Pull ups are extremely challenging exercises. While hanging on the bar, you must use the muscles of your upper body to pull your body up to the bar. Then, slowly lower your body down. When mastered, however, pull ups can potentially yield excellent results. They tone your arms and give you the V-shape on your back that swimmers develop. Always consult your doctor before beginning this or any new exercise regimen.
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The main action involved during a pull up is shoulder adduction, moving the upper arm bone into the body. The muscle mainly responsible for this action is the latissimus dorsi. The latissimus dorsi is shaped like a triangle. The base of the triangle represents the origin of the muscle, adjoining your middle and lower spine. The top of the triangle represents the insertion of the latissimus dorsi, stretching out to your upper arm bone. During a pull up, the latissimus dorsi tugs on the upper arm bone, adducting it into the body. As a result, your body rises to the bar.
Teres Major, Infraspinatus and Teres Minor
The infraspinatus spans the entire backside of your shoulder blade and stretches out to your upper arm bone. Below the infraspinatus is the teres minor, and below the teres minor is the teres major. Both the teres minor and teres major commence on the shoulder blade and stretch out to the upper arm bone. Together these three muscles assist the latissimus dorsi in shoulders adduction during the pull up. This is the action that moves your upper arm bone into your body, lifting your body up to the bar.
Pectoralis Major and Coracobrachialis
Imagine a Japanese hand fan turning 90 degrees. The top of the fan represents the origin of the pectoralis major, which spans the length of breastbone. The bottom of the fan represents the insertion of the pectoralis major, stretching out to the upper arm bone. The coracobrachialis starts on the front of the shoulder joint and stretches out to the upper arm bone. Conjointly, the these two muscles assist the latissimus dorsi in shoulder adduction during the pull up. This action moves your upper arm bone in, and lifts your body up to the bar.
Long Head of the Triceps
The triceps muscle is made up of three heads: lateral, long and medial. All three straighten the arm at the elbow, because they all lengthen down the upper arm bone and into the elbow joint. However, the long head is the only head of the triceps that commences at the shoulder blade. Therefore, it performs an additional action that the lateral and medial heads do not: shoulder adduction. Accordingly, the long head of the triceps contributes to the movement of the upper arm bone into your body as well as lifting your body up to the bar during the pull up.