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The shoulder muscles and tendons

Many muscles are attached to different parts of the shoulder, and they are used to move the arm about in space. The deltoid muscle is attached to the shoulder blade along the spine of the scapula (the acromion) and also attaches to part of the clavicle. This muscle crosses the shoulder joint and attaches to the humerus halfway down the bone. The deltoid is the strongest shoulder muscle. It lifts your arm up and away from the side of your body. Doctors call this type of motion "abduction."

Underneath the deltoid muscle is the rotator cuff of muscles. The rotator cuff is a group of four muscles that form a strong cuff around the shoulder joint and are the muscles that help to control the rotation and position of the arm. Each of these muscles has a tendon at the end that attaches to the humerus by growing directly into the bone. These four muscles are:

  • The subscapularis
  • The supraspinatus
  • The infraspinatus
  • The teres minor

The subscapularis muscle is attached to the deep surface of the scapula and then travels in front of the humeral head. It fits into a bump of bone on the humerus called the lesser tuberosity. This muscle is used to internally rotate the arm and to bring the arm down to the side of the body (a motion that is called "adduction"). The subscapularis also plays a very important role in preventing the shoulder from slipping out of the front of the joint.

The three other muscles of the rotator cuff are all attached to the back of the scapula and travel behind the humeral head. The supraspinatus, infraspinatus, and teres minor all insert on a bump of bone called the greater tuberosity. These three muscles together are called the posterior rotator cuff. They externally rotate the arm and also help to bring the arm down to the side of the body (adduction). The supraspinatus muscle and tendon can be considered the weak link in the shoulder, since the supraspinatus muscle is the most commonly injured part of the rotator cuff. This muscle begins on the top and back of the shoulder blade and travels along the top of the head of the humerus. The ceiling of the space that this muscle travels in is formed by the acromion, and it is in this space that the muscle is very vulnerable to wear and tear and injury.

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