The shoulder girdle consists of  a whole bunch of bony joints and muscles that connect the upper limbs to the rest of the skeleton and provide an impressive range of movement.
The three bones which form the Shoulder Girdle are the Scapula, the Clavicle, and the Humerus, and the most important feature of the shoulder is the large range of movement that it allows, which is of prime importance to daily life.

Trapezius.

The Trapezius Muscle has many functions:

• Moving the shoulder blade in toward the spine.
• Moving the shoulder blade up and down.
• Bringing the head and neck in a backward direction.
• To rotate and side bend the neck.
• Assists in breathing.
• Rotates the shoulder blade so that the upper-most part of the upper arm faces up.

The Effects on the Neck:

Because the Trapezius Muscle works to move the neck in several directions, its degree of tightness or looseness affects neck flexibility.

For people who work at desks and computers, or who spend many hours driving, the Upper Trapezius is that muscle above your shoulder that becomes very sore and painful.

The Trapezius Muscle is in four parts:

The upper fibres of the cervical vertebrae is the weakest part of the muscle and only provides minor elevation of the clavicle.

• The area commonly known as’ Upper Traps’ is a strong elevator, rotator and retractor of the Scapula.
• The mid-portion of the Trapzius is mainly responsible for scapula retraction.
• The lower fibres of Trapezius assists in retraction and rotation.

Working together they have the effect of simultaneously lifting and retracting the Scapulae. The Trapezius is used most commonly to fix the Scapula to allow the Deltoid to move the Humerus.

Action:

• Laterally rotates, elevates and retracts Scapula. If Scapula is fixed, extends and laterally flexes neck.

Typical uses:

• Shrugging shoulders.
• Overhead movements.

Levator Scapulae.

Shrugging the shoulders (scapula elevation) involves the use of Levator Scapulae and Trapezius.

Fixation of the scapula by other muscles, allows the Levator Scapulae muscles to work together to aid cervical extension, or independently to laterally flex (side bend) the neck towards the side of the working muscle.

Action:

• Elevates the scapula and gives lateral flexion of the cervical spine (each side independently)
• Also responsible for the extension of the cervical spine (each side independently).

Typical use.

• Shrugging shoulders.
• Carrying a heavy shopping bag.

Rhomboids.

There are two rhomboid muscles – Rhomboid Major and Rhomboid Minor.

Rhomboid Major is larger and positioned below Rhomboid Minor.  Chins and dips are excellent activities for developing these muscles.

Action:

• Retracts Scapula (bringing the shoulder blades together). Rotates Scapula downwards to rest position.

Typical use.

• Pulling a draw open.

Pectoralis Minor.

The Pectoralis Minor muscle is the smallest of the two pectoral (chest) muscles.

It works together with the Serratus Anterior  which protracts and rotates the Scapulae upwards.

When the two work together, pure protraction (without rotation) is produced.

Actions:

• Elevates ribs if scapula fixed, protracts scapula (assists Serratus Anterior).
• Rotation of the scapula downwards.

Typical use.

• Pushing a door open.

Serratus Anterior.

The Serratus Anterior muscle is used in activities which draw the scapula forwards.

It is used strongly in push-ups and bench presses. Winged Scapulae are an indication of a weak Serratus Anterior.

Actions:

• Laterally rotates and protracts Scapula.

Typical use.

• Reaching up to open a high window.

Sternocleidomastoid.

Sternocleidomastoid can clearly be seen when you turn your head to one side, on the opposite side of the neck.

Actions:

• Flexes and laterally rotates cervical spine. Protracts head when acting together . Extends neck when neck already partially extended

• Contraction on one side only: Laterally flexes (side bends) to the same side and rotates to the other.

Typical use:

• Looking at the floor.
• Looking over your shoulder.
• Holding the phone between your ear and shoulder.
• Generally being a sticky beak.

The shoulder joint is one of the body’s most flexible and allows the arm to be positioned and moved in just about any direction.

The Shoulder Joint also allows the arm to be rotated inward (internal rotation) or outward (external rotation) to help put the bent forearm in a favorable position to perform a required task.

The head of the Humerus is almost spherical in shape and rotates freely in the glenoid fossa of the Scapula.
The shoulder joint’s flexible ligaments, the Rotator Cuff, and the other muscles that cross this joint all add to the shoulder’s stability and its incredibly amazing range of movement.

Deltoid.

The Deltoid muscle is used in all side lifting movements and any movement of the humerus on the Scapula. It is divided into three portions, anterior, middle and posterior, with the fibres having different roles due to their orientation.

Actions:
• Anterior portion – Shoulder flexion and internal rotation.
• Posterior portion – Shoulder extension and external rotation.
• All fibres – Shoulder abduction.

Typical Use:
• Lifting.

Supraspinatus.

The Supraspinatus muscle is one of the four muscles which make up the rotator cuff. Its main function is to stabilise the upper arm by holding the head of the humerus in position.
It is important in throwing motions to control any forward motion of the head of humerus.

Actions:
• Abduction .
• Stabilisation of the humerus and shoulder joint.

Typical use:
• Holding shopping bags away from the body

Infraspinatus.

The Infraspinatus muscle assists the lifting of the arm while turning the arm outward (external rotation). It is the main external rotator of the shoulder joint.

It is one of the four rotator cuff muscles crossing the shoulder joint and is commonly injured.

Actions:
• Shoulder abduction.
• External rotation.
• Stabilises the shoulder joint

Typical use:
• Brushing hair.

Teres Major.

The Teres Major is only functional when the Rhomboids fix the scapula. This muscle mainly helps the Latissimus Dorsi.

Action:

• Medially rotates and adducts arm.
• Stabilises the shoulder joint.

Typical use:
• Tucking your shirt into the back of your jeans.

Teres Minor.

The Teres Minor is one of the four rotator cuff muscles surrounding the shoulder. Its main action, along with the Infraspinatus  is to externally rotate the shoulder joint. It assists the lifting of the arm during outward turning (external rotation) of the arm.

There are two Teres muscles, the other being Teres Major.
The other muscles whose tendons form the rotator cuff are:
•    The infraspinatus muscle, which (like the teres minor) helps in the outward turning (external rotation) of the arm.
•    The supraspinatus muscle which is responsible for elevating the arm and moving it away from the body; and
•    The subscapularis muscle, which moves the arm by turning it inward (internal rotation).

Actions:
•    External rotation.
•    Shoulder abduction and stabilises shoulder joint.

Typical use:
•    Brushing hair.

Subscapularis.

The Subscapularis moves the arm by turning it inward (internal rotation).

The Subscapularis is one of the four rotator cuff muscles which cross the shoulder joint. The muscle also acts to hold the head of the humerus in position and prevents it moving forwards. It is shown here with a couple of ribs cut away at the front.

“Subscapularis” means under (sub) the scapula (the wingbone). The subscapularis muscle originates there, beneath the scapula.

Actions:
• Internal rotation.
• Adduction.
• Stabilises shoulder joint.

Typical use:
• Tucking your shirt into the back of your jeans.

Latissimus Dorsi.

The Latissimus Dorsi muscle is one of the largest in the body.

It is a powerful extensor muscle of the arm and is used extensively in chinning and climbing and its basic function is to pull the shoulders downward and towards the back.

It extends from the spine, hip and ribs to the upper arm.

They are commonly known at ‘the lats’.

Actions:
• Extends, adducts and medially rotates arm.
• Helps with deep inspiration and forced expiration

Typical use:
• Pushing down on the arms of a chair when standing up.

Pectoralis Major.

Pectoralis major is the largest and most superficial of the two chest muscles. Pec Major and the anterior fibres of Deltoid work closely together. Pec-fly and push-up exercises are good for the Pectoralis major.

Actions:
• Clavicular head:flexes and adducts arm.
• Sternal head: adducts and medially rotates arm .
• Accessory for inspiration.

Typical use:
• Applying a roll-on deoderant.