FINDING FINAL POSTURES

In this study, a model of movement planning (Rosenbaum, Engelbrecht, B ushe, & Loukopoulos, 1993a, 1993b; Rosenbaum, Loukopoulos, Meulenbroek , Vaughan, & Engelbrecht, 1995), in which movements are generated on t he basis of the efficacy of different possible goal postures, was test ed. The model predicts which limb segments will be used and how the se gments will be combined in reaching. The model's predictions were comp ared with observations from a study in which seated participants reach ed for targets in a sagittal plane, using the hip, shoulder, and elbow . Estimates of 4 free parameters-an expense factor for each of the 2 c ontributing joints, and a 4th parameter that specified the relative we ight of spatial accuracy versus effort minimization, were used for fit ting the model to the observed joint angles. The model accounted for 9 6% of the variance of the observed joint angles and did a better job a ccounting for the data than several alternative models. A key new find ing was that balance constraints play a more important role in determi ning joint contributions than was previously recognized.