A THEORETICAL MODEL FOR THE SHOULDER GIRDLE APPLICATIONS IN SPORT

Abstract

The shoulder system is one of the most solicited in the human body due both to its large range of mobility and to the magnitude of forces which are acting over. Only to give an example, using a confirmed finite element model for the shoulder girdle (Pronk, 1990; van der Helm, 1992) during unloaded abduction-adduction movement the maximum force in the glenohumeral joint is about 400 N (!), increasing to about 1,100 N in the 10 kg loaded abduction-adduction. According to our knowledge in the literature exists a lack concerning theoretical models for this system and the existing ones are not able to explain the complete range of motion. In the same time, our experience in working with high-performance athletes, told us that many injuries appear in this joint, most of them in throwing events. In our paper we try to present an analytical model for the shoulder girdle and to predict the force distribution over the mechanism for javelin throw. The technique of a group of 6 high-performance athletes, was analyzed using common video methods and non-professional equipment. The trajectories of the collected points on video tapes are analyzed using classic interpolation methods and prepared for model input. In the present model we have eliminated the hypothesis of closed-chain mechanism and we have replaced the multi degrees of freedom joints by collection of one degree of freedom joints using the Gruebler-Harisberger equivalence theorem. The weakest point of this model is to specify the muscle forces and to introduce them in the model. After that it was followed the classic method for positional, kinematic and dynamic analyzes, using a matrix method based on Denavit-Hartenberg matrix representation. The comparison between these two methods- theoretical and collected on videa tapehas provided good results for position and kinematic analyzes, but not reliable for dynamic analyzes. The advantage of such model could be that in this way it is possible to simulate all types of movements and it is possible to predict the sportive behavior to abnormal solicitations.