Improvements, Validation and Adaptation of a Shoulder Model

Doctoral thesis, 1999

A series of biomechanical investigations aimed at contributing to the understanding of shoulder function and improvements of a computer based shoulder model is presented. In the thesis the structure of shoulder models is revised. The problem of validation of biomechanical models is discussed in view of the fact that undisturbed muscle forces in living subjects cannot be measured. A new tool for development as well as validation purposes is presented and applied to the Gothenburg shoulder model. The tool consists of equipment making it possible to measure strength profiles, i.e. the maximal force a subject can produce in all directions in a plane perpendicular to the upper arm. Such profiles are closed (convex) diagrams, dependent in shape on humeral orientation. The information contained in such measured profiles is very large, which may be employed in validation but also in adjusting model parameters to different individuals or population groups. Roles of passive elements, control objectives and constraints imposed by external influences have also been investigated. It was found that the anterior deltoid muscle, known to be active in forward flexion, place a major part in stabilising the joint between the clavicula and the shoulder blade. The modelling of the coracoclavicular ligaments presented in the thesis shows that these ligaments are similarly very effective in this stabilisation. Since maximal strength is independent of control objectives, strength profiles were used to develop the shoulder model, and it was shown feasible to adjust muscle parameters to individuals or populations. Another important improvement presented concerns the interplay between the orientations of the shoulder bones (the shoulder rhythm). The rhythm was previously extrapolated outside of the range where it had been measured. Implementation of ligament properties and attachment sites made it possible to improve on this extrapolation. Two sets of experiments have been designed to study of the (controlled) end point stability of the hand operating a handheld tool with different requirements on precision. The results from the different set-ups were consistent, showing that shoulder muscle tension was unchanged or slightly decreased, the spontaneous vibration spectrum narrowed and acceleration levels increased as higher precision was demanded.