3-DIMENSIONAL PUSHRIM FORCES DURING 2 SPEEDS OF WHEELCHAIR PROPULSION

Upper limb pain frequently occurs in manual wheelchair users. Analyzin g the pushrim forces and hub moments occurring during wheelchair propu lsion is a first step in gaining insight into the cause of this pain. The objectives of this study were as follows: to describe the forces a nd moments occurring during wheelchair propulsion; to obtain variables that characterize pushrim forces and are statistically stable; and to determine how these variables change with speed. Convenience samples (n = 6) of paralympic athletes who use manual wheelchairs for mobility and have unimpaired arm function were tested. Each subject propelled a standard wheelchair on a dynamometer at 1.3 and 2.2 m/s. Biomechanic al data were obtained using a force-and moment-sensing pushrim and a m otion analysis system, A number of variables that describe the force a nd moment curves were evaluated for stability using Cronbach's alpha. Those measures found to be stable (alpha > 0.8) at each speed were the n examined for differences associated with speed. The tangential, radi al, and medial-lateral forces were found to comprise approximately 55, 35, and 10% of the resultant force, respectively. in addition to dura tion of stroke and propulsion, the following variables were found to b e stable and to differ with speed (1.3 m/s +/- SD; 2.2 m/s +/- SD): pe ak force tangential to the pushrim (45.9 +/- 17.9 N; 62.1 +/- 30 N), p eak moment radial to the hub (9.8 +/- 4.5 N.m 13.3 +/- 6 N.m), maximum rate of rise of the tangential force (911.7 +/- 631.7 N/sec; 1262.3 /- 570.7 N/sec), and maximum rate of rise of the moment about the hub (161.9 +/- 78.3 N.m/s; 255.2 +/- 115.4 N.m/s). This study found stable parameters that characterize pushrim forces during wheelchair propuls ion and varied with speed, Almost 50% of the forces exerted at the pus hrim are not directed toward forward motion and, therefore, either app ly friction to the pushrim or are wasted. Ultimately this type of inve stigation may provide insight into the cause and prevention of upper l imb injuries in manual wheelchair users.