POSTURAL DEPENDENCE OF MUSCLE ACTIONS - IMPLICATIONS FOR NEURAL CONTROL

The neural control of reaching entails the specification of a precise pattern of muscle activation distributed across the many muscles of th e arm. Musculoskeletal geometry limits the possible solutions to this problem. Insight into the nature of this constraint was obtained by qu antifying the postural variation in the mechanical actions of six huma n shoulder muscles. Estimates of muscle mechanical actions were obtain ed by electrically stimulating muscles to the point of contraction and recording the resulting forces and torques with a six-degree-of-freed om force-torque transducer. In a given experiment, data were obtained for up to 29 different arm postures. The mechanical actions of each mu scle varied systematically with arm posture, regardless of the frame o f reference used to define these actions. The nature of this dependenc e suggests that a relatively simple strategy can be used by the nervou s system to account for the changing mechanical actions of arm muscles .