This simple isometric exercise is harder than it looks, and it seriously works out your quads. Since it requires a wall, it’s best done at a pit stop or in the airport/train station. All you need to do is squat with your back and head firmly pressed against a wall and hold it for as long as you can—no more than a minute. Your knees should be shoulder-width apart and bent at a 90-degree angle. Do three reps for a solid burn.