What is it?
Basically any pain in the lower legs is referred to as shinsplints by the majority or runners. The unfortunate thing is, this pain could be a variety of different things. What you're feeling could be anything from some inflamed tissues to a stress fracture. The most common type of shin pain, however, is medial (inside of the leg) pain. Other types of shinsplints can be felt on the anterior side (outside) of the leg. In some cases, it will actually be a stress fracture and the pain is more often radiating around the leg. However, all of these pains are usually brought on by one thing, too much too soon.

What caused it?
A sudden increase in mileage or intensity usually brings about these symptoms, yet this is often an underlying cause to these different types of pain. First step is to identify what you are actually suffering from.

• Posterior Shinsplints
  This is the most common pain which runners experience in their lower legs. It is felt on the inside of their leg, ranging from a small point of sensitivity to the entire side of the leg filled with pain. If one tries to pin-point the pain, it is usually in the tissues and not in the bone. Pronation is usually the cause of the inflamation. The pronation of one's foot, if not controlled with the right shoes or orthodics, puts lots of stress on the tissues in the area. Specifically, the tibialis posterior is a common location for the inflamation. As this is usually soft-tissue damage, it will be more painful in the morning when you wake up.
• Anterior Shinsplints
  In this case, pain is felt around the outside of the leg more often. The location of the pain is usually in the tissue around the bone too, but can feel like it is on the bone itself. It is generally agreed that this is caused by the calf muscles being too strong or too tight for the shin muscles. This puts too much stress on them, resulting in pain. Like Posterior Shinsplints, this is soft-tissue damage so is more severe in the morning.
• Compartment Syndrome
  This is not really a form of shinsplints, but feels very similar to anterior shinsplints. In this case, there is swelling of muscles in closed compartments, creating too much pressure. The resulting pain can be very severe. It can be very hard to distinguish if one has compartment syndrome instead of Anterior Shinsplints. One must see a doctor for specialized tests to measure the pressure. On the bright side, however, this injury is not nearly as common as shinsplints.
• Stress Fracture
  A stress fracture is felt around the leg, as more of a radiating pain. If one can locate the pain, it usually travels around the leg horizontally as opposed the the generally vertical line of sensitivity for other types of shinsplints. This can be caused by many different things, but over training is almost always a leading cause. Since this is damage to the bone itself, it will feel better in the morning because the bone has had a chance to rest. A stress fracture requires immediate attention and must be diagnosed by a doctor and usually a bone scan.

What to do?
The best immediate response to any type of shin pain is to stop or greatly decrease one's training. From there, seeing a doctor is probably best, so as to accurately identify what the injury is and the causes. In the mean time, icing will always help to reduce the inflamation. If one is suffering from Posterior Shinsplints, stretching the achilles will usually help to solve the problem. Stretching the calves is more beneficial for Anterior Shinsplints. For both of these situations however, strengthening the shins and muscles surrounding them is a must. Flexing your foot upwards, bringing your toes towards your shin, with some resistance is the easiest way to achieve this. For the other injuries though, medical attention is usually important and one should contact a sports medicine physician as soon as possible.