Water and electrolyte fluxes during exercise and their relation to muscle fatigue

Muscle fatigue - defined as impaired contractility - coincided in rat skeletal muscle during in vitro experiments with a decrease in intracellular potassium concentration, [K], and a decrease in resting membrane potential, RMP. In man changes in intra- and extracellular electrolyte concentrations and RMP were calculated from muscle biopsies and blood samples during voluntary contractions. At exhaustion in maximal exercise the intracellular [K] decreased by 20-40 mM from a resting value of 165 mM. Simultaneously extracellular [K] increased by 1-2 mM, and also the intra- as well as extracellular [Na] tended to increase. The calculated RMP therefore decreased from a resting value of -89 mV to -75 mV. During prolonged submaximal exercise similar changes occurred although the rate of these changes was slower. The K lost from the exercising muscles was being taken up by other tissues, and during recovery the fatigued muscles regain the previously lost K.

the K-gradient across the muscle membrane decreased significantly during maximal as well as submaximal exercise. At exhaustion this change is of an order of magnitude, which may well impair the excitability of the muscle membrane and thereby the contractility of the muscle fibres.