Relationship between resultant force at the pushrim and the net shoulder joint moments during manual wheelchair propulsion in elderly persons

Objective: To determine the relationship between the resultant force at the pushrim and the net shoulder joint moments during manual wheelchair propulsion in elderly persons. Design: Convenience sample. Setting: Motion analysis laboratory. Participants: Older manual wheelchair users (N = 14; age, 68.2 +/- 5.2y) were tested. Interventions: Kinematic and kinetic data were collected during manual wheelchair propulsion at a speed between 0.96 and 1.01m/s for 10 seconds and at a power output around 22.4W on a wheelchair ergometer. Main Outcome Measures: Net shoulder joint moments were computed with an inverse dynamic model. The mechanical use of the forces at the pushrim and the mechanical fraction of effective force were measured during propulsion. Results: Mechanical use and mechanical fraction of effective force had a positive and significant correlation with the net internal (P<.05) and external (P<.001) shoulder rotation moment, the net flexion (P<05), and extension (P<001) moment in the sagittal plane, and the net flexion (P<.001) moment in the horizontal plane. Conclusions: The results suggest that because the resultant force at the pushrim has a greater tangential component and a greater proportion of the maximal voluntary force, most of the net moments around the shoulder are higher. Thus the optimal way of propelling, from a mechanical point of view (ie, tangential), may not be advantageous for manual wheelchair users.