Biomechanical model of the shoulder joint
A biomechanical model of the glenohumeral joint has been developed to investigate muscle and joint loading during real life three-dimensional activities. Based on a rigid body mechanics approach, the model incorporates algorithms to correct for curved muscle paths and bone geometry, providing realistic muscle orientation over a wide range of limb positions. An optimization routine has been incorporated, minimizing overall maximum muscle stress in the 26 individual muscle elements considered. The model utilizes anatomical muscle and bone data, subject anthropometric data, kinematics measured using a 6 camera Vicon motion analysis system and hand loading measured using a force-plate and mobile six-component strain gauged force transducer developed for this project. Model stability and sensitivity to input data uncertainties have been investigated. Data used for this was actual subject activity data. Random uncertainties of a known statistical distribution were generated using a Monte Carlo data perturbation technique and superimposed on the subject data. No model instability or unacceptable error magnification was demonstrated in this investigation. A study of real life three-dimensional activities has been conducted using five male subjects. Normalized, averaged muscle and joint forces were calculated for each activity. Using the same five subjects, electromyographic (EMG) muscle activation was measured for the same five activities. Both surface and intra-muscular fine wire electrode techniques were used. Eight muscles including infraspinatus, subscapularis and supraspinatus were instrumented. The resulting EMG data was normalized and averaged for each activity. Muscle activation appears in good agreement with published EMG and our own EMG study. Overall joint compressive and shear forces of up to 7 and 2 times body weight respectively have been calculated. Results of the study indicate glenohumeral joint forces for athletic activities can be as high as 7 times those forces previously predicted in other studies for simple abduction and flexion.