Introduction

The brachialis is an important flexor of the forearm at the elbow.[1] The brachialis provides flexion of the elbow at all physiologic positions and is considered a "purse flexor" of the forearm at the elbow.[2] The head is more superficial, and with greater volume has a biomechanical advantage that allows it to be the main actor in the elbow flexion. The smaller head, being oblique and deep, facilitates the beginning of the elbow flexion from a position of complete articular extension. The brachialis muscle is in the anteroinferior area of the arm and is deeper than the biceps brachialis muscle; the brachialis contributes to the upper part of the cubital fossa floor in the elbow joint.

Structure and Function

The brachialis is an elbow flexor that originates from the distal anterior humerus and inserts onto the ulnar tuberosity. The brachialis is one of the largest elbow flexors and provides pure flexion of the forearm at the elbow.[2] It does not provide any supination or pronation of the forearm. Within the literature, there are conflicting reports of the detailed anatomy of the brachialis. Traditionally, the brachialis has been described as a single-head muscle, although cadaver assessment has demonstrated that brachialis muscle may actually have two heads, one superficial and one deep.[3] The superficial head forms the major head, which originates from the anterior mid-shaft humerus and the lateral intermuscular septum, and inserts onto the ulnar tuberosity.[3] In detail, the insertion of the deep head has three portions: a medial and lateral aponeurosis, and muscular contractile fibers that attach directly to the ulna. The deep head forms a smaller muscle that originates from the anterior humerus and the medial intermuscular septum and inserts mainly into an aponeurosis that branches to the ulna.[3] Furthermore, different anatomic variants have been described in the literature, with the potential for real clinical implications (see "Physiologic Variants" and "Surgical Considerations"). 

The brachialis muscle is the strongest flexor of the elbow in the absence of supination, as with supination and flexion, its mechanical momentum becomes more disadvantaged than the biceps brachialis muscle.

Embryology

As a skeletal muscle, the brachialis ultimately develops from the mesoderm layer.[4] The upper extremity musculature develops from a common muscle of origin, muscle primordia, that later develops into specific muscle heads.[5][6] Specifically, the muscle primordia develop from dorsolateral somite cells that migrate into limb buds around day 28 of development.[5] The muscle primordia later split into separate flexor and extensor components. This division is controlled by signaling from connective tissue deriving from the lateral plate mesoderm.[5][6] Notably, anterior-posterior development is under the control of downstream signaling of sonic hedgehog protein, secreted at the zone of polarizing activity (ZPA) in the posterior limb bud.[5][6] Dorsoventral differentiation is under the control of WNT7A and downstream signaling pathways thereof.[5][6] Differences in these complex developmental pathways can ultimately lead to different anatomic variants. These variants can also have a significant influence on surrounding neural and vascular structures (see "Physiologic Variants" section). 

Blood Supply and Lymphatics

The blood supply of the brachialis is ultimately from tributaries of the brachial artery and the radial recurrent artery. There are occasionally other arteries that may supply the brachialis, including branches of the ulnar collateral arteries. The venous drainage from the brachialis muscle is ultimately the brachial vein, later joining with the basilic vein tributary and forming the axillary vein. The upper limb contains both superficial and deep lymphatic channels. The superficial channels generally follow superficial vasculature and perforate into the deep lymphatics at various points, particularly near the cubital fossa.[7] The deep lymphatic channels generally follow main vessels and ultimately drain into the axillary lymph nodes.[7] It bears mentioning that research has documented various anatomic variants of this relevant vasculature.[8][9][10][8]

Nerves

The brachialis often has dual innervation, being innervated medially by musculocutaneous nerve and laterally by the radial nerve.[11] However, the musculocutaneous nerve provides the majority of the motor supply to the muscle.[11] Other anatomic variants, including individual innervation by the musculocutaneous nerve, have also been described.[11]

The musculocutaneous nerve passes between the biceps and brachialis muscles, wherein the terminal branch of the lateral antebrachial cutaneous nerve emerges. The radial nerve passes between the brachialis and brachioradialis muscles in the lateral arm after spiraling and emerging from the spiral groove of the humerus. However, there is documentation of anatomic variants of the brachialis muscle, which have influenced nerve location and innervation patterns (see "Physiologic Variants" section). 

The median nerve can also innervate the brachialis muscle.

