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I showed a difference is what is seen in videos and a frame showing
separation of the upper arm from the chest for Federer. Most ATP players do not do that but have the chest press onto the humerus for a longer time than Federer. That is true.
Feliciano Lopez also shows a similar early use of the shoulder joint for his drives. When I first watched his backhands in a match to see his drives, there were hardly any 1HBH drives. See one of his games. Lopez hit few top spin backhand drives relative to other ATP players and Justine Henin. His backhand slice was his preferred backhand.
Place Federer's and Lopez's 1HBHs with Justine Henin's 1HBH in between and go frame-by-frame. When I compared 1HBHs, I selected camera angles that showed 'chest press' from a favorable observation angle.
Another observation is whether the upper arm (humerus) angle turns with the line between the two shoulders or turns more independently. This issue was illustrated.
Try it yourself by doing extra chest press and no shoulder joint motion, slower and easy to be safe. Many other strokes are two-phase also. That is because on the forehand and 1HBH you first use uppermost body turn and then use the shoulder joint for forward velocity. See Djokovic forehand from the side camera view. Uppermost body turn then add shoulder joint.
You get more racket head speed by first accelerating the shoulder mass, arm and racket and then second using the shoulder joint. Clearly seen in videos.
For the majority of forum readers - that do not like to analyze high speed videos - give it a try for 3 hours. Then ask yourself if you have learned anything.
Video observations on sub-motions of the ATP 1HBH & Justine Henin's.
https://tt.tennis-warehouse.com/ind...and-waht-force-to-start-forward-swing.462997/
Chest press was realized in post #51.
The rest of the thread has many video observations of sub-motions of the ATP 1HBHs and Justine Henin's.