Brachialis muscle

The brachialis muscle is a strong, broad, and flat muscle located in the anterior compartment of the arm, lying deep to the biceps brachii. It is the primary flexor of the elbow joint and is essential for forearm movements regardless of the position of the hand. Unlike the biceps, which is involved in both flexion and supination, the brachialis acts purely on flexion, making it biomechanically crucial in lifting and pulling tasks.

Location

The brachialis originates from the distal anterior surface of the humerus and inserts onto the coronoid process and tuberosity of the ulna. It is located in the middle and lower regions of the anterior arm, directly in contact with the humerus, and lies deep to the biceps brachii throughout its course.

Structure

• Shape: Flattened fusiform muscle with a broad origin and narrow insertion.
• Fascial relations: Deep to the biceps brachii; separated from it by a fascial plane.
• Insertion site: The tendon of insertion inserts wide across the coronoid process and tuberosity of the ulna, giving it strong mechanical leverage.

Function

• Primary flexor of the elbow joint: Active during all forearm positions (supination, pronation, and neutral).
• Workhorse of elbow flexion: Unlike the biceps brachii, its action is unaffected by the orientation of the forearm.
• Synergist muscle: Assists the biceps during rapid or forceful elbow flexion.

Physiological Role(s)

• Provides stable and powerful flexion during tasks like lifting, climbing, pulling, and holding heavy objects.
• Active during both isotonic (movement) and isometric (stabilizing) contractions of the elbow.
• Acts continuously during elbow flexion activities, regardless of supination or pronation.

Development

The brachialis muscle arises from the ventral muscle mass of the upper limb bud during embryonic development. It differentiates from mesodermal myoblasts that migrate to the anterior compartment of the arm. Innervation is established early via the musculocutaneous nerve. The muscle becomes fully functional by birth and continues to mature during early childhood, especially with the development of fine motor skills and limb strength.

Relations

• Anterior: Biceps brachii, brachial fascia, skin.
• Posterior: Humerus (directly in contact).
• Lateral: Radial nerve and radial recurrent artery (lateral margin).
• Medial: Brachial artery and median nerve run medially but are separated by the biceps brachii.

Clinical Significance

• Brachialis strain: Overuse during repetitive lifting or intense weight training can cause localized tenderness and weakness in elbow flexion.
• Musculocutaneous nerve injury: May result in weakened flexion due to loss of brachialis and biceps activation; forearm sensation may also be reduced.
• Radial nerve variation: In some individuals, a lateral portion of the brachialis is innervated by the radial nerve, which may complicate nerve injury presentation.
• Compartment syndrome: In cases of trauma or crush injury, swelling of the anterior compartment may compromise brachialis function.
• Bicipital aponeurosis injury: May obscure brachialis injury diagnosis, as the aponeurosis lies superficial to it.

Surface Anatomy and Palpation

• The brachialis is not directly palpable in most individuals due to its deep location.
• During resisted elbow flexion in pronation (which minimizes biceps involvement), a bulge on either side of the distal biceps may indicate brachialis contraction.
• Ultrasound and MRI are commonly used to visualize the brachialis in diagnostic or post-traumatic evaluation.

Imaging

• Ultrasound: Demonstrates the muscle’s depth and fiber orientation, particularly useful in dynamic assessment.
• MRI: Best modality for evaluating partial or complete tears, hematomas, or muscle atrophy in chronic nerve injury.

Anatomical Variations

• The brachialis may be double-headed, with superficial and deep layers.
• Accessory slips to the biceps tendon or brachioradialis have been documented.
• Occasionally, a tendinous arch forms over the brachial artery, potentially contributing to neurovascular compression.