Balance recovery from medio-lateral perturbations of the upper body duringstanding

Postural control strategies have in the past been predominantly characteriz ed by kinematics, surface forces: and EMG responses (e.g. Horak and Nashner , 1986, Journal of Neurophysiology 55(6), 1369-1381). The goal of this stud y was to provide unique and novel insights into the underlying motor mechan isms used in postural control by determining the joint moments during balan ce recovery from medio-lateral (M/L) perturbations. Ten adult males receive d medio-lateral (M/L) pushes to the trunk or pelvis. The inverted pendulum model of balance control (Winter et al., 1998, Journal of Neurophysiology 8 0, 1211-1221) was validated even though the body did not behave as a single pendulum, indicating that the centre of pressure (COP) is the variable use d to control the centre of mass (COM). The perturbation magnitude was rando m, and the central nervous system (CNS) responded with an estimate of the l argest anticipated perturbation. The observed joint moments served to move the COP in the appropriate direction and to control the lateral collapse of the trunk. The individual joints involved in controlling the COP contribut ed differing amounts to the total recovery response: the hip and spinal mom ents provided the majority of the recovery (similar to 85%), while the ankl es contributed a small; but significant amount (15%). The differing contrib utions are based on the anatomical constraints and the functional requireme nts of the balance task. The onset of the joint moment was synchronous with the joint angle change, and occurred too early (56-116 ms) to be result of active muscle contraction. Therefore, the first line of defense was provid ed by muscle stiffness, not reflex-activated muscle activity. (C) 1999 Else vier Science Ltd. All rights reserved.