Running downhill, even at slight grades, creates extra stress on the shin bone, which can lead to shin splints, those pesky and painful microtears in tissue where muscle connects to the shin bone. Not a problem on the treadmill? Think again.
A zero incline on a treadmill mimics a slight downgrade — a negative incline, if you will. To avoid this problem, run with at least some incline when on a treadmill; we suggest keeping the incline set to at least one percent — this setting better simulates the environmental factors of running outside.
Another factor: the amount of time you're spending in the gym. If you have recently increased your mileage on a treadmill, then be careful. A treadmill surface is consistent, whereas roads and trails vary, even when they are flat. As a result, every stride on a treadmill is the same, which can lead to overuse/repetitive injuries like shin splints. Try some preventative exercises, like walking on your tiptoes, to keep this common injury at bay.