Grip force control during gait initiation with a hand-held object

When walking with a hand-held object, grip force is coupled in an anticipatory manner to changes in inertial force resulting from the accelerations and decelerations of gait. However, it is not known how grip and inertial forces are organized at the onset of gait, and if the two forces are coupled in the early phases of gait initiation. Moreover, initiating walking with an object involves the coordination of anticipatory postural (e.g., ground reaction force changes) and grasping adjustments. The aim of this study was to investigate the relationship of ground reaction, grip, and inertial force onsets, and the subsequent development of the coupling of grip and inertial forces during gait initiation with a hand-held object. Ten subjects performed gait initiation with a hand-held object following predictable and unpredictable start signals. We found that ground reaction and grip force onsets were closely linked in time regardless of the predictability of the start signal. In the early period of gait initiation, the grip force started to increase prior to inertial force changes. While the strength of the coupling of grip and inertial forces was moderate in this early phase, it increased to values observed during steady-state gait after the swing foot left the ground. The early grip force increase and the coupling of grip and inertial forces represent an anticipatory control process. This process establishes an appropriate grip-inertial force ratio to ensure object stability during acceleration after foot-off and maintains this increased ratio thereafter. The results suggest that grasping and whole body movements are governed by a common internal representation.