Compensatory mechanisms of balance to the scaling of arm-swing frequency

The present study investigated the contribution of the Hof (2007) mechanism 1 (M1-moving the center of pressure (COP) with respect to the vertical projection of the center of mass (COM_Total)); and mechanism 2 (M2-rotating the trunk and upper limbs around the COM_Total) to postural control and the stability of COP-COM_Total cophase as a function of lateral arm-swing frequency. Young adults were instructed to stand still on a force platform while alternating their arm swinging from above the head to the side of their thigh to create perturbations to postural control. Scaling the frequency of arm-swing (random step changes of 0.2Hz within a bandwidth of 0.2 to 1.6Hz) increased the SD of COP but decreased the SD of COM_Total. Increments in arm-swing frequency induced a progressive increase in M1 and decrease in M2 in terms of their relative contribution to postural stability. The cophase between COP and COM_Total became more tightly in-phase over increments of arm-swing frequency. These findings show an adaptive compensatory role of M1 and M2 within the stability of COP-COM_Total coupling in the regulation of human balance control.