Pullups are an excellent exercise for strengthening many of the muscles of your back and shoulders, including the rhomboids, middle and lower trapezius, teres major and latissimus dorsi muscles. They also strengthen several of your arm muscles, including the biceps brachii, brachialis and brachioradialis, as well as the pectoral muscles of the chest.
Back Muscles
Pullups strengthen the muscles that pull the shoulder blades down and in toward the spine, including the rhomboids as well as the middle and lower trapezius muscles. The rhomboids run from the spine to the inner edges of the shoulder blades. Their job is to retract the shoulder blades, or pull them toward the spine. The middle and lower fibers of the trapezius attach to the thoracic spine and insert onto the collarbones and shoulder blades. They retract the shoulder blades and depress them, or pull them down the back.
Back and Shoulder Muscles
The latissimus dorsi and teres major muscles together draw the upper arm bones back and down when you do a pullup. The latissimus dorsi is a broad muscle that runs from the pelvis, the lower back, the lower thoracic spine and the lower ribs to attach to the upper arm bone, or humerus. The teres major is a much smaller muscle that assists the latissimus dorsi. It originates on the side of the shoulder blade and also inserts onto the humerus.
Arm Muscles
Pullups also work the flexors of the elbow, which include the biceps brachii, brachialis and brachioradialis. The biceps runs from the shoulder blade to one of the forearm bones, the radius. The brachialis originates on the front of the humerus and inserts onto the other forearm bone, the ulna. The brachioradialis attaches to the lower part of the humerus and inserts near the wrist. Together, these muscles flex your elbow as you pull yourself up to the bar in a pullup.
Chest Muscles
Although pullups principally strengthen the back muscles, they also involve the chest muscles, or the pectorals. The lower fibers of the pectoralis major assist the latissimus dorsi with the action of pulling the arms back and down in a pullup. Those fibers originate from the sternum and middle ribs and attach to the humerus. The pectoralis minor lies under the pectoralis major and runs from three of the upper ribs to the shoulder blade. It assists in depressing the shoulder blade.
References
- "Strength Training Anatomy"; Frederic Delavier; 2001
- "Trail Guide to the Body"; Andrew Biel; 1997
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