Shivering when nervous or anxious is a common thing.
But, shivering or trembling sometimes also occurs when we are performing a work which requires high accuracy. In such case, our whole body doesn't shivers but a part (usually hand) which is involved with that work shivers. The most common example is threading a needle. It requires great accuracy and you might have noticed that when you are doing so, your hands are continuously shaking which prevents you from accomplishing that task flawlessly. Even when we aren't in a state of nervousness, our hands tremble. It's un-resistable.
Yet another quick example - Try to stretch your one arm and try to keep it straight (have it loose, don't try to force it straight), you might notice that your arm is actually shaking a little bit. You can't hold it exactly straight without moving even a little bit.
It's also noticeable when we try to pin-point a small spot with our hand(finger) without actually touching it. (that finger and the hand as a whole starts trembling)
So, what's the reason? It isn't anxiousness as we shiver even when we are calm. Perhaps, it's gravity which is pulling our hand down and we are trying to lift it continuously which creates a imbalance? But, then we should shiver always?