When running through nature, especially on trails in wooded or hill-like areas, you will use muscles that you wouldn’t normally use on a flat surface. You may shift your weight from going uphill or downhill and you may activate parts of your calf muscles that you typically wouldn’t. Muscles in your arms will also be used as you counter-weight your body while running on an ever-changing terrain. Trail running is a great example of this, and any time you change the elevation of your run, you’re going to use different muscles in your body.