Brachialis Muscle: Location and Actions

The brachialis muscle is located on the front part of the upper arm, nearest the elbow. Along with the biceps brachii and the brachioradialis, it is one of the primary flexors of the elbow. It gets its name from the Greek words brachialis and brachion, pertaining to the (upper) arm. It is important not to confuse these words with the Greek brachy which means “short.” Although not as large as the biceps brachii, the brachialis is a relatively large and wide muscle and these two muscles, along with the coracobrachialis, make up the anterior (front) compartment of the upper arm. Unlike the biceps brachii, which attaches to the radius, the brachialis attaches to the ulna, making it suited for flexion of the elbow only, since it can only pull on the ulna and the ulna does not rotate. However, it provides strong elbow flexion in both supination and pronation. 1Doyle, James R., and Michael J. Botte. Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 102-103.,2Simons, David G., Janet G. Travell, Lois S. Simons, and Janet G. Travell. Travell & Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999. 660-665.

Structure and Function of the Musculoskeletal System - 2e
Recommended: Structure and Function of the Musculoskeletal System by James Watkins

In fact, the brachialis has been called a “Workhorse Elbow Flexor” working intricately with the biceps brachii but doing much of the work that we usually attribute to the biceps. The biceps is large and superficial to the brachialis, which lies beneath it, making it is easily overlooked. Based on cross-sectional analysis of the major elbow flexors, the brachialis appears to provide 47% of the torque for elbow flexion while the biceps provides 34% and the brachioradialis contributes 19%. The muscle may also contribute to stabilization of the elbow joint. 3Doyle, James R., and Michael J. Botte. Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 102-103.,4Simons, David G., Janet G. Travell, Lois S. Simons, and Janet G. Travell. Travell & Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999. 660-665. The image below illustrates the location of the brachialis relative to the biceps.

Brachialis and biceps muscles illustration
Biceps Superior to Brachialis on Anterior Arm

When the shoulder joint is fixed, as during a curling exercise, the brachialis moves the ulna, and thus forearm, toward the humerus. When the forearm is fixed, the muscle moves the humerus toward the forearm, as during pullups or chinups. If a heavy object were dropped into the outstretched palm, the brachialis would contract in an eccentric action to decelerate the lowering of the forearm, along with the biceps. 5Simons, David G., Janet G. Travell, Lois S. Simons, and Janet G. Travell. Travell & Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999. 660-665.

The brachialis originates on the anterior part of the distal humerus, having a long and wide area of attachment, starting in the area where the deltoid inserts and ending closer to the elbow on the anterior humerus where the origin forms an inverted “V” shape, just above the elbow joint. The way the muscle is situated on the distal humerus, below the midpoint of the shaft, makes it partially envelop the bone on the anterior, lateral, and medial aspects so that around two-thirds of the bone’s circumference is covered. The belly of the brachialis is relatively flat and becomes concave on the front and convex on the back as it extends towards its insertion at the ulna. The middle fibers run vertically, the medial fibers run obliquely from medial to lateral, and the lateral fibers run obliquely from lateral to medial. These portions become tendinous at different points before converging to insert on the anterior aspect of the base of the coronoid process of the ulna.

labeled brachialis muscle illustration
Brachialis Muscle with Its Relationship to Deltoid Insertion and Biceps Tendon

There are several variations and anomalies possible, including the muscle being divided into two or more separate heads or bellies, with each head originating at a different point, anterior and posterior to the deltoid insertion. The distal insertion may become more variable in this case, with additional or different insertional sites including the radius or radius and ulna, and other variations. 6Doyle, James R., and Michael J. Botte. Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 102-103.

Brachialis Origin, Insertion, and Action

Origin: Distal half of anterior portion of the humerus, covering a long and wide area starting near the deltoid insertion and ending close to the elbow on the distal humerus.

Insertion: Coronoid process of the ulna bone.

image by robswatski via flickr

Action: Strong flexion of the elbow, the only pure flexor of the joint. Many of the activities which use and work the biceps brachii also work the brachialis. The muscle can be somewhat preferentially isolated by pronating the forearm, which makes the biceps somewhat less effective. However, the interplay between the brachialis, biceps and brachioradialis during resisted elbow flexion is not nearly as predictable as most sources claim, being quite variable even in one individual on repeated subsequent efforts, making true and consistent isolation of any one elbow flexor unlikely. 7Simons, David G., Janet G. Travell, Lois S. Simons, and Janet G. Travell. Travell & Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999. 660-665.,8Floyd, R. T., and Clem W. Thompson. Manual of Structural Kinesiology. Dubuque, IA: WCB/McGraw-Hill, 1998.,9Doyle, James R., and Michael J. Botte. Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 102-103.

Synergists: The brachialis is synergistic with the biceps brachii as a prime mover in elbow flexion. It is often claimed to be a secondary mover or assistant to the biceps but it is proper to call it a chief mover, along with the biceps, if not the main mover. Both muscles have large swing components and small stabilization components. They are assisted by weaker synergists of flexion including the brachioradialis, pronator teres, and the wrist and finger flexors flexor carpi ulnaris, flexor carpi radialis, and flexor digitorum sublimis, which have small swing components (the brachioradialis being the largest swing muscle of these) and strong stabilizing components so that these muscles mostly stabilize the elbow during flexion and extension while providing a little assistance to the brachialis and biceps brachii for elbow flexion. 10Watkins, James. Structure and Function of the Musculoskeletal System. Champaign, IL: Human Kinetics, 2010.,11Simons, David G., Janet G. Travell, Lois S. Simons, and Janet G. Travell. Travell & Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999. 660-665.

Sources   [ + ]

1, 3, 6, 9. Doyle, James R., and Michael J. Botte. Surgical Anatomy of the Hand and Upper Extremity. Philadelphia: Lippincott Williams & Wilkins, 2003. 102-103.
2, 4, 5, 7, 11. Simons, David G., Janet G. Travell, Lois S. Simons, and Janet G. Travell. Travell & Simons’ Myofascial Pain and Dysfunction: The Trigger Point Manual. Baltimore: Williams & Wilkins, 1999. 660-665.
8. Floyd, R. T., and Clem W. Thompson. Manual of Structural Kinesiology. Dubuque, IA: WCB/McGraw-Hill, 1998.
10. Watkins, James. Structure and Function of the Musculoskeletal System. Champaign, IL: Human Kinetics, 2010.