The relationships between plasma potassium, muscle excitability and fatigue during voluntary exercise in humans.

The relationships between extracellular potassium elevation and EMG variables in relation to muscle fatigue were investigated during handgrip exercise in humans. Acid-base state, lactate, potassium ([K+](v)) and sodium in venous plasma, as well as variables of surface voluntary and evoked (M-wave) EMG were determined during repeated dynamic (DE) and static (SE) exercise (1 min exercise, 4 min rest). The different rises of [K+](v) were induced by randomly varied workloads. After 15 min of warming up, the M-wave area increased to 124.9 +/- 19.6% (P < 0.001) in comparison with the control value. Simultaneously, the [K+](v) decreased from 4.1 +/- 0.3 to 3.6 +/- 0.3 mmol l(-1) (P < 0.01). During both SE and DE, there were marked intensity-dependent signs of fatigue. The [K+](v) correlated with changes of the integrated EMG (r = 0.87, P < 0.001 for both DE and SE). Changes in the M-wave area during the exercise bouts correlated inversely with the [K+](v) (r = -0.73, P < 0.001). The M-wave area did not decrease below the control value at any intensity. The median frequency of the EMG decreased during exercise, depending on the exercise intensity (r = -0.73 for SE, r = -0.47 for DE, P < 0.001) with a maximal decrease to about 80% after SE with the maximal workload. The muscle action potential propagation velocity changed in the range of about +/-2%. For the first time, a negative relationship between venous potassium and M-wave area was shown during voluntary exercise. However, there was no evidence that the decrease in muscle performance was mainly caused by a decrease in sarcolemmal excitability resulting from a high extracellular [K+].