Anatomy and supply
The Brachialis is a long and strong muscle of the upper arm. It originates at the distal half of the anterior side of the humerus. In addition, the origin tendon attaches to the medial and lateral intermuscular septa of the arm, two dividing membranes separating the flexor from the extensor muscles. Distally the muscle inserts at the tuberosity of the ulna where its' fibers are also connected to the joint capsule. The nerve supply comes from the musculocutaneous nerve (C5-C7), however in 70-80% of people, the muscle has double innervation with the radial nerve (C5-C6).
For the most part, the brachialis lies under the biceps brachii and is therefore not easy to palpate from the surface. The muscle barely has superficial parts found at its' lateral border and distally. Even though it is located deep in the upper arm, the brachialis muscle still contributes indirectly to the surface anatomy as its' large belly makes the biceps brachii look much larger on the surface than it actually is. (“Behind every great biceps brachii is a great brachialis”.)
The brachialis is the strongest flexor of the elbow joint. It is even a stronger flexor than the biceps brachii, because it is closer to the joint axis and furthermore only stretches over one joint in contrast to the biceps brachii. A small contraction of the muscle consequently leads to a larger flexion in the elbow. Another function of the brachialis is helping with maintenance of tension found on the joint capsule, whereby it prevents damages to the capsule during hyperextension.
In cases of extreme strain of the elbow, inflammation of the brachialis tendon (brachialis tendonitis) or injuries of the joint capsule often occur. Too many pull-ups, excessive tennis practice or other inappropriate strain of the elbow are among the typical causes. As climbers are commonly affected by this condition, it is clinically also referred to as the “climber’s elbow”. The classic symptoms include swelling, redness as well as restricted and painful movements of the elbow.