Fatigue after submaximal intensive stretch-shortening cycle exercise

Objective: The aim of the present study was to examine some sites of neurom uscular fatigue after submaximal intensity stretch-shortening cycle exercis e. Methods: Twelve male subjects performed consecutive sledge jumps at 60% of maximal height until exhaustion (mean duration 443.7 s +/- 304.9 s, mean +/- SD). Results: During the exercise, the blood lactate increased from 1. 8 +/- 0.6 mmol.L-1 (before exercise) to 6.1 +/- 1.7 mmol.L-1 (P < 0.001) an d serum creatin-kinase from 248 +/- 142 IU.L-1 to 584 +/- 344 IU.L-1 (P < 0 .001). Electrical stimulation of the vastus lateralis and quadriceps femori s muscles to induce isometric knee extension resulted in decreased peak tor que during single and double twitch after workout (from 22.1 +/- 6.3 Nm to 17.3 +/- 8.0 Nm, P < 0.05, and from 96.6 +/- 15.4 Nm to 76.2 . 19.8 Nm, P < 0.001, respectively), whereas there were no significant chan es in contrac tion and relaxation times. Torque during 20-Hz stimulation decreased signif icantly (from 23.7 +/- 9.2 to 16.1 +/- 7.8 Nm. P < 0.01) but not at 100-Hz stimulation. During maximal voluntary isometric knee extensions, the rate o f torque development was significantly (P < 0.01) more impaired than maxima l torque (from 1619 +/- 390 Nm.s(-1) to 1004 +/- 360 Nm.s(-1) and from 185 +/- 30.7 Nm to 151 +/- 32.3 Nm, respectively, both P < 0.001). At the same time, the muscle activation level increased by 15.8 +/- 24.1% (P < 0.05). T he mean EMG amplitude of vastus lateralis during MVC increased by 34.9 +/- 39.2% (P > 0.05). Conclusion: It was concluded that after submaximal stretc h-shortening exercise, the low-frequency fatigue occurred, very likely caus ed by lower Ca2+ release per single action potential.