Sarcolemmal excitability as investigated with M-waves after eccentric exercise in humans

It has been shown that intensive eccentric muscle actions lead to prolonged loss of muscle force and sarcolemmal damage. This may lead to a reduction in the excitability of the sarcolemma and contribute to the functional deficit. Experiments were carried out to test sarcolemmal excitability after eccentric elbow flexor exercise in humans. Electrically elicited surface compound muscle action potential (M-wave) properties from 30s stimulation trains (20Hz) were analyzed in biceps brachii muscle immediately after, 1h and 48h after the exercise. M-wave area, amplitude, root mean square and duration were reduced immediately after the eccentric exercise. However, no such reduction could be observed 48h after the exercise, although the maximal voluntary isometric and eccentric torques were still depressed by 12.2+/-9% (P<0.001) and 17.7+/-9% (P<0.001), respectively. Acute increase in plasma concentrations of K(+) and Ca(2+) were also observed after the eccentric exercise. These findings suggest that eccentric exercise may acutely decrease sarcolemmal excitability, which seems to be partially related to increased extracellular ion concentrations. However, disturbance of sarcolemmal excitability is not the major factor determining eccentric exercise induced prolonged loss of muscle strength, because no prolonged impairment was observed in any of the studied M-wave parameters.