Hallux is the Latin term for the big toe of the foot. Situated along the inner side of the foot, it is the largest of the toes, although not always the longest. The hallux is the toe humans push most of their weight off when walking, and it contains two bones, the proximal and distal phalanges, which permit the movements of flexion and extension at the joint between them, the interphalangeal joint, as well as at the joint between the proximal phalanx and the foot, the metatarsophalangeal joint.
This toe, referred to in anatomy as the first digit of the foot, is made up of two bones known as phalanges, whereas the four smaller toes have three apiece. The nearest phalanx of the hallux is known as the proximal or first phalanx, while the farthest bone is known as the distal or second phalanx. Between them is a ginglymoid or hinge joint called an interphalangeal joint, which is capable of the movements of flexion and extension, or the curling and straightening of the toe. This joint is held together by the plantar and collateral ligaments, with the plantar ligaments stretching between the bones on the underside of the foot and the collateral ligaments running as a pair between the bones on either side of the joint. The dorsal or top of the interphalangeal joint is held together by the tendons of the muscle that extends the toe.
At the near end of the first phalanx, the hallux meets the first metatarsal bone, one of the long bones of the midfoot, at a joint known as the first metatarsophalangeal joint. This joint features an oval-shaped surface — the distal end of the metatarsal — that inserts into a cavity of a corresponding shape at the proximal end of the first phalanx. These two bones are also held together by ligaments, a plantar and a pair of collateral ligaments. The shape of this joint allows movement in two planes: flexion and extension, and abduction and adduction, or the spreading and drawing in of the hallux, respectively. The latter movements are only very slight, because actually the smaller toes move laterally with respect to the big toe.
To the outside of the two phalangeal bones are the tendons of the muscles that attach to the hallux. The flexor hallucis longus muscle curls the toe. This muscle originates on the back of the fibula bone in the lower leg and attaches its tendon to the plantar surface of the distal phalanx. The muscle that extends the toe at both joints is the extensor hallucis longus, which similarly begins on the front of the fibula and attaches via a tendon to the dorsal surface of the distal phalanx of the hallux. Preventing this muscle from hyperextending the big toe are the plantar and collateral ligaments of both joints, which therefore help prevent toe injury during walking, running, and jumping movements.