Effects of stimulated hip extension moment and position on upper-limb support forces during FNS-induced standing - A technical note

This study explores the effects of active hip extension moment produced by electrical stimulation on the support forces the arms must exert through an assistive device during quiet erect standing with functional neuromuscular stimulation (FNS) in individuals with spinal cord injuries (SCI). A static sagittal plane biomechanical model of human standing was developed to pred ict the effects of stimulated hip extension moment and sagittal plane hip a ngle on the arm support necessary to maintain an upright posture. Two indiv iduals with complete thoracic SCI were then tested while they stood with co ntinuous stimulation to the knee and trunk extensors. The steady-state acti ve extension moment exerted at the hip was varied by activating different c ombinations of hip extensor muscles with continuous stimulation while stead y-state support forces applied to the arms and feet during standing were me asured. The steady-state support forces imposed on the arms during quiet st anding decrease with increased stimulated hip extension moment and are high ly dependent upon hip flexion angle, as predicted by the biomechanical simu lations. Experimentally, the combination of gluteus maximus and semimembran osus stimulation produced three times more steady-state hip extension momen t than did stimulation of the gluteus maximus and adductor magnus. This res ulted in a ten-fold decrease in body weight supported on the arms. More ver tical postures (smaller hip flexion angles) improve the effectiveness of th e hip extensor muscles in reducing the support forces placed on the arms. A single Newton-meter of stimulated hip extension moment with the hips fixed at 5 degrees of flexion results in almost five times the reduction in arm support forces as with the hips at 20 degrees. To minimize the forces appli ed by the arms on an assistive device for support while standing with FNS, these preliminary results suggest that (1) efforts should be made to assume the most erect postures possible and (2) muscles and stimulation paradigms that maxiize active hip extension moment should be chosen.