Originally Posted by LeeD
I thought most of the service power comes from the trophy position onwards and upwards, before the lats are stretched.
The lats might affect followthru, but I don't see it affecting the initial throwing motion.
I don't know about the various contributions to power during the entire service motion but for the last acceleration of the racket toward the ball for racket head speed the main contributor is internal shoulder rotation. I don't know the relative contribution of the lat and pec to ISR but I believe that the lat is probably the most important since it is the largest of the internal shoulder rotator muscles.
See image with lat shown in red.
Now picture what happens when the lat - in red - shortens rapidly while the arm stays up as in the serve. The lat is attached to the front of the arm and the upper arm, and with it the entire arm, rotates rapidly.
The result is the twitch-like arm rotation seen in the video thumbnail.