PUSHRIM FORCES AND JOINT KINETICS DURING WHEELCHAIR PROPULSION

Objective: To investigate pushrim forces and joint kinetics during whe elchair propulsion and to discuss the differences between inexperience d and experienced wheelchair users. Design: Cohort study. Setting: Hum an engineering laboratory at a state university. Subjects: Four men wh o use manual wheelchairs for mobility and four nondisabled men who did not have extensive experience pushing a wheelchair; all subjects were asymptomatic for upper extremity pain or injury. Methods: Subjects pu shed a commonly used wheelchair fitted with a force-sensing pushrim on a stationary wheelchair dynamometer. Video and force data were collec ted for 5 strokes at one speed of propulsion. Pushrim forces and net j oint forces and moments were analyzed. Main Outcome Measures: Pushrim forces, radial (F-r) and tangential (F-t), were analyzed and compared for both groups in relation to peak values and time to peak values and as ratios of overall forces generated. Net joint forces and moments w ere analyzed in a similar fashion. Results: Pushrim forces and joint m oments were similar to those previously reported, with radial forces a veraging between 34 and 39N and tangential forces ranging on average b etween 66 and 95N. Tangential forces were higher than radial forces, a nd mean ratios of tangential forces to the resultant force were approx imately 75%, whereas mean radial force ratios were approximately 22%. All subjects showed higher joint moments at the shoulder than at the e lbow or wrist. A large component of vertical reaction force was seen a t the shoulder. Significant differences (p < .05) were found between g roups for peak tangential force and time to peak tangential and peak v ertical forces, with wheelchair users having lower values and longer t imes to reach the peak values. Conclusions: Discrete variables from th e force-time curves can be used to distinguish between wheelchair user s and nonusers. The experienced users tended to push longer, used forc es with lower peaks, and took longer time to reach peak values. This p ropulsive pattern may have been developed to reduce the chance of inju ry by minimizing the forces at the joints, as a means of maximizing ef ficiency or as a combination of these factors. More work investigating 3-dimensional forces and the influence of seating position and variou s conditions of propulsion such as speed changes, ramps, and direction al changes on injury mechanisms needs to be completed. (C) 1996 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation