Lie face up, with your back flat on an exercise mat, knees bent at 90 degrees and your arms extended above you, parallel to your thighs. This is your start position. Proceed to extend both legs and lower them towards the floor. Simultaneously extend your arms directly behind you and overhead towards the floor. Be sure to keep your core braced throughout and not let your lower back arch as you lower your legs and arms. Pause at the bottom of the movement, with both your legs and arms hovering an inch off the floor. Reverse the movement and bring both legs and arms together back to the start position. Repeat for the required number of reps (or time).