Set the bar on a Smith machine to 3 to 4 feet from the floor. Lie on your back with your chest directly below the bar. Reach up and grab it with a shoulder-width, underhand grip so you're hanging from the bar and your body forms a straight diagonal line from your heels to your shoulders. Keeping your core tight, pull yourself up until your chest meets the bar (shown). Lower and repeat for 10 to 12 reps. If you don't have access to a Smith machine, do regular pushups instead.