Background. For the tennis serve consider the forearm-racket angle when the racket impacts the ball. The forearm-racket angle
only at impact is of interest for this thread.
The forearm-racket angle will vary a lot as seen from different camera views (see comment & demo at end).
The forearm-racket angle can be viewed as having two separate components, as seen in a side view and a behind view.
This component of the forearm-racket angle, resulting in a near vertical racket at impact as seen from the side, is necessary to keep the serve in.
1)
Side View. Viewed from the side, the racket at impact is roughly vertical, give or take a few degrees. This angle changes very rapidly around impact and even
during impact. As a first estimate, the angular rotation rate for the racket around impact is on the order of 1° per millisecond. What are the variations of this component for the flat, slice and kick serve? It would be helpful if some of the replies had high speed video links or, better yet, single impact frames with the camera viewing from the side (perpendicular to the ball's trajectory is best.)
Examples of the view from the side of the ball's trajectory. Note the forearm-racket angle component from the side -
----------------------------------------------------------------------------------------------------------
2)
Behind View. Viewed from behind, the well known angle,
β, between the forearm and racket that is necessary for the internal shoulder rotation (ISR) serve is very clearly displayed. It is the other component of the forearm-racket angle.
Thread on this angle,
β, and ISR.
http://tt.tennis-warehouse.com/showthread.php?t=361610
Keep in mind that these angles change
very rapidly after the arm extends - angular rotation rate is on the order of 1° per millisecond as a first estimate. Also, that these rotations are never forced or controlled, 'muscled', but flow as a result of practicing the serving technique using the kinetic chain with sequential
stretch-shortening cycles.
This component of the forearm-racket angle is necessary to develop racket head speed when the arm is axially rotated by ISR.
Examples of view looking along the ball's trajectory from behind to show this 2nd component of the forearm-racket angle. (The forward tilt of the arm seen in the side view does not show very well.)
Kick serve -
Kick serve -
There are many common comments to describe how to hit the ball for the various types of serves:
1) kick serve - 'hit up on the ball 7 to 1 o'clock'
2) slice serve - 'hit across the back of the ball'
3) slice serve - 'hit the side of the ball with an angle on the racket face'
4) kick serve - Rafter's incomprehensible comment to 'hit 5 to 11 o'clock' or that it feels like 5 to 7 o'clock. ?
5) general serve - 'hit up the mountain'
6) general serve - 'hit as if your were throwing a racket'
7) general serve - others, please supply some of the many instructions
I have been trying to understand these verbal instructions without getting a very clear idea of what they mean.
I believe that the forearm-racket angles as viewed on high speed videos at impact will have a strong correlation with each type of serve.
Flat serve - smaller forearm-racket angle component as seen from behind.
Slice serve - smaller forearm-racket angle component as seen from behind.
Kick serve - larger forearm-racket angle component as seen from behind, as for the examples above.
These angles will also play a part in hitting
'up' on the ball. For the kick serve, the large forearm-racket angle component will allow the racket to impact the ball while traveling up and also to the right (for a RH server), a diagonal path.
Grips. Another issue to consider is the grip. The grips influence the forearm to racket angle. Moving from a continental to a 'strong' continental or backhand grip, in particular, changes the forearm to racket angle.
http://tt.tennis-warehouse.com/showthread.php?t=483360
(
Interpretation of Videos. Because the serve is a very 3D motion and the camera gets a 2D projection of it, different camera viewing angles can show most anything for the forearm-racket angle from various view points. As a demo, take a paper clip and straighten it out. Put a 45° angle in it. Hold one end and rotate the paper clip while looking at it. The apparent angle will go from 0° to 45° as it is rotated. A similar effects occurs as a camera views a forearm-racket angle from an undefined viewpoint. That's why it's useful to limit the camera viewing locations to: 1) the side and 2) behind and along the ball's trajectory.)