From simulation to stimulation: a predictive synergy-based control strategy for functional electrical stimulation...

Functional electrical stimulation (FES) cycling stimulation strategies are often derived from heuristics, or able-bodied kinematics and electromyography, which may be unsafe and non-representative for people with complete spinal cord injury. Optimally controlled predictive musculoskeletal simulations with synergy implementation offer a way to generate individualized, physiologically plausible muscle trajectories without prior experimental data. Building on a previous proof of concept, this study developed and experimentally validated a simulation-derived stimulation strategy for FES cycling.

A two-stage musculoskeletal predictive simulation framework was implemented using OpenSim Moco. First, a speed-tracking optimal control problem optimized the model's kinematics and individual muscle trajectories. Second, muscle excitations were decomposed into functional synergies using non-negative matrix factorization, producing coordinated activation patterns for quadriceps and hamstrings muscle groups. Subject-specific constraints were incorporated into the model. The resulting activation profile was then processed and converted to a crank-angle lookup table for real-time control. Experimental validation was conducted in one subject with complete spinal cord injury (SCI), comparing the simulation-derived control signal (CS) with a refined empirical bang-bang (BB) protocol used in the Cybathlon 2024 edition.

Simulations generated stable synergy-based muscle trajectories aligned with expected cycling dynamics. Experimentally, performance varied with trial order, with both CS and BB enabling unassisted FES cycling. BB produced higher average power and cadence, while CS delivered more symmetrical left-right balance and showed partial recovery in performance in higher stimulation intensities.

Although BB outperformed CS in power and cadence, the CS approach proved to be feasible and has methodological advantages. The consistent and physiologically structured behavior of the CS signal demonstrates that predictive simulation can provide a safe, reproducible baseline for individualized FES cycling control, particularly for new users or research groups.

This was the first implementation in cycling of optimal control-driven FES combined with muscle synergies, generating feasible, physiologically grounded stimulation profiles for FES cycling in an individual with complete SCI. While not surpassing a highly refined empirical strategy, the simulation-derived signal performed reliably and represents a promising foundation for model-based personalization of FES control.

Universidade de Brasília Ethics Committee, 75441323.4.0000.8093.