Biomechanical and kinetic constraints in the control of limb movement

Most of the emphasis in the study of the control of arm movements in three dimensions has been on kinematic factors. Some of the questions that have been taken up extensively are the kinematic transformations between the location of a target point in space and the corresponding posture of the arm, as well as the kinematic factors that may influence the trajectory the arm takes in moving from one point to another. The most commonly accepted model for limb movement is a serial one in which the initial stages are devoted to planning a trajectory in hand or joint coordinates. The subsequent stages would then generate the requisite joint torques via appropriate patterns of muscle activation; either by a computation of inverse dynamics or by taking advantage of the viscoelastic properties of muscles. Kinetic and biomechanical factors influencing trajectory planning as well as the patterning of muscle activation have received much less attention and this paper focuses on these aspects of the control problem. The author shows that energy minimization criteria are capable of predicting the posture of the arm at the conclusion of a pointing movement. The author also presents results of recent physiological investigations concerning the manner in which the pulling directions of shoulder muscles change with arm posture