Your tendons need training too.

Just read a fascinating article from peak performance issue 186 sep 2003 explaining that tendons can have a huge impact on sports performance.

The main concept is the heavy weight training will increase the stiffness of a muscle tendon unit, while flexibility exercises will increase compliance.

the idea is that stiff achilles tendons store and return kinetic energy better and help endurance runners.

In turn heavy weight training without flexibility training may reduce the efficiency of the snap during a serve.