Femoral loads during passive, active, and active–resistive stance after spinal cord injury: a mathematical model

Objective. The purpose of this study was to estimate the loading environment for the distal femur during a novel standing exercise paradigm for people with spinal cord injury. Design. A mathematical model based on experimentally derived parameters. Background. Musculoskeletal deterioration is common after spinal cord injury, often resulting in osteoporotic bone and increased risk of lower extremity fracture. Potential mechanical treatments have yet to be shown to be efficacious; however, no previous attempts have been made to quantify the lower extremity loading during passive, active, and active–resistive stance. Methods. A static, 2-D model was developed to estimate the external forces; the activated quadriceps forces; and the overall bone compression and shear forces in the distal femur during passive (total support of frame), active (quadriceps activated minimally), and active–resistive (quadriceps activated against a resistance) stance. Results. Passive, active, and active–resistive stance resulted in maximal distal femur compression estimates of not, vert, similar45%, not, vert, similar75%, and not, vert, similar240% of body weight, respectively. Quadriceps force estimates peaked at 190% of body weight with active–resistive stance. The distal femur shear force estimates never exceeded 24% of body weight with any form of stance. Conclusions. These results support our hypothesis that active–resistive stance induces the highest lower extremity loads of the three stance paradigms, while keeping shear to a minimum.