This article was originally published on November 24, 2015.An actual first-world problem: Basically everyone who sits a lot has a lazy ass. The glute muscles deactivate, because they get used to being underused and simply stop pulling their own weight. "For a lot of people, it’s hard to even activate the glutes in the first place,” says NYC-based celebrity trainer Josh Holland, CPT . He suggests a little test: From standing, try squeezing your glutes together for 10 reps. Now, try squeezing one cheek and then the other. See what he’s talking about?So why is having lazy butt muscles such an issue? The glutes are (or should be) a major initiator of most lower-body movements, including walking, climbing stairs, running, and jumping. “If you have well-developed glutes, you’ll move better,” Holland explains. And, of course, along with improved function comes improved aesthetic.Plus, having stronger muscles around the hips can greatly reduce your risk of knee injury. The hips are responsible for keeping your knees in line when you're walking or running, and preventing those knees from going out of whack in the event of a sudden, unexpected movement — such as missing a curb or being jostled on the train.To kick your butt into shape, try our 30-day challenge. It incorporates movements in multiple directions, to hit all the muscles of the hips and butt. For the moves separated by a comma, do all reps before moving on to the next exercise. For moves connected with a plus sign, do the exercises one after another. With multiple sets, denoted by x2 or x3, rest one minute before repeating. And enjoy those rest days — you’ll need ‘em for recovery between the harder workouts.