The shoulder joint consists of the humerus, scapula and clavicle, which are attached by a five articulations and a system of muscles, tendons and ligaments. Because the shoulder joint is a ball-and-socket joint and has the highest range of motion (RoM), it has the highest risk for injury, such as dislocation, bursitis and rotator cuff damage. All joints work together with the trunk, spine, hips and legs to perform a wide range of feats?pitching a baseball, performing a handstand and lifting a suitcase, for example.
Articulations
The acromioclavicular joint and the glenohumeral joint link the three shoulder bones together, while the two joints that attach the shoulder complex to the rest of the body are the sternoclavicular and the scapulothoracic joints. The glenohumeral joint provides the main movement and range of motion of the shoulder while the other joints provide stability to the shoulder's structure and assists the glenohumeral joint during movement.
Flexion and Extension
Front movement in the sagittal plane is shoulder flexion and is usually 180 degrees, while back motion (shoulder extension in the sagittal plane) can reach 70 degrees. This movement is commonly seen in softball pitches and underhand throws.
Abduction and Adduction
Abduction refers to a joint that is moving away from the body while adduction is moving towards the body. In the shoulder joint, there are two types of abduction and adduction. <br /><br />Abduction of the arm to the side typically reaches 170 degrees, but some people, who are more mobile, can flex and abduct the shoulder more than 180 degrees. Jumping jacks is an example shoulder abduction. Horizontal abduction, where the arm is extended in front of you and is moved horizontally backward and outward from a starting position of 90 degrees of shoulder flexion, reaches between 120 to 130 degrees. A backhand punch in kung fu is an example of horizontal abduction. Shoulder adduction with your arm to the side reaches about 45 degrees across your body. Horizontal adduction, where your the arm moves horizontally forward and inward from a starting position of 90 degrees of shoulder flexion, reaches 30 degrees across your body.
Rotation
The shoulder can externally and internally rotate at the glenohumeral joint. When the arm is alongside the torso with the elbow flexed 90 degrees, internal rotation is limited by the hand reaching the body, while external rotation is limited to 90 degrees by tightening of the soft tissues crossing the anterior aspect of the shoulder. Similarly, external rotation is limited to 50 degrees.
Considerations
When strengthening or stretching the shoulder, incorporate the entire body rather than isolating the shoulder joint only. It has fascia (connective tissues between the skin and muscles) and nerves that connect to the torso and hips that affect how much force, control and range of motion the shoulder can generate. An example would be a baseball pitch, where the pitcher generates power from his legs, through his torso and out to his shoulder and arm to throw the ball.
References
- Basic Biomechanics; Susan J. Hall; 2003
- Impingement Syndrome; Chris Gellert; 2003
- Facilitated Stretching; Rober McAtee, Jeff Charland; 2007
