Standing with functional neuromuscular stimulation: effect of foot placement and feedback variables

The long-term goal of this investigation is to enhance the functional abilities of individuals that stand using functional neuromuscular stimulation (FNS). A target-acquisition paradigm was used to evaluate two postural variables, center of pressure (COP) and the position of the pelvis (POP) for the sensing of postural adjustments. These variables were examined under two different foot placements: normal and modified tandem stance. Subjects with spinal cord injury (SCI) stood using FNS and made postural adjustments using voluntary upper body control to move a cursor that represented either their COP or POP to a given target displayed on a monitor. Performance and strategy indexes were compared for four stance conditions. Results indicate that the POP feedback provided improved quality of control for many movement directions. Measurements of reaction forces at the hands did not indicate consistent changes across stance conditions. When using the POP feedback, subjects effectively utilized a load/unload strategy in which weight was shifted from one foot to the other in order to execute the posture shift. Thus, sensing and controlling the translation of the pelvis may prove to be a useful approach to enhance the functionality of FNS standing systems.