Running on flat terrain results in fast times, but it may not be the most realistic environment for road and trail runners. Most non-track running courses include some hills, which can cause your running times to vary. Knowing the impact of hills on your training will help you become a better runner and ensure that you are able to train for upcoming races appropriately.
Uphill
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Jack Daniels, author of "Daniels' Running Formula," estimates that for every percent of incline you experience in an uphill, your running time will slow by 12 to 15 seconds per mile. Building hill running into your training will help to reduce this and make it easier for you to tackle any incline.
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Downhill
Daniels estimates that downhills will help you improve your time by approximately eight seconds per mile for every percent gradient of decline. Even so, it's important to acclimate your muscles to running downhills. Downhill running requires heavy use of your quad muscles. Building these muscles during training will reduce muscle soreness and potential for injury while racing.