ANTICIPATORY POSTURAL ADJUSTMENTS IN STANCE AND GRIP

The reactive forces and torques associated with moving a hand-held obj ect between two points are potentially destabilising, both for the obj ect's position in the hand and for body posture. Previous work has dem onstrated that there are increases in grip force ahead of arm motion t hat contribute to object stability in the hand. Other studies have sho wn that early postural adjustments in the legs and trunk minimise the potential perturbing effects on body posture of rapid voluntary arm mo vement. This paper documents the concurrent evolution of grip force an d postural adjustments in anticipation of dynamic and static loads. Su bjects held a manipulandum in precision grasp between thumb and index finger and pulled or pushed either a dynamic or a fixed load horizonta lly towards or away from the body (the grasp axis was orthogonal to th e line of the load force). A force plate measured ground reaction torq ues, and force transducers in the manipulandum measured the load (tang ential) and grip (normal) forces acting on the thumb and finger. In al l conditions, increases in grip force and ground reaction torque prece ded any detectable rise in load force. Rates of change of grip force a nd ground reaction torque were correlated, even after partialling out a common dependence on load force rate. Moreover, grip force and groun d reaction torque rates at the onset of load force were correlated. Th ese results imply the operation of motor planning processes that inclu de anticipation of the dynamic consequences of voluntary action.