Back Exercises
It is essential when exercising to keep your bodies muscular frame as symmetrical as possible. This is to say, if you're developing the muscles of the abdominal region, you should also develop the muscles around the lower back.
Incorrect muscular balance (simple working the abs) will lead to poor posture, whereby the abdominal muscles will pull your body trunk forward, causing strain on the lower back.
Whenever you complete any abdominal workout, aim to finish with some simple back care / strengthening exercises, followed by a good stretching routine to help ease the tension within the muscles.