Fasciculations are fascinating

Muscle fibres do not normally contract all at once, as most people imagine. Instead they are organized into groups called “motor units,” one per motor nerve. Rather than firing all at once, the groups alternate their contractions, like pistons. At any given time, thousands of motor units are in different phases of contraction and relaxation. The units are so small and the switching system is so fast that their coordinated action seems to be completely smooth to us.

There is an interesting exception, though: if you get tired enough that a lot of motor units start failing to contract, the switching system fails because there aren’t enough motor units available for smooth contraction. This is why muscles start to shudder and quiver with very intense exertion! Cool, eh The switching system is mind-boggling in its efficiency and complexity, and a fantastic example of how much more physiologically complex muscle tissue is than most people realize.