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

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/l. Femoral venous plasma lactate rose to a peak level of 10.0 +/- 0.8 mmol/l 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.