Sit at the cable row machine. Grab it as if you're going to do seated cable rows but don't pull with your upper body, just your lower back. It is similar to a good morning but seated and using a cable for added resistance throughout the motion. Go very light and focus on form and squeeze.
This is an all lower back exercies, doesn't involve hammys, glutes, or any muscle group other than what you're focusing on. The full range starts with your chest just over your knees and ends with your upper body making an angle about 20 degrees past 90.