Upper Arm Muscles and Ligaments

Ligaments hold your bones together, reinforcing your joints. Several ligaments attach your upper arm bone, or humerus, to your shoulder blade, while others attach the lower end of the humerus to the bones of your forearm. Many muscles attach to the humerus, enabling your arm to move freely in all directions: up, down, backward, forward and side to side. Other muscles of your arm are responsible for moving your elbow.

Shoulder and Elbow Ligaments

Several ligaments connect the humerus to the shoulder blade, including the glenohumeral ligaments and the coracohumeral ligament. Shoulder ligaments tend to be loose, which allows for free movement but can also create instability. Ligaments on either side of the elbow joint join the humerus to the bones of the forearm. The medial collateral ligament reinforces the inner elbow and the lateral collateral ligament strengthens the outer elbow. The medial collateral ligament is often injured from a fall onto an outstretched hand.

Deltoids and Shoulder Muscles

The deltoid muscles cover the shoulder and upper arm. They originate on the collar bone and shoulder blade, and insert onto the humerus. The front deltoids lift your arms forward, the side deltoids lift your arms sideways, and the rear deltoids lift your arms backward. Exercises to train your deltoids include overhead presses and lateral raises. The rotator cuff muscles also run from your shoulder blade to your humerus. They reinforce your shoulder ligaments, stabilizing the shoulder joint.

Elbow Flexors and Extensors

The muscles on the front of the arm -- the biceps brachii and the brachialis -- bend the elbow. The biceps arises from the shoulder blade and attaches to one of the forearm bones, the radius. When it contracts, it not only flexes the elbow but turns the palm up, supinating the forearm. The brachialis is the most powerful elbow flexor. It runs from the humerus to the other forearm bone, the ulna. Exercises such as curls and rows train the elbow flexors. Consisting of three heads, or muscle bellies, the triceps brachii muscle covers the back of the upper arm, with one head that also attaches to the shoulder blade. The triceps inserts onto the ulna and straightens the elbow. Pushups and dips strengthen the triceps.

Other Arm Muscles

Several other muscles also attach to the upper arm bone. The pectoralis major, the large muscle of your chest, pulls your upper arm across your body. Bench presses and flys work the pectorals. The latissimus dorsi, a large muscle of your back, pulls your upper arm bone down. Pull-downs and pull-ups target your latissimus dorsi, along with a smaller muscle with a similar function, the teres major. The coracobrachialis, another small muscle that runs from your shoulder blade to the humerus, pulls your arm in.