This is what I think: that is cool.
Adding resistance to the strokes in shadow practice could be very beneficial: especially in younger players who don't have as much of the muscle development yet, or for female players who need some extra strength. Ideally the resistance to the arm would be from the exact direction of the stroke which would actually change through the stroke rather than being from a constant place.
How could you get that kind of resistance directly opposing the movement? A simple idea: do it under water. A swimming pool, at the part where the water is a height where, once you are in your stance, the water is just about the height of your shoulders. The neat thing is, the water would also resist the movement of the legs and the rotation of the hips. Unfortunately having the luxury to do that kind of thing in a pool with the water the exact right height is probably not as easy as paying and training someone to hold the bungee chord.
Another idea for resistance for shadow strokes that would be pretty simple would be a racket that weighed 300-400 grams total weight; so, approximately 2 times heavier than a standard racket.
I used to do shadow drills with a 3 lb weight that was shaped like a disk and called a "SmartBell". It doesn't duplicate holding the grip of the racket though.
One more detail, you wouldn't want to do too much of the resistance version if you were an adult and already fairly strong.
For an adult who is already decently strong but not fast enough, the shadow training done as fast as you could while maintaining good form would be more useful for developing speed, reset and reaction time.
Next time I get to do my shadow training I am going to have to have someone film a small part of it so you can see how I roll! [emoji2][emoji236]
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