Control of the wrist in three-joint arm movements to multiple directions in the horizontal plane

In a reaching movement, the wrist joint is subject to inertial effects from proximal joint motion. However, precise control of the wrist is important for reaching accuracy. Studies of three-joint arm movements report that the wrist joint moves little during point-to-point reaches, but muscle activit ies and kinetics have not yet been described across a range of movement dir ections. We hypothesized that to minimize wrist motion, muscle torques at t he wrist must perfectly counteract inertial effects arising from proximal j oint motion. Subjects were given no instructions regarding joint movement a nd were observed to keep the wrist nearly motionless during center-out reac hes to directions throughout the horizontal plane. Consistent with this, wr ist muscle torques exactly mirrored interaction torques, in contrast to mus cle torques at proximal joints. These findings suggest that in this reachin g task the nervous system chooses to minimize wrist motion by anticipating dynamic inertial effects. The wrist muscle: torques were associated with a direction-dependent choice of muscles, also characterized by initial recipr ocal activation rather than initial coactivation to stiffen the wrist joint . In a second experiment, the same pattern of muscle activities persisted e ven after many trials reaching with the wrist joint immobilized. These resu lts, combined with similar features at the three joints, such as cosine-lik e tuning of muscle torques and of muscle onsets across direction, suggest t hat the nervous system uses similar rules for muscles at each joint, as par t of one plan for the arm during a point-to-point reach.