The aim of this study was to determine, by simulation on real data, the effect of modifying the direction or effectiveness of a given force amplitude on the load sustained by the shoulder estimated by joint forces and moments. Kinematics and kinetics data were recorded on 14 manual wheelchair users 68.2+/-5.2 years for 10 s at sub-maximal speed (0.96-1.01 m/s). The simulation consisted in modifying force effectiveness at the pushrim while maintaining the same initial force amplitude. Shoulder kinetics were computed for simulated resultant forces from radial to tangent directions and also for initial force effectiveness. The results show that as the force was simulated tangent to the wheel, there was a significant increase in the average proximal and anterior shoulder joint forces. Also, significant increases in average internal rotation, flexion in the sagittal and horizontal plane moments were reported. Higher shoulder kinetics could accelerate the onset of fatigue and increase the risk of injury. A single-case analysis revealed an improvement window for force effectiveness ( approximately 10%) in which shoulder kinetics were not substantially increased. Our results provide useful information on what would happen to shoulder kinetics if we were able to teach manual wheelchair users to modify their force pattern at the pushrim. The results suggest that for an elderly population, it is not wise to aim at producing a mechanically optimal resultant force at the pushrim (i.e., tangent). Smaller increases of the initial force effectiveness would be preferable.