Rectifying postures reconstructed from joint angles to meet constraints

Postures are often described and modeled using angles between body segments rather than joint coordinates. Models can be used to predict these angles as a function of anthropometry and postural requirements. Postural represen tation, however, requires the joint coordinates. The use of conventional fo rward kinematics to derive joint coordinates from predicted angles may viol ate task constraints, such as the placement of a hand on a target or a foot on a pedal. Errors arise because the anthropometry or other motion charact eristics of a subject, for which the prediction is to be made, may differ f rom the data from which the prediction model was derived. We describe how t o rectify model-predicted postures to exactly satisfy such task constraints . We require that the model used for predicting the angles also produce est imates of the variation in these predictions. We show how to alter the init ial angle predictions, with the amount of perturbation at each angle depend ent on the accuracy of its estimation, so as to exactly satisfy the joint c oordinate constraints. Finally, we show in an empirical example that this c orrection usually produces better overall predictions of posture than those obtained initially. (C) 1999 Elsevier Science Ltd. All rights reserved.