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

Functional electrical stimulation (FES) cycling is a rehabilitation exercise that improves health outcomes following spinal cord injury. However, the ability of FES cycling to promote positive bone changes remains ambiguous, which might be caused by inappropriate joint loading. Using a neuromusculoskeletal model, we estimated hip and knee moments, and joint contact forces of uninjured subjects using three different simulated FES regimes with increasing levels of personalization and compared them to voluntary cycling. The simulated FES regimes included a generic pattern commonly used clinically and two patterns based on a neuromusculoskeletal optimization approach; all FES regimes used quadricep, hamstrings, and glute muscle stimulation. For the hip contact force orientation, all three FES regimes were significantly different to the voluntary solution over most of the crank cycle, but generic FES was most different, also resulting in hip edge loading. Similarly, among the evaluated FES regimes, the generic pattern produced the hip and knee contact force magnitudes that were most different to voluntary cycling. Overall, the results suggest that FES regimes could be personalized to the individual to optimize both cycling performance and joint loading and could thereby be tuned to create mechanical stimuli aimed at promoting positive bone adaptations.