ReviewMuscle fatigue – from motor units to clinical symptoms
Introduction
On the occasion of the 2011 Congress for the International Society of Biomechanics held in Brussels, I was recognized for career achievements in biomechanics and honored with the prestigious Muybridge Award. In recognition of the honor, I dutifully began the Muybridge Lecture by acknowledging the contributions to the field of biomechanics of the pioneer after whom the award was named, Eadward James Muybridge (1830–1904). The Society describes Muybridge as the “father of cinematography”. Although he did accumulate 20,000 images of animal and human locomotion over an 8yr period, he was more interested in the representation of motion from an artistic perspective than in analyzing its attributes. Rather, it was a contemporary of his who had the same initials and was born and died in the same years who contributed more than Muybridge to the scientific study of human motion. His name was Etienne-Jules Marey (1830–1904). The substantial contributions to the study of human motion by the venerable professor at the Collége de France and member of the Academy of Sciences are described in an excellent biography written by Marta Braun (1992). Marey and Muybridge were aware of each other's work, and their relatively brief interaction is described in the proceedings of a conference to celebrate the 100 yr anniversary of the beginning of cinema (Delimata, 1996). Accordingly, the lecture was dedicated to EJM (1830–1904).
The purpose of the lecture was not to provide an historical account of key developments in biomechanics, but rather to discuss the difficulties associated with changing the ideas that define a contemporary issue. The topic chosen for the occasion was muscle fatigue, and the approach was to proceed from ideas on muscle fatigue that should be discarded through to challenges for the future. This was accomplished by discussing motor unit types and fatigue, myoelectric manifestations of fatigue, and the distinction between fatigue and fatigability.
Section snippets
Motor unit types and fatigue
The motor unit denotes the basic functional element of the central nervous system and muscle that produces movement. It comprises a motor neuron in the ventral horn of the spinal cord, its axon, and the muscle fibers that the axon innervates (Duchteau and Enoka, 2011, Sherrington, 1925). The central nervous system controls muscle force by varying the activity of the motor units in the muscle. The force exerted by each motor unit depends principally on the number of muscle fibers that are
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 interneurons, and peripheral afferent feedback. After these inputs have been integrated by the motor neurons, the activation signal sent from the spinal cord to muscle comprises trains of action potentials for the motor units that have been recruited for the action. The net motor unit activity, therefore, is reflected in the magnitude of the activation
Fatigue and fatigability
The fatigability of muscle is classically quantified as the decline in MVC force after performing some form of demanding physical activity (Enoka and Duchateau, 2008, Gandevia, 2001). At least since the observations of Mosso (1906), it has been known that the mechanisms responsible for the decline in MVC force depend on the characteristics of the task that induced the fatigue (Enoka and Stuart, 1992). Although the evidence clearly indicates that fatigue cannot be attributed to any single
Conflict of interest statement
None declared.
Acknowledgements
Among the many colleagues with whom I have been privileged to work, I would like to acknowledge the significant contributions of Professor Jacques Duchateau (Université Libre de Bruxelles) and Professor Dario Farina (Georg-August University of Göttingen) to the ideas and findings described in this paper.
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