Effect of start-up kinetics on wheelchair pushrim dynamic analysis

Many investigators have examined pushrim forces and moments and joint kinetics during wheelchair propulsion, but rarely are start-up kinetics performed in the analysis. Kinetic data were collected as nine spinal cord injured experienced wheelchair users propelled their wheelchairs with instrumented pushrims on a stationary dynamometer. The subjects pushed from rest to at least 1.8 meters/second (m/s) for 20 seconds. A paired t-test was used to determine significant differences between steady state propulsion and start-up propulsion. Significant increases (p<0.05) in Fx (horizontal force), Fy (vertical force), Mz (moment about the wheelchair wheel axis), and F (three-dimensional resultant force) were found between start-up and steady state propulsion. Steady state analysis of wheelchair biomechanics will underestimate the magnitudes of forces and moments at the pushrim. Investigation of wheelchair propulsion biomechanics as a risk for injury should include start-up kinetics.