The "fitness" industry is gold mine for scams. Think about it! Where does the information comes from? It has to originate with the athletes. Boxing. Long, long ago, people figured out the endurance plays a key role in fights, thus running was born in boxing. If it doesn't come from the athletes themselves, I'm not buying it.
You have to be good with Boolean logic to understand what I'm saying. Even if it comes from the athletes themselves, I may not buy it. Flexibility. Dancers own flexibility.
NYC Ballet Workout: Fifty Stretches And Exercises Anyone Can Do For A Strong, Graceful, And Sculpted Body Paperback – January 13, 1997
by Peter Martins (Author)
Am I going to "buy" it, figuratively and literally? Maybe not. I read one of the review on Amazon that they take basic yoga poses and put them in the book. Unless the exercises are used by the dancers themselves as they were learning ballet, I'm not buying it.
The "gym" was created by an art, bodybuilding, look up Frank Zane. It is a perfect art that all athletes can tailor to their needs. That's ⅓. You need endurance and flexibility. You can learn from triathletes and ballet dancers respectively. Tailor, like tailors. Making a suit that fits your unique dimensions in 3D space. You can take weight training, endurance training, and flexibility training to "suit" you. Unless tennis players themselves are doing the tailoring, I consider it guessing (nothing wrong with guessing).
That's another thing. Learn where the arts come from. I'm pretty sure donkey kicks came from bodybuilding (not that arts don't learn from other arts). If you see a ballet dancer doing donkey kicks:
3 possibilities.
1. Dancers always did donkey kicks. (I don't think so)
2. Dancers now incorporate donkey kicks. (maybe)
2. They rip off bodybuilding and selling it to the public. (most likely)
Here's a good video to help you understand.