Feasibility of optimizing muscle activation of individuals with spinal cord injuries to reduce caregiver effort in assisted transfers

Spinal cord injury (SCI) significantly affects the quality of life of affected individuals and their caregivers. Assisted transfers after SCI are physically demanding and one of the leading causes of musculoskeletal injuries among caregivers. While mechanical lift systems can reduce injury risk, their adoption is limited by high costs, large size, and portability. Functional neuromuscular stimulation (FNS) presents a promising alternative for facilitating transfers by activating the paralyzed muscles below the level of injury. However, determining the correct and optimal stimulation profiles during transfers is challenging with heuristic methods alone. To address this, we utilized a three-dimensional musculoskeletal model in OpenSim to estimate muscle activation profiles for knee and hip extension during an assisted partial sit-to-stand (STS) maneuver of an individual with SCI by running predictive and inverse dynamics simulations with OpenSim Moco tool. To verify the simulation results, an open-loop controller delivered the predicted neural stimulation to two individuals with SCI (injury level, T10 and T8) during the STS portion of assisted transfer activities with a single physical therapist acting as a caregiver for consistency. The caregiver was able to complete the partial STS with stimulation, but could not complete the prescribed lifting maneuver without activation of the paralyzed muscles. The ground reaction forces and kinematic data from the experiment (with and without stimulation) were used in the OpenSim MocoInverse tool for inverse dynamics computation to estimate joint reaction forces of the caregiver model. Optimized stimulation assisted knee and hip extension of the participants with SCI reduced the predicted joint reaction forces experienced by the caregiver, significantly decreasing peak total shoulder joint reaction forces magnitudes by 23.8% (T10) and 33.2% (T8). For one participant (T8), predicted hip and lumbar spine joint reaction force magnitudes of the caregiver were reduced by 27.5%, and 35.3%, respectively. The results indicate that optimizing neural stimulation profiles for individuals with SCI to facilitate assisted transfers is feasible and has the potential to reduce the physical effort and therefore the likelihood of injurious joint loads experienced by the caregiver.