Foot Anatomy and Biomechanics (Arch, Windlass Mechanism, Gait Cycle)

The hindfoot (talus, calcaneus) meets the midfoot (navicular, cuboid, three cuneiforms) at the transverse tarsal (Chopart) joint, and the midfoot meets the forefoot (metatarsals, phalanges) at the tarsometatarsal (Lisfranc) joint. The bones form a medial longitudinal arch (calcaneus-talus-navicular -cuneiforms-medial three metatarsals; higher, more mobile), a lateral longitudinal arch (calcaneus-cuboid-lateral two metatarsals; lower, flatter, contacts the ground) and a transverse arch (across the cuneiforms/metatarsal bases). The arch is held by three layers: bony (wedge-shaped bones, with the talar head/navicular the medial keystone); ligamentous - the plantar fascia, the spring (plantar calcaneonavicular) ligament supporting the talar head, and the long and short plantar ligaments; and muscular/dynamic - the intrinsics and especially tibialis posterior (whose failure causes adult acquired flatfoot), with peroneus longus and the long toe flexors.

The plantar fascia (aponeurosis) runs from the medial calcaneal tubercle to the bases of the proximal phalanges, passing beneath the metatarsal heads. When the metatarsophalangeal joints dorsiflex - as the heel rises and the toes extend at push-off, the hallux being most important - the fascia is wound around the metatarsal heads like a cable around a drum (a 'windlass'), which shortens the distance between the calcaneus and the metatarsal heads, elevates the medial longitudinal arch, and inverts the hindfoot, stiffening the foot into a rigid propulsive lever. Clinically, passively dorsiflexing the hallux and watching the arch rise is Jack's (Hubscher) test, used to assess a flexible flatfoot and the integrity of the windlass.

The arch is not only a windlass but an elastic arch-spring: the plantar fascia and the other arch tissues stretch and store strain energy as the arch flattens under load in early-to-mid stance, then recoil and return that energy at push-off, improving the efficiency of walking and running. High-speed imaging shows the plantar fascia is not perfectly isometric - it strains through stance - yet still behaves as a near-ideal windlass during propulsion, with its shortening timed to enhance arch recoil at push-off. This energy-storage role also explains why the fascia is highly loaded (and prone to overload/plantar fasciitis) at the medial calcaneal tubercle.

• Plantar fasciitis: overload of the highly stressed plantar fascia at the medial calcaneal tubercle (modelling shows peak fascia tension late in stance, increased substantially by the windlass).
• Pes planus (flatfoot): failure of arch supports - tibialis posterior dysfunction (dynamic) and/or spring-ligament failure (static) - the hindfoot stays pronated and the arch collapses; Jack's test and the double/single heel-rise test assess flexibility and tibialis posterior.
• Pes cavus: a high, rigid arch (often neuromuscular) overloads the heel and metatarsal heads.
• Hallux rigidus / first-ray problems: loss of MTP dorsiflexion impairs the windlass and the forefoot rocker.
• Surgery: arch-restoring procedures (e.g. medialising calcaneal osteotomy, tendon transfers, lateral-column lengthening) and fusions are designed around restoring this bony-ligamentous-muscular balance and the flexible-to-rigid mechanics. (See our Plantar Fasciitis, Adult Acquired Flatfoot / PTTD, Pes Cavus and Hallux Rigidus topics.)