Robotic trunk perturbation magnitude affects standing postural responses in individuals with motor complete spinal cord injury receiving epidural stimulation

After a severe spinal cord injury (SCI), the human spinal circuitry receiving epidural stimulation can generate lower limb postural responses to sensory inputs associated with trunk perturbation. Here, we assessed the effects of different trunk perturbative forces on standing postural responses in six individuals with chronic, motor complete SCI receiving epidural stimulation to facilitate standing. The robotic upright stand trainer (RobUST) provided constant assistance for pelvic control and delivered precise trunk perturbations with different magnitudes (10 ± 4%BodyWeight (BW) -Low, 14 ± 4%BW -Mid, and 18 ± 4%BW -High) and directions (left, right, front, back). Trunk excursion, ground reaction forces (GRF), and electromyography (EMG) amplitude were assessed. Perturbation magnitude significantly affected trunk excursion and GRFs in all tested directions (p ≤ 0.002), with larger modulations elicited by higher perturbation magnitudes. Lower limb EMG amplitude responses were less consistent. In some instances, perturbation magnitude did not affect EMG amplitude. In others, high-magnitude perturbations promoted larger EMG amplitude modulation compared to weaker perturbations, resulting in higher or lower levels of activation depending on muscle and direction. These findings contribute to characterize the residual postural control potential of the human spinal circuitry below the level of SCI, which should be considered when defining postural training protocols with spinal cord neuromodulation.