Virtually all movements involve the recruitment of motor units, the basic functional units of the neuromuscular system, from multiple muscles. Given the functional diversity of motor units, the effective production of specific movements undoubtedly depends upon some principles to organize the ensemble of active motor units. The motor units of a skeletal muscle may be recruited according to different strategies. From all possible recruitment strategies nature selected the simplest one: in most actions of vertebrate skeletal muscles the recruitment of its motor units is by increasing size.
Early neurohistologists recognized that neurons within the central nervous system vary widely in size, but had not realised the functional significance of this basic observation. The largest cells have surface areas which are at least 100, perhaps 1,000, times greater than those of the smallest cells. Correspondingly, the diameters of axons in the central and peripheral portions of the nervous system range from less than .25 p. to more than 20 c-c. This broad spectrum of physical dimensions invites a search for functional correlates.
The motor unit, defined as an a-motoneuron and all the muscle fibres that it innervates, represents the functional quantum by which the nervous system regulates the development of muscle force. This regulation occurs by concurrent variation in the number of active motor units and their rate of activation.
The size principle theory of motor unit recruitment provides a very robust framework with which motor unit recruitment patterns can be predicted. This principle is commonly understood to describe the process whereby motoneurons are recruited in an orderly manner from low to high axonal conduction velocity and motor units are recruited from small to large, slow to fast, weak to strong, and fatigue resistant to fatigue susceptible isometric contractions.
Motor unit size has been shown to vary both within and between muscles with a trend for smaller units to be composed of slower twitch muscle fibres and larger motor units to be composed of faster twitch fibres. In addition, smaller motor units have smaller diameter nerve axons, which result in slower action potential conduction velocities along them (Bawa et al., 1984). The size principle therefore predicts that, based on both contractile properties and action potential conduction velocities, faster motor units will be recruited after slower motor units have been activated and will be the first motor units to be derecruited.
In 1957, Elwood Henneman published a brief report in Science demonstrating that single flexor motor axons recorded extracellularly in a ventral root filament were recruited in order of spike height as the intensity of electrical stimulation to the ipsilateral sciatic nerve was increased. Furthermore, the individual axons ceased firing in the inverse order that they were recruited after the stimulus, i.e., first recruited, last derecruited.
It was further hypothesized that this rule might apply ubiquitously in the nervous system because the properties of all nerve cells appeared at that time to be, as stated by Henneman, "remarkably alike."
The size principle theory was developed following work on stretch reflexes in decerebrate cats (Henneman 1957). The stretch reflex causes a stretched muscle to contract and its primary sources of input are mechanoreceptors, sensitive to muscle length changes, called muscle spindles. Muscle spindles contain a small number of intrafusal muscle fibres, which are supplied by sensory nerves (Ia and II afferent fibres) and a-motoneurons. The correlation between motor unit axonal conduction velocity and fused tetanic tension is weak when data are collected from large mixed fibre muscles, with the lack of correlation most apparent when numerous faster contracting motor units are present. This finding may be a result of the weaker monosynaptic Ia afferent feedback supplied to faster motor units, compared to feedback supplied to slower motor units.
It is important to recognize that at the conception of the size principle, Henneman specifically stated that the independent variable was the intensity of the stimulus and the dependent variable was the recruitment of higher-threshold (larger) motor units. Force was not the prerequisite for recruitment; force was the result of a more intense stimulus. The level of effort in voluntary muscle actions determines the degree of motor unit activity. The fundamental neuromuscular concept is that all voluntary muscle actions initiate in the brain and action potentials are generated, which is analogous to the external electrical stimulus generated in Henneman's studies: the greater the stimulus (internal action potentials generated by the brain or external stimulus generated in vitro), the greater recruitment of motor units through the size principle. Each individual's effort determines motor neuron activation.
The relationship between motor-unit recruitment and activation-rate modulation undoubtedly varies between muscles but the result is always an ability to finely grade muscle force, particularly at low forces. A skeletal muscle together with the motoneurons controlling it consists of several hundred motor units of different sizes. In the human medial gastrocnemius muscle there are about 300 motor units with tetanic forces ranging from 0:63 to 203:5 g (Garnett et al. 1979).
The force output of a muscle is determined by the sum of the force outputs of the active motor units. Due to the large number and different properties of the motor units, their order of recruitment must be specified in a suitable way or the task of determining the sequence of motor unit recruitment could be onerous.
The different schemes by which motoneurons might be grouped in orderly recruitment were clearly formulated by Wyman et al. They used the term ''motor pool'' to designate a functionally coordinated group of motoneurons exhibiting the orderly recruitment sequence predicted by the size principle, and suggested that the motor pool might consist of ''all the motoneurons innervating a single muscle, all the motoneurons in a given ventral root (probably subdivided into flexor and extensor groups), or all the motoneurons activated by a given stimulus.''
Orderly, size-related recruitment of motoneurons illustrates how hundreds of cells operate as a functional entity to produce a highly deterministic output. This task is autonomously performed by the motoneuron pool in the spinal cord. The pool as a whole receives input from the central nervous system and from peripheral receptors and distributes it to its individual motoneurons. If the common input to the pool exceeds the threshold of a motoneuron, this motoneuron will fire action potentials and consequently activate the muscle fibers which it innervates.
The coherent action of the pool depends largely on the distribution of input to its members through the connections of afferent fibres. Spike-triggered averaging has been utilized to study these connections. Impulses were recorded from dorsal root filaments (DRFs) containing single la fibres from the medial gastrocnemius muscle and used to trigger the sweep of an averager. The tiny, generally indistinguishable, excitatory postsynaptic potentials (EPSPs) elicited in an motoneurons by each impulse were fed to the input of the averager. By summating a few hundred of these small responses, each time-locked with the signal that triggered the sweep, larger, so-called excitatory postsynaptic potentials 'single-fibre' EPSPs were generated, indicating that a direct, functional connection existed between the afferent fibre and the motoneurons.
Impulses in individual la afferents elicit 'single-fibre' EPSPs in 94 % of homonymous motoneurons. Functionally absent la connections are due to transmission failure. The larger the diameter of the afferent fibre, as judged by its conduction velocity (CV), the greater was the mean amplitude of the EPSP (Mendell & Henneman, 1971).
This widespread projection, suggesting that the majority of the motoneurons in a pool are excited by each afferent impulse at about the same time, obviously helps to explain the coherent action of the motoneurons pool. In 1979 it was discovered that the projection probability of single la fibres rose to about 100 % immediately after low spinal transection (Nelson, Collatos, Niechaj & Mendell, 1979), suggesting that la fibres project anatomically to all homonymous motoneurons.
Mendell was properly cautious in interpreting this surprising finding, which suggests that axonal conduction or synaptic transmission may fail somewhere in the projection of la fibres to motoneurons. Regardless of the site of failure or its cause, the occurrence of transmission failure and its relief indicate that functional connectivity is by no means fixed or invariant, but reflects dynamic, state-dependent processes.
Orderly recruitment of motor units has been proposed to have several functional advantages. The physiological reasons of recruiting slower motor units first can be separated into two main categories.
First, it is thought to simplify central nervous system control of muscle contractions. Orderly recruitment ensures that the slowest, most fatigue resistant, motor units are recruited first for any given task. These motor units can provide sufficