Estimating shoulder maximum muscle forces in individuals with C5-C6 tetraplegia

Individuals with spinal cord injury (SCI) at the C5-C6 level, lose voluntary control over some of the shoulder muscles, preventing them from performing activities such as transfers or reaching above horizontal. Rehabilitation interventions such as functional neuromuscular stimulation can be used to restore lost function in this population, but due to the mechanical complexity of the shoulder, implementing such interventions is a difficult task. Musculoskeletal modeling can be used to gain a better understanding of shoulder function following cervical SCI and to aid in the design of rehabilitation interventions that restore lost function. Following spinal cord injury, some muscles will become completely paralyzed and the strength of the muscles remaining under partial voluntary control will vary across subjects depending on the level and extent of the injury. In order to reflect SCI, the musculoskeletal model needs to be customized for each individual, specifically to include maximum muscle forces. An algorithm to estimate shoulder maximum muscle forces based on experimental measurements of shoulder moments and muscle activation has been developed and applied to simulated trial data. The algorithm could successfully retrieve the maximum muscle forces from the simulated data. The effect of practical issues such as the difficulty of measuring muscle activity in deeper muscles was also evaluated.