Effects of trunk perturbation magnitude and direction knowledge on standing postural responses in individuals with spinal cord injury receiving epidural stimulation

Objective. Individuals with motor complete spinal cord injury (SCI) receiving tonic spinal cord epidural stimulation to facilitate standing (scES) can generate lower limb postural responses and can improve standing postural control following robotic postural training. Here, we assessed the effects of (i) trunk perturbation magnitude and (ii) participants’ knowledge of perturbation direction on standing postural responses. Methods. Six individuals with chronic motor complete SCI, already implanted with a scES unit, participated in this study. Lateral, front, and back trunk perturbations were delivered by the robotic upright stand trainer (RobUST) while participants were standing with scES and free hands. Three perturbation magnitudes (10±4% -Low, 14±4% -Mid, and 18±4%BodyWeight -High) were tested 4 times for each direction in a randomized order; perturbation direction was not disclosed. Participants also received 4 perturbations (14±4%BodyWeight) for each cardinal direction while already knowing where RobUST would pull their trunk. Trunk displacement, ground reaction forces (GRF), and EMG activity of lower limb muscles were assessed. Results. Perturbation magnitude significantly affected trunk displacement and GRFs in all tested directions (p values between 0.002 and <0.001), with larger modulations elicited by higher magnitudes. For example, during lateral perturbations, trunk displacement with Mid- and High-magnitude was 82% and 163% larger than with Low. Similarly, ipsilateral vertical GRF with Mid- and High-magnitude was 14% and 23% larger than with Low. Lower limb EMG amplitude responses were less consistent. No significant differences were observed between Low- and Mid-magnitude. Conversely, High-magnitude could promote either larger EMG responses (e.g., Tibialis Anterior, Back perturbation) or somewhat disrupted responses resulting in lower EMG amplitude (e.g., ipsilateral Vastus Lateralis, Lateral) compared to weaker perturbations. Disclosure of perturbation direction promoted significantly smaller trunk displacement in Lateral (-23%) and Back (-54%) perturbations, smaller GRF modulation in Lateral, and no effect on EMG. Conclusion. Trunk displacement and GRF showed relevant modulations that were consistent with changes in perturbation characteristics. Conversely, lower limb EMG could show opposite amplitude responses to High perturbations, and were not affected by the knowledge of their direction. These findings contribute to characterize the residual postural control potential of the spinal circuitry below the level of SCI and should be considered when defining postural training protocols.