WRIST ACTION AFFECTS PRECISION GRIP FORCE

When moving objects with a precision grip, fingertip forces normal to the object surface (grip force) change in parallel with forces tangent ial to the object (load force). We investigated whether voluntary wris t actions can affect grip force independent of load force, because the extrinsic finger muscles cross the wrist. Grip force increased with w rist angular speed during wrist motion in the horizontal plane, and wa s much larger than the increased tangential load at the fingertips or the reaction forces from linear acceleration of the test object. Durin g wrist flexion the index finger muscles in the hand and forearm incre ased myoelectric activity; during wrist extension this myoelectric act ivity increased little, or decreased for some subjects. The grip force maxima coincided with wrist acceleration maxima, and grip force remai ned elevated when subjects held the wrist in extreme flexion or extens ion. Likewise, during isometric wrist actions the grip force increased even though the fingertip loads remained constant. A grip force ''pul se'' developed that increased with wrist force rate, followed by a sta tic grip force while the wrist force was sustained. Subjects could not suppress the grip force pulse when provided visual feedback of their grip force. We conclude that the extrinsic hand muscles can be recruit ed to assist the intended wrist action, yielding higher grip-load rati os than those employed with the wrist at rest. This added drive to han d muscles overcame any loss in muscle force while the extrinsic finger flexors shortened during wrist flexion motion. During wrist extension motion grip force increases apparently occurred from eccentric contra ction of the extrinsic linger flexors. The coactivation of hand closin g muscles with other wrist muscles also may result in part from a gene ral motor facilitation, because grip force increased during isometric knee extension. However, these increases were related weakly to the kn ee force. The observed muscle coactivation, from all sources, may cont ribute to grasp stability. For example, when transporting grasped obje cts, upper limb accelerations simultaneously produce inertial torques at the wrist that must be resisted, and inertial loads at the fingerti ps from the object that must be offset by increased grip force. The mu scle coactivation described here would cause similarly timed pulses in the wrist force and grip force. However, grip-load coupling from this mechanism would not contribute much to grasp stability when small wri st forces are required, such as for slow movements or when the object' s total resistive load is small.