I know this topic has been discussed before, but I could not find it any more using search. When you serve, you want your arm to be completely relaxed. When you hit your forehand and backhand, if you have plenty of time, you can transfer your weight from your legs, hip and shoulder to your arm and use this kinetic energy to hit the ball. Your arm can be relaxed as well. However, when you are late in your groundstrokes, you just have to muscle the ball back. When your opponents' shots catch you off balance, you just have to hit an one-dimensional shot to get the ball back, your arm can't be relaxed in this case. My question is how hard should we grab our racket when hitting groundstrokes?