Biomechanics Blog Post #3

The clinical relevance of the Scapulohumeral Rhythm is vital to understanding and measuring the motion in the shoulder joint. There is a ratio of movement between the scapula and humerus and when this ratio is disturbed it can cause implications with our clients. There is a 2:1 ratio for range of motion in the shoulder joint. If a client moves their shoulder 3 degrees, the humerus will move 2 degrees and the scapula will move 1 degree. For example, a client with full 180 degrees of shoulder abduction, the humerus would move 120 degrees and the scapula would only move 60 degrees. There is a synchronization between the movements of the humerus and scapula. One implication involved in a disrupted scapulohumeral rhythm is impingement syndrome. This is when the subacromial space in the shoulder joint is compressed and the supraspinatus tendon is trapped and cannot function like it normally would. This would cause severe pain for our client. Another relevance for the scapulohumeral rhythm is to promote an optimal length-tension relationship between muscles. The length-tension relationship ensures a muscle will be at the optimal length to produce a strong contraction. It allows for the most actin and myosin to cross-bridge to form the contraction. An impaired scapulohumeral rhythm could produce an active insufficiency between the muscles. Lastly, another relevance of the scapulohumeral rhythm is the promotion of joint congruency. The head of the humerus articulates with the glenoid fossa of the scapula in such a way to promote the greatest range of motion (ROM). If there is no rhythm between the movements of the scapula and humerus this will cause the congruency between this joint to be lost.