Comparative anatomy of the flexor digitorum brevis in hominids

The flexor digitorum brevis (FDB) is a key intrinsic foot muscle that contributes not only to toe flexion but also to energy absorption and generation during bipedal locomotion in humans. Despite its functional importance, the homology among hominid species and morphological alterations during evolution in each species of the FDB remain insufficiently understood. A more detailed morphological analysis, particularly a comparative analysis of origin sites and innervation, is necessary to clarify the homology of FDB among hominids. In this comparative anatomical study, the feet of eight humans, three chimpanzees, and three orangutans were analysed. In humans, the FDB typically originates from the calcaneal tuberosity and plantar aponeurosis and is innervated by the medial plantar nerve (MPN). However, in one case, dual innervation from both the medial and lateral plantar nerves (LPNs) was observed. The accessory muscle belly from the flexor digitorum longus tendon, found in four human cases, was inserted variably into digits IV and/or V, similar to the FDB tendon, and was innervated by the MPN in all cases. In chimpanzees, the FDB originates from two sites, the calcaneus and flexor digitorum tibialis (FDT) tendon, both innervated by the MPN in all cases. In one orangutan specimen, the FDB was divided into two bellies originating from the calcaneus: one inserted into digit II and was innervated by the MPN, while the other merged with the flexor digitorum fibularis tendon to digit III and received dual innervation from both the medial and LPNs. In another orangutan case, the FDB originated from the calcaneus as a single muscle inserted into digits II-IV and was innervated by the MPN, with the accessory belly from the FDT tendon innervated by the LPN. The third orangutan case showed a single FDB into digits II-IV innervated by the MPN and dual innervation of the muscle belly originating from the FDT tendon via the medial and LPNs. These findings suggest that the dual-origin sites and innervation of the FDB, which was previously considered a variation in humans, may be a common morphological feature shared by hominids. The combination of dual-origin sites and innervation accounts for the greater diversity of the muscle compared with those with a single origin site or innervation. The characteristics in humans may be consistent with the morphological alterations of the foot, which are adaptations to habitual bipedal locomotion, such as a robust and posteroinferiorly elongated calcaneal tuberosity and a medial shift of the functional foot axis.