The ready position is the position you want to be in when you're waiting for your opponent to hit you the tennis ball. It's the foundation for tennis movement in general. From this position you can employ the various footwork patterns that let you move around the tennis court correctly. The ready position is very similar to an athletic stance in basketball. Your feet are a little bit wider than shoulder-width apart, your knees are slightly bent and the weight should be on the balls of your feet.
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This video falls under tennis footwork.
Footwork is arguable the most important part of the game. Moving to the ball and recovering after each shot correctly is absolutely critical — great forehand or backhand technique means nothing if you can't get to the ball. Great footwork is a skill shared by all of the top pros on tour.