As many as 1.2 million Americans rely on wheelchairs for mobility, and this number is increasing. Even though upper extremity (UE) musculoskeletal problems are common among wheelchair users, the scientific literature on the biomechanics of wheelchair propulsion is limited. In particular, the relationship between upper extremity loads and the risk of musculoskeletal injury is poorly understood. A systematic evaluation of the loads on the upper extremities of wheelchair users will do much to improve our understanding, treatment and ability to prevent musculoskeletal injuries in this population. In this study the investigators will use experimental techniques that they have developed and validated in conjunction with an analytical model of wheelchair propulsion that they have already developed. This combination will allow them to predict the maximum propulsion moment about the wheel axle for a given wheelchair configuration as a function of a subject's anthropometric dimensions and strength capacities at the shoulder, elbow, and wrist joints. In this study, the investigators propose to examine the effects of seat position and handrim diameter on the mechanics of wheelchair propulsion. In particular, three measures of wheelchair propulsion effectiveness will be used. These measures include the wheelchair propulsion strength ratio (WPSR), the fraction effective force (FEF), and the propulsion moment generated about the wheel axle (Mw). Each of these quantities has potential importance and will be analyzed while subjects propel a wheelchair on level ground and up a 12:1 (run: rise) ramp to address the following specific aims and hypotheses: Hypotheses 1 - The seat height will be """"""""optimal"""""""" (i.e., minimum WPSR; maximum FEF and Mw) when the elbow angle is 120 degrees (180 degrees=full elbow extension) with the hands is at the top-dead center of the handrim, (as contrasted to angles of 100 degrees and 140 degrees.) Hypothesis 2 - The neutral position of the axle (2.5cm forward of the most extreme posterior position) will be more optimal that the most posterior or anterior positions (i.e., minimum WPSR; maximum FEF and Mw). Hypothesis 3 - With the axle in the neutral position and the seat providing the optimal elbow angle, a medium handrim diameter (40cm) will be a better choice as compared to smaller (28cm) and larger (51cm) rim diameters (i.e., minimum WPSR; maximum FEF and Mw).
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