Comparison of new approaches to estimate mechanical output of individual joints in vertical jumps.

Conventional calculation of joint power is not effective in order to assess translational motions of human body. Two new approaches were developed in this study to estimate translational mechanical outputs from individual joints. They were applied to the analysis of vertical jump motions of six male subjects. In both cases, body of subjects were modeled with a four mass-points system model, and joints were regarded as motion generators. In one approach, increase of vertical component of ground reaction force (GRF) was decomposed into push-off force of three joints (ankle, knee, and hip joints). This procedure gave an estimation of impulse exerted per leg, as 83-92 N s (95% confidential interval) for squat jump, which was similar to half of the impulse provided to mass center of the body calculated from GRF, 82-88 N s. In the other approach, amount of the power exerted by each joint vertically was estimated by calculating the scalar product of joint reaction force and relative velocity vectors of adjacent segments. This approach gave estimation of vertical work provision per leg, as 201-226 and 141-181 J for squat and counter movement jumps, which were not so different from half of the work provided to mass center of the body calculated from GRF, 209-227 and 137-175 J, respectively. As these approaches make it possible to calculate translational mechanical outputs specifically, they are useful and consistent concerning the analysis of translational motion of human body.