Roger M. Enoka. Department of Kinesiology. University of Colorado, Boulder, CO 80309-0354. U.S.A.
PmChronic activity patterns, such as strength training, limb immobilization, and aging, produce marked adaptations in both the muscular and nervous systems. In this brief review, some of the involved mechanisms are examined as they are revealed through studies on the maximality, specificity, and pattern of the neural drive tomuscle. The studies on maximality indicate that it is difficult to activate maximally a muscle by voluntary command, the capacity varies across muscles, tasks, and training, and the maximum discharge rates of motor neurons decreases with immobilization and increases with strength training. The data on specificity demonstrate that: strength can be increased by training with imagined contractions; the velocity specificity of isokinetic training is evident with intended contractions; the strength training influences the untrained homologous muscle in the contralateral limb; the bilatral deficit can become a bilateral facilitation with appropriate training; and that eccentric contractions appear to involve a different activation scheme compared to isometric and concentric contractions. Finally, the literature on the pattern of the neural drive suggests that: coactivation varies with training and often decreases as skill level increases; measures of motor-unit synchronization reveal changes in neuronal connectivity with physical training; the reflex potentiation varies across muscles, individuals, and activity patterns; the modulation of the H-reflex amplitude with training involves changes in the motor neuron; and the motor neurons exhibit a bistable, excitability property that may be influenced by exercise. Despite the breadth of this evidence, there remain substantial gaps in our knowledge, particularly regarding the symmetry of adaptations with increased and decreased chronic physical activity.
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.