Shoulder Joint

The shoulder joint is the most mobile joint in the body.

That amazing range of motion requires a great deal of stability and is therefore a very injury-prone joint.

The joint is made up of 3 bones, the scapula, the clavicle and the humerus. These bones are connected to each other and to the trunk through the glenohumeral, acromioclavicular, sternoclavicular and scapulothoracic joints (1,2). There must be a correct functioning of the joints so that the movement is normal and harmonious (2).

The shoulder has 3 degrees of freedom and allows us to mobilize the arm in the 3 planes of space (1).

• Transverse axis: Allows flexion-extension movements. The axis is in the frontal plane and performs the movements in the sagittal plane.

• Anteroposterior axis: Allows ABD (abduction) and ADD (adduction) movements. The axis is in the sagittal plane and performs movements in the frontal plane.

• Vertical axis: Perform horizontal flexion-extension exercises. Perform the movements in the transverse plane.

The fourth axis

There is a fourth longitudinal axis of the humerus, this axis allows internal and external rotation movements of the arm. Within this axis there are two types of rotation: Voluntary rotation, which only occurs in 3-axis joints and is due to the contraction of the rotator muscles and automatic rotation, which appears without the need to perform any action voluntary. It is also known as “Codman’s paradox.”

The characteristics of this joint allow us to perform a movement called circumduction. This movement is carried out by the combination of the previous axes described.

The biomechanics or function of the shoulder joint can be compromised when some of the characteristics previously mentioned do not work in the correct way.

Musculature:

The musculature in the shoulder, as in the other structures of the body, is the one that performs the movement actions and the one that offers active stability.

It is important to know the muscles of the shoulder girdle and determine which muscles can be involved in swimming to a greater or lesser degree and their probability of injury.

Internal Rotator Muscles:

• Latissimus dorsi
• Major round
• Subscapular
• Pectoralis Major
• External Rotator Muscles:
• Infraspinous
• Minor round

The external rotator muscles can be disadvantaged by the increased number of internal rotation muscles and create decompensation. This decompensation can lead to shoulder injuries such as scapular dyskinesia, tendonitis, and glenohumeral joint instability.

Abductor muscles:

• Deltoid
• Supraspinatus
• Long portion of the biceps. (its rupture causes a loss of 20% of the abduction force

Motor muscles of the shoulder girdle:

• Trapezius: Prevents the fall of the arm and separation of the scapula from the thorax. Important when carrying heavy weights.

• Rhomboids: Elevate the scapula and fix the lower angle of the scapula against the ribs.

• Elevator of the scapula: performs the action of raising the shoulders. Important when carrying heavy weights.

• Serratus anterior: Fixes the scapula against the thorax.

• Pectoralis minor: Lower the stump of the shoulder and move the scapula outwards.

• Subclavian: The clavicle descends and the shoulder stump coapts the sternocostoclavicular joint.

Kapandji AI. Joint physiology. Senior member: Editorial Médica Panamericana; 2000.