The tendons of these muscles merge to be inserted onto the calcaneus as the Achilles tendon. The plantaris originates on the femur proximal to the lateral head of the gastrocnemius and its long tendon is embedded medially into the Achilles tendon. The triceps surae is the primary plantar flexor. Its strength becomes most obvious during ballet dancing. It is fully activated only with the knee extended, because the gastrocnemius is shortened during flexion of the knee. During walking it not only lifts the heel, but also flexes the knee, assisted by the plantaris.In the deep layer of posterior muscles, the tibialis posterior arises proximally on the back of the interosseous membrane and adjoining bones, and divides into two parts in the sole of the foot to attach to the tarsus. In the non-weight-bearing leg, it produces plantar flexion and supination, and, in the weight-bearing leg, it proximates the heel to the calf. The flexor hallucis longus arises on the back of the fibula on the lateral side, and its relatively thick muscle belly extends distally down to the flexor retinaculum where it passes over to the medial side to stretch across the sole to the distal phalanx of the first digit. The popliteus is also part of this group, but, with its oblique course across the back of the knee, does not act on the foot. Intrinsic
On the top of the foot, the tendons of extensor digitorum brevis and extensor hallucis brevis lie deep in the system of long extrinsic extensor tendons.
The inside support area is marked by strong greyish material to support the weight when a person lands on the outside foot and then roll onto the inside foot. Underpronation (supination)
An individual who underpronates also initially strikes the ground on the lateral side of the heel. As the individual transfers weight from the heel to the metatarsus, the foot will not roll far enough in a medial direction. The weight is distributed unevenly across the metatarsus, with excessive weight borne on the fifth metatarsal, toward the lateral side of the foot. In this stage of the gait, the knee will generally, but not always, track laterally of the hallux. Like an overpronator, an underpronator does not absorb shock efficiently – but for the opposite reason. The underpronated foot is like a diving board that, instead of failing to spring someone in the air because it is too flimsy, fails to do so because it is too rigid. There is virtually no give. An underpronators arches or ankles dont experience much motion as they cycle through the gait. An individual whose bone structure involves internal rotation at the hip, knee, or ankle will be more likely to underpronate than one whose bone structure has external rotation or central alignment. Usually – but not always – those who are bow-legged tend to underpronate. An individual who underpronates tends to wear down their running shoes on the lateral (outside) side of the shoe toward the rear of the shoe in the heel area.