To determine how arm movements influence postural sway in the upright position after stroke, interactions between arm, trunk, and center of pressure (CoP) displacements in the sagittal direction were investigated in participants with hemiparesis and healthy subjects. Participants swung both arms sagittally in either of 2 directions (in-phase, anti-phase) and at 2 speeds (preferred, fast) while standing on separate force plates. Variables measured included amplitude and frequency of arm swinging, shoulder and trunk range of motion, CoP displacements under each foot and of the whole body, and the relationships between the arm, trunk, and CoP displacements. CoP displacements under the non-paretic leg were greater than those under the paretic leg, which may in part be related to the larger amplitude of swinging of the non-paretic arm. CoP displacements under each foot were not related to arm swinging during in-phase swinging at the preferred speed in healthy subjects. When speed of arm swinging was increased, however, the CoP moved in a direction opposite to the arm movement. In contrast, in individuals with hemiparesis, CoPs and arms moved in the same direction for both speeds. During anti-phase swinging in healthy subjects, the trunk counterbalanced the arm movements, while in participants with hemiparesis, the trunk moved with the affected arm. Results show that stroke resulted in abnormal patterns of arm-trunk-CoP interactions that may be related to a greater involvement of the trunk in arm transport, an altered pattern of coordination between arm and CoP displacements, and an impaired ability of the damaged nervous system to adapt postural synergies to changes in movement velocity.
K.I. Ustinova is with the Research Institute of Neurology in the Russian Academy of Medical Sciences, 123098, Moscow, Russia. V.M. Goussev, R. Balasubramaniam, and M.F. Levin are with Center for Interdisciplinary Research in Rehabilitation (CRIR), 6300 Darlington, Montreal, Quebec, H3S 2J4, Canada. In addition, Ustinova and Levin are with the School of Rehabilitation at the University of Montreal, Montreal, Quebec, Canada, and Balasubramaniam is with Sensory Motor Neuroscience in the School of Psychology at the University of Birmingham, UK.