Scapulohumeral rhythm: relationship between motion velocity and rhythm.

The relative contributions of scapulothoracic and glenohumeral motion at different rates of shoulder motion were studied through adduction to abduction in the scapular plane. Nineteen shoulders of 10 healthy individuals (all men, 24-30 years of age) were analyzed using an image intensifier and a high-resolution digital video system. High- and low-speed motion consisted of 2 and 4 seconds per one cycle, respectively, from abduction to adduction in the scapular plane. Glenohumeral and scapulothoracic ratios were fixed at low speed and these results agree with the finding of other researchers. Ratios at high speed were not fixed and differed significantly from those at low speed. Ratios were high at the beginning of abduction or adduction and at angles beyond 40 degrees abduction, then decreased according to the arm movement. Glenohumeral motion at high speed was more dominant at the beginning of abduction or adduction beyond the setting phase, then became less dominant according to the arm movement, compared with the motion at low speed.