John423 (is that a Biblical handle?),
Back extensions IMO do little or nothing. Try this:
Lay on your back, with your feet elevated on an exercise ball. Now SLOWLY raise your butt off the ground so your body forms a more or less straight line from your feet to your shoulders. You should go up and down without shifting side to side. SLOWLY lower your butt to the mat. Repeat 10 times. If you can't find an exercise ball, you can instead have your knees bent and 'build a bridge' that way.