Stress fracture risk increased in underweight female runners

There’s been a long-held belief amongst the running community that a lighter frame results in a faster performance – but a study from the Ohio State University’s Wexner Medical Center suggests that if this is taken too far, there can be a price to pay in the long term.

According to the study, involving dozens of female athletes over a period of three years, women with a body mass index of 19 or below are more likely to develop stress fractures, due to their bodies being unable to handle the constant shock stresses of running.

The study utilised a bespoke stress fracture classification system in order to identify the factors that put female runners at an increased risk of developing a stress fracture – and according to the results, one of the most important factors was a low body mass index.

Dr Timothy Miller, Assistant Professor of Clinical Orthopaedic Surgery and Sports Medicine at Ohio State University, pointed out that running involves a series of repetitive pounding on hard surfaces – and without enough lean muscle mass for dissipation of impact forces, the bones of the legs become vulnerable to injury.

"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.” says Dr Miller. “Until some muscle mass is developed and BMI is optimised, runners remain at increased risk of developing a stress fracture."

During the course of the study, all injuries incurred by the study group were evaluated on the bespoke system, with the severity of the injuries graded and the time it took to return to practice measured. The research team discerned that among those suffering from grade 5 stress fractures - the most severe - women whose BMI was 19 or higher took about thirteen weeks to recover, while those with a lower BMI took more than 17 weeks to return to running: a full month longer.

So how does one retain an optimum weight whilst indulging in a pursuit which naturally works the pounds off? As Dr Miller pointed out, it's imperative that women know their BMI and work to maintain a healthy level - his suggestion is that female athletes should aim to maintain a body mass index of 20-24. He pointed towards a regime of resistance training, geared towards strengthening the lower legs, in an attempt to build shock-absorbing muscle mass.

Another way of increasing leg muscle mass is to revise one’s diet. If you’ve not done so already, cutting down on empty calories such as processed and snack foods and ramping up on foods rich in protein are a must, along with considering mixing muscle-building supplements into your diet.

Obviously, if you’re already a serious runner, you’re pretty adept at knowing what shape your body is in and what you need to do to keep it in optimum condition. But if you feel your regime is starting to take its toll on your knees, it’s time to step back and check your body mass.