Mecanim: How to make the hand close?

Most animations need some tweaking in order to fit your purposes, like taking an animation where the hands are in their relaxed position and make them to make a fist or grab some object, like a broomstick for instance.

When rigging the humanoid character, there is an option to configure the rigging where you can control most of the muscles of the model. Hence, my question is how to gain access to open or close the hands procedurally.

Thank you for your response!