This is my favorite exercise for building back strength.
You can perform this exercise from a few different starting positions, depending on what is comfortable for you. Start either by lying forward over a physio-ball, with hands and toes touching the floor to stabilize you, on the floor on your hands and knees, or lying flat on your tummy, with arms outstretched overhead.
Now raise your right leg a few inches of the floor, while at the same time raising your left arm up off the floor. Hold for 10 seconds, and then repeat with opposite leg and arm. Repeat 10 times.
The more advanced version is to lie flat of your tummy, and raise both arms and legs off at the same time, holding as long as you can – up to 2 minutes.
Keep your neck neutral during this exercise, and maintain a strong core while performing it – that means belly tight.