The activity in the three regions of the trapezius under controlled loading conditions—an experimental and modelling study

Background. There is a degree of conflict in the literature regarding the biomechanical role of the three regions of trapezius. It is suggested that some of this may result from a lack of experiments using a well defined loading regime. Objectives. To study the activity of the three regions of trapezius under controlled load in order to gain further insight into its role in shoulder biomechanics. Design. The emg activity of the three regions of trapezius was studied on five subjects using a specially designed test system which allowed the application of either a uniaxial shrug force or a pure couple in the coronal plane. Methods. Test rigs were designed and constructed to measure isometric loads applied by shoulder shrugging and isometric pure moments produced in coronal plane abduction and adduction. Surface emg of the three regions of trapezius was performed simultaneously with loading. Results. The activity of upper trapezius was always present but changed little with the type of loading. Middle trapezius was dominant during both shrug and abduction whereas the lower fibres predominated during adduction. Conclusions. The use of a test system applying defined loading at the shoulder allows the detail analysis of trapezius muscle activity. The reproducible technique can be extended to other shoulder muscles.