Sarcoplasmic reticulum function and muscle contractile character followingfatiguing exercise in humans

1. This study examined the alterations in calcium release, calcium uptake a nd calcium ATPase activity of skeletal muscle sarcoplasmic reticulum in res ponse to a bout of intense dynamic knee extensor exercise, and the relation ship between these changes and alterations in muscle contractile characteri stics in the human quadriceps. 2. In biopsy samples taken from the vastus lateralis, sarcoplasmic reticulu m calcium release and calcium uptake were significantly depressed (P < 0.01 and 0.05, respectively) immediately following the exercise with no alterat ion in the sacroplasmic reticulum Ca2+-ATPase activity. 3. A 33 % reduction in the maximum voluntary isometric torque was found fol lowing the exercise, with reduced torques from electrically evoked isometri c contractions at low frequencies of stimulation (10 and 20 Hz) but not at higher frequencies (50 and 100 Hz). 4. The depressed calcium release was correlated (P<0.05) with a decreased r atio of torques generated at 20:50 Hz, indicating an involvement in low fre quency fatigue; however, no correlations between the muscle relaxation time s or rates of change of torque and calcium uptake were observed.