Instructions
Position your upper back on a padded bench so that your body runs perpendicular to the bench.
Align your feet so they are about shoulder width apart, with the soles flat on the box with your back and hips elevated from the floor.
Slowly lower your body into the gap between the bench and the box allowing your toes to point upward as you do so.
Lift your body upwards using the muscles in your lower back and glutes all the way to the top of the extension.
Tense you glutes holding the top position before lowering your body back down for the next rep.