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.
Department of Neurology, Children's Hospital of San Francisco, University of California.
R S
RS
Moussavi
P J
PJ
Carson
M D
MD
Boska
M W
MW
Weiner
R G
RG
Miller
D000200:Action Potentials; D000328:Adult; D004576:Electromyography; D015444:Exercise; D005528:Foot; D006225:Hand; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism / Q000502:physiology*