"In a wall sit, you're basically sitting without a chair," says Holly Rilinger , a certified personal trainer and former professional basketball player. But, unlike plopping in a chair, when you do a wall sit, you have to use your butt and quads to support and hold your body up. Exercises that involve holding your body in one position, instead of doing several repetitions, are called isometric exercises. Even though you're supposed to stay totally still, when you hold isometric moves, like a plank or glute bridge , it requires the maximum amount of effort from your muscles, she says. "An isometric exercise requires the full exertion of the muscle," Rilinger says. "And that's key for building strength."