In order to strengthen your back--and any other vital area--it's never wrong to do cardiovascular exercises. When you are feeling sudden back pain, walking should be the cardiovascular exercise of choice. Walk at a moderate pace for at least 15 minutes. Walking on a softer surface, such as a running track at a high school or college, is much better for your back than walking on hard pavement.
Get on all fours. Put your head down and then take your right leg and extend it until it is parallel to the floor while you point your toes to the back of the room. Hold the extension for a count of five. Put your right foot back, and then extend your left leg and point the foot to the back of the room. Hold it for a count of five. Do 10 extensions with each foot, take a 30-second break and repeat the set.
Lie on your back and raise your knees to a 45-degree angle. Take hold of your right leg right behind the knee. Pull it up to your chest and hold it for 3 seconds. Return your leg to the 45-degree starting angle and then do the exercise a total of 10 times. Do the same exercise with the left leg. Take a 30-second break and repeat the set.