Summary

Recent epidemiologic literature reviews show that working with the arms raised increases the likelihood of developing musculoskeletal disorders (MSD) of the shoulders. Although the origin and mechanisms of shoulder MSD have not been demonstrated in the scientific literature, performing a task with the arms up increases the load on the shoulder muscles. However, there is little to explain why this raised arm posture is awkward or how it affects the upper limb in movement. Musculoskeletal modelling of the human body is now an effective and non-invasive way to obtain information on the internal forces applied to the muscles, tendons and bones.

The overall objective of this research project was to develop (1) a kinematic model and (2) a musculoskeletal model of the shoulder girdle and arm in order to study shoulder biomechanics during handling and (3) to test its sensitivity to task conditions and masses. Laboratory simulations of several handling tasks were done to evaluate musculoskeletal stress on the shoulder during such tasks. Eighteen subjects moved instrumented boxes of three different masses (6 kg, 12 kg and 18 kg) over the same distance at hip, shoulder and eye level. The first part of the report contains a kinematic analysis describing the joint strategies used by the subjects. Increased box mass produces more efficient motor strategies in the subjects, such as bringing the load to be lifted closer to the trunk during the movement. Lifting a box to eye level when moving a load increases the shoulder girdle’s involvement in the movement compared to lifting it to shoulder level. The height of the box to be handled also affects glenohumeral elevation.

In the second part of the report, a musculoskeletal model was developed for a preliminary estimate of the internal forces applied to the upper limb when the box is moved. The model is sensitive to load mass variations and the different stages of the movement. However, the specific tasks performed do not seem to change the maximum forces and muscle activations. The last part of the report describes the electromyographic (EMG) activity of ten shoulder girdle and arm muscles. This experimental EMG measurement identified the main muscles involved in moving a box. For instance, the biceps were not affected by the final height of the load to be lifted, but by increased box mass. On the other hand, the upper trapezius seems to be decisive when the handling tasks require lifting a box to a significant height, although the role it plays in this kind of task has yet to be documented. In addition, EMG activity of the anterior and median deltoids is significant regardless of box height and mass. One recommendation that can be made on the basis of this report is that a task that involves moving a 6 kg or 12 kg box between hip and shoulder level should be preferred to the same movement between shoulder and eye level. Taking a posture with the elbows bent and the load held as close to the trunk as possible should be encouraged until research has validated better techniques for preventing shoulder injury when transferring a load at a height.