Head Position in Pro Tennis: The Serve

[doctorhl]

It's a shame there isn't significant pro data readily available for shoulder flexibility( as well as other measurements). Sampras style would seem to require it to avoid injury.

[flatflat]

Regarding head position, I follow the excellent example shown in an old hard copy of Tennis magazine showing Karolina Pliskova keeping her head up well after the ball has been struck.
I personally do the same thing, and also created a visual cue for myself. I want to see the palm of my tossing hand fully outstretched with palm facing backward (like you're supposed to),the ball rising into the hitting zone, and finally, I want to see the ball disappear! Bingo!
A comment on "fully extending your racquet arm". To me, fully extending your racquet arm means "like hanging from a pull-up bar". This will lock up the free movement of your arm! Try 95% to 97% of this length instead.