If you’re not a runner, walking is a great low-impact way to work up a sweat. But when you want to kick it up a notch, increasing the incline on the treadmill is a surefire way to get your heart pounding. Just ask Lauren Giraldo, a TikTok star who came up with the viral “12-3-30” workout that the fitness community can’t stop raving about.

The 12-3-30 workout is simple: You walk on the treadmill at a 12 percent incline and 3 mph pace for a total of 30 minutes. Giraldo first shared the details about the workout in a video that’s now garnered 2.8 million likes, saying it’s the only form of exercise she’s done to reach her fitness goals. “I used to be so intimidated by the gym, and it wasn’t motivating,” she said. “But now I go and do this one thing, and I can feel good about myself.”

Obviously a whole lot of people got excited about being able to get in better shape by walking alone. And while walking itself is already beneficial, doing so at an incline really does check out. Aside from providing a cardio boost, it also strengthens your muscles.

“There are benefits to elevating the treadmill incline, including improved cardiovascular function, higher caloric burn, and glute strengthening,” says Holly Roser, a personal trainer and owner of Holly Roser Fitness. “This is the best possible form of cardio outside of running, as you’re using your entire body. That’s unlike when you’re on a bike or elliptical, where the machine is helping your movement, causing less effort on your part. It’s also low-impact, which is easier on your joints—especially for those who are having knee pain or low back pain.”

“There are benefits to elevating the treadmill incline, including improved cardiovascular function, higher caloric burn, and glute strengthening.” —Holly Roser, trainer

Don’t be fooled, though: This workout is anything but easy. The 12 percent incline can be incredibly challenging, and that’s why Roser says you shouldn’t jump right in if you’re new to working out. “Many people, especially beginners, cannot tolerate such a steep incline for such a long period of time,” she says. “Doing so can increase knee pain, calf tightness, flare up any old injuries, and if you’re not wearing correct shoes for your feet, increase plantar fasciitis pain.”

If you’ve already been working out for three to six months, you’re good to go with the 12-3-30 workout. If not, start by doing hills on the treadmill in bursts. That way, you can work your body up to the ultra-challenging 12 percent incline. “Start by getting up to 5 percent incline, then dropping down to 2 percent, repeating that for 15 minutes,” she says. “Then go from an 8 percent incline (which is still really hard), to 3 percent, repeating that for 10 minutes. Then go from a 10 percent incline to 5 percent incline. This gives your lungs, knees, and hips recovery time.”

If you want to run instead, try this treadmill hill workout:

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