On a beautiful day, Kendra goes out for a run. A casual few miles at a relatively easy pace. She has been training on the River Walk religiously after work. The serenity of the sound of her stride on the boardwalk, the wind off the river. is the perfect way to melt away the stress of work and think about her next marathon. Kendra visualized the finish line, picturing clocking in just ahead of her goal time, getting that medal, all of her hard work paid off. Suddenly her daydreams are cut short by a vague ache in her left foot. It seems in perfect cadence with the sound of that foot hitting the boardwalk. She wonders what happened. It doesn't seem that bad though, so she finishes her run.

The next day, she gets out of the shower and notices that her left foot looks a little swollen. There is a hint of color on the top of her foot, not quite a bruise. She starts to worry. Later that day she has another run planned. But by the end of the day, she finds herself taking her left shoe off at her desk. She wonders why it aches whenever she walks down the hall. That evening, after work she heads out for a run, but the aching turns into a throbbing pain only a mile into the run. She turns around and runs back to the car, wondering if she will have to cancel the hotel room and try to get a partial refund for the entry fee.

This is the classic story of a stress fracture. They happen with all kinds of athletes ranging from pathologically over-trained gymnasts to casual runners. The reason is simple. The amount of stress applied exceeds the body's ability to withstand the load. And something has to give.

If you are a physics geek, you know about Wolf's Law. This law states that when the load applied to a particular bone increases, the external cortical (load-bearing) portion of the bone becomes thicker and stronger as a result. In short, the bone will remodel itself over time, becoming stronger and better able to resist that sort of stress. This increases the bone's capacity to withstand longer and longer runs throughout a period of distance run training. It is also the rationale behind weight-training, and other exercise programs to fight the gradual bone-loss associated with osteoporosis. This of course is a good thing. However, it is possible for this to backfire. And when it does, a stress fracture will result.

When someone comes in to my office with a stress fracture, they usually have some vague pain in the top of the mid-foot, but can't seem to pinpoint the discomfort. They have some swelling, but not a great deal. They seem to remember some bruising, but only when prodded. They usually don't connect the bruising with the onset of pain. They do always seem to discern that running makes it significantly worse. Interestingly, they have also continued to run on it anyway.

Some are smarter than others, and some are tougher than others. I see the full range. The sensible ones come in after only a few aching runs, and having noted some swelling that seems to get worse walking around at work. Some will run until they can barely walk. I had one of theses hobble in after running on it for eight weeks. It was a mess. After some creative surgery and a bone stimulator he is almost back in action, yet the season is all but over. However, it does not have to be that way.

Stress fractures are prevented, first and foremost, by sensible training. Your bones must have the right combination of load (gradually increasing long runs), rest (including light days and adequate sleep), and good nutrition in order for Wolf's Law to prevail. Otherwise your body cannot increase the strength of the bones fast enough to stave off the tiny little crack in the bone that is a stress fracture.

When a stress fracture first occurs, (like in Kendra's case) the crack is not even visible on X-Ray. It can take 4-10 weeks for it to actually show up. If however you continue to run, all of that pounding causes the tiny little crack to become a big crack and then fracture all the way through. This leads to two pieces of bone that may or may not want to get together again. This is clearly visible on X-Ray, but not good. This can lead to surgery (which by the way, is lots of fun for me, but not so much for the patient).

Characteristics that are proven risk factors for stress fractures include high arches, being of female gender, having a wider pelvis, and a limb length discrepancy (meaning one leg is longer than the other). You can increase this risk by running only one side of the road with a steep shoulder (like consistently running against traffic), wearing an inappropriate type of running shoes (not enough cushion/too much motion control), and running on hard surfaces. Runners who might get stress fractures are more likely to encounter problems who are more prone to shin splints, ignore their rest days and have a tendancy to run hard on light days, and add lots of hill training.

I personally believe that a Type-A personality peppered with a combination of blind determination and a penchant for goal attainment is also a solid risk factor, but this has never been studied. The reality is that all distance runners have some measure of these traits or they wouldn't think it is a good idea (much less fun) to train for a marathon. But that is why we follow a plan...with planned rest days...to protect ourselves from the innate desire to train more, run longer and "ensure success through greater effort."

If you think you might have a stress fracture (pain worse with running, goes away with rest, maybe some swelling and/or bruising) get checked out. It is easy to fix when it first starts without too much lost training. If however you ignore that aching, sometimes throbbing, pain with each stride, you may find yourself in Kendra's position: thinking of canceled hotel rooms and partial refunds. Instead, play it safe so you can be back on the road, dreaming of the finish chute, friends and family screaming encouragement, a new shiny medal hanging around your neck.