One of the most common injuries in runners is a stress fracture of the lower leg (tibia) because running fast causes the foot to hit the ground with tremendous force that can shatter bones. A study from the University of Minnesota shows that women with stress fractures do not have weaker bones, they have smaller and weaker calf muscles (Medicine & Science in Sports & Exercise, December, 2009). Another study from Iowa State University in Ames, in the same journal, shows that longer strides cause the greatest foot strike forces that increase bone fracture risk.
Strong muscles may help to prevent bones from breaking by absorbing more force from the foot hitting the ground during running. Most distance runners do not use weight machines to strengthen their leg muscles. They strengthen their calf muscles by running very fast no more often than three times a week.
In the Iowa study, reducing stride length by ten percent reduced force of the foot striking the ground and therefore reduced force on the tibia.
Shortening your stride will not slow you down. When your foot hits the ground, your Achilles tendon contracts to store up to 60 percent of your foot strike force. Then when you step off that foot, your Achilles tendon releases the stored energy to drive you forward. Over-striding deprives you of some of this stored energy. Since many runners take strides that are too long, shortening stride length usually allows them to increase cadence and will help to increase speed and endurance.
Stress fractures are small cracks on the surface of the bones. When you run, your feet hit the ground with a force equal to or greater than three times body weight, which can shatter bones. The largest bones are usually the strongest, and people with the largest bones have the largest muscles. So runners with the smallest circumference around their calf muscles are the ones most likely to suffer stress fractures (1). Stress fractures usually start as a minor discomfort in the foot, lower leg or pelvis, that occurs near the end of a long run. Usually the pain goes away as soon as you stop running. On the next day, it hurts in the same spot earlier in the run. If it hurts to touch a spot on a bone and does not hurt an inch away, you probably have a stress fracture. Most of the time, you don't need a cast, but should stop running for the three to six weeks it takes for you to be able to run without pain. If you have to exercise, ride a bike or swim. X-rays are usually not sensitive enough to diagnose stress fractures. If your doctor wants to prove the fracture, he will order a bone scan. Other risk factors for stress factors besides having small calf muscles include restricting food and having irregular periods (2). Women with stress fractures that do not heal may need to take a bone strengthener (3).
By Gabe Mirkin, M.D., for CBS Radio News
1a. Medicine & Science in Sports & Exercise, December, 2009
1) KL Bennell, SA Malcolm, SA Thomas, SJ Reid, PD Brukner, PR Ebeling, D Wark. Risk factors for stress fractures in track and field athletes - A twelve-month prospective study. American Journal of Sports Medicine 24: 6 (NOV-DEC 1996): 810-818.
2) KL Bennell, SA Malcolm, SA Thomas, PR Ebeling, PR Mccrory, JD Wark, PD Brukner. Risk factors for stress fractures in female track-and-field athletes: A retrospective analysis. Clinical Journal of Sport Medicine 5: 4 (OCT 1995): 229-235.
3) CA Haberland, D Seddick, R Marcus, LK Bachrach. A physician survey of therapy for exercise-associated amenorrhea: A brief report. Clinical Journal of Sport Medicine 5: 4 (OCT 1995): 246-250.
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