A biomechanical model to quantify shoulder load at the work place.

A simple two-dimensional biomechanical model of the shoulder has been developed to quantify shoulder muscle load, joint load and endurance time in work situations. The model is applicable to the analysis of working postures requiring elevated upper arm positions in the plane of the scapula, with the trunk upright, and the elbow flexed at 90°. The only input variable of the model, to be measured in the work situation, is the angle between the upper arm and the vertical line. In addition, the total body weight and the upper arm length should be known. For a given arm position, the model predicts the individual forces in the deltoid and supraspinatus muscles, and the reaction force at the glenohumeral joint. Furthermore, the model estimates the endurancetime of the given arm position, which is the period of time until the shoulder muscles become fatigued. The results show that maximum muscle forces occur at 80° of arm elevation, when the deltoid force is about 22% of its maximum force, and the supraspinatus 25%. The corresponding glenohumeral joint force is about 43% of total body weight. At 80° of arm elevation, the endurance time is about 5 minutes. If a working posture is maintained for more than 1 hour, the model suggests that the elevation angle between the upper arm and the trunk should be less than 15°, in order to prevent muscular fatigue in the shoulder muscles.