Here's another effective back strengthening exercise: the bent-over row with a body bar. You can buy a body bar at most sporting goods or fitness stores. Keep your back straight throughout this move to get the best results.
Assume start position as shown by bending right knee, keep left leg straight.
Hold one end of bar, keep other end on floor.
Pull bar up toward shoulders by lifting elbow up toward ceiling.
Make sure to keep your back straight.