Function and fatigue monitoring in neuromuscular electrically‑evoked exercise after spinal cord injury

INTRODUCTION: Functional electrical stimulation (FES) can restore or augment movement in individuals with spinal cord injury (SCI), yet rapid neuromuscular fatigue limits training duration and the feasibility of closed‑loop control. AREAS COVERED: We review FES control‑system components and sensory feedback modalities used to monitor function and fatigue in SCI, synthesizing evidence from kinetic and kinematic sensors, namely joint angle, torque, center of pressure, and muscle‑level sensors such as surface/evoked electromyography, mechanomyography, ultrasound, and electroencephalography. We outline how stimulation parameters i.e. amplitude, frequency, pulse width, are modulated within open‑ and closed‑loop architectures and summarize strategies to mitigate fatigue - frequency‑aware controllers, asynchronous/multiped stimulation. EXPERT OPINION: Limited focus on fatigue is concerning, as effective fatigue management is crucial for developing closed-loop FES systems that support longer training sessions and improve long-term effectiveness and usability of rehabilitation interventions. Fatigue management must be embedded, not appended, within closed‑loop FES. Priority areas include frequency‑aware controllers, asynchronous/ multipad stimulation, and robust muscle‑state sensing, particularly mechanomyography and ultrasound, that is less vulnerable to stimulation artifacts than surface EMG.