Load on the shoulder in low intensity wheelchair propulsion

Objective. To assess the mechanical load on the glenohumeral joint and on shoulder muscles during wheelchair propulsion at everyday intensities. Design. Model simulations based on experimental input data Background Virtually nothing is known about the mechanical load on the upper extremity during wheelchair propulsion. Hand rim wheelchair propulsion is a significant risk factor for shoulder pain and injury among wheelchair users. A musculoskeletal model of the upper extremity during wheelchair propulsion will quantify the stresses placed on anatomic structures and may provide insight into the source of symptoms and injuries. Methods. Three experienced wheelchair users underwent wheelchair exercise tests at combinations of two load levels (10 and 20 W) and two velocities (0.83 and 1.39 m(.)s(-1)) during which input data were collected for a musculoskeletal model of the upper extremity. The model was then used for the estimation of the glenohumeral contact force, as well as individual muscle forces. Results, Peak glenohumeral contact forces were between 800 and 1400 N (100-165% body weight) and differed significantly between load levels. Averaged over the push phase, these forces were 500-850 N. In absolute terms the m. deltoideus and rotator cuff muscles were highly active (>100 N). In relative terms the load on the m. supraspinatus was high, with peak values of over 50% of its maximum attainable force. Conclusions, Low intensity wheelchair propulsion does not appear to lead to high contact forces. The muscle forces in the rotator cuff and especially in the m. supraspinatus are high. This might indicate a risk for muscle damage and the subsequent development of shoulder complaints. such as rotator cuff tears.