Finding final postures.

In this study, a model of movement planning (Rosenbaum, Engelbrecht, Bushe, & Loukopoulos, 1993a, 1993b; Rosenbaum, Loukopoulos, Meulenbroek, Vaughan, & Engelbrecln, 1995), in which movements are generated on the basis of the efficacy of different possible goal postures, was tested. The model predicts which limb segments will be used and how the segments will be combined in reaching. The model's predictions were compared with observations from a study in which seated participants reached 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 contributing joints, and a 4th parameter that specified the relative weight of spatial accuracy versus effort minimization, were used for fitting the model to the observed joint angles. The model accounted for 96% of the variance of the observed joint angles and did a better job accounting for the data than several alternative models. A key new finding was that balance constraints play a more important role in determining joint contributions than was previously recognized.