Tone and lift your backside with this workout that'll strengthen your glutes, which is great for your back and knees, and help you rock your bikini bottoms. For day 25 of our Better-Body Challenge, we start with some fat-blasting intervals on the treadmill that play with both speed and incline, which will work your booty even more. We end the workout with 10 minutes of strength training that focuses on the glutes. If you're not a fan of running, here's an elliptical plan to use instead. Now get to it!