STRETCH-INDUCED ENHANCEMENT OF MECHANICAL POWER OUTPUT IN HUMAN MULTIJOINT EXERCISE WITH COUNTERMOVEMENT

The relation between the eccentric force developed during a countermov ement and the mechanical power output was studied in squatting exercis es under nominally isotonic load (50% of 1-repetition maximum). The su bjects (n = 5) performed squatting exercises with a countermovement at varied deceleration rates before lifting the load. The ground reactio n force and video images were recorded to obtain the power output of t he body. Net muscle moments acting at hip, knee, and ankle joints were calculated from video recordings by using inverse dynamics. When an i ntense deceleration was taken at the end of downward movement, large e ccentric force was developed, and the mechanical power subsequently pr oduced during the lifting movement was consistently larger than that p roduced without the countermovement. Both maximal and mean power outpu ts during concentric actions increased initially with the eccentric fo rce, whereas they began to decline when the eccentric force exceeded s imilar to 1.4 times the sum of load and body weight. Video-image analy sis showed that this characteristic relation was predominantly determi ned by the torque around the knee joint. Electromyographic analyses sh owed no consistent increase in time-averaged integrated electromyograp h from vastus lateralis with the power output, suggesting that the enh ancement of power output is primarily caused by the prestretch-induced improvement of an intrinsic force-generating capability of the agonis t muscle.