Muscles

The brachialis muscle relates anteriorly to the biceps muscle, the brachioradialis muscle, the pronator teres muscle, and the vascular-nerve bundle of the arm.

Physiologic Variants

There have been some reports of anatomic variants of the brachialis muscle. One case report discussed an accessory brachialis muscle found during cadaver dissection at Harvard Medical School in the year 2003.[12] The accessory brachialis muscle was found to originate from the mid-shaft of the humerus and the medial intermuscular septum and crossed both the median nerve and brachial artery before inserting into the common tendon of the elbow flexor muscles.[12] Another report described an accessory brachialis muscle that included a fibrous/muscular tunnel containing the brachial artery and median nerve, suggesting that this anatomic variant could yield nerve compression symptomatology with muscle contraction.[13]

Surgical Considerations

The brachialis typically gets split during anterior and anterolateral surgical approaches of the humerus.[11] Such methods are common for humerus fractures, particularly supracondylar humerus fractures.[14] Of note, supracondylar fracture of the humerus is one of the most common pediatric injuries of the elbow region. Estimates are that it accounts for between 15 to 17% of childhood extremity fractures.[15] Given the importance of the brachialis muscle and the proximity to various neurovascular elements, this approach requires extreme care. Notably, cadaver studies have demonstrated that anterolateral splitting of the brachialis yields a significant probability of damaging lateral branches of nerve supply, even when the brachialis receives dual innervation (e.g., from both the musculocutaneous and radial nerves).[11] Given anatomic variability, it may be difficult to predict the exact location of essential nerves within these muscle compartments, further complicating the surgical approach.[11] Some have described utilizing a lateral approach that does not split the brachialis muscle, although there is a concern that the required manipulation and dissection may increase the risk of post-operative nerve palsy.[16] 

Clinical Significance

Given that the brachialis muscle is an important elbow flexor, it is of clinical relevance. Impaired elbow flexion can result from several etiologies, including neurologic, neurovascular, muscle rupture, or traumatic causes. Generally, injury to the biceps brachii is more commonly the cause of elbow flexor trauma, although there is also documentation of isolated injuries to the brachialis muscle.[17][18][19][20] The majority of these cases were attributable to either overuse injuries or strenuous weight loading injuries. Magnetic resonance imaging (MRI) is generally the most accurate means of diagnosing these isolated muscle ruptures, although there are also suggestions regarding the use of ultrasound for diagnosis as a low-risk and cost-effective alternative.[19] Although the literature is limited, some have suggested that conservative, non-surgical management is sufficient for uncomplicated, isolated brachialis injury.[19]

Additionally, there has been documented use of brachialis tendon transfer, specifically to reconstruct the flexor digitorum profundus and the flexor pollicis longus after brachial plexus injuries.[21] Since forearm muscles are not always available for use, the brachialis is an alternative donor for such applications. This study demonstrated that this donor strategy yielded excellent results in achieving "key pinch" and "hook grasp" in these patients.[21] 

For a manual evaluation of the strength of the brachialis muscle, the operator puts his resistance on the patient's wrist, while the latter holds the elbow extended and with the palm of the hand forward. The patient will have to flex the elbow without supination; in this way, we are evaluating the flexor muscle stronger than the elbow.

Other Issues

Brachialis syndrome is the result of a permanent injury to the median nerve, following a poor patient positioning during surgery, in particular, due to a decompression of the median nerve in the antecubital fossa.

According to the literature, the brachialis muscle could be one of the causes of the alteration of the arthrokinematics of the shoulder (thanks to the anatomic-myofascial continuum), causing pain to the movement and disturbance to the rotator cuff of the shoulder. The presence of trigger points in the brachialis muscle could be a clue. In some patients, the injection of drugs in the brachialis muscle can solve the problem of the shoulder.

Myositis ossificans in the sporting field, generally, involves specific muscle areas, including the brachialis muscle. The approach is often conservative, and the athlete has excellent chances of returning to his sport.

Angiosarcoma formation can affect the brachialis muscle, in particular, in deep tissues. Angiosarcoma may present as a deep swelling or hematoma; a bioptic test is required to understand the nature of the swelling.

Tendinopathy of the brachialis muscle at its insertion is a rare event. Generally, the symptom is a pain in the antecubital area, and a conservative therapy (steroid injection) is the recommended approach to management.