Henneman's size principle

Henneman’s size principle states that under load, motor units are recruited from smallest to largest. In practice, this means that slow-twitch, low-force, fatigue-resistant muscle fibers are activated before fast-twitch, high-force, less fatigue-resistant muscle fibers. It was proposed by .

The size principle states that as more force is needed, motor units are recruited in a precise order according to the magnitude of their force output, with small units being recruited first, thus exhibiting task-appropriate recruitment. This has two very important physiological benefits. It minimizes the amount of fatigue an organism experiences by using fatigue-resistant muscle fibers first and only using fatigable fibers when high forces are needed. It also permits fine control of force at all levels of output.

Before Henneman’s size principle, it was known that neurons varied greatly in size; however, the functional significance of this was not yet known. Henneman's early papers on the motor units of the cat soleus and gastrocnemius muscles found that the diameter of a motor nerve fiber relates to number of muscle fibers it innervates; in other words, the size of the motor unit is proportional to the amount of muscle fibers it innervates.^, The evidence in previous studies had found that nerve impulse action potentials directly relate to motor neuron size. Henneman concluded that larger impulses represent the firing of larger motor neurons, and he used this information to determine the size of the motor neurons as they fired.

Henneman compared the order of individual motoneurons firing during a stretch of the triceps muscle of a decerebrate cat to the amplitude of their impulses, in order to determine if and when each size motoneuron fired. He found that the smallest motor neurons represented by the smallest impulse amplitude had lower thresholds for stretch and fired first, while larger motor neurons had higher thresholds and fired last. This order of recruitment held true for all but 2 out of 165 cases. Henneman also looked at what order the motor neurons were inhibited as the stretch was released and found that it was the reverse order in which they were recruited. This held true for all but 2 of 236 cases. Knowing that the size of the impulses recorded directly relates to cell size, these results show a significant correlation between cell size and threshold for firing. Henneman called this phenomenon the “size principle”, which mandates the order of recruitment during a stretch.

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