Lie face up, with your back flat on an exercise mat, knees bent at 90 degrees and your arms by your sides. This is your start position. Proceed to extend your knees and lower your legs towards the floor. Be sure to keep your core braced throughout and not let your back arch as you lower your legs. Pause at the bottom of the movement, with your legs hovering an inch off the floor. Reverse the movement and repeat for the required number of reps (or time).