Henneman's size principle

Henneman’s size principle is helps us understand muscle activation during various tasks. Essentially, it says that motor units are recruited in a specific order, starting from the smallest to the largest, based on the force required for a task.

Lifting Light Loads:
When muscles contract under light loads, low-threshold motor units are the first to be activated. These units govern smaller muscle fibers that are fatigue-resistant but exert minimal force. Such fibers are ideal for tasks requiring endurance without the need for high force, such as maintaining posture or sustained, low-intensity activities.

Lifting Heavy Loads:
Conversely, under heavy loads, the muscles need to generate a greater force. This demands the recruitment of high-threshold motor units that control larger muscle fibers. These fibers are capable of producing significant force, but they fatigue more rapidly than their smaller counterparts. This is why during heavy lifting or high-intensity tasks, muscles tire more quickly.

Light Loads to Failure:
An interesting phenomenon occurs when lifting light loads to muscular failure. As the muscle fatigues from repeated contractions, even with a light load, it requires additional force to continue the activity. This simulates the demands placed on the muscle as if it were lifting heavier loads. Thus, towards the end of such a workout, the high-threshold motor units (associated with larger fibers) are recruited, even though the load remains light, mimicking the muscular recruitment patterns seen with heavier loads.