Kinematics of shoulder abduction in the scapular plane. On the influence of abduction velocity and external load.

In this paper the kinematics of arm abduction in the scapular plane of 38 healthy experimental subjects is reported. The ratio of the glenohumoral and the scapulothoracic components of the motion were determined and the influence of the abduction speed and of the external load on it were investigated. The investigation of the effect of abduction speed involved all 38 subjects. Each of them performed one slow and one fast abduction. Statistical analysis showed that there were large differences between individuals, but that for any one individual the abduction process is essentially reproducible. For the one individual there is a strong, linear relationship between glenohumeral and scapulothoracic rotation. The slope of the regression of the glenohumeral component on total arm abduction varied from 0.75 to 0.5 with a sample mean of 0.66. It means that only two-thirds of arm abduction occurs in the glenohumeral joint, the remaining third taking place via scapular rotation. In slowly performed abductions the slope of the regression was significantly greater than in the high-speed movements, but differences were very small. In the investigation of the effect of external load, statistical analysis indicated that the slope of the intraindividual regression is largely independent of the load. The possibility of observer bias was analysed, too. It was found that the standard deviation of the abduction parameters determined by different observers was between 6 and 10% of that between different experimental subjects. RELEVANCE: A knowledge of the kinematics of the shoulder joint is necessary to understand subacromial pathology and in particular impingement problems. The scapulohumeral rhythm depends on the balanced and coordinated function of the muscles involved. The activation pattern of the abductor muscles, as reflected in shoulder kinematics, seems to be individual and stored as an engram.