Muscle weakness following dynamic exercise in humans.

Electrical stimulation of the triceps surae in five healthy male subjects showed that following 1-2 h level running and uphill walking, at submaximal voltages of stimulation, exercise enhanced the twitch and tetanic responses, but the supramaximal time to peak tension (TPT), twitch (Pto) and tetanic tensions (Po) at 10 and 20 Hz were reduced by 16 ms (-12.6%), 11 (-8.9%), 163 (-17.5%), and 230 N (-18.1%), respectively. High-frequency (50 and 100 Hz) tetanic stimulation produced qualitatively similar changes to the 20-Hz response, but the stimulus response curve for the two frequencies was different and the ratio of 20- to 50-Hz response (20/50) (cf. Edwards et al., J. Physiol, London 272: 769-778, 1977) was voltage dependent. The reduction in Po at 100 Hz was associated with a decrease in maximal voluntary contraction (MVC). The effects of exercise on Pto and Po at 10, 20, 50, and 100 Hz were short lived and recovered within approximately 2 h. In contrast box-stepping produced a greater fall in Pto and Po at 10 and 20 Hz, which was long lasting (at least (22 h), and there was a consistent fall in the 20/50 ratio. a 2-min "fatigue" test showed that the muscles were weaker but not more fatigable after exercise. Our results seriously question the validity of using submaximal stimulation voltages and ratios for testing human muscle function and suggest that long-lasting muscle weakness is not associated with recovery from prolonged walking, running, and only observed after box-stepping exercise.