Set up a squat rack station or a Smith Machine so that a barbell is somewhere between waist and knee height; the more advanced you are the lower you can set the bar. Lie on your back under the bar. Reach up and grip the bar with an underhand grip with both hands. Your hands should be shoulder width apart. Brace your core and lift your hips from the floor, so you have one unbroken line from your head to your toes. Keeping your body straight and hips lifted, pull yourself up towards the bar by pulling your elbows as far back as you can and trying to touch the bar with your chest. Exhale as you do so. Hold for a moment at this top position, bracing your core and engaged your back. With control, lower yourself back to the start position. Inhale as you do so. If required, regress the movement by performing the exercise with bent legs.