Where is my Infraspinatus?
Your Infraspinatus is a muscle in the posterior aspect of your shoulder complex, specifically on the back of your shoulder blade (scapula). In anatomy, it is closely related to the Teres minor muscle and has nearly the same origin, insertion, and function.
Posterior Shoulder, in the infraspinous fossa of the scapula
"infra-"= meaning under;
"-spin-"= referring to the spine of the scapula;
"-us"= referring to the muscle
All together: “the muscle under the spine of the scapula.”
Why This Matters
Application for Workouts
The Infraspinatus is considered a primary external rotator of the upper arm. It is part of the rotator cuff group of muscles, also known as the “S.I.T.S.” muscles. The four rotator cuff muscles are: Supraspinatus, Infraspinatus, Teres minor, and Subscapularis. The rotator cuff as a whole, has the important job of stabilizing the head of the humerus (upper arm bone) in the shoulder joint through out different movements. In this manner, the group helps to position the humerus correctly so that other larger muscles can complete their motions effectively. The rotator cuff muscles are also credited with initiating movement, i.e. the first 1/3 of a lateral raise.
It is nearly impossible to complete any upper body motion without using your rotator cuff in some manner, especially if you are paying attention to form. Think about the different ways you can position your grip for a shoulder press. Underhand, over hand, using dumbbells, or the famous Arnold Press, which heavily recruits your infraspinatus to perform external rotation as you press. Even doing a triceps exercise with solid form requires stabilizing your arm in the shoulder socket to prevent rounding of the shoulders. This uses other muscles such as your traps and rhomboids as well, but you better make sure you keep your arm properly rotated using the rotator cuff.
On a Shoulder day:
Infraspinatus + Teres minor: Isolated external rotations using cable, or laying on the side using a dumbbell.
Infraspinatus + Supraspinatus + Anterior Deltoids: Any variation of a shoulder press, but especially the arnold press.
Infraspinatus + Trapezius + Posterior Deltoids: Posterior Deltoid Flys, especially those with dumbbells that use an external rotation as you move toward the top.
Emphasis on Bodybuilding
Infraspinatus is the most important muscle of the rotator cuff for bodybuilding because it is the only one of the 4 that is significantly visible; the other 3 S.I.T.S. muscles are deep to/hidden under other muscles and cannot be seen. The Infraspinatus is located directly beneath the posterior deltoid, and just lateral to the lower trapezius. This muscle group is best visualized from the back. Examples include: in a rear double biceps pose, a tricep’s pose, a side or transition pose while peering back over the shoulder, or a standard back pose in bikini.
If you fail to stretch the Infraspinatus and/or over work it, you could lose some degree of internal rotation, especially if you do a lot of pressing movements. Maintaining strong, yet flexible rotator cuff is important for shoulder health and longevity while lifting, as neglecting to do could lead to impingement.
1) Infraspinatus: Infraspinous fossa and scapular fascia.
The infraspinatus sits on the posterior of the scapula (shoulder blade) below the spine. it attaches towards the medial border of the scapula. There is also a thick fascia overlying and surrounding the infraspinatus, and similar to many forearm muscles, the infraspinatus also partially inserts into this thick, tendon-like fascia.
1) Infraspinatus: Lateral aspect of Greater tubercle
The rotator cuff muscles Supraspinatus, Infraspinatus, and Teres minor all insert onto the greater tubercle of the humberus. Infraspinatus specifically inserts onto the lateral aspect of it. If you feel the top-most part of your arm bone just under your shoulder bone (acromion process) you can actually feel where it attaches.
1) Infraspinatus: External Rotation of the humerus
The primary action of the Infraspinatus is to outwardly rotate the humerus within the glenohumeral joint. The anatomical term for this is External Rotation of the Humerus, or Lateral Rotation of the arm AT the shoulder. Think of bending your elbows, while keeping your arms at your sides, then while holding your elbows in position, spread your hands out wide. There are only a couple pure examples of external rotation, and the exercises are usually referred as such. External Rotation or Abducted External Rotation or Face Pulls.
When a muscle acts as antagonist, it is trying to resist the action of a different muscle. The natural antagonist of the Infraspinatus is the Subscapularis. These muscles are both part of the rotator cuff, and as expected the Subscapularis is responsible for internal rotation aka medial rotaion. There are other internal rotators depending on the arms position such as Pectoralis major and Latissimus dorsi.
The Infraspinatus and Supraspinatus are both innervated by the suprascapular nerve. It is often mentioned how the suprascapular nerve passes under the transverse scapular ligament, under the scapular notch.
This nerve originates from the C5-6 Superior division of the Brachial Plexus. The Brachial Plexus is the main set of nerves that innervate the shoulders and arms. Occasionally, the suprascapular nerve can become pinched, or entraped, in the spinoglenoid notch, where it passes from the supraspinous fossa around the spine to the infraspinous fossa.
The Infraspinatus is primarily supplied by the suprascapular artery, but realistically the shoulder is rich in anastomoses. (anastomoses are different blood vessels that link up to supply an area from multiple angles. Other arteries that particpate in this anastomosis are: scapular circumflex artery and dorsal scapular artery. It is often mentioned how the suprascapular artery passes under the transverse scapular ligament, under the scapular notch.
Blood Flow: Heart -> Aorta -> (Brachiocephalic Trunk) -> Subclavian -> Thyrocervical Trunk -> Suprascapular Artery (can also come directly off subclavian in some cases).