Postural responses during standing in subjects with spinal-cord injury

The purpose of this study is to establish the arm as a postural limb in subjects with spinal cord injury (SCI) who stand using their arms for balance. This study examines whether repeatable responses are elicited in the arm muscles of subjects with SCI while standing and performing cued arm raises, and whether the muscle activity results in adequate postural correction as compared to able-bodied standers. Four subjects with thoracic-level SCI performed standing trials using orthoses or functional neuromuscular stimulation and while using one hand for balance. Under two stereotypical conditions (quiet standing and rapid arm raises in response to a cue), EMG responses and force-plate sway measures were obtained. During the arm-raise trials the EMG signals from the muscles of the support arm were analysed for their onset times, revealing consistent sequences of muscle activity for a given subject, irrespective of the mode of standing, but showing variability from person-to-person. These results were interpreted as ‘personal strategies’ for postural control. These strategies involved choosing an initial body position and maintaining a level of cocontraction of the wrist and elbow musculature to provide stiffness about the joints. The effectiveness of these postural strategies was quantified by looking at the mean velocity of sway of the centre of pressure. These results were compared to 15 young, able-bodied subjects. Subjects with SCI showed mixed degrees of postural stability as compared to the ablebodied subjects. Factors that may contribute to the effectiveness of fine postural control are discussed, including the lack of sensory feedback from the lower extremities, the possible biomechanical disadvantage of the arm in providing precorrecting movements of the body, and the comfort level exhibited by the subjects.