Motor neurons with large cell bodies tend to innervate fast-twitch, high-force, less fatigue-resistant muscle fibers, whereas motor neurons with small cell bodies tend to innervate slow-twitch, low-force, fatigue-resistant muscle fibers.
Both likely contribute and reflect the astounding coordinated development of neural circuit and cellular properties in motor neurons and muscle.
An experiment of the quadriceps femoris found that motor units are in fact recruited in an orderly manner according to the size principle.
[12] 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).
[14] 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.
[14] 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.
[16] 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.
Smaller neurons have higher membrane resistance and require lower depolarizing current to reach spike threshold.
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.