Muscles of the Upper Limbs
Arm (Shoulder to Elbow)
Anatomists refer to the upper arm as just the arm or the brachium. (The lower arm is the forearm or antebrachium.) There are three muscles on the upper arm that are parallel to the long axis of the humerus, the biceps brachii, the brachialis, and the triceps brachii.
The biceps brachii is on the anterior side of the humerus and is the prime mover (agonist) responsible for flexing the forearm. It has two orgins (hence the “biceps” part of its name), both of which attach to the scapula bone. Biceps brachii has two heads, called the short head of biceps brachii on the medial aspect and the long head of biceps brachii on the lateral aspect. It inserts on the radius bone. The biceps brachii has two synergist muscles that assist it in flexing the forearm. Both are found on the anterior side of the arm and forearm. One of these is the brachioradialis muscle which is largely on the forearm (see the next section) and the other is the brachialis, which is largely on the upper arm. The brachialis muscle is deep to the biceps brachii and both its origin and its insertion are more distal to the shoulder than its equivalents on the biceps brachii. Like the biceps brachii the origin of the brachialis is on the humerus. Parts on the brachialis can be seen peeking out from under the biceps brachii, especially lower on the arm. The locations of these three muscles are shown in.
On the posterior side of the arm is the triceps brachii muscle. It the antagonist to the biceps brachii. When the triceps brachii contracts it extends the forearm, undoing any flexing brought about by contractions of the biceps brachii. As a result, when the triceps brachii is contracted, the biceps brachii and its synergists must be relaxed, and vice versa. The triceps brachii has three origins, called the long head (medial aspect), the lateral head (lateral aspect), and the medial head (deep to the long and lateral heads). It is easiest to view the triceps brachii from the posterior, but the medial head and its origin are deep to the lateral head and the long head, and so is the medial head of the triceps brachii is partially obscured from the posterior.