The Reaction Principle
This principle highlights the importance of temporal movement and the effect of twisting in the early phase. The predominant motion of the backswing, and the movement of the impactor, is carried out with an arm and torso movement in the same direction. The stroke has always been regarded as an act of the whole body. The strong muscles of the trunk and legs when powered up, will produce kinetic energy needed to achieve the stroke. This task is called kinetic movement. The muscles involved are called kinetic. Trunk muscles, hips and knees joints hold an initial force, respectively.
A tennis player needs a power of around 4000 watts (Tusker, 1994). 1 kg of lean body mass produce about 150 watts. The mass of a human arm, depending on age and sex and trained muscle is between 4kg and 8 kg. Since the arm works synergistically and only half the arm muscles are used, the arm is reached to a force of not more than 600 watts of power. Therefore a groundstroke must be involved other muscles of the trunk and legs. This is to activate the muscle power needed. The power needed is about 26 to 27 kg. According to Ben Kibler, 51% of the kinetic energy, 54% of the muscles of the trunk and the upper body and only 15% by wrist arm is required in a stroke.
What Really Happens?
During the rotation of the upper body the internal muscles of the body respond. The external oblique abdominal muscles as well as the rotatores of the back muscles also respond. That means that the backswing and the immediate upper body rotation in the direction of the ball, forwarded by the acceleration of the arm and racket, are the most important components in the modern tennis technique.
In early striking movement, hip movement works before the arm movement with approximate 0,20-0.25 seconds. In a groundstroke, we follow to gain a big acceleration of the arm and racket at the impact point of the racket with the ball. Hence the acceleration is followed by the extension of the arm forward. Then it is finally followed by the bending of the elbow and the finish of the movement and it should be over the shoulder.
Acceleration of the racket
Is obtained by the translational movement of the body weight in the direction of the ball and rotation. The possible shift of body weight forward to attack the ball and step in, is good for a better control of the groundstroke. It is also good for maintaining balance and for the acceleration of the speed play.