Intramuscular nerve distribution pattern in the human tibialis posterior muscle.

The human tibialis posterior muscle (TPM) has developed to maintain the foot arches for adopting bipedal locomotion. The insertion tendon of this muscle is U-shaped in a cross section, and the fibular part of the muscle whose muscle fibers originated from the fibula has a unique architecture. To understand the developmental history of the human TPM, distribution pattern of intramuscular nerves was investigated in ten sides of eight cadavers. Muscular branches entering the TPM could be classified into five types according to the distribution pattern in the muscle. The nerves innervating the part deeper to the insertion tendon ran transversely in the medial direction toward the tibia. Muscular branches innervating the fibular part ran medioinferiorly, and in four cases had communicating rami with the nerve innervating the other parts of this muscle. Muscular branches innervating the superficial layer whose muscle bundles originated from the tibia ran laterally toward the fibula. These results suggest that the fibular part might develop from the common primordium of the TPM, and the deeper layer of the muscle might expand laterally during the development to acquire a new attaching space on the interosseous membrane and fibula. The superficial layer might also develop laterally to acquire the additional attaching spaces on the fascia of the flexor digitorum longus muscle and flexor hallucis longus muscle, thus the insertion tendon might become U-shaped. With these measures, the TPM might be able to increase muscle volume to yield stronger power.