Human neuromuscular fatigue is associated with altered Na⁺-K⁺-ATPase activity following isometric exercise

This is the authors' abstract. We don't add key points for this paper.

The purpose of this study was to investigate the hypothesis that reductions in Na+-K+- ATPase activity are associated with neuromuscular fatigue following isometric exercise. In control (Con) and exercised (Ex) legs, force and electromyogram were measured in 14 volunteers [age, 23.4 +/- 0.7 (SE) yr] before and immediately after (PST0), 1 h after (PST1), and 4 h after (PST4) isometric, single-leg extension exercise at ~60% of maximal voluntary contraction for 30 min using a 0.5 duty cycle (5-s contraction, 5-s rest). Tissue was obtained from vastus lateralis muscle before exercise in Con and after exercise in both the Con (PST0) and Ex legs (PST0, PST1, PST4), for the measurements of Na+-K+-ATPase activity, as determined by the 3-O-methylfluorescein phosphatase (3-O-MFPase) assay. Voluntary (maximal voluntary contraction) and elicited (10, 20, 50, 100 Hz) force was reduced 30-55% (P < 0.05) at PST0 and did not recover by PST4. Muscle action potential (M-wave) amplitude and area (measured in the vastus medialis) and 3-O-MFPase activity at PST0-Ex were less than that at PST0-Con (P < 0.05) by 37, 25, and 38%, respectively. M-wave area at PST1-Ex was also less than that at PST1-Con (P < 0.05). Changes in 3-O-MFPase activity correlated to changes in M-wave area across all time points (r = 0.38, P < 0.05, n = 45). These results demonstrate that Na+-K+- ATPase activity is reduced by sustained isometric exercise in humans from that in a matched Con leg and that this reduction in Na+-K+-ATPase activity is associated with loss of excitability as indicated by M-wave alterations.