The relationship between plasma potassium, muscle membrane excitability and force following quadriceps fatigue

Abstract

To examine the simultaneous changes in plasma [K⁺], muscle excitability and force during fatigue, ten male adults (mean age = 22 ± 0.5 years) held an isometric contraction of their right quadriceps muscle at an intensity of 30% maximum voluntary contraction (MVC) for 3 min. Femoral venous and brachial arterial [K⁺] were determined from serial samples drawn before, during, and for 15 min following the 3-min contraction. Each blood sample was synchronized with a maximal stimulation of the right femoral nerve to evoke a twitch and compound muscle action potential (M-wave). Immediately post-exercise, twitch torque was only 42% of baseline and femoral venous plasma [K⁺] had increased significantly from 4.02 ± 0.08 mmol/l to 5.9 ± 0.22 mmol/1. Femoral venous plasma lactate rose to a peak level of 10.0 ± 0.8 mmol/1 at 1 min post exercise. The recovery of the twitch torque was exponentially related to the recovery of femoral venous plasma [K⁺] (r² = 0.93, P < 0.01). There was no evidence for any loss of muscle membrane excitability during the period of increased extracellular [K⁺], in fact, the M-waves tended to be potentiated in the early phases of the recovery period. These results suggest that muscle membrane excitability is maintained in spite of increased extracellular [K⁺] following fatigue induced by a sustained submaximal quadriceps contraction. However, the strong relationship between twitch torque and femoral venous plasma [K⁺] suggests that K⁺ may be exerting its effect distal to surface membrane action potential propagation, most likely in the T-tubular region.