The weighted back extension is an exercise targeting the spinal erectors, as well as the glutes and hamstrings. Back extensions are often done for high reps without weight, but when done with weight, they can be an excellent accessory movement for deadlift strength as well as an overall posterior-chain strength builder. Because your back is the limiting factor, it's still an exercise that is usually done for moderate to high reps, such as 8-12 reps per set or higher. You can weight it with a plate or by holding a dumbbell or other weight in your hands.