Effect of system tilt and seat-to-backrest angles on load sustained by shoulder during wheelchair propulsion

This study determined the effect of system tilt angle (STA) and seat-to-backrest angle (SBA) changes on the load sustained by the shoulder during manual wheelchair propulsion. Fourteen elderly participants (mean +/- standard deviation age 68.2 +/- 5.2 years) were recruited. Combinations of three STAs (0 degrees, 5 degrees, and 10 degrees) and three SBAs (95 degrees, 100 degrees, and 105 degrees) were randomly tested. The initial position of the wheel axle was held constant with respect to the participant's shoulder position in each condition (horizontal: 4 cm forward of shoulder, vertical: 110 degrees to 120 degrees elbow extension). The shoulder load was estimated by the joint moments. The analysis did not reveal any significant differences between shoulder joint moments (average and peak) for the various STA and SBA combinations. Changing the seat angle while keeping the wheel-axle position constant maintained the shoulder load at the same level. Thus, seat angle can be determined with the goals of user comfort and pressure modulation at the seat interface for alleviating pressure ulcers without increasing risk of overuse shoulder injuries.