The muscles of the body are held in place by a tissue known as fascia. They are fibrous sheets of connective tissue that what really hold the body together. Normally there is some movement or give in this fascial tissue so that when exercising and muscles expand the fascia moves to accommodate that expansion. In some people the fascia is actually so tight that when exercising and the muscles try to expand they cant and this becomes painful. In the very worst cases of this the expansion of the muscle in the tight compartment results in the blood supply being cut off. This can be very serious. It can also happen in cases of trauma if the muscle expands in a too tight compartment. In runners the most common place to get a compartment syndrome is in the front of the leg where it is called anterior compartment syndrome. Basically it hurts on the front of the leg when running as the muscles expands and stops hurting when you stop running. It is not like a tendon injury where the symptoms persist after stopping the exercise. A clinician can organise testing of the pressures in the compartment to confirm the diagnosis.
Traditionally conservative measure do not work very well to treat this and most cases either resulting in the person just giving up running or having surgery. The surgery involved cut the fascial compartment so that the muscles have room to move. More recently there has been a big trend to move the runners from being a more traditional heel striker to start contacting the ground with the midfoot. This has been shown in two studies to be a very successful way to manage anterior compartment syndrome. The reason for this is that the actual muscle that is involved doe not have to work so hard when forefoot striking compared to heel striking.