The talonavicular joint and its surrounding connective tissues form the keystone of the medial longitudinal arch. Several non-muscular structures contribute to maintaining the overall structure and height of the medial longitudinal arch, including the plantar fascia, the spring ligament, and the first tarsometatarsal joint. Among these, the plantar fascia is the most critical passive structure supporting the medial longitudinal arch. This fascia is composed of robust longitudinal and transverse fibrous bands rich in collagen, forming a complex structure that covers the entire plantar surface and extends to the lateral aspects of the foot.
The plantar fascia can be divided into superficial and deep fibrous layers. The superficial fibers primarily attach to the thick dermis layer, while the broader deep plantar fascia originates from the medial tubercle of the calcaneus posteriorly. As it progresses anteriorly, it divides into three main fiber bundles: medial, lateral, and central. These fibers are tightly integrated with the first layer of intrinsic foot muscles, creating a cohesive structure. Among these, the central fibers are the largest and most functionally significant. They extend toward the heads of the metatarsals and firmly attach to the plantar plates (ligaments) of the metatarsophalangeal joints and the fibrous sheaths of the flexor tendons. This anatomical configuration allows the central fibers of the deep fascia to stretch during active toe extension, generating additional tension in the medial longitudinal arch. This mechanism is especially vital during activities such as standing on the toes or the push-off phase of gait.
In a normal standing posture, body weight is transmitted through the talonavicular joint and effectively distributed along the medial longitudinal arch in an anterior-posterior direction. Ultimately, this force is transferred to the ground via the fat pads and thick dermal tissues of the heel and forefoot (Figure A). Typically, the heel region experiences approximately twice the compressive force of the forefoot region, with the highest pressure observed under the heads of the second and third metatarsals.
During weight-bearing in an upright position, the talus is compressed downward, exerting a flattening effect on the medial longitudinal arch. This stress increases the distance between the calcaneus and the metatarsal heads. At this point, the tensile force generated in the stretched connective tissues, particularly the deep plantar fascia, acts as a semi-elastic tie-rod, effectively limiting the excessive flattening of the arch (represented as the stretched spring in Figure A). This truss-like structure is essential for efficiently supporting and absorbing body weight. Cadaveric studies have demonstrated that the deep plantar fascia plays a crucial role in maintaining the height of the medial longitudinal arch, as severing this structure reduces the arch's structural stability by approximately 25%.
When the arch is loaded, the heel typically exhibits slight eversion, observable from the rear as the calcaneus tilts slightly outward relative to the tibia. Once weight is removed from one foot during gait, the elastic and flexible arch structure returns to its original height, while the calcaneus returns to a neutral position with slight inversion. This restoration prepares the foot to absorb impact during the subsequent weight-bearing phase.
In healthy feet under comfortable standing conditions, the activation of intrinsic or extrinsic foot muscles is generally minimal. As illustrated in Figure A, the shape and height of the medial longitudinal arch are primarily maintained by the passive resistance of connective tissues. Active muscular support comes into play only in specific situations requiring a "secondary line of support," such as when lifting heavy objects or when the inherent support of the connective tissues is compromised due to excessive elongation.