Personalized mapping of inhibitory spinal cord circuits in humans via noninvasive neural decoding and in silico modeling

Studying human motoneuron activity through electromyography (EMG) can yield insights into the operation of fundamental spinal cord microcircuits. Traditional surface and needle EMG methodologies have limited capacity to shed light on the diversity of motor unit (MU) control strategies that may be unique to each individual. Here, we used high-density surface EMG (HDsEMG) to sample multiple MUs per participant to investigate the features of inhibitory spinal microcircuits in both upper and lower limb control. We characterized the net inhibition as a function of individual MU firing rates, revealing participant-specific relationships. In silico modeling replicated these experimental characteristics and suggested that properties of the inhibitory currents rather than motoneuron size are responsible for net functional inhibition. Our results show that HDsEMG can highlight distinct control strategies across circuits and motor pools, revealing participant-specific properties of inhibitory spinal microcircuits.