What is the Henneman size principle?

The Henneman size principle is a foundational concept within muscle physiology. According to Andrew Huberman, it describes how motor units are recruited in a sequential manner from low threshold units to high threshold units during muscular contractions. This means that when you perform any physical activity requiring muscle contraction, you start by using the smallest and least force-generating motor units. As the demand for force increases, larger motor units are progressively recruited to meet the required force output.

This principle dictates that lighter weights can also be effective for building muscle strength and hypertrophy, contrary to the common belief that only heavy weights are effective. The engagement of higher threshold motor units, necessary for muscle growth and strength, does not strictly require heavy weights; it can also be achieved through specific training parameters that involve lower weights but are sufficiently challenging to escalate to the recruitment of these larger motor units 1.