The foot and ankle are composed of multiple bones with multiple articulations that function in complex ways. However, in biomechanical studies that do not focus specifically on foot motion, the foot is often considered as a single unit. Multi-segment models are becoming more common in the biomechanical studies of gait (walking/running) and foot function.
There are several ways to anatomically and functionally divide the foot.
Phalanges (singular: phalanx) are the 56 finger and toe bones. Each of the toes has three phalanges (proximal, middle, and distal) while the big toe (hallux) has two (proximal and distal). In total, over one-quarter of the bones in the human body are phalanges.
There are seven tarsal bones in the foot. In the rear foot, the talus articulates above with the concavity formed by the union of the distal tibia and fibula of the shank to form the ankle joint. Below, the talus articulates with the calcaneus to form the subtalar joint. The axes of these and other major joints of the foot run oblique to the cardinal planes; therefore, they have motions that cross all three planes.
The ankle joint axis runs mostly medial-lateral and primarily allows sagittal plane up-and-down movement of the foot. The subtalar joint axis runs nearly anterior-posterior and is also tilted upward. The upward tilt of approximately 45° rising from posterior to anterior allows movement and associated forces to transfer between the horizontally oriented foot and the vertical tibia. The primary motion of the subtalar joint is frontal plane inversion and eversion, where the sole faces inward and outward, respectively. Numerous ligaments surround the ankle and subtalar joints, binding the bones of the leg to each other and to those of the foot.
