The efficacy and physiological bases of small muscle mass exercise in health and disease

The conventional approach to aerobic exercise prescription involves large muscle mass exercise and the manipulation of variables such as training intensity, duration and frequency to promote desired adaptations. However, during whole-body exercise, central limitations (i.e., neural, pulmonary and/or cardiac) constrain exercise tolerance and limit the increase in muscle blood flow and the degree of intramuscular metabolic perturbation incurred. Consequently, even during high-intensity large muscle mass exercise, a substantial peripheral reserve remains, potentially diminishing the adaptive stimuli that drive improvements in peripheral function and, in turn, exercise tolerance. In contrast, these central constraints are markedly attenuated during small muscle mass aerobic exercise, such as single-leg cycling or knee extension. As a result, muscle activation, blood flow, work rate and the magnitude of metabolic perturbation per unit of muscle are considerably greater during small compared with large muscle mass exercise. Because many of these responses are thought to represent key triggers initiating peripheral adaptations, such as angiogenesis and mitochondrial biogenesis, small muscle mass exercise might confer unique advantages for enhancing peripheral vascular and metabolic function. This review outlines the key physiological differences between small and large muscle mass exercise, their relevance to peripheral adaptations, and current evidence on the efficacy of small muscle mass exercise in improving peripheral function and exercise tolerance in performance, health and disease.