The main problem with this muscle is that it is a support muscle that gets weaker as all of the muscles around it get stronger. This muscle originates on the back of the tibia and goes down the back of the tibia and inserts on the medial cuneform. An easy way to spot tibialis posterior problems is to look for a little "puffy spot" on the inside of the arch.
Treatment is easy. The problem is you may have to do it for as long as you don't want to have problems as you are predisposed to it. First, take a towel and place on the floor, take your barefoot and place on the towel and scrunch your toes toward your heel thus bunching up the towel. Next, get a red or green theraband about 5 feet long, tie it in a knot, take a seat, cross one leg over the other, place your other foot on the band with one leg, (with shoes on) place the band around the inside of the crossed foot. Allow band to pull your foot down and out, now sweep your foot in and toward you and repeat 15 times. I recommend twice a day. Repeat this on the other leg/foot because the problem is lurking over there.