Effects of acute isometric knee extension exercise at 40% MVC on muscle mechanical properties and Ca2+ pump function

Abstract

To investigate the effects of intermittent isometric knee extension exercise on muscle mechanical properties and sarcoplasmic reticulum (SR) Ca2+‐ATPase activity, 8 healthy males performed 30 minutes of single‐legged knee extension exercise at 40% maximal voluntary contraction (MVC) with a 50% duty cycle (5 sec. contraction, 5 sec relaxation). Quadriceps isometric contractile properties were measured during voluntary and evoked contractions before (PRE) and after (POST) exercise and on recovery day 1 (R1), 2 (R2), 3 (R3) and 6 (R6). Exercise caused a reduction (p<0.05) in the maximal twitch force POST when compared with PRE (141 ± 15 vs. 74 ± 8, N) that had recovered by R1 (135 ± 11, N). MVC force was also depressed (p<0.05) POST when compared to PRE (557 ± 58 vs. 313 ± 16), but unlike evoked force, did not fully recover until R6 (p<0.05).Measurements of maximal Ca2+‐ATPase activity (uM/mg protein/min) were made in homogenates that were prepared from muscle biopsy samples taken before and after exercise and throughout recovery. Compared with PRE, maximal Ca2+‐ATPase activity was not different at any time point. In conclusion, muscle fatigue caused by intermittent isometric knee extension exercise at 40% MVC is not associated with intrinsic alterations in SR Ca2+‐ATPase function in humans. Supported by NSERC