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 De:Elwood Henneman.
Benefits of the Size Principle
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.
From the time of Henneman and his discovery of size principle, many studies have been done to see if his theory holds up to the results of multiple experiments. An experiment of the quadriceps femoris found that motor units are in fact recruited in an orderly manner according to the size principle. The study looked at average motor unit size and firing rate in relationships with force productions of the quadriceps femoris by using a clinical electromyograph (EMG). Results showed the size of motor units increased linearly with increased force production, and firing rate remained constant to 30% maximum force and then increased with greater generation of force. When viewing motor unit potential during muscle contraction on an EMG, as the force generated increases, the amplitude (strength) and frequency (firing rate) increases. The motor units are being recruited in an order from slow, low force to fast, high force.
Size Principle and EMGs
The concept of size principle can be applied to therapeutic techniques. It was shown that the use of electrical stimulation of muscles for motor control would stimulate large, fatigable motor unit first. For many years it has been believed that the use of electromyostimulation (EMS) to stimulate muscle contraction creates a reversal of the general size principle recruitment order, due to the larger motor unit axons having a lower resistance to electric current. Recently, however, the results of the studies purporting this theory have come under some minor contention. In an article titled “Recruitment Patterns in Human Skeletal Muscle During Electrical Stimulation”, Professors Chris M. Gregory and C. Scott Bickel propose instead that the muscle fiber recruitment induced by EMS is non-selective pattern that is both spatially fixed and temporally synchronous. They back this claim with physiological data, metabolic data, mechanical data, and even by re-examining the results of other studies which claimed the reverse size principle paradigm.
Despite the debate, orderly recruitment of motor units can be achieved under optical control in vivo. Thus, the use of optical control with microbial opsins has been shown to promote normal physiological order of recruitment.
Experiments Relating to Size Principle
In 1986, a study comparing factors such as conduction velocity, twitch torque, twitch rise time, and half-relaxation of stimulated tibial muscle found evidence that the conduction velocity of individual muscle fibers types may be another parameter to include in the size principle. The data from the experiments showed a high degree of correlation between the four factors, which were consistent with a similar study performed several years prior. In that study, an increase in muscle fiber conduction velocity was observed when there was a higher level of voluntary muscle contraction, which agrees with the gradual recruitment of higher-force muscle types.
In Wistar rats, it was found that cell size is the crucial property in determining neuronal recruitment. Motor neurons of different sizes have similar voltage thresholds. Smaller neurons have higher membrane resistance and require lower depolarizing current to reach spike threshold. The cell size contribution to recruitment in motor neurons during postnatal development is investigated in this experiment. Experiments were done on 1- to 7-day-old Wistar rats and 20- to 30-day-old Wistar rats as well. The 1- to 7-day-old Wistar rats were selected because early after birth, the rats show an increase in cell size. In 20- to 30-day-old Wistar rats, the physiological and anatomical features of oculomotor nucleus motor neurons remain unchanged. Rat oculomotor nucleus motor neurons were intracellularly labelled and tested using electrophysical properties. The size principle applies to the recruitment order in neonatal motor neurons and also in the adult oculomotor nucleus. The increase in size of motor neurons led to a decrease in input resistance with a strong linear relationship in both age groups.
- Motorneuron mapping. (n.d.). Retrieved from http://www.eng.mu.edu/wintersj/muscmod/nms-func-physiology/nm-map.htm
- Henneman, E., Somjen, G. & Carpenter, D. O. (1965). Functional significance of cell size in spinal motoneurons. J. Neurophysiol. 28, 560-580.
- Henneman, E., Wuerker, R. & McPhedran, A. (1965). Properties of motor units in a homogeneous red muscle (soleus) of the cat. J. Neurophysiol. 28, 71-85
- Henneman, E., Wuerker, R. & McPhedran, A. (1965). Properties of motor units in a heterogeneous pale muscle (m. gastrocnemius) of the cat. J. Neurophysiol. 28, 85-99
- R.A, C., D, S., B, T., M, M., W.F, B., & E.J, M. (n.d). The relationship of motor unit size, firing rate and force. Clinical Neurophysiology, 1101270-1275. doi:10.1016/S1388-2457(99)00054-1
- Video: https://www.youtube.com/watch?v=pC3NJZ1cjuM
- Llewellyn, M. E., Thompson, K. R., Deisseroth, K., & Delp, S. L. (2010). Orderly recruitment of motor units under optical control in vivo. Nature Medicine, 16(10), 1161-1165. doi:10.1038/nm.2228
- Gregory, C. M., & Bickel, C. S. (2005). Recruitment patterns in human skeletal muscle during electrical stimulation. Physical Therapy, 85(4), 358-364. Retrieved from http://www.physther.net/content/85/4/358.short
- Andreassen, S., & Arendt-Nielsen, L. (1987). Muscle fibre conduction velocity in motor units of the human anterior tibial muscle: a new size principle parameter. The Journal of Physiology, 391, 561-571. Retrieved from http://jp.physoc.org/content/391/1/561.short
- Carrascal, L., Nieto-González, J. L., Torres, B., & Nunez-Abades, P. (2011). Diminution of voltage threshold plays a key role in determining recruitment of oculomotor nucleus motoneurons during postnatal development. PLOS One, Retrieved from http://www.plosone.org/article/info:doi/10.1371/journal.pone.0028748