Simulated effect of reaction force redirection on the upper extremity mechanical demand imposed during manual wheelchair propulsion.

Background: Manual wheelchair propulsion is associated with overuse injuries of the shoulder. Reaction force redirection relative to upper extremity segments was hypothesized as a means to redistribute mechanical load imposed on the upper extremity without decrements in wheelchair propulsion performance.

Methods: Two individuals performed wheelchair propulsion under simulated inclined (graded) conditions using self-selected control strategies. Upper extremity kinematics and reaction forces applied to the wheel were quantified and used as input into an experiment-based multi-link inverse dynamics model that incorporates participant-specific experimental results. Reaction force direction was systematically modified to determine the mechanical demand imposed on the upper extremity (elbow and shoulder net joint moments and net joint forces) during wheelchair propulsion. Results were presented as solution spaces to examine the upper extremity load distribution characteristics within and between participants across a range of reaction force directions.

Findings: Redirection of the reaction force relative to the upper extremity segments provides multiple solutions for redistributing mechanical demand across the elbow and shoulder without decrements in manual wheelchair propulsion performance. The distribution of load across RF directions was participant specific and was found to vary with time during the push phase.

Interpretation: Solution spaces provide a mechanical basis for individualized interventions that aim to maintain function and redistribute load away from structures at risk for injury (e.g. reduce demand imposed on shoulder flexors (reduce shoulder net joint moment) or reduce potential for impingement (reduce shoulder net joint force).