In order to get a more flexible back you can do the 'bridge' exercise. For that, lie on a soft surface and put your arms and legs shoulder-width apart. Now support yourself with your arms and legs and push yourself up to the 'bridge' position. Remain in this position for 15 seconds and repeat the exercise after a break of 30 seconds. Hint: For a lighter version of this exercise you can spread out one leg while being in the 'bridge' position.