Accessory Belly of the Biceps Brachii Identified During Cadaveric Dissection: A Literature-Based Case Review

The biceps brachii is classically described as a two-headed flexor–supinator in the anterior compartment of the arm, but multiple studies show that variants with three or more heads are common in human populations.^{1, 2} Accessory bellies (supernumerary heads) typically arise from the humerus and join the main muscle mass or distal tendon, altering muscle bulk, line of pull and the local relationships to the terminal branches of the brachial plexus.

The recent meta-analysis by Benes et al. pooled 6,955 upper limbs and reported accessory heads in 9.6% of limbs (95% CI 8–11%), with a single humeral accessory head being the dominant configuration (8.4%).¹ These variants intersect intimately with the musculocutaneous nerve, median nerve and brachial artery, and can therefore generate compressive neuropathies or unexpected findings in trauma and reconstructive surgery.^{2, 7}

This case-review article synthesises published cadaveric reports and clinically documented accessory biceps bellies, placing them in the context of larger prevalence studies and highlighting the recurrent patterns of neurovascular risk relevant to surgeons, radiologists and anatomists.

A focused literature search was undertaken in PubMed, Scopus and Google Scholar (coverage to 8 December 2025) using combinations of the terms “accessory head” and “biceps brachii,” “supernumerary head” and “biceps,” “third head of biceps brachii,” “four-headed biceps brachii,” and “accessory bicipital aponeurosis” with “median nerve entrapment.”

Inclusion criteria were: (1) human cadaveric or clinical studies; (2) a clearly described additional muscular belly or tendinous slip attributable to the biceps brachii; and (3) at least one of the following: origin, course, insertion, innervation or explicit relationship to the median nerve, musculocutaneous nerve or brachial artery. Large cadaveric series and the systematic meta-analysis by Benes et al. were treated as quantitative backbone data for prevalence estimates, whereas individual case reports and small series were used to map the morphological extremes and clinical consequences.^{1, 2}

For each eligible report, data were extracted on limb side, sex, number of biceps heads, precise humeral or fascial origin, distal attachment, innervation pattern, and relationship to surrounding neurovascular structures. Clinically oriented cases were additionally coded for symptom pattern (pain, neuropathy, vascular compromise), imaging findings and surgical outcome where reported.

The meta-analysis by Benes et al. reported accessory heads of the biceps brachii in 9.6% of limbs overall, with a dominant single humeral accessory head and rarer multiple-head configurations.¹ Rodríguez-Niedenführ et al. revisited 175 cadavers and found supernumerary humeral heads in 15.4% of cases, emphasising their frequency and proposing a three-type classification (superior, inferomedial and inferolateral humeral heads).²

Regional series broadly support these pooled estimates. Radhika et al. observed accessory heads in 10% of 50 upper limbs in a South Indian cohort,³ while Kosugi et al. identified supernumerary heads in approximately 12% of Japanese arms and documented a strong association with branching variants of the musculocutaneous nerve.⁴ Collectively, these data indicate that accessory bellies are common variants of the anterior arm rather than isolated curiosities.

Across cadaveric case reports and series, a consistent pattern emerges: most accessory bellies arise from the anteromedial humeral shaft between the insertion of coracobrachialis and the origin of brachialis and then join the common bicipital mass or tendon distally.^{2, 9} Less common origins include the anterolateral humerus, the medial intermuscular septum, slips from brachialis fibres, or fascial expansions related to pectoralis major.^{2, 5}

The accessory head usually blends with the main muscle belly or inserts into the bicipital tendon and/or aponeurosis, increasing local muscle bulk proximally or near the elbow joint.^{2, 3} While three-headed biceps configurations are most frequent, four-headed biceps and rarer higher-order arrangements have been repeatedly documented.^{3, 10}

Manjatika et al. recently reported third heads in 20.2% of dissected upper limbs in a Southern African cohort and highlighted substantial variability in shape (rectus, triangular, flat and round profiles), underlining how accessory bellies can occupy different spatial niches within the anterior arm compartment.⁹

In most anatomical studies, accessory bellies receive branches from the musculocutaneous nerve, consistent with the usual innervation of the biceps brachii.^{2, 3} However, variant innervation patterns are clinically important. Benes et al. noted median nerve innervation in a minority of accessory heads across pooled data, correlating with unusual motor patterns and potential entrapment sites.¹ Al-Kushi described a third head innervated by the median nerve in human dissection, providing further evidence of developmental overlap between median and musculocutaneous motor territories.⁵

Several case reports describe accessory bellies coexisting with duplicated or communicating musculocutaneous and median nerves, particularly in Japanese and South Asian series, which may complicate nerve blocks or nerve transfer procedures.^{4, 11} Arterial supply generally arises from muscular branches of the brachial artery, mirroring the main biceps supply, as demonstrated in both classical classifications and more recent cadaveric work.^{2, 12}

Clinically oriented case reports cluster into a few reproducible mechanisms of neurovascular interaction. First, accessory bellies may cross superficially to the median nerve, as in the original description by Mas et al., forming a potential entrapment site in the mid-arm.⁸ Second, robust slips from an accessory head can form a tunnel through which both the median nerve and brachial artery pass, creating a fixed fibro-muscular canal that narrows in elbow flexion, as described in bilateral four-headed biceps cases.¹⁰

Third, accessory bicipital aponeuroses in the cubital fossa have been implicated in proximal median neuropathy; Kraemer et al. reported symptom resolution after surgical division of such an accessory aponeurosis.⁷ Fourth, in some three-headed biceps configurations the musculocutaneous nerve runs between or through variant heads, increasing its vulnerability during approaches to the coracobrachialis–biceps interval and during fracture fixation of the humeral shaft.^{4, 11}

Finally, as highlighted by Heo et al., supernumerary heads can substantially remodel the topography of the anterior arm muscles without overt neuropathy, reminding clinicians that many variants remain subclinical yet still relevant to surgical and imaging interpretation.¹²

Classifications by Rodríguez-Niedenführ and others divide supernumerary heads into superior, inferomedial and inferolateral humeral types, based largely on their origin and relationship to coracobrachialis and brachialis.^{2, 9} Most case reports in this review, including four-headed variants, fit within the inferomedial pattern, arising deep to the short head and coursing distally towards the common bicipital mass. Functionally, such accessory bellies increase the physiological cross-section of biceps and shift the local force vectors for elbow flexion and forearm supination.^{2, 12}

From a clinical standpoint, attention should focus on variants that create rigid tunnels or tight grooves around the median nerve, musculocutaneous nerve or brachial artery. These are likely to become symptomatic in individuals performing repetitive elbow flexion or resisted forearm movements, and they explain “high” median neuropathies not accounted for by lacertus fibrosus alone.^{7, 10} High-resolution MRI and ultrasound of the anterior arm and cubital fossa should therefore be interrogated specifically for anomalous bicipital slips in cases of unexplained neuropathy.

For surgeons, awareness that accessory heads are present in roughly one in ten limbs means they should be considered part of the expected spectrum of anatomy during approaches to the humerus, tendon transfers around the elbow and decompression of the median nerve. For regional anaesthetists, variant musculocutaneous nerve courses associated with accessory bellies provide a plausible explanation for occasional block failure when standard landmarks are used.^{4, 11} For anatomists, documenting these variants during dissection and correlating them with nerve trajectories reinforces the concept that the textbook two-headed biceps is only one of several recurrent human patterns.

Accessory bellies of the biceps brachii are common anatomical variants, with meta-analytic prevalence around 8–10% and some populations showing rates above 15%.^{1, 3} Morphologically, they most often arise from the anteromedial humerus and blend into the common bicipital mass, but clinically important minority patterns include median nerve innervation and multi-headed configurations that remodel the anterior arm.

While many of these variants are incidental findings at dissection or imaging, a subset forms fixed fibro-muscular tunnels in the arm and cubital fossa that predispose to neuropathy of the median nerve or compression of the brachial artery during elbow motion.^{7, 10} Surgeons, radiologists and neurologists should therefore regard accessory biceps bellies not as rare curiosities but as predictable variants that warrant systematic consideration in anterior arm surgery, high median neuropathy and atypical imaging appearances.