SLOW FORCE RECOVERY AFTER LONG-DURATION EXERCISE - METABOLIC AND ACTIVATION FACTORS IN MUSCLE FATIGUE

To investigate the roles of metabolic and nonmetabolic factors in huma n muscle fatigue, two relatively brief nonexhausting exercise protocol s that produced similar levels of moderate fatigue were used: short-du ration exercise (SDE; 2-min sustained maximal voluntary contraction) a nd long-duration exercise (LDE; 15- to 20-min intermittent exercise). After exercise and during recovery, multiple potential mechanisms of f atigue were studied from measurements of voluntary, twitch, and tetani c forces; intracellular metabolites (using P-31-nuclear magnetic reson ance spectroscopy); and electromyographic signals. The major findings were as follows. 1) After SDE, fatigue closely correlated with increas ed [P(i)]. Both force and [P(i)] recovered within approximately 5 min after exercise. 2) After LDE, force recovered slowly, with significant fatigue beyond 15 min after exercise; however, recovery of [P(i)] was not slowed. 3) Electromyographic signals were little affected by eith er protocol. These findings suggest that multiple mechanisms contribut e to moderate fatigue. Fatigue from SDE may arise primarily from metab olic mechanisms, whereas fatigue from LDE involves an additional slowl y recovering nonmetabolic mechanism that may arise from impaired activ ation, beyond the cell membrane, at the level of excitation contractio n coupling.