Biomechanical evolutionary advantage of flexor pollicis longus accessory head and topographical association with neurovascular structures.

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The human upper limb has undergone various evolutionary myologic changes, accompanied by corresponding modifications in the anatomical course of neurovascular structures. In this study, we aimed to elucidate the emergence of the accessory head of the flexor pollicis longus (AHFPL) muscle as a beneficial biomechanical evolutionary development and its topographical relationship with adjacent neurovascular structures. In this pursuit to understand this phenomenon, dissections were conducted on sixty-two upper limbs from thirty-one cadavers. We found a strong positive correlation between the length of the AHFPL and the distance of the branching point of the brachial artery, ulnar artery, and median nerve into the anterior interosseous nerve. Additionally, we noted a significant negative correlation between the width of the AHFPL muscle and the angle formed between the long axes of the FPL and AHFPL. This increases the angle between the resultant vector and the FPL muscle, decreasing the angle between the AHFPL and the resultant force. Consequently, the resulting force shifts from the initial radial position to a slightly ulnar side, balancing the muscle forces at the same point. The authors hypothesize that this resultant vector provides a biomechanical advantage for the thumb, enhancing emphatic grip precision, providing extra power, and enabling meticulous cupping grip when using tools. Rather than considering the presence of AHFPL as a mere anatomical variation, it should be viewed as an advantageous evolutionary biomechanical development.