Nonmetabolic fatigue in exercising human muscle.

Human muscle fatigue, assessed by declining maximum voluntary contraction (MVC), may be caused by impairment of excitation-contraction coupling as well as by metabolic alterations within the muscle. To distinguish between these 2 possibilities, we assessed changes in excitation-contraction coupling by measuring twitch tension (TT) and twitch potentiation (P-TT) of the human adductor pollicis and tibialis anterior, while we analyzed metabolic changes using 31P magnetic resonance spectroscopy. Low-intensity exercise produced a much greater fall of TT and P-TT than MVC, high energy phosphates, or intracellular pH (pHi). In addition, the recovery of TT and P-TT was much slower than recovery of MVC, high energy phosphates, or pHi. These results suggest that the greater fall of TT and the delayed recovery following low-intensity exercise are caused by impaired excitation-contraction coupling.