The arch of the back, bending of the knees and use of the whole body is essential for a great service.
But this is a movement that looks so simple but is often very difficult to achieve.
To use all of your body in the service stroke it is important that you do not “think” about bending your knees. The knees will bend as a bi-product.
If you consciously think “Bend the knees” the lower body is usually isolated, and the service loses rhythm as the lower and uper body are not connected.
Even at the highest level this separation of the lower and upper body can be a problem. If you look at the services of Maria Sharapova and Serena Williams and think about this seperation it becomes clear that the smooth rhythm of Serena’s service is absent from Sharapova’s delivery.
The result is that Sharapova has good and bad days on the service while Serena is remarkably consistent.