You guys are missing a key word Iv been using, its called: Adrenaline!
Your muscles are made up of millions of strands of muscle fibres. your body, under normal conditions, splits all those millions of fibres up into groups that will all contract together, which we call motor units. when you flex a muscle, only a few of those motor units are actually contracting, and they only contract for a split second, then relax - but because thousands of motor units are contracting one after the other, and overlapping, your muscle as a whole contracts, and smoothly closes/opens your joints.
when adrenaline is released, it stimulates a bunch of receptors on the heart, the lungs, the muscles etc etc and makes your body assign many more muscle fibres to each motor unit - meaning that each motor unit is more powerful, but because there are fewer motor units overall, as they all contract at different times, the movement is far less smooth, and is rather jerky - which is why many people shake when you scare them, or inject them with adrenaline.
adrenaline = more muscle fibres firing overall