Around 1952, Harold Edgerton, a leading researcher in high speed imaging including stroboscopic photography, photographed Gussie Moran's serve using stroboscopic techniques with light flashes of microseconds duration that are repeated.
http://edgerton-digital-collections.org/?s=hee-nc-49001#hee-nc-49001
See numbers on the picture.
#1 Her foot stays on the ground. Prior to 1962 the rules of tennis required that one foot remain on the ground. All serving techniques prior to that time required that one foot remain on the ground. The leg thrust/jump used in the current tennis serve is now much greater and that affects comparisons to serves before the rule change.
#2 The racket is oriented 'edge on' to the ball, a checkpoint for a high level serving technique. An estimated, 50% of active tennis players do not have their rackets oriented edge on and therefore have a lower performance serving technique. Most don't know.
#3 The racket head has rotated both forward and into the page between #2 and #3. The apparent length of the racket has shortened between #2 and #3 because the racket is viewed at an angle in #3. Place a paper on the screen to measure the apparent racket length difference. Between #2 and #3 the arm and racket are accelerated to high rotation rates into the page by the joint motion of
internal shoulder rotation (ISR). There is some evidence that ISR stops accelerating before impact. ? At impact one estimate is that 40% of racket head speed is caused by ISR and 60% is caused by the other swinging motions. The significant part played by ISR was not understood for the tennis serve until about 1995, when researchers B. Elliott, R. Marshall and G. Noffal began publishing research results. In the 1970s, I had carefully studied similar strobe photos in Vic Braden's
Tennis for the Future, but I had somehow missed noticing the important racket edge orientations produced by internal shoulder rotation. Upper arm rotation at the shoulder (ISR) never crossed my mind until 2011. Most tennis players don't understand what is shown in the strobe picture.
#4 Peak of the toss.
#5 Ball location at impact.
#6 Racket position after impact.The high speed arm rotation and racket head continue forward after impact probably from momentum with little ISR muscle forces.
#7 Ball trajectory with a downward projection angle as for nearly all serves including kick serves.
#8 Compare the hand and racket face orientations in #2 and #8. The thumb is up in both #2 and #8. There has been about 180° of rotation. That 180° rotation probably came from both ISR and pronation.
Leading up to impact, muscle forces cause ISR to accelerate the arm and racket very rapidly. But ISR continues into the follow through at high speed after the muscle forces have stopped.
You can see similar numbered serve positions in pictures and videos of current serves, except now both feet will be in the air.
