Postural responses to multidirectional perturbations to the hand during stance

Humans are easily able to maintain their balance while applying force with their hands to move or stabilize objects. Based on Newton's laws, the applied force must be counteracted by ground reaction force (GRF) to maintain balance. However, because the GRF is partitioned between the two legs there is no unique solution. Furthermore, central nervous system (CNS) can employ an infinite number of muscle activation patterns to achieve ground reaction force (GRF) vectors needed to satisfy both the task-level goal and balance. This study examines the postural response when hand position must remain stable as an external force is applied in different directions during normal stance. We investigated whether the CNS uses an invariant strategy to compensate for forces acting in different directions.