Neural adaptations with chronic physical activity (Fragment)

Roger M. Enoka. Department of Kinesiology. University of Colorado, Boulder, CO 80309-0354. U.S.A.




The performance capabilities of the human neuromuscular system are affected significantly by the amount and type of daily physical activity. For example, a body builder who exercises several hours a day will have more lean body mass and greater strength than a less active person. Conversely, an individual who is confined to bedfor a few weeks or who has a limb immobilized in a cast will experience muscle atrophy and a decline in strength. It is evident from these types of observations that themorphological and functional characteristics of the neuromuscular system can be affected by chronic levelsof physical activity. To improve the management of these effects in both health and disease, the physiological mechanisms that mediate such adaptations have been the subject of much scrutiny. The adaptations that occur in the neuromuscular system with chronic levels of physical activity can be assessed in a variety of ways. From a human-movement perspective, the most common approach is to distinguish among the neural and muscular mechanisms that influence muscle fatigue, power production, and strength. Since the classic paper of Merton (1954), for example, the predominant opinion on the cause of muscle fatigue has been that it is the result of factors located in muscle. However, more recent evidence has identified a significant role for neural mechanisms in muscle fatigue (Enoka, 1995; Gandevia et al., 1995). Similarly, it has been recognized for quite some time that neural factors contribute significantly to increases in muscle strength, especially at the beginning of a training program (Enoka and Behm, 1996; Sale, 1988). The purpose of this review is twofold: (1) to describe some of the evidence that supports a significant role for neural mechanisms in the adaptations that accompany chronic changes in physical activity; and (2) to evaluate the types of neural mechanisms that appear to contribute to these adaptations. Within this construct, the emphasis is on recent work performed on human subjects.