CSCS Henneman Size Principle of Motor Unit Recruitment

Apr 14, 2023

Edited by: Danielle Abel

Henneman Size Principle Explained

You've probably heard of Type I and Type II muscle fibers, but did you know that there's a sequential process that the body follows to activate different types of muscle fibers?

Muscle fibers are activated first by the nervous system - the brain, spinal cord, and the associated nerves that originate from the spinal cord.  

Fiber Types

Within each muscle, there are different types of muscle fibers, each of which is designed to serve different purposes. Within each muscle fiber there are smaller subunits called myofibrils. 

  • Type I fibers
  • Type IIa fibers
  • Type IIx fibers

Within fiber types, there are smaller and larger sets of myofibrils. For example, there are more myofibrils in Type II muscle fibers compared to Type I muscle fibers. For example, Type II muscle fibers may have 100 myofibrils, compared to Type I with 50 myofibrils organized into motor units. When nerves activate a muscle fiber, they activate all of the myofibrils within that motor unit. 

  • Motor units = a muscle fiber, consisting of multiple myofibrils, controlled by the same nerve
  • All or nothing principle = the entire motor unit must be recruited 

If you think about cutting the bicep muscle in half and then looking at the crosssection of the muscle, you would see a mosaic distribution of myofibrils within the muscle itself. A mosaic distribution is representative of the different types of fibers themselves. 

Progressive Recruitment

From a motor unit recruitment perspective, the activation occurs based upon the intensity of the recruitment volume and the intensity of the force. 

  • The less force, the less recruitment of smaller fiber type (Type I)
  • As force increases, more fibers are recruited (Type IIa)
  • As force increases, more fibers are recruited (Type IIx)

All Type I fibers are recruited first, and then Type II are added on to increase the force production. 

Type IIa and Type IIx help increase force production, but they fatigue quickly. 

Selective Recruitment

Well-trained athletes who have high rates of neural development may be able to recruit Type II fibers before Type I fibers with a high rate of force. 

Fiber Transitions

Training can actually cause fiber transitions to occur. For example, as training endurance improves Type IIx fibers may transition into Type IIa fibers with high recruitment, high force development, and improved fatigue resistance. Type IIa fibers are considered intermediary fibers because they are high functioning fibers with crossover to many athletic applications. 

Muscle Group Fiber Distributions

Different types of muscles will have different percentages of fiber types based upon the needs of the muscle. 

  • Gastrocnemius - high amounts of Type II fibers for things like sprinting
  • Quadriceps - high amounts of Type II fibers for movements like squats 

These larger muscles don't need to possess fine motor control functions compared to other smaller muscles like those in your eyes and fingers. These smaller muscles need more Type I fibers to assist with fine motor control and fatigue resistant.

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