Most exercise results in some skeletal muscle damage. However, unaccustomed exercise andlor eccentric exercise can cause extensive damage. This exercise-induced muscle damage causes a response that can be characterized by a cascade of metabolic events. Within 24 to 48 hours, delayed onset muscle soreness and weakness, the most obvious manifestations of the damage, peak. Increased circulating neutrophils and interleukin-1 occurs within 24 hours after the exercise, with skeletal muscle levels remaining elevated for a much longer time. There is a prolonged increase in ultrastructural damage and muscle protein degradation as well as a depletion of muscle glycogen stores. These metabolic alterations may result in the increased need for dietary protein, particularly at the beginning of a training program that has a high eccentric component such as strength training. The delay in muscle repair and glycogen repletion following damaging exercise should cause coaches and athletes to allow an adequate period of time between competition for complete recovery.
William J. Evans is with the USDA Human Nutrition Research Center on Aging, Tufts University, 711 Washington St., Boston, MA 02111.