The bicycle is a classic abdominal-strengthening exercise, but anyone with a bad back knows that if you do it incorrectly -- or before your body is strong enough -- it can cause pain in your lower back. The key is to raise your extended leg high enough so it does not cause your back to overarch. You'll know if you are overarching if your rib cage pokes up and the space under your waist increases when you straighten your leg.
1 Lie on your back with your knees bent and your feet parallel to each other. Place your hands behind your head. Exhale, contract your abdominals to lift your head, neck and shoulders off the floor. Keep your shoulders moving away from your ears. Inhale.
2 On the next exhale, keep your abdominals firm as you stretch your left leg out toward a 45-degree angle. Bring your left shoulder toward your right inner knee. It's OK if you don't fully straighten your left leg. Focus on contracting your abdominals, and press your navel down toward your spine as you twist. Inhale, bend your knee and return to the start position. Repeat on the other side. Continue alternating sides until you have done 8 to 12 reps on each side.