Most problems of the foot and ankle can be drawn back to lack of ankle dorsiflexion, and tibialis posterior tendonitis is no exception. It can be excruciating, and mistaken for other pathologies, even knee pain or meniscus tears. The origin of the muscle is just below the knee, runs down the shin medially (inside), takes a 90 degree turn behind the ankle, and inserts into the arch. It is an ankle inverter and plantarflexor, but more importantly, it helps form the arch.

If you are missing ankle dorsiflexion, your foot will turn outward when you walk. As you are toeing off, your heel whips back to make up for the outward position. This motion biases overuse of posterior tibialis.

Furthermore, if you are flatfooted, the inside ankle puts pressure on the tendon like a bowstring, causing more irritation. Now, this is a bit of a chicken or the egg scenario. Because of the role this muscle in creating an arch, you can make the case that an inefficient tibialis posterior causes a flat foot. Or, that a flat foot creates overuse of tibialis posterior and puts pressure on it. Either way, the treatment is the same. Reduce pain first, so you can function. Isometrics are good at cooling down tendonitis. Try this simple exercise:

Then you want to restore dorsiflexion. Try these two exercises:

Also, having a flat foot is not a life sentence. You can improve your arch through strengthening tibialis posterior when the pain is gone, and by strengthening the foot intrinsic muscles. Try this out:

You can try an orthotic initially, to form an arch and reduce pressure on the muscle. However, you should aim to rebuild your arch through exercise and eventually ditch the orthotic. 

Want to work on your ankles at home? Check out our ankle program: