The 10 percent rule is the simplest of all tricks forinjury free running. It’s less relevant once you start running 20 or 30 miles/week, but a critical one when you first start out. In the early days of running, the muscles tend to beef up pretty quickly, while the tendons, ligaments and bones take a while to catch up. This imbalance causes all sorts of problems including shin splints, top of the foot pain, stress fractures, etc. The real trouble is the strengthening muscles give you a false sense of headiness that you can just crank up the distance.