There are few injuries runners dread more than stress fractures. Not only are they painful, but they also often take months to heal. The time spent resting and recovering can mean precious time away from competition and ruined training regimens.

But there are also few injuries among runners — or at least among competitive track and field athletes — that are more common than stress fractures. Due to the high-intensity, high-volume training required, research suggests stress fractures occur in more than 1 in 5 competitive runners.

[See: 8 Lesser-Known Ways to Ruin Your Joints.]

As a former captain of the Ohio State University track and field and cross-country teams, I saw firsthand how stress fractures could impact the performance of our team as a whole and even end the careers of some of my teammates. I was well aware that stress fractures could occur in almost anyone at any time. Today, as a team physician, I am dedicated to better understanding why they occur and how to prevent them.

Predicting who is at risk for a stress fracture is not easy. Even the fittest, most careful runners are vulnerable. But I recently published a study shedding light on one factor that may be at play: Female runners, I found, seem to be at higher risk of these injuries if they are underweight.

My colleagues and I at The Ohio State University Wexner Medical Center spent three years studying Division I collegiate athletes, identifying 18 women from the same team who suffered stress fractures. We found that those who had a body mass index of less than 19 were not only more likely to have been injured, but their recovery times were also considerably longer. In fact, those with low BMIs were more likely to have been injured than those with average BMIs (22) or even higher BMIs, over 25.

To classify the degree of injury in the various stress fractures, we used the Kaeding-Miller classification system, a grading scale I developed with my colleague Dr. Christopher Kaeding. This system is unique in that it characterizes injuries on a scale of severity from 1 to 5. Additionally, it takes into consideration not only the runner’s symptoms but also utilizes X-rays, bone scans, computed tomography images and magnetic resonance imaging scans.

Among those with grade 5 stress fractures — which are the most severe — we found that women with normal BMIs or higher took 13.7 weeks to recover. Those with low BMIs (below 19) took 17 weeks to recover — a full month longer.

[See: 8 Health Challenges Facing Olympic Athletes.]

The reason women who weigh less for their heights were injured more often and more severely is a matter of physics. Running and other track and field events require training that subjects athletes to repetitive loading and biomechanical stress. Constant pounding on hard surfaces takes a toll on the legs — especially the shinbone, or tibia.

If a woman is light for her frame, there commonly is not sufficient muscle mass or soft tissue in her legs to dissipate the shock of impact on a hard surface like asphalt. The force is then absorbed and concentrated directly onto the bones. Over time and without adequate time for recovery, microfractures can develop in the bone, eventually progressing and coalescing into a complete fracture line across the bone.

Male runners and track and field athletes are at risk for similar injuries, but we female athletes are much more vulnerable. Because of that, it is imperative that women know their BMI and work to maintain a healthy level. They should also train to strengthen the lower leg to prevent injury, even if that means adding some weight from increasing muscle mass.

[See: 9 Useful Gym Machines for Women.]

Aiming to gain weight may seem counterintuitive to many runners, who assume that carrying extra pounds means adding more time to their race performance. This is not always the case, however, since what they view as “extra pounds” might actually add up to their ideal performance weight. Either way, whether you’re a collegiate athlete or a weekend warrior, the potential of adding a few seconds to your time surely beats missing months of training and racing while you recover from a stress fracture.

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How Skinny Is Too Skinny to Run? originally appeared on usnews.com