Flexing the knees: tennis players tend to move a lot around the court at a fast pace in order to keep up with serving and returning the ball to the opponent. The knee is a hinge joint and therefore a very crucial joint for this game. If the player's waist is rigid and cannot move to its full potential, all the stress goes to the knees. Loosening the hips and flexing the knee is one of the most important yoga lessons that will help the tennis player prevent injuries. The body must be flexible in order to move in any possible direction.