The brachialis is a thick muscle on the front of your upper arm that sits underneath the biceps. Its sole job is bending your elbow, and it actually generates more flexion force than the biceps in many arm positions. Because it’s hidden beneath the more famous biceps brachii, most people don’t know it exists, but it plays a central role in arm strength, arm appearance, and certain elbow injuries.
Where the Brachialis Sits
The brachialis originates from the front surface of the humerus (your upper arm bone), roughly at mid-shaft, and runs down to attach on the ulnar tuberosity, a small bump on the larger of your two forearm bones. This means it crosses only one joint: the elbow. The biceps, by contrast, crosses both the elbow and the shoulder, which is why the brachialis is considered a purer elbow flexor.
Cadaver studies have shown the brachialis often has two heads rather than the single head described in older textbooks. The larger, superficial head attaches to the ulnar tuberosity directly. A smaller, deeper head inserts through a fan of connective tissue that branches onto the ulna. Both heads work together, but the two-head structure helps explain the muscle’s broad, flat shape and its ability to produce force across a range of elbow angles.
Why It Matters More Than the Biceps for Elbow Flexion
The brachialis is a pure flexor. Because it attaches to the ulna (which doesn’t rotate when you turn your palm up or down), the brachialis pulls with roughly the same mechanical advantage regardless of whether your forearm is pronated (palm down), supinated (palm up), or neutral (thumb up). The biceps, on the other hand, is also a forearm supinator, so its contribution to bending the elbow changes depending on hand position.
Electromyography research confirms this relationship. When subjects added supination torque on top of elbow flexion, biceps activity increased substantially while brachialis activity actually decreased modestly. The two muscles essentially trade off: when the biceps is most active, the brachialis and brachioradialis are least active, and vice versa. This is why a neutral or pronated grip shifts more of the flexion workload onto the brachialis.
Nerve Supply
The brachialis receives its primary nerve supply from the musculocutaneous nerve, which runs through the front of the arm. But dissection studies have found that about 82% of people also have a secondary branch from the radial nerve supplying part of the muscle. In most of those cases, the radial nerve branch enters the lower outer portion of the brachialis. This dual innervation is clinically relevant: even if one nerve is damaged, the muscle may retain partial function through the other.
How It Shapes Your Arm
Because the brachialis sits directly beneath the biceps, its size affects how your upper arm looks from the front and sides. A well-developed brachialis pushes the biceps upward and outward, making the arm appear wider and denser. It also adds visible thickness at the outer edge of the upper arm, particularly when viewed from behind or in a relaxed position. For people focused on arm aesthetics, the brachialis is often the difference between an arm that looks round only when flexed and one that looks thick all the time.
Exercises That Target the Brachialis
Since the brachialis works hardest when the biceps is mechanically disadvantaged, the most effective exercises use a neutral or pronated grip. Hammer curls (dumbbell curls with your thumbs pointing up) are the most common choice. The neutral grip reduces biceps involvement and shifts demand to the brachialis and brachioradialis.
Two variations increase the isolation further. Chest-supported hammer curls, performed face-down on an incline bench, eliminate momentum and reduce shoulder involvement, forcing the brachialis to do more work. Preacher hammer curls pin your elbows in front of your body against a pad, which prevents you from swinging and keeps tension on the target muscles throughout the range of motion. Reverse curls (palms facing down on a standard barbell or EZ bar) also emphasize the brachialis by placing the biceps in its weakest rotational position.
Brachialis Injuries and Climber’s Elbow
Brachialis injuries typically happen in one of two ways: a sudden eccentric load (the muscle lengthening under force, like catching yourself during a fall or resisting a forced elbow extension), or chronic overuse from repetitive pulling motions.
Acute strains usually present as pain and swelling on the front of the arm just above the elbow. You’ll likely have difficulty fully straightening the elbow, and bending it against resistance will hurt. Finger movement and forearm rotation typically remain normal, which helps distinguish a brachialis injury from problems with the biceps tendon or forearm muscles. Because the brachialis sits deep beneath the biceps, bruising is uncommon, and the muscle is hard to palpate directly, though a palpable lump sometimes forms with more significant tears.
The chronic version has earned the nickname “climber’s elbow.” Rock climbers spend long periods with their arms in a semi-bent, pronated position while gripping holds and pulling upward. This places repeated stress on the brachialis muscle and its tendon attachment, leading to tendinopathy (gradual tendon breakdown) or low-grade strains. The same pattern can develop in anyone who does high volumes of pull-ups or chin-ups, particularly with a pronated grip. Symptoms build gradually: a dull ache in the front of the elbow that worsens with pulling and improves with rest.
Most brachialis strains are partial tears within the muscle belly rather than complete ruptures at the tendon insertion, though insertional tears do occur. Lower-grade strains involve swelling within the muscle without actual fiber disruption, while more significant injuries show partial or complete tearing of muscle or tendon fibers.
Anatomical Variations
Some people have an accessory brachialis, an extra slip of muscle tissue alongside the standard one. One documented case found an accessory brachialis that crossed over the median nerve and brachial artery before joining the common tendon of the elbow flexors. Another case described an accessory muscle that formed a tunnel around those same structures. This matters because contraction of the accessory muscle could compress the nerve or artery, potentially causing numbness, tingling, or reduced blood flow in the forearm and hand. These variants are uncommon but worth knowing about if you experience unexplained nerve symptoms in the arm that worsen with elbow flexion.