Smallest motor units first, largest last
The Henneman size principle describes the orderly recruitment of motor units according to size. As the demand for force rises, the nervous system activates smaller, lower-threshold motor units first, then progressively larger, higher-threshold ones. The smaller units tend to control fatigue-resistant slow-twitch fibers, while the larger units control more powerful, faster-fatiguing fast-twitch fibers.
This ordering means light efforts mainly use the small, enduring units, and only heavier or faster efforts call in the large, powerful ones. It is a key reason that lifting heavy or moving explosively is generally needed to recruit the highest-threshold fibers.
Implications and caveats
The principle explains why load matters for which fibers are trained, and why high-threshold units can also be reached late in a set taken close to failure as smaller units fatigue. Recruitment order is robust, though firing rate (rate coding) and the speed of the intended movement also shape force output.
This is general educational information about muscle physiology, not training or medical prescription.