Anticipatory postural adjustments in stance and grip.

The reactive forces and torques associated with moving a hand-held object between two points are potentially destabilising, both for the object's position in the hand and for body posture. Previous work has demonstrated that there are increases in grip force ahead of arm motion that contribute to object stability in the hand. Other studies have shown that early postural adjustments in the legs and trunk minimise the potential perturbing effects on body posture of rapid voluntary arm movement. This paper documents the concurrent evolution of grip force and postural adjustments in anticipation of dynamic and static loads. Subjects held a manipulandum in precision grasp between thumb and index finger and pulled or pushed either a dynamic or a fixed load horizontally towards or away from the body (the grasp axis was orthogonal to the line of the load force). A force plate measured ground reaction torques, and force transducers in the manipulandum measured the load (tangential) and grip (normal) forces acting on the thumb and finger. In all conditions, increases in grip force and ground reaction torque preceded any detectable rise in load force. Rates of change of grip force and ground reaction torque were correlated, even after partialling out a common dependence on load force rate. Moreover, grip force and ground reaction torque rates at the onset of load force were correlated. These results imply the operation of motor planning processes that include anticipation of the dynamic consequences of voluntary action.