Running can have a big impact, both on your waistline and on your joints. According to MayoClinic.com, running at a fast 8-mph clip can burn 861 calories per hour for a 160-pound person. However, you're also jarring your knees with every step. Some surfaces are softer on your knees than others, but treadmill and asphalt are about the same hardness. The jury's still out on which surface is better for your knees, since both have pros and cons.
How Hard Is It?
A hard surface makes high-impact exercises such as running even higher impact. Your knees act as a type of shock absorber, and the harder the surface, the more shock they must absorb -- up to three times your body weight with each step, according to "Runner's World." A soft surface such as grass and dirt -- as long as it's not hard-packed -- absorbs more of the shock and gives you more bounce in your step. As far as firmness goes, a treadmill and asphalt are about the same. A "Runner's World" list of the best running surfaces gives treadmills a 6.5 and asphalt a 6. Concrete, however, is much harder, earning a rating of 2.5 -- avoid it if possible when you're running. That means dodge the light-colored sidewalk and stick to asphalt road surfaces if it's safe to do so or look for asphalt-paved paths. These paths will be dark gray, almost black, with solid surfaces, compared with concrete sidewalks that are nearly white with evenly spaced, built-in cracks.
Watch Out!
One benefit treadmills have over asphalt is that they offer a solid, continuous surface. Asphalt can be pitted with potholes or scattered with rocks and gravel, making traversing the surface treacherous. Stepping wrong on an unpredictable asphalt surface can cause you to twist your ankle or jam your knee, making it potentially worse for your knees than a treadmill. However, navigating the small changes in the asphalt surface makes you step differently than running on a safe treadmill surface, which can strengthen the muscles all around your knee and help prevent future injury.
Running Up and Down
Programming an incline into your treadmill workout is a pro and a con. It's a pro because most asphalt inclines -- which are, of course, usually found outdoors -- typically are followed by an asphalt decline. Running downhill can stress your knees more than running uphill or over flat surfaces. However, running continually uphill, as you would on a constant treadmill incline, can strain your ankles. As your ankle muscles weaken during your workout, your knees tend to pick up more of the load, leading to more jarring action.
Mix It Up
Instead of sticking only to running -- on any surface -- give your knees a break a couple of times a week by switching to a different form of cardio. Try riding a bike, swimming laps or hitting the elliptical machine at the gym. These low-impact options get your heart pumping without straining your knees, keeping them in tip-top shape for your next running workout.