Medial tibial stress syndrome is an overuse injury that is common in runners. The pain from this is usually felt when you run the finger and press hard down the inside of the tibial bone. The actual pathology is unclear as it probably varies from person to person. The problem is related the the attachment of the muscle to the bone, the periosteum which is the covering of the bone and is probably triggered by a bending or twisting of the bone with repeated stress of an activity like running.
The problem generally comes on when there is a too rapid increase in the number of miles run. if the increase is gradual and slow, then the muscle and bone can adapt to the increase in load. If it is too rapid the tissues do not adapt and things can break down leading to medial tibial stress syndrome. Typically there might be a problem with the running biomechanics that might be playing a role in the cause of the problem. Worn or old shoes have also been implicated at potentially being a problem.