Stress Fractures Caused by Weak Muscles and Over-Striding
Posted Dec 18 2009 11:34am
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.