22/09/2026
A healthy foot is not rigid. Foot biomechanics matter!
Biomechanics of the Medial Foot & Tibialis Posterior Mechanism
The image illustrates the medial longitudinal arch and the mechanical role of the tibialis posterior tendon in supporting and controlling the foot. The tendon passes behind the medial malleolus and inserts extensively in the medial midfoot, giving it an important role in controlling pronation, supporting the arch, and producing a more rigid foot during propulsion.
In the first illustration (a), the line of pull of the tendon is shown along the medial aspect of the foot. When the tibialis posterior contracts, it generates a combination of plantarflexion and inversion torque. Because its line of action lies medial to the subtalar and midtarsal regions, it can oppose excessive eversion/pronation and contribute to maintaining the height of the medial longitudinal arch.
The second illustration (b) emphasizes the importance of the tendon moment arm and its line of pull. The ability of a muscle to rotate a joint depends on the perpendicular distance between its line of action and the joint axis. Therefore, changes in foot position alter the effective moment arm and consequently change the amount of force required from the tibialis posterior to control the foot.
During pronation, the calcaneus everts and the talus tends to adduct and plantarflex relative to the foot. This combination can lower and lengthen the medial longitudinal arch. The tibialis posterior acts eccentrically during appropriate loading to help control the rate and magnitude of pronation, rather than simply preventing all pronation.
The third illustration (c) highlights the consequences of excessive mechanical stress around the medial foot. Repeated loading combined with insufficient muscular or ligamentous support can increase deformation of the arch. The tibialis posterior works together with the spring ligament, plantar fascia, intrinsic foot muscles, and other extrinsic muscles to resist this deformation.
The plantar surface illustrations demonstrate how the center and distribution of plantar pressure can change with foot mechanics. During normal gait, loading progresses from the heel toward the forefoot. The foot initially behaves as a relatively compliant structure for shock absorption and adaptation, then becomes progressively stiffer as the body moves toward push-off.
The windlass mechanism also becomes important during terminal stance. As the toes dorsiflex, particularly at the first metatarsophalangeal joint, the plantar fascia winds around the metatarsal heads. This shortens the functional distance between the heel and forefoot, elevates the medial arch, and increases foot stiffness. Tibialis posterior activity complements this mechanism by helping maintain medial-column stability.
The final illustration (d) can be interpreted as the relatively preserved arch configuration in comparison with the more deformed/loading conditions shown elsewhere. Importantly, a healthy foot is not rigid. Effective biomechanics require a balance between mobility for shock absorption and stiffness for propulsion.
