Structural and functional analysis of bilateral five-headed biceps brachii muscles with clinical insights

Introduction: The biceps brachii (BB) muscle is a two-headed muscle of the arm composed of one long head (LH) and one short head (SH). Accessory heads have been described and classified; however, reports of three concurrent accessory heads are especially rare, and their structural and functional analyses still require investigation. This study aims to analyze the structural and functional properties of a bilateral case of BB muscles each with three accessory heads. Methods: A case of bilateral BB muscles with three accessory heads were discovered during routine human cadaver dissection. The muscles were carefully cleaned of extraneous tissue and photographed in situ. All five heads were examined bilaterally for gross structural features, and sarcomere states were examined via light microscopy. Maximal isometric force (F_max) and contralateral comparability (δ) were calculated for each head. Results: The three accessory heads originated sequentially from the mid-shaft of the humerus. The proximal accessory head inserted into the distal end of the SH, while the middle and distal accessory heads inserted separately into the adjoined LH and SH muscle mass just before the common tendon. All accessory heads were structurally similar (δ ≤ 0.17; mass = 2.10 g; length = 10.33 cm; PCSA = 0.23 cm²; F_max = 5.24 N) and were innervated by the musculocutaneous nerve. The left and right LH and SH collectively generated a F_max of 49.91 N and 53.05 N while the left and right accessory heads collectively generated a F_max of 18.04 N and 13.38 N, respectively. Conclusions: The bilateral BB muscles with their accessory heads generated an almost equivalent total F_max (67.95 N vs. 66.43 N, respectively), ∼23.35% of which was generated by the accessory heads. Despite the conceivable advantage the accessory heads may offer to generating additional force, accessory muscles can also cause serious clinical implications. The gross imaging, biomechanical analyses, contralateral comparability data, and relevant clinical insights presented in this study may help guide diagnosis and treatment in related clinical cases.