Overhead technique
I did various wall drills today .................. overhead smashes...........................................
..................................................................
https://www.youtube.com/watch?v=glY2ZLXLX9s&list=UUuFVCRX5zNr10dxmbeM1B9Q
........................................................
A few years ago I decided that I had to have more pop on my volleys. I tried adding as much power as I could with about a 90° angle in the elbow.
DON'T TRY THIS. On one of the first few attempts at this new volley technique - that I came up with without much research -I gave myself a Golfer's Elbow injury (torn tendon). My volley technique was very close to your overhead smash technique.
I did not know at the time, but that motion is
internal shoulder rotation. It is powered by the lat and pec, probably the two largest muscles attached to the arm.
I have looked very briefly at a few high speed videos of pro smashes. At first look, these overhead smashes appear to be abbreviated serves - with some motions over less of a range of motion than for the serve. Both use internal shoulder rotation in the same way with a nearly straight arm. Also, there may be some other overhead smash techniques being used successfully in high level tennis. ?
On the serves, it is OK to apply heavier force using ISR because the arm is straight (and not with a 90° angle at the elbow), the arm and racket's moment of inertia is low about the
arm's axis, and there is not the stress that produced my Golfer's Elbow with the 90° angle in the elbow. (I've seen a few references that say Golfer's Elbow injuries might be associated with the forehand or serve - no details. ) Baseball pitchers also use a similar elbow angle and ISR in part of the pitching motion but they don't have the racket extending out.
I'd stop doing that overhead technique unless you can find it being done by high level players in high speed videos.