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2 3 contact with the ground (mid-stance phase), and we then push off with the medial side of the foot and big toe (push-off phase). As the individual moves through the mid-stance phase of the gait cycle, the leg moves over the planted foot in a diagonal direction. This diagonal plane movement produces a combination of dorsiflexion, eversion, and abduction of the foot called pronation. When an individual is too far onto the medial side of their foot during mid-stance phase, they are overpronating. The PT plays a primary role in resisting overpronation. Because the PT is a primary inverter of the foot, it resists the strong foot eversion during pronation through eccentric action of the muscle. It is this strong eccentric load that often leads to chronic overuse of the PT muscle-tendon unit. Another crucial function of the PT is its role as the primary dynamic stabilizer of the medial longitudinal arch. The longitudinal arch is created by the shape of the foot bones as well as the dynamic support from muscles and tendons (Image 3). It is critical for shock absorbency and propulsion of the body. When the PT is not functioning properly, the muscle becomes weak and the arch collapses. The collapsed arch is commonly referred to as flatfoot deformity, and dysfunction in the PT is the most common cause. Between 5 percent and 15 percent of people have flatfoot deformity, which would suggest that this condition is more common than many would think. 1 The plantar calcaneonavicular ligament (also called the spring ligament) is also an important support for the arch. When the PT is not functioning with its proper strength, the spring ligament must take on a much greater load in the arch. Overload on this ligament can lead to multiple biomechanical problems in the foot. In addition, excess body weight can overload the PT, as well as the spring ligament, and lead to a collapsed longitudinal arch. Loss of integrity in the arch structure can lead to numerous foot and lower extremity problems. Arch collapse is linked to disorders such as forefoot abduction, plantar fasciitis, stress fractures in the leg or foot, calcaneal or hallux valgus, and tarsal tunnel syndrome. 2 It is important to look at the role of the PT in these conditions because treatments may be aimed only at a local factor without taking into consideration the role of this muscle-tendon unit in the problem. Distal attachments of posterior tibialis. Image is from 3D4Medical's Complete Anatomy application. The medial longitudinal arch. Image is from 3D4Medical's Complete Anatomy application. Yo u r M & B i s w o r t h 2 C E s ! G o t o w w w. a b m p . c o m / c e t o l e a r n m o r e . 93 Distal tendon insertions The posterior tibialis is a major support for the longitudinal arch. MUSCLE-TENDON DYSFUNCTION Chronic overload of the PT tendon can lead to damage, weakness, and inflammation of the tendon around the medial ankle, called posterior tibial tendon dysfunction (PTTD). Women appear to develop this condition more frequently than men, although it is not clear why. Tendon degeneration and muscle weakness begin long before symptoms appear. The problem often becomes evident when the flat foot becomes apparent, along with foot or leg pain. It is not clear why the condition progresses so far before the symptoms are recognized. Small tears in the tendon can decrease its pulling strength, so it is likely there is a feedback loop between tendon fiber damage and decreased muscle stimulation. This combination can then lead to the eventual muscle weakness and collapsed longitudinal arch. Once the arch begins to collapse, more permanent foot deformities may result. For example, calcaneal valgus frequently occurs along with the flatfoot deformity and PT weakness (Image 4). Calcaneal valgus is a postural distortion in which the distal end of the calcaneus tilts in a lateral direction. Once the foot collapses into