Being as light as possible may seem like an advantage for runners, but it may actually make female runners more prone to stress fractures, according to researchers at the Ohio State University Wexner Medical Center.

“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,” Dr. Timothy Miller, assistant professor of clinical orthopaedic surgery and sports medicine, explained. “Until some muscle mass is developed and BMI is optimized, runners remain at increased risk of developing a stress fracture.”

Over a three-year period, Miller and his team analyzed dozens of female track runners and their injuries, and found that of the women with the most severe stress fractures the runners with a BMI below 19 were at a higher risk and took a full month longer to heal than those with a BMI of 19 or higher. Without enough lean muscle mass to dissipate the force of the foot hitting the ground, leg bones are left vulnerable and the repetitive pounding is more likely to cause injury.

“It’s imperative that women know their BMI and work to maintain a healthy level,” Miller said. “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.”