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.