleagues indicate that the relation between surface EMG amplitude and muscle force is not constant during fatiguing contractions and that neither the amount of muscle activation (number of muscle fiber action potentials) nor the level of neural drive (number of motor unit action potentials) can be reliably estimated from EMG amplitude during fatiguing contractions.
Fatigue and fatigability:
The fatigability of muscle is classically quantified as the decline in MVC force after performing some form of demanding physical activity
Motivated by the lack of progress on translating observations on the mechanisms underlying laboratory measures of fatigability to the impairments experienced by individuals during activities of daily living, it seems necessary to question our current conceptualization of fatigue. A more global perspective seems appropriate in which fatigue can be influenced by both perceptions of fatigue and the mechanisms the establish levels of fatigability. Perhaps such a framework can provide a more effective foundation from which to establish the functional relevance of the diverse observations on fatigue.