Lie on your back with your knees bent and feet flat on the floor.
Pull your belly-button towards your spine and clench your buttock muscles to roll the tail bone up off the floor.
Continue this movement, rolling your spine off the floor until you have a straight line from your knees to your shoulders.
The majority of the effort should come from your abdominal and buttock muscles.
Reverse the movement, to lower your hips back down, ensuring you keep your buttock muscles clenched until the end of the movement.
