Muscle fatigue--from motor units to clinical symptoms

Reductionist approaches have provided little insight on the fatigue experienced by humans during activities of daily living. Some of the reasons for this lack of progress include the persistence of outdated concepts, the misinterpretation of experimental recordings, and a failure to embrace a global perspective on fatigue. This paper summarizes the three examples of these limitations that were discussed in the 2011 Muybridge Award lecture: motor unit types and muscle fatigue, myoelectric manifestations of fatigue, and fatigue and fatigability. Although the motor units in a population do exhibit a range of fatigability values, there are not distinct groups of motor units and the concept that some motor units are resistant to fatigue emerged from protocols in which motor units were activated by electrical stimulation rather than voluntary activation. The concept of distinct motor unit types should be abandoned. The second example discussed in the lecture was the use of surface EMG signals to assess fatigue-related adjustments in motor unit activity. The critical assumption with this approach is that the association between surface EMG amplitude and muscle force remains constant during fatiguing contractions. Unfortunately, the relation does not remain constant and a series of computational studies demonstrate the magnitude of the discrepancy, including the absence of an association with the activation signal emerging from the spinal cord and that received by the muscle. The third example concerned the concepts of fatigue and fatigability. It has long been recognized that fatigue involves both sensations and impairments in motor function, and the final part of the lecture urged the integration of the two constructs into a single scheme in which fatigue can be modulated either independently or by interactions between perceptions of fatigue and the mechanisms that establish levels of fatigability. The expectation is that such critical evaluations of the concepts and approaches to the study of fatigue will provide a more effective foundation from which to identify the factors that contribute to fatigue in health and disease.