INFRASPINATUS (External Rotators of Rotator Cuff) :
ORIGIN: Medial 2/3rd of the Infraspinous fossa of the scapula and to the adjacent infraspinatus fascia
INSERTION: Middle facet of the greater tuberosity of the humerus and shoulder joint capsule superiorly & posteriorly
INNERVATION: Suprascapular nerve (C5 & C6)
BLOOD SUPPLY: Suprascapular artery & circumflex scapular artery
ACTIONS: Infraspinatus along with Teres minor laterally rotate the humerus with arm in any position and stabilize the humeral head in glenoid fossa.
- During the shoulder abduction, both these muscles have increased activity during initial 70° elevation, as they assist in downward sliding of humeral head and between 70° to 115° they produce humeral lateral rotation. After 115° there activity decreases.
- These are required to have smooth shoulder abduction and avoid jamming of humeral head against acromion process.
- Both these muscles eccentrically decelerate the shoulder internal rotation.
- The infraspinatus & teres minor work synergistically with posterior fibers of deltoid to laterally rotate the shoulder.
- The infraspinatus & teres minor as part of rotator cuff work synergistically with supraspinatus & subscapularis to dynamically stabilize the glenohumeral joint.
- Infraspinatus is quite superficial muscle. The superomedial portion of the muscle is covered by the lower trapezius and its tendon by the deltoid.
- The fascia covering the infraspinatus is called as the infraspinatus fascia, a strong membrane which also provides origin to the muscle.
- The fascia is attached firmly above to the lower border of scapular spine covering teres minor and infraspinatus and continues with deltoid fascia laterally & blends with fascia of rhomboids medially along the vertebral border of scapula.
- The Myofascial line- Deep Back Arm Line which originates from upper thoracic & 7th cervical vertebra passes down and out with the rhomboid muscles to the vertebral border of the scapula & continues around the scapula with the rotator cuff, specifically infraspinatus & teres minor.
- Upper body posture significantly affects the recruitment of anterior and posterior musculature of shoulder region and may cause altered scapulo-humeral rhythm and biomechanical mal-alignment.
- According to Neumann, tightness in posterior capsule of the shoulder may cause anterior translation of the humeral head. The infraspinatus & teres minor may contribute to tautness in the posterior capsule. But as they are placed posteriorly on the humerus and attached to greater tuberosity they may impart posterior translatory force.
- According to Lawerence et al. & Scovazzo et al., pain & dysfunction did not result in superior glide but an increase in anterior glide during flexion and inferior glide during abduction. This can be thought of due to excessive activity of infraspinatus and teres minor muscles that contribute to the excess anterior translation & inferior glide.
- The infraspinatus tendon can be damaged due to trauma, wear & tear from overuse, especially during various sports like swimming, tennis, overhead throwing games, skiers and among the individuals who are doing excessive overhead arm movements.
- There can be infraspinatus tendonitis or rupture of tendon leading to muscle weakness or inhibition, which will consequently affect the glenohumeral joint stability and may lead to various shoulder pathologies.
- There are many common activities that can lead to overuse and overloading of infraspinatus muscle. Due to this, the muscle may suffer repetitive strain and trauma and may undergo prolonged contraction resulting into development of trigger points within it.
- These activities as a part of routine life or sports require over head arm movements for long periods, reaching back repeatedly for retrieving wallet or fasten a bra or zip a dress, typing on a computer without elbow support, etc.
- Lying on the affected side or even lying on the back can cause severe pain and cause sleep disturbances.
- Infraspinatus trigger points occur on the upper part of the muscle &/or along inner scapular border.
- The trigger points that are below scapular spine refer pain to the front of the shoulder, down the biceps, forearm. Wrist, hand & thumb. Pain may be present in some cases in the back of the neck on same side. Sometimes pain near biceps attachment is mistaken for biceps tendonitis.
- Trigger points along inner scapular border may refer pain between shoulder blades on same side.
- Norkins C, Levangie P. “Joint structure & function.” 4th ed.
- Myers T. “Anatomy Trains. Myofascial meridians for movement and manual therapists.” Churchill Livingston. Second edition.
- Infraspinatus- Wikipedia
- gustrength.com- infraspinatus