Back extensions are a low impact, low-intensity exercise which will target your posterior chain, consisting of the back, glutes, and hamstrings.
Start by lying face down on the floor or on a yoga mat. Keep your chin tucked in so that your neck is in line with your spine at all times. Place your finger tips by your temples and, keeping your legs on the floor, raise your upper body up off the floor. You may not get more than an inch or two off the floor, but this is fine, it is a small but intense movement. Squeeze your glutes as tight as possible and slowly lower yourself back down. Try not to let your elbows track back, keep them in line with your shoulders.