Have posted this in the past numerous times. This image is a representation of (equally-timed) data points for a Sampras serve. The blue dots show the position of the tip of Sampras’ racket as recorded by a video camera.
Note that the racket in this image was
added to the plot of points to show where contact was made. From this we can see that RHS was greatest (data points further apart) immediately prior to contact. Immediately after contact, we see that RHS has decreased (as a result of contact).
Further, we see that the tip of Pete’s racket continues to move upward (slightly) after contact. This is a sure sign that he’s imparted some topspin to this serve. Note that this may very well be a 2nd serve.
This serve image & data was taken from tennis research study conducted by Dr Jani Pallis, Dr Duane Knudsen and other notable tennis scientists. John Yandell may have participated in this research study. The image above was taken from a TennisServer article that show data representations for 6 different Pete Sampras serves. Two, if not three, of the serve data shown is for flat serves.
For NONE of the Sampras serves shown, is the racket moving in a downward direction at contact. At contact, the racket tip is still moving upward. In 2 or 3 images, however, the swing path is nearly horizontal at contact — but NEVER down.
Tennis Science, Engineering and Technology by Dr. Jani Macari Pallis.
www.tennisserver.com