Most treadmill runners fall into one of two camps: They’ve never touched their treadmill?s incline buttons (wait, you can adjust more than speed?) or they beep their way up to max incline every chance they get (higher equals harder equals better, right?)

Neither approach is going to score you the results you want. That?s because changing up your treadmill?s incline changes the muscles you emphasize during your workout, says Jason Fitzgerald, a USA Track & Field-certified coach and the founder of Strength Running. ?Running a variety of inclines forces the body to engage different muscles [particularly in the calves, quadriceps, and glutes], increases the aerobic demand of the run [helping you develop more endurance], and boosts muscular strength, which can help prevent injuries.?

Here?s everything you need to know about running from low to high treadmill inclines?and how to mix them up for better fitness gains.

Your Body on Low vs. High Inclines
When you run on a treadmill with zero-percent incline, your leg muscles?including your calves, glutes, and quads?work pretty much as they do when you?re running outside on flat surfaces, says Fitzgerald. (Sure, they don?t have to work quite as hard as they might have to when you?re outside?since you don?t have wind to contend with on the treadmill?if you?re training for an outdoor race, Fitzgerald recommends setting your treadmill to one or 1.5 percent incline to make up for that difference.) If you?re just trying to improve your running skills, running on a flat treadmill allows you to focus on your form without throwing complicating factors like ramps into the mix, he says.

But?if you really want your legs to burn, turning up the incline can make it happen. ?As the incline increases, the muscles are forced to do more work as the body must produce more power to propel itself not only forward?but also up against gravity,? says Fitzgerald. Ipso facto, you burn more calories and build more muscle.

For instance, in one Gait & Posture study, when people walked at a nine-percent incline, they increased the activation of their gastrocnemius (a calf muscle) by 175 percent, their biceps femoris (a quad muscle) by 635 percent, and their gluteus maximus (the main muscle in your booty) by 345 percent, compared to when they walked at zero incline. While you work all of these muscles at low inclines, in the study, as the treadmill?s steepness increased, so did how hard the muscles had to work with every step.

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Still, steeper isn?t always better. ?If you have any issues with hip flexor tightness, high inclines can cause irritation to those muscles,? says Joy R. Miles, an endurance coach with Fitness Formula Clubs?Indoor Triathlon series in Chicago and a USA Triathlon-certified coach.

Plus, more often than not, when people try walking or running at steep inclines, they lean back and hang onto the rails for dear life, messing up their posture and gait. By reducing activation of the leg muscles, hanging on essentially defeats the purpose of increasing incline. Whether you are walking, running, or sprinting, you should never set the incline or speed so high that you can?t move hands-free, your body forming a straight line. You should bend forward slightly at your ankles, says Fitzgerald.

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?The only reason to take the incline to its max?most treadmills top out at 15 percent?is when you are training for something specific, such as a very steep hike,? says Miles. ?If the goal is to prepare for a hilly race course, workouts should mimic the demands of that course with similar incline levels and speeds,? says Fitzgerald. ?Higher inclines are more difficult than lower inclines, and?of course, slower speeds are easier than higher speeds. Both variables can be changed in an almost unlimited manner to vary the workout and type of stress the runner is looking to experience.?

Exactly. The best workout routine is going to vary things regularly?you want to keep your body guessing. After all, you aren?t going to be running up a hill?or on a perfectly flat stretch of pavement?forever.

How to Switch Up Your Inclines
Based on what you?re training for, you might want to alternate between running on low-to-zero inclines on some days?and on higher ones on others. (Fair warning: If you?re new to inclines, Fitzgerald recommends mastering moderate inclines of two to four percent before moving up.)

Or?if you want to vary inclines throughout your run?which is a great option if you only run once per week or you’re training to master extremely varied terrain?settle into your typical pace for the distance you?re running?and then increase the grade by 0.5 percent every one to two minutes, he says. See how high you can go (again, while keeping good form and your hands off of the rails), and then slower to flat, decreasing your incline in 0.5 percent increments every one to two minutes. You can also try increasing your incline to two to five percent and running at that grade for one to three minutes before lowering back to flat ground for the same amount of time.

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