The infraspinatus muscle is one of the four rotator cuff muscles in the shoulder, playing a crucial role in stabilizing the shoulder joint and allowing for various movements of the arm. It is a thick, triangular muscle, which helps in lateral rotation of the arm and stabilizes the shoulder by holding the head of the humerus in place.
Location
The infraspinatus muscle is located on the posterior aspect of the scapula, below the scapular spine. It occupies the majority of the infraspinous fossa, a concave surface on the back of the scapula. The muscle fibers converge to form a tendon that inserts into the greater tubercle of the humerus.
Anatomy
The infraspinatus muscle is a crucial component of the shoulder’s rotator cuff, with detailed anatomy that enables its function in shoulder stabilization and movement.
Origin
The infraspinatus muscle originates from the infraspinous fossa of the scapula, which is the large, concave surface on the posterior (back) side of the scapula, below the scapular spine.
Insertion
The fibers of the infraspinatus muscle converge as they move laterally toward the shoulder joint, forming a thick tendon that inserts onto the middle facet of the greater tubercle of the humerus. This location is just below where the supraspinatus muscle inserts and is crucial for the muscle’s role in lateral rotation of the arm.
Structure
- The muscle is thick and triangular in shape. The surface facing the scapula is covered by the supraspinatus muscle and the trapezius muscle, making the infraspinatus part of the deeper layer of shoulder muscles.
- The tendon of the infraspinatus is integrated with the joint capsule of the shoulder, contributing to the strength and stability of the shoulder joint.
Blood Supply
The blood supply to the infraspinatus muscle primarily comes from the suprascapular artery and the circumflex scapular artery, branches of the subclavian artery. These blood vessels provide the necessary nutrients and oxygen for muscle function.
Innervation
The infraspinatus muscle is innervated by the suprascapular nerve, which originates from the upper trunk of the brachial plexus (C5 and C6 nerve roots). This nerve enters the muscle at the suprascapular notch, a notch in the superior border of the scapula.
Relations
- Superiorly: The infraspinatus is bordered by the supraspinatus muscle and the scapular spine.
- Medially: It is bordered by the spine of the scapula and the rhomboid muscles.
- Laterally: The muscle extends toward the humerus and is adjacent to the deltoid muscle when the arm is in a neutral position.
- Inferiorly: It lies above the teres minor muscle, another component of the rotator cuff.
Functional Compartment
The infraspinatus is part of the posterior compartment of the shoulder muscles and plays a significant role in the rotator cuff, contributing to the stability and mobility of the shoulder joint.
Function
The infraspinatus muscle has several important functions related to the movement and stabilization of the shoulder joint:
- Lateral (External) Rotation of the Arm: The primary function of the infraspinatus muscle is to externally rotate the humerus at the shoulder joint. This movement is essential for a range of motions and activities, such as reaching behind the back or throwing.
- Stabilization of the Shoulder Joint: The infraspinatus, as part of the rotator cuff, helps stabilize the glenohumeral (shoulder) joint by keeping the head of the humerus firmly within the shallow socket of the scapula, known as the glenoid fossa. This stabilizing effect is crucial during arm movements to prevent dislocation and ensure joint integrity.
- Assisting in Horizontal Abduction: Though not its primary role, the infraspinatus assists in horizontal abduction of the arm (moving the arm away from the body in a horizontal plane), particularly when the arm is in external rotation.
Clinical Significance
Rotator Cuff Tears and Tendinitis: The infraspinatus is one of the rotator cuff muscles frequently involved in shoulder injuries, such as tears and tendinitis. These conditions can lead to pain, weakness, and limited range of motion in the shoulder, significantly impacting daily activities and quality of life.
Impingement Syndrome: The infraspinatus can be affected by shoulder impingement syndrome, where the space between the shoulder bones decreases, causing pinching or irritation of the muscle and tendon. This condition often results in pain and movement restriction.
Frozen Shoulder: Also known as adhesive capsulitis, this condition involves stiffness and pain in the shoulder joint. The infraspinatus muscle, among others, can become stiff and contribute to the limited movement seen in this syndrome.
Rehabilitation and Physical Therapy: Understanding the anatomy and function of the infraspinatus is crucial in designing effective rehabilitation exercises for shoulder injuries. Strengthening and stretching the infraspinatus and other rotator cuff muscles are common components of recovery programs to restore function and prevent future injuries.
Diagnostic Imaging: The infraspinatus muscle is often examined through diagnostic imaging, such as MRI or ultrasound, in cases of shoulder pain or injury. These images help in diagnosing conditions like tears, tendinitis, or muscle atrophy.
