Brachialis Muscle: Anatomy, Function, Training, and Importance

What is the muscle between bicep and arm?

The muscle situated deeply between the biceps brachii and the humerus (upper arm bone) is the brachialis muscle. It is the primary and most powerful pure flexor of the elbow joint.

Understanding the Brachialis Muscle

When discussing arm muscles, the biceps brachii often takes center stage due to its prominent appearance. However, beneath the biceps lies a crucial, often overlooked muscle that plays a foundational role in elbow flexion: the brachialis. This muscle is anatomically positioned directly on the anterior surface of the humerus, making it deeper than the biceps.

Anatomy and Location

The brachialis muscle is a broad, flat muscle that forms the floor of the cubital fossa (the triangular area on the anterior aspect of the elbow).

• Origin: The brachialis originates from the distal half of the anterior surface of the humerus, extending down to within about 2.5 cm of the elbow joint. It also originates from the intermuscular septa, which are fibrous partitions that separate muscle compartments.
• Insertion: Its fibers converge to insert onto the coronoid process and the tuberosity of the ulna, which is the larger of the two bones in the forearm.

Unlike the biceps brachii, which crosses both the shoulder and elbow joints, the brachialis only crosses the elbow joint. This anatomical characteristic is key to understanding its primary function.

Primary Function: Pure Elbow Flexion

The brachialis muscle is often referred to as the "workhorse" or "true flexor" of the elbow joint. Its unique anatomical attachments mean that its sole action is to flex the elbow, regardless of the forearm's position (supinated, pronated, or neutral).

• Independent Action: While the biceps brachii is a powerful elbow flexor, it also supinates the forearm and assists in shoulder flexion. The brachialis, however, is unaffected by forearm rotation. Whether you are performing a supinated (palm up) curl, a pronated (palm down) curl, or a neutral grip (hammer) curl, the brachialis is actively engaged in flexing the elbow.
• Constant Engagement: The brachialis is always recruited during any elbow flexion movement, making it the most consistently active elbow flexor. This contrasts with the biceps, which is less active when the forearm is pronated.

Why is the Brachialis Important?

The brachialis muscle is critical for overall arm strength, function, and even the appearance of a well-developed arm.

• Strength and Power: As the primary elbow flexor, a strong brachialis contributes significantly to movements requiring pulling, lifting, and carrying. It provides the foundational strength for all arm flexion tasks.
• Muscle Mass and Aesthetics: While hidden beneath the biceps, a well-developed brachialis can push the biceps forward, contributing to the overall size and "peak" of the arm. It adds thickness and fullness to the upper arm.
• Injury Prevention: A balanced development of all elbow flexors, including the brachialis, can help distribute stress across the joint and reduce the risk of overuse injuries.

Training the Brachialis

While the brachialis is active in all elbow flexion exercises, certain movements can emphasize its recruitment more effectively.

• Pronated Grip Curls (Reverse Curls): When you perform curls with a pronated (palm-down) grip, the biceps brachii is put at a mechanical disadvantage for elbow flexion. This forces the brachialis and brachioradialis to take on a greater share of the load, making reverse curls an excellent exercise for directly targeting the brachialis.
• Neutral Grip Curls (Hammer Curls): With a neutral grip, the biceps still contributes significantly, but the brachialis and brachioradialis are also heavily recruited. Hammer curls are effective for building overall arm mass, including the brachialis.
• Forearm-Supported Curls: Exercises like preacher curls or concentration curls, where the arm is stabilized, can help isolate the elbow flexors and reduce involvement from other muscle groups, allowing for a more focused contraction of the brachialis.
• Slow and Controlled Movements: Emphasize a full range of motion and controlled eccentric (lowering) phases to maximize time under tension for the brachialis.

Differentiating from Other Arm Muscles

It's important to distinguish the brachialis from its more superficial neighbors:

• Biceps Brachii: Located on top of the brachialis, the biceps has two heads and is involved in elbow flexion, forearm supination, and shoulder flexion.
• Brachioradialis: Located on the lateral (thumb side) aspect of the forearm, the brachioradialis is also an elbow flexor, but it is strongest when the forearm is in a neutral (hammer grip) position. It originates from the humerus and inserts on the radius.

Common Issues and Injuries

While generally robust, the brachialis can be susceptible to:

• Strains: Overuse or sudden, heavy lifting can lead to muscle strains, causing pain and weakness in elbow flexion.
• Tendinopathy: Inflammation or degeneration of the brachialis tendon, often due to repetitive stress.
• Referred Pain: Dysfunction in the brachialis can sometimes refer pain to the forearm or wrist.

Proper warm-up, gradual progression in training, and listening to your body's signals are crucial for maintaining brachialis health.

Conclusion

The brachialis muscle, though often overshadowed by the more prominent biceps, is a fundamental component of upper arm anatomy and function. As the primary and most consistent elbow flexor, it is indispensable for everyday tasks and athletic performance. Understanding its role and incorporating targeted training can lead to improved arm strength, enhanced muscle development, and better overall joint health. Don't overlook this crucial "workhorse" of the arm.