Effects of elastic recoil on maximal explosive power of the lower limbs

The maximal explosive power during a two legs jump was measured on four competitive athletes [mean age 24(SD 4.3) years; height 1.79 (SD 0.09) m; body mass 68.7 (SD 12.8) kg] at different starting knee angles (70, 90, 110, 130 and 150 degrees). The experiments were performed on a newly developed instrument with which both force and speed could be measured using a force platform and a wire tachometer, respectively, and on a conventional force platform. At the smallest knee angle (70 degrees) the mean power output (W in watts per kilogram) developed during the jump was found not to differ significantly between the two methods (P > 0.1). At the larger knee angles W was 18.4% (90 degrees), 34.5% (110 degrees), 47.4% (130 degrees) and 19.4% (150 degrees) higher using the conventional force platform (P < 0.05 throughout). The difference of W between the two methods was attributed to the recovery of elastic energy due to the counter movement which immediately preceded the jump on the conventional platform, but not on the newly developed instrument. Indeed because of a mechanical arrangement which prevented the subject from moving towards the platforms, eccentric work (W-) could not be performed on the newly developed instrument; whereas W- on the conventional force platform was almost negligible at 70 degrees knee angle [mean 1.7 (SD 2.3 J)] reached a maximum of 13.1 (SD 7.9) J at 130 degrees and decreased again to a mean 4.7 (SD 3.6) J for the largest angle (150 degrees). Furthermore, on the conventional force platform, the force at the onset of the positive speed phase (Fi) was an increasing function of W- (r2 = 0.519, P < 0.001); and the difference of W between the conventional and new instruments was larger the larger the difference of Fi (r2 = 0.391, P < 0.01).