Seated knee lifts can be a challenging workout for your abs and hip flexors. Sit towards the front of your seat with your knees pressed together and your back straight, and grip the front of the chair. Lift your legs toward your chest, keeping your core engaged and your knees locked in a 90-degree angle—try not to slouch. Lower your legs, but don’t touch the floor until you’ve finished a set. Do three sets of 15 to 20 reps for a real burn.