Relative contributions to the net joint moment for a planar multijoint throwing skill: early and late in practice

The purpose of this investigation was to determine, for a planar, multijoint throwing skill, if the relative contributions of the components of the net joint moment (NJM) at the elbow and shoulder change after practice. Each participant (N = 7) performed 200 throwing trials equally distributed across 5 consecutive days. Each participant threw a 0.15-kg ball as far as possible using the nondominant arm while the motion of the throwing arm was restrained to a horizontal plans. From video data and body segment inertial estimations, NJMs and NJM components (i.e., generalized muscle moments and motion-dependent moments) were calculated for selected early and late practice trials. Performance (throwing distance) showed an expected improvement from early to late practice. The dynamics analysis indicated that participants increased average NJMs and NJM components at both joints. However, the relative contribution of NJM components, expressed as ratios of those components to the NJM at each joint, did not change after extended practice. Restraining the throwing arm to a horizontal plane may partly explain why no changes were found in the relative contributions of NJM components. The lack of change in moment ratios support a motor strategy of scaling joint moments for faster movements.