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Underweight female runners have higher risk of stress fractures

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Female runners who are underweight are more likely to become injured, and take longer to heal, according to a study published in Current Orthopaedic Practice.

The study suggests that female track athletes who have a body mass index (BMI) of less than 19 are at a higher risk of developing stress fractures than women with a BMI of 19 or higher. It also found that lighter women who suffered stress fractures took longer to recover from them than other runners.

Researchers at The Ohio State University Wexner Medical Center studied tibial stress injuries in female athletes over a period of three years.

“We found that over time, we were able to identify the factors that put female runners at an increased risk of developing a stress fracture,” said Dr Timothy Miller, assistant professor of clinical orthopaedic surgery and sports medicine. “One of the most important factors we identified was low body weight, or low body mass index.”

Miller explained that runners endure repetitive pounding on hard surfaces and, without enough lean muscle mass for dissipation of impact forces, the bones of the legs are vulnerable.

“When body mass index is very low and muscle mass is depleted, there is nowhere for the shock of running to be absorbed other than directly into the bones. Until some muscle mass is developed and BMI is optimised, runners remain at increased risk of developing a stress fracture,” Miller said.

The Kaeding-Miller stress fracture classification system was used to grade all injuries and to compare the time to return to practice.

Among those with grade 5 stress fractures — the most severe — the research team found that women whose BMI was 19 or higher took about 13 weeks to recover. Those with a lower BMI took more than 17 weeks to recover and return to running — a full month longer.

“It’s imperative that women know their BMI and work to maintain a healthy level,” Miller added. “They should also include resistance training in their training regimen to strengthen the lower leg to prevent injury, even if that means adding weight from additional muscle mass.”

Miller suggests that female athletes aim to maintain a body mass index of 20-24.