Body Mass, Training History, and Stress Fractures

September 3, 2012

Stress fractures are a problem with many types of athletes and with military trainees. These injuries occur to the lower extremity, typically the tibia , femur, or metatarsals and are the result of the training stimulus exceeding the bone’s ability to recover from the training. This is the type of injury experienced by distance runners, some sprinters, and many military recruits.

Moran et al studied stress fractures in male military recruits for the Israeli Defense Forces in the September issue of the Journal of Strength and Conditioning Research. From their perspective, being able to predict if a recruit was at greater risk of a stress fracture from training would be a highly desirable thing. There are a number of variables that would logically seem to be related to stress fractures in males; lack of fitness, running surfaces, running footware, bodyweight (i.e. heavier combined with the variables listed), nutritional intake, etc.

The authors studied 116 soldiers in two different companies. The first company was used to develop a predictor model, the second company was used to validate the model. Some 77 different variables were collected and analyzed to help develop the model as the researchers followed the recruits through four months of basic training and six months of advanced training.

During that training program, between 37-40% of the recruits suffered a stress fracture, most occurring in the tibia followed by the metatarsal and the femur. The results are interesting, The prediction model uses three variables to predict the presence or absence of more than 85% of the stress fractures; basically the odds are related to the amount of time spent doing aerobic training per session before basic training, the number of aerobic training sessions per week before basic training, and the soldier’s waist circumference.

Those soldiers that employed aerobic training of 40 minutes in duration or greater, per workout, that had fewer training sessions (1-2 times per week at the longer duration), and that had narrower waists were 16.6 times more likely to suffer a stress fracture in training! This runs counter to what I’d think, I would think this type of individual was well adapted to the training. The authors recommend shorter, more frequent training (4-6 times per week for 20-30 minutes of duration).

It’s possible that the lack of body mass implied by the narrower waists means less stress on the bones to stimulate remodeling. It’s also possible that combined with the relatively infrequent nature of the training (1-2 times per week) also isn’t enough to stimulate strengthening of the bone. Then to throw this individual into a situation where they are training several times a day, and marching, all of which on less than ideal conditions and footwear could be a recipe for an injury.

Moran, D.S., Finestone, A.S., Arbel, Y., Shabshin, N., and Laor, A. (2012). A simplified model to predict stress fracture in young elite combat recruits. Journal of Strength and Conditioning Research, 26(9), 2585-2592.