The serve is the most complex stroke in tennis and the stroke that allows for the greatest degree of personal idiosyncrasy. Ideally, each person should tailor his serving style to his individual build, strength and physical configuration.
If there is one characteristic that differentiates club players from professionals, it is the degree to which the knees and back flex during the serve. Most average players have a slight back arch when they serve, and this produces a generally adequate flat or slice serve. A powerful flat serve or stronger slice serve requires a high degree of flex-the pro posture. As for the American twist service, an even more extreme flex is needed. Only then can you impart the necessary spin that gives the serve its characteristic high bounce.
The more you can comfortably increase your flex, the harder your serve will be. "Bend" to the task a little more. Every player should strive to find his or her maximum comfortable bend.

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