Anticipatory postural adjustments associated with lateral and rotational perturbations during standing

We studied the role of different leg and trunk muscle groups in the generat ion of anticipatory postural adjustments (APAs) prior to lateral and rotati onal perturbations associated with predictable and self-triggered postural perturbations during standing. Postural perturbations were induced by a var iety of manipulations including catching and releasing a load with the righ t hand extended either in front of the body or to the right side, performin g bilateral fast shoulder movements in different directions, and applying b rief force pulses with a hand against the wall. Perturbations in a frontal plane ("lateral perturbations") were associated with significant asymmetrie s in APAs seen in the right and left distal (soleus and tibialis anterior) muscles; these asymmetries dependent on the direction of the perturbation. Rotational perturbations about the vertical axis of the body generated by f ast movements of the two shoulders in the opposite directions were also ass ociated with direction-dependent asymmetries in the APAs in soleus muscles. However, rotational perturbations generated by an off-body-midline force p ulse application were accompanied by direction-dependent asymmetries in pro ximal muscle groups, but not in the distal muscles. We conclude that muscle s controlling the ankle joint play an important role in the compensation of lateral and rotational perturbations. The abundance of muscles participati ng in maintaining vertical posture allows the control system to use differe nt task-dependent strategies during the generation of APAs in anticipation of rotational perturbation. (C) 2001 Elsevier Science Ltd. All rights reser ved.