Postural adjustments to voluntary head motions during standing are modified following stroke

Abstract

Objective. To compare postural adjustments to voluntary head motions during standing between stroke and healthy subjects. Design. Quasi-experimental design involving two groups: stroke patients ( n=8) and age-matched healthy controls ( n=5). Background. Massive sensory stimulation from the visual, vestibular and neck proprioceptive systems is conveyed during a voluntary head motion. It remains to be investigated how stroke patients integrate this sensory information and generate appropriate postural adjustments to maintain standing balance. Methods. Eight stroke patients and five healthy age-matched subjects performed fast head motions (up, down, right, left, or none) in random sequence during standing. Body kinematics in 3-D, horizontal displacement of the center of pressure and the body center of mass, as well as bilateral vertical ground reaction forces and activation of tibialis anterior, medial gastrocnemius, vastus lateralis and semitendinosus muscles, were acquired and analyzed. Results. Stroke patients moved their heads at lower velocities while maintaining similar range of motion as compared to healthy subjects. Stroke patients showed larger difference in center of pressure and center of mass excursions and larger root-mean-square values for center of pressure and center of mass displacement and velocity during all directions of head motions. Stroke patients presented disrupted loading patterns to avoid weight bearing of the paretic limb, in contrast to the typical and direction-specific loading patterns observed in healthy subjects during head motions. Conclusions. Stroke patients manifest altered postural adjustments to voluntary head motions during standing. It suggests that sensorimotor integration processes for stance and balance are impaired by a stroke. Relevance Head motions are expected to produce little displacement in body center of mass, as the mass of the head is relatively small as compared to the rest of the body. However, the complex information channeled from vestibular, visual and neck proprioceptive afferents must be processed by the central nervous system in relation to the motor commands, such that head motions can be executed smoothly and rapidly while maintaining standing balance. The altered postural adjustments to head motions observed in stroke patients suggest an underlying impairment in sensorimotor integration.