Lower but still normal bone mineral density scores and shorter limb length are predictive of bone stress injuries (BSI) in collegiate distance runners, according to a new study conducted by researchers at Houston Methodist and the University of the Kansas.

The study, which analyzed the medical records of student athletes over a recent 7-year period, also found that lower fat mass was associated with higher risk in males and less lean muscle in the legs was associated with higher risk among females.

"Regardless of what's clinically normal, the runners who had lower bone mineral density scores were significantly more likely to experience bone stress injuries," says Brad Lambert, Ph.D., manager of the Center for Human Performance Orthopedic Biomechanics Research Laboratory at Houston Methodist and the study's principal investigator. "Clearly, relative bone density is important, which is why specialized benchmarks for this population are needed."

The study, published in the Orthopaedic Journal of Sports Medicine in February, represents the first time bone mineral density, body composition and anthropometric parameters before initial collegiate injury have been compared between runners with BSI and their uninjured counterparts.

Researchers identified key risk factors and predictors of BSI based on the scores of 79 student athletes from the 2013-2019 seasons.

The injuries occur when repetitive, load-bearing activities such as running, dancing and marching place more strain on the skeletal system than a bone or joint can withstand, resulting in a range of issues from pain and swelling to stress fractures.

Lower limb BSI involving the pelvis, tibia and femur are especially common among collegiate track and field athletes. Long-distance runners sustain the highest rates annually.

Both intrinsic factors, such as bone strength and body characteristics, and extrinsic factors, including training volume and intensity, are thought to play a role in the development of these overuse injuries.

Comparing apples to apples

"In sports medicine, we naturally have an interest in studying the human body and finding ways to predict and prevent injuries, but general population norms and criteria for assessing risk are not always applicable to athletes," says Lambert.

For instance, athletes who run 50 miles a week will have significantly higher baseline bone mineral density (BMD) scores than average individuals who walk five miles a week.

When you limit studies to a specific athletic group, norms can be established, and useful patterns and findings emerge.

"You have to compare athletes against their exact peers: people who train the same amount, at the same level, with the same activities," says Lambert. "We've done similar studies on professional ballet dancers, who also have very high rates of bone stress injuries, and professional soccer players, who also run a significant amount per week, but their values are completely different."

Does density predict destiny?

In the retrospective case-controlled study, male and female distance runners from the University of Kansas track and field team who underwent total body dual x-ray absorptiometry (DXA) scans prior to the start of the fall season were sorted into two groups: BSI (those who received an MRI-confirmed lower limb BSI during the course of the season) and no-BSI.

Measurements included total bone mass, BMD (total body, spine, pelvis, femoral and tibial), percentage body fat, fat mass, lean mass (overall, legs, trunk and arms) and skeletal dimensions (shoulder width, trunk length, arm length, leg length and leg/trunk length ratio).

The overall rate of BSI among study participants was 23%, with 17% of males and 29% of females experiencing lower limb BSI.

Among those in the no-BSI group, total body, spine, pelvis and leg BMD measurements were significantly higher (p<.001 for all) than in the BSI group.

Although the injured group had relatively low bone density scores compared to the uninjured group, their age matched total BMD Z score (-0.1 ± 0.6) is considered clinically normal.

Additional risk factors for bone stress injuries

Besides bone mineral density, characteristics associated with an increased risk of BSI include shorter arm and leg length and lower leg/trunk ratio in both sexes; lower percent body fat in males; and lower lean muscle mass of the leg in females.

While the researchers aren't certain why these factors were independently predictive, they suspect the sex-based differences have to do with how the body responds during times of low energy availability.

As far as the anatomical differences are concerned, the research team believes biomechanical factors related to loading and stride frequency may be at play, though more research is needed to confirm.

Practical implications for runners

An important focus for the research team was creating an easy way to identify risk so that appropriate interventions could be put in place. To that end, they developed a field screening test using standard body measurements that can predict BMD within a four-point margin of error.

"Not everybody has access to a DXA scanner, and even if you do, it may not be covered by insurance," Lambert says. "In our paper, we discuss the model and the demographic variables that go into it, but we also plan to further develop the equation for practical use in the field, to make it easy for anyone to get a quick risk assessment without needing a DXA scan."

And if an athlete is identified as at risk? Does it mean their running season is over before it even began?

"It's not that you have to stop running or training, because this is just showing you're at risk, not that you have an actual injury," Lambert says. "But you should start taking preventive measures."

There are effective supplements to strengthen bones. You also can have a nutritionist and strength and conditioning coach work with the athlete to mitigate against the risks.

"It's definitely in the runner's best interest and the team's best interest to prevent these injuries before they occur," Lambert says. "The only way to heal them is to rest, and that can sometimes take a long time. Also, once they occur, they're more likely to recur."

The investigators recommend looking at the effectiveness of various interventions as a logical next step for the research. They also plan to study other groups of runners to validate their findings on a larger and more diverse sample set.