Center of pressure measures to assess standing performance

Functional neuromuscular stimulation (FNS) systems have provided individuals with spinal cord injury (SCI) varying levels of standing capability. Our long-term goal is to enhance an FNS user's ability to perform activities of daily living by providing sufficient standing stability. However, control of standing posture with complete SCI is compromised by a lack of sensory feedback from receptors in the feet. The goals of this study were: a) to document a variety of center of pressure (COP) measures during standing, b) to compare able-bodied and FNS standing, and c) to relate various measures to overall quality of standing performance. Two subjects with complete SCI that stand with FNS (one with a surface system and one with an implanted stimulation system) and five able-bodied subjects stood quietly on force platforms. Reduced hand support forces were coincident with anterior/posterior (A/P) mean COPs that were further posterior in the base of support and medial/lateral (M/L) mean COPs that were more centered between the feet. In addition, M/L COP excursions, maximum A/P COP velocities, and maximum M/L COP velocities were all significantly greater for FNS users than for able-bodied standers. Furthermore, maximum A/P COP velocities and mean A/P COP frequencies were significantly greater during FNS standing trails that required increased support from the hands. COP measures such as these may be useful tools to assess standing performance and to determine strategies for adjusting posture that can reduce an FNS user's reliance on upper extremity support while maintaining stable balance.