Mammalian muscle fiber types and their adaptability

Abstract

The neuromuscular system can adapt to a variety of different kinds of chronic workloads. The specificity of these adaptations is reflected in the specific components of the neuromuscular system that are overloaded. An examination of changes that occur during normal development can serve as a basis for efforts to understand the mechanisms which are responsible for the maintenance of a given histochemical, biochemical and physiological profile of a motor unit. The normal process of muscle development can be retarded by spinal transection or immobilizing a neonatal muscle and some of the neonatal features can be reintroduced in adult animals by these same procedures. In general the speed related properties of fast skeletal muscle are not altered markedly by any type of physiological overload. There is some evidence that immobilization or intense endurance trained slow muscle is faster than the normal muscle. The metabolic properties related to maintenance of tension during prolonged work are readily adaptable to chronic low resistance-highly repetitive contractions. Factors that determine the specificity of the response to neuromuscular overload seem to be motor unit recruitment frequency, the tension produced within the musculature and the duration of the tensions. © 1978 by the American Society of Zoologists.