Sudden release during a pulling task: the effect of release load on stance perturbation and recovery.

Ten subjects performed a horizontal pulling task with sudden release for three randomly allocated loads of 20, 40, and 60 N to examine the effect of sudden release load on centre of pressure (COP) motion and the response strategy with respect to load, and also to determine critical loads at which the response strategy must involve stepping to prevent a fall. Sudden release produces COP deflection directed posteriorly and laterally towards the anteroposterior midline. The magnitude of these deflections increase with release load (P<0.001), as does the time taken to re-establish normal stance following release (P<0.001). Interaction between lateral and anteroposterior shifts in the COP are seen following release, and establishing COP over a favoured limb may be an important factor in the response strategy. This may be so, because if the perturbation is large enough to provoke a protective step, then the COP will have to be located towards one limb before the step can take place. Linear regression of the anteroposterior limit of the COP indicates a wide range of critical loads that will result in a necessary preventive step backwards being taken (80-257 N, mean 153 N), although these values are likely to be overestimated somewhat.