Potassium kinetics in human muscle interstitium during repeated intense exercise in relation to fatigue

Accumulation of K⁺ in skeletal muscle interstitium during intense exercise has been suggested to cause fatigue in humans. The present study examined interstitial K⁺ kinetics and fatigue during repeated, intense, exhaustive exercise in human skeletal muscle. Ten subjects performed three repeated, intense (61.6±4.1 W; mean±SEM), one-legged knee extension exercise bouts (EX1, EX2 and EX3) to exhaustion separated by 10-min recovery periods. Interstitial K⁺] (K⁺]_interst) in the vastus lateralis muscle were determined using microdialysis. Time-to-fatigue decreased progressively (P<0.05) during the protocol (5.1±0.4, 4.2±0.3 and 3.2±0.2 min for EX1, EX2 and EX3 respectively). Prior to these bouts, K⁺]_interst was 4.1±0.2, 4.8±0.2 and 5.2±0.2 mM, respectively. During the initial 1.5 min of exercise the accumulation rate of interstitial K⁺ was 85% greater (P<0.05) in EX1 than in EX3. At exhaustion K⁺]_interst was 11.4±0.8 mM in EX1, which was not different from that in EX2 (10.4±0.8 mM), but higher (P<0.05) than in EX3 (9.1±0.3 mM). The study demonstrated that the rate of accumulation of K⁺ in the muscle interstitium declines during intense repetitive exercise. Furthermore, whilst K⁺]_interst at exhaustion reached levels high enough to impair performance, the concentration decreased with repeated exercise, suggesting that accumulation of interstitial K⁺ per se does not cause fatigue when intense exercise is repeated.