The Henneman size principle dictates that motor units are recruited in a staircase pattern from low to high threshold, conserving energy.

“There's an important principle of muscle physiology called the Henneman size principle. The Henneman size principle essentially says that we recruit what are called motor units. Motor units are just the connections between nerve and muscle in a pattern that staircases from low threshold to high threshold. What this means is when you pick up something that is light, you're going to use the minimum amount of nerve to muscle energy in order to move that thing. Likewise, when you pick up an object that's heavy, you're going to use the minimum amount of nerve to muscle connectivity and energy in order to move that object. It's basically a conservation of energy principle.”