The Brachialis Muscle (brachialis anticus), located deep in the anterior region of the upper arm, is the primary muscle responsible for elbow flexion. It lies beneath the biceps brachii and forms the floor of the cubital fossa. Extending from the humerus to the ulna, this muscle provides over 50% of elbow flexion, acting as a more powerful flexor than the biceps.

Anatomy

Morphology and Connection Points

This muscle originates from the anterior surface of the distal half of the humerus. Its origin is generally below the deltoid tuberosity and extends approximately 2.5 cm proximal to the elbow joint surface. The muscle inserts on the tuberosity of the ulna and the anterior surface of the coronoid process. This placement creates a vector that enhances the muscle's effect on elbow flexion. Regarding adjacent structures, the biceps brachii lies superiorly, while the structures that form the floor of the cubital fossa lie inferiorly. Furthermore, this muscle is in close contact with important nerve structures: the musculocutaneous nerve is superficially connected to the muscle, and the radial nerve is deep in contact with the muscle.

Embryological Origin and Classification

The brachialis muscle develops from the ventral mesodermal mass as part of the anterior flexor muscle group of the upper extremity. As such, it is located among the flexor muscles and is one of the primary structures that perform flexion in the forearm. Its embryological development and functional characteristics make its primary connection to the musculocutaneous nerve significant.

Nerve Innervation and Vascular Supply

The primary innervation of the muscle is provided by the musculocutaneous nerve (C5–C7). However, in approximately 70–80% of individuals, the muscle is additionally innervated by the radial nerve (C5–T1). This phenomenon, referred to as "dual innervation," is clinically significant in electrophysiological evaluations. Vascular supply is provided by the muscular branches of the brachial artery and the recurrent radial artery. These arteries provide the oxygen and nutrient supply necessary for the muscle's functional maintenance.

Function and Clinical Relationships

The brachialis muscle is the strongest muscle responsible for elbow flexion and performs this function regardless of the position of the forearm. This characteristic makes it a more stable flexor than the biceps brachii. It also contributes to the stabilization of the humeroulnar joint. Clinically, weakness or damage to this muscle leads to significant loss of strength in elbow flexion. Electromyography (EMG) and nerve conduction studies (NCS) are used for neural evaluation, and magnetic resonance imaging (MRI) is used for structural evaluation. The muscle is generally innervated by the musculocutaneous nerve. Anatomical variations around the brachialis muscle are of surgical importance. Variations such as duplication and anomalous attachments to the radius or adjacent muscles (e.g., supinator or pronator teres) should be considered in surgical planning. These variations may increase the risks associated with neurovascular structures, particularly during interventions in the elbow region.