Voluntary and reflex responses to fatigue with stretch-shortening exercise.

The effects of 50 depth-vertical jumps (DJ) on counter-movement vertical jump (CMJ), squat jump (SJ), and DJ, isometric knee extension force (MVC), and patellar tendon response were examined in 12 men. There was no reduction in the height of the 50 DJs, but the contact time lengthened by 16%. Height of CMJ and DJ did not change after the 50 DJs, but the activity duration of the leg muscles averaged 13.2% longer. Conversely, height of SJ declined by 11.3%. MVC decreased by 20.1%. No changes occurred in reflex latency and peak-to-peak duration of the compound muscle action potential (AP), but reflex time, AP duration, and AP amplitude increased by 6.4, 6.1, and 23.3%. Thus, subjects compensated for the loss of voluntary force by lengthening the activity duration of the prime movers, potentiating muscle fiber action potentials, and/or enhancing muscle spindle sensitivity.