The tibia is the “shin bone”.  Posterior means behind.  The posterior tibial tendon sits on the inside of the left, just behind the shin bone.  It extends down the leg and wraps under the medial malleolus (the bump on the inside of your ankle) and finally attaches to the inside/bottom of the foot.  Tendinitis means inflammation, swelling and pain – so it may hurt anywhere along the tendon’s course.