If you dig your thumbs or fingers deep right off the tibia (shin bone) on the medial (inner) side, you'll hit Posterior Tibialis.
Its fibers are mostly layered behind the tibia, right next to the bone, deep under the calf muscles.
When an athlete adds too much training too soon, it's common for the Posterior Tib to tear off the bone (aka shin splints). This type of injury requires athletes to stop training, and start strengthening their ankles, their calf muscles, and of course the Posterior Tibialis.
Posterior Tib stabilizes the foot and the ankle. Its tendon extends around the medial ankle bone and attaches at several foot bones (Navicular and Medial Cuneiform) on the bottom of the foot.
Its main actions are to flex the foot down (Plantarflex) and turn the foot inward (inversion). These actions make your foot and ankle feel steady and not wobble.
Occasionally, the Posterior Tib tendon gets injured by too much repetitive motion or bearing too much weight.
Long-Term Fix: Strengthen It.
Massage is effective at sending blood flow to Posterior Tibialis and speeding up recovery.