The tendon is posterior to ankle axis and medial to the STJ axis which makes it a plantarflexor and invertor. Additionally, it directly opposes the action of the peroneus brevis muscle by acting as an adductor of the forefoot. It is the primary dynamic stabilizer of the medial longitudinal arch.
The closed chain foot adduction exercise is one of the more effective ways of strengthening the posterior tibialis. In 2004, Kulig, et al. conducted a study using magnetic resonance transaxial imaging to find the signal intensity of the posterior tibialis muscle during 3 exercises (closed chain foot adduction, unilat. heel raise, and open chain foot supination). What they discovered was that in individuals with a normal arch index, the posterior tibialis was most activated during closed chain foot adduction when compared to the other two exercises mentioned above.
To perform this exercise, place a theraband around the leg of a table/therapy plinth and the patient's foot. Instruct them to keep their foot on the ground throughout the exercise. Teach them to perform foot adduction. Be sure to look for substitutions as they are often common when prescribing this exercise.