The shoulder plays a very important role during manual wheelchair propulsion. Unfortunately, substantial numbers of manual wheelchair users eventually develop shoulder injury or pain. Recently, studies have begun to investigate the etiology of wheelchair user shoulder injuries. This study compared three coordinate systems used to represent the shoulder during wheelchair propulsion. Our results show statistically significant differences between the three shoulder representations analyzed. Differences are seen for individuals and for the subjects as a group. Based upon our results, the fixed-z model appears preferable over the other representations due to its simplicity, low hardware requirements, and the similarity of the results to the free representation. This article also provides some insight into maximal shoulder joint forces and moments recorded during manual wheelchair propulsion. Future work should include more sophisticated models of the shoulder complex.