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Using your legs when serving
Although on the surface it looks as if the arm does all the work while you're serving, advanced players know how to use their entire bodies, including their legs, in their serves.
Newton's third law states that for every action there is an equal and opposite reaction. When you push against the ground, the ground pushes back. This drives your racket shoulder up and helps place the racket in the proper position behind your back. At the same time, it stretches your muscles and stores energy like a pulled rubber band. In essence, pushing off the ground allows you to generate a more powerful serve.
There are two major benefits to using your legs in this way. One, you call upon the strongest muscle groups, your leg and upper-body muscles – instead of just your arm – to generate force. Second, you're less likely to get injured because the stresses are being shared by these large muscle groups.
Estimate how much you use your legs by noting how much your knees bend during the serve. Research has shown that most top players have an average kneed bend of approximately 70 degrees.
Source: Taken from 'Lesson from Isaac Newton', Paul Roetert
