The effect of voluntary arm abduction on balance recovery following multidirectional stance perturbations.

The goal of this study was to investigate how voluntarily abducting one arm, 90 degrees at onset of a rotational perturbation of the support surface, influences the recovery of upright stance. Young adults were tested under four stance conditions: abducting one arm to the horizontal only (AO); perturbation of stance using a support surface rotation only (PO); combined support surface rotation and abduction of the downhill arm, ipsilateral to tilt (IPS); and fourth abduction of the uphill, contralateral arm (CON). Simultaneous auditory and visual trigger cues were used for arm raising. Perturbations consisted of six directions of combined support surface roll and pitch rotation (7.5 degrees and 60 degrees/s). Outcome measures were whole body centre of mass (COM) movements and body segment angular displacements recorded with a motion analysis system, as well as leg, trunk, and arm EMG responses. Arm raises contralateral and ipsilateral to the direction of support surface roll were more rapid than in the AO condition and significantly reduced or increased, respectively, COM lateral displacements relative to the PO condition. The changes in COM displacements and velocities during combined CON arm raise and perturbation were greater than expected from the sum of displacements for AO and PO conditions alone, but less for the IPS condition. Arm raising increased trunk roll in a direction opposite arm raising was more than for the AO and PO conditions. Robust effects were also observed for hip abduction but not for leg flexion. Early balance correcting activity was enhanced on the side opposite arm raising and later stabilising activity reduced bilaterally in lower trunk muscles compared to summed activity for the AO and PO conditions. Similar effects were observed in gluteus medius muscles but effects were weak in ankle muscles. EMG onsets in muscles of the raised arm were earlier than in the AO conditions. We conclude that triggered arm abduction, contralateral to the direction of support surface rotation, had significant stabilization benefits for young adults and ipsilateral arm movements had destabilizing effects. The arm raises could be simultaneously executed with balance corrections. These results provide insights into the integration of balance corrections and voluntary commands into one automatic reaction that may be useful in training fall avoidance.