Selection of functional electrical stimulation patterns affects hip and knee mechanical loads during semi-recumbent cycling

Functional electrical stimulation (FES) cycling improves health outcomes following spinal cord injury. However, the ability of FES cycling to improve bone mineral density remains unclear, which might be caused by inappropriate loading of the joints and underlying bones. A neuromusculoskeletal model was used to estimate hip and knee joint contact forces in uninjured participants using three different simulated FES regimes and compared them to voluntary cycling. The simulated FES regimes included a generic pattern commonly used clinically and two patterns based on neuromusculoskeletal optimization approaches. Compared to voluntary cycling, all three FES regimes generated significantly different hip contact force orientations over most of the crank cycle. Notably, generic FES was most different to voluntary cycling, producing a mean difference of 48.34 degrees in orientation over crank angles 0-43° and 81-360° (P < 0.001), produced hip edge loading, and large differences in hip (mean difference -0.583 body weight, 90-122° and 237-331°, P < 0.001) and knee (-0.348 body weight, 75-116° and 205-267°, P < 0.001) contact force magnitudes. These results suggest FES regimes could and should be personalized to the individual to optimize cycling performance, joint loading, and mechanical stimuli aimed at promoting positive bone adaptations.