One issue for the serve is that 99%+ of the videos are from ground level cameras or those slightly above. There are very few videos from the overhead camera view. All cameras take 2D images of 3D space. The components of object motion
up or down in a frame or
side to side in a frame are reasonably well shown. But the component of object motion
toward or away from the camera is shrunken in the video. It is the same for our eye.
In the picture below, the camera is not above the server but it is high enough and shows the large angles that exist in a high level serve. These angles would show more accurately from the above camera view or from a camera on the ground looking up. Unfortunately, there are not many videos from cameras placed above or below high level servers.
There is a very strong and unfortunate tendency to describe tennis strokes in, say , three words that are easy to remember, for example, '
hit up and out' on the tennis serve. The definition of the word 'hit' is left to the reader. Does 'hit' apply to the racket head or the hand? Does 'hit' apply to the
path of the hand or racket head? Does it apply only to the 4 milliseconds of impact or to the path of the hand or racket? Near impact or farther away? High speed videos show all this information but because of the 2D image vs 3D space issue, described above, it takes careful selection of camera angles and interpretation to see what is going on. Just forget about any accuracy using the three word descriptions of sub-motions of tennis strokes, like
hit up and out. In the picture below, notice that the racket is moving
in relative to the hand path, but still out relative to the ball's trajectory. What do readers understand from 3 words?
This picture, from an elevated camera viewing along the plane of the curved hand path, shows some very large angles. These angles are often completely left out of discussions that try to describe complex 3D motions in a few words. These easy-to-remember words act as traps, wrong turns in the road, that cause the serve to be misunderstood and many of its most important sub-motions to be hidden. I missed ISR for 35 years and the majority of active tennis players are missing it now. Tennis researchers missed it until 1995. The Tennis Serve Nuthouse.
In the picture below there are several angles:
1) ball trajectory, (I tend to think that this is the best reference direction for other angle measurements.)
2) the path of the hand is roughly circular and the plane containing that circle is at a significant angle to the ball's trajectory.
3) the path of the center of the racket head just before and after impact. ~45 d. to ball trajectory?
4) the path of the racket head overall cannot be described with any simple words because it includes significant rotation from internal shoulder rotation (ISR).
5) the angle on the face of the racket relative to the ball's trajectory at impact is another independent variable. It changes rapidly from ISR and will affect the side-to-side placement more than the simplified path of the racket head, 3) above.
6) the angle of the hand and wrist (extended for slice and flat, kick?) ) for impact for each grip variation. Relative to what? Forearm?
7) there are many other angles at instants during the service motion.
The short word descriptions can't describe sub-motions of the serve. A high speed video is the best description available short of
motion capture systems.
Slice serve.
I believe that these angles and paths are developed by the server by practice and feel. But when these angles are not there in the high level technique and cannot be confirmed by videos, sometimes with checkpoints, this is one reason why the serve is so hard. Or rather, these features are rarely observed for feedback and the serve won't work without them.
The picture below shows all the angles present but from a side view camera. But this view camera make the serve look much more 2 dimensional with height and forward direction shown well but the direction away from the camera is shrunken.s
It looks like a simple forward swing. But there is always a clue, look at #2 and #3 carefully. The racket appears shorter in #3 than #2. Why? Because the dimension away from the camera is shrunken and the racket is angled away from the camera, so the racket appears shorter. Gussie Moran was using a high level technique in the 1950s for her ISR approach to the ball. The #2 vs #3 apparent racket lengths and the racket head orientations shown here, including racket edge on to the ball, is actually a swing path of the racket head very similar to the first picture.
If you have a different serving technique, you are completely on your own developing that serving technique and have basically no references or videos for how to do it. Most important today is the Waiter's Tray technique, that the majority of active tennis players are using. The Waiter's Tray serve leaves out
internal shoulder rotation (ISR) and so the observed angles will be different. I have never seen a Waiter's Tray serve from the above camera view. I expect that the racket head shows little rotation approaching the ball and that the angle between the racket path and ball trajectory is smaller. There would be other differences.
When you look at high level serves from the same camera angles, while it is still difficult to
measure angles,
seeing differences is something that anyone can do.
Fuzzy Yellow Balls has videos from above for Frank Salazar's serves, for kick, flat and slice serves. ( Toly's great composite pictures of these serves may have their links recently stopped by Tinypics or Photobucket. ) ?