In tennis we create force in a different way
 
We are not pushing or lifting; instead, we’re swinging (in a controlled way), and the objects we’re dealing with – the racquet and the ball – are not heavy. That’s why the key to creating force is speed rather than strength. (After all, except perhaps Rafael Nadal, most professional tennis players are not muscular and bulked up!). Speed is also a much more important factor in creating kinetic energy (Ek) than mass:
 
Ek = (mass x speed2) / 2
 
If we increase mass by two times, the Ek will increase by two times, but if we increase the speed by two times, the Ek will increase by four times!
 
And kinetic energy is the energy that is released at contact from the racquet to the ball. A faster ball is a result of a faster racquet head that transfers more kinetic energy to the ball; therefore, the key is to increase speed.
 
Speed is created by a different process in the body than the usual force with tensing our muscles that we’re familiar with, and once you get a good hold of it, it’s fairly effortless compared to “muscling the ball”.