The effort
You can not only take a cup and bring it to a mouth, without having spilled contents, when you drink, the weight of a cup decreases, but all of you it is equally correct and coordinating hold it.
With it you are helped by the mechanisms described above.
But still also work of muscles, their ability to develop a certain effort matters.
The effort is result of reduction of muscles.
Expressiveness and an orientation of force of reduction of muscles too are regulated by the centers of a brain.
.