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Getting faster is hard work – long training runs; muscle searing track sessions; hill repeats – runners are used to enduring the fatigue and discomfort that come with a hard workout and most are willing to put up with the common aches and pains of an active lifestyle. But, sometimes that nagging discomfort gets worse after running on it. The deep aching pain of a stressed bone is unlike that of muscle soreness or fatigue and, as many have learned, running through the signs of an overloaded bone can lead to a stress fracture.
Simply put, bone is an active structure that responds to the stress placed on it. Microscopic damage created by the stress of running is usually quickly repaired, but in the case of stress fractures, repetitive trauma overwhelms the bone’s repair response, leading to a more serious injury.
Running makes bones stronger over time, but too much stress with too little time to recover can cause bones to fatigue and fail. Because of this, lower body stress fractures are a common injury among runners, representing 20% of all running related injuries.
Too Much, Too Soon
Sudden increases in mileage and volume are a leading cause of stress fractures, and a great deal of advice for the prevention of stress fractures is, rightfully, devoted to the external factors that can leave one susceptible to overuse injuries. However, intrinsic factors such as muscular strength and fatigue resistance can also play a big role in preventing the injury.
Dr. Jordan Metzl, author of The Exercise Cure, sums up the risk for stress fracture injury as the intersection of three circles; bone density; biomechanics; and training factors. “While training errors and bone health play a major role in risk, biomechanical problems can contribute significantly to the development of stress fractures,” Metzl says.
Although traditional stress fracture recovery programs place an emphasis on rest and cross-training, recent recommendations from sports medicine researchers at the University of Washington focuses not only recovering from a stress fracture, but prevention of future stress injuries as well.
A comprehensive program to prevent or recover from a stress fracture injury should include the following elements:
1. Resistance Training
The evidence for the incorporation of strength training into stress fracture rehabilitation programs begins with the observation that injured runners exhibit diminished leg muscle size and strength. For instance, female runners with stress fractures were found to have significantly lower levels of quad strength. Additionally, in a study of 136 military recruits, those with diminished knee extension strength had 5 times greater risk for stress fracture development.
Researchers hypothesize that lowered quad strength causes runners to adopt a straighter knee at heel strike, resulting in increased stress to the lower leg bones and raising the risk of stress fracture by a factor of 5.
The calf muscles — the dominant muscle group during the push off portion of the running stride — can also influence the risk of a stress fracture injury. Because studies have shown both an increase in bone strain and a decrease in calf muscle activity after prolonged running and marching, experts suggest that a strong calf can counter some of the tibial (shin) bone stress of running.
A 2009 research study from the University of Minnesota examined the size of runner’s calf muscles and lower leg bones to determine if a group of injured female runners were predisposed to stress fracture injury because of smaller bones. They were surprised to learn it was not the size of the lower leg bone that predisposed runners to injury, but rather the size of their calf muscles. Those with larger, and supposedly stronger calf muscles were less likely to suffer a stress fracture to the tibia, or lower leg.
2. Muscular Endurance Training
It may come as no surprise that tired muscles can’t provide the same support for tendons, ligaments and bones that fresh legs can. As one of the primary functions of muscle is the absorption of the impact of running, tired and fatigued muscles are less able to absorb that force, transmitting that stress to adjacent bone. Dr. Anthony Luke, director of the University of California, San Francisco’s RunSafe program emphasizes, “You can hear the difference in the foot strike of fatigued runners whose running technique has begun to break down” adding, “That louder sound is bones losing the protection of the shock-absorbing muscle.”
Delaying the onset of muscular fatigue through the development of muscular endurance through the incorporation of high repetition, low load resistance exercise can lessen the chance of fatigue-related changes in a runner’s capacity to absorb impact with every stride. A 2012 study in the journal Medicine and Science in Sports and Exercise observed a reduced tolerance for impact in runners whose lower leg muscles were fatigued.
3. Hip Stability Training
Pick up any fitness or running magazine and chances are you’ll read about the importance of hip strengthening. While it may not be a panacea, a stable and efficient hip is important for athletic performance by providing the foundation for the coordination and efficient movement of the legs. In a study focusing on female runners with a history of tibial stress fracture, researchers found that excessive inward motion of the knee and hip during the running gait was a predictor of tibial stress fracture. Strengthening the outer hip muscles (gluteus medius) can control these forces with running.
4. Gait Retraining
As stress fractures are thought to be related, in part, to the overall intensity of impact, changing a runner’s gait to reduce these forces has potential to lessen the chance of injury. Dr. Michael Fredericson, team physician for the Stanford University Cross Country team stresses, “Increasing muscle strength and endurance won’t have any effect on injury risk unless it is translated into improved running mechanics.”
A 2011 study in the journal Orthopedics and Biomechanics determined that a 15% increase in stride frequency lowered the overall impact of running in a group of recreational runners. A more recent review article by Fredericson recommends a modest change in stride rate, stating, “increases of 10% or less above an athlete’s preferred (step) rate being adequate to reduce impact loading.”
While no one has directly studied the long term effects of changing gait on the incidence of stress fracture injuries, changing gait mechanics to encourage a greater turnover and reduce reaching forward with your stride offers promise for reducing impact and injury. Note, however, that adjusting cadence may reduce performance and efficiency — permanent, effective gait changes are best accomplished by improving your stride mechanics with strategic mobility, strength and muscle activation work.
With the strong correlation of sudden changes in training to overuse injuries, smart training choices are a critical factor in avoiding stress fractures. However, you can further reduce your risk by following these straightforward, research – proven, preventative measures.