Received October 31, 2006
Revised January 2, 2007
Accepted after revision April 10, 2007

¹ Hanover Medical School

^* To whom correspondence should be addressed. E-mail: chouchakov.vladimir{at}mh-hannover.de.

	Abstract

Abstract 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 to the control. 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 AP propagation velocity changed in the range of about ±2%.For the first time, a negative relation between venous potassium and M-wave area was shown during voluntary exercise. However, there was no evidences that the decrease in muscle performance was mainly caused by a decrease in sarcolemmal excitability due to a high extracellular [K⁺].

Key Words: Electrical activity, Muscle fatigue, Potassium