Spatial and temporal relation of kinematics and muscle activity during unstable sitting

Recent work suggests that kinematics-based electrical stimulation may restore dynamic trunk stability following spinal cord injury. However, to ensure fatigue-resistant control, knowledge of the relation between body motion and the activity of relevant muscles during non-impaired, unstable sitting may be beneficial. Therefore, our objective was to quantify the spatial and temporal relationships between (1) characteristic angular kinematics (i.e., the kinematics characterizing trunk stabilization) and (2) trunk and upper leg muscle activity in unstable sitting as elicited via a wobble board. Wobble board motion and bilateral electromyograms from fourteen trunk and upper leg muscles were recorded in fifteen non-disabled participants (i.e., individuals with no history of neurological or musculoskeletal impairments or pain, gait or balance difficulties, or use of a walking aid) sitting on a wobble board. The relationship between wobble board tilt and the electromyograms was quantified using cross-correlation analysis. During unstable sitting, the trunk was stabilized through direction-specific activation of the trunk and upper leg muscles, preceding wobble board displacement by 110-230 ms. Direction-specific activation implies the presence of active neural control, while preceding activation may be needed to account for known torque generation time delays. Furthermore, the specific muscles activated for each direction of wobble board displacement suggest the use of stiffness control in the anterior-posterior, but not medial-lateral direction. Future work will use the gained insights in defining the muscle stimulation patterns of kinematics-based neuroprostheses that can restore trunk stability following impairment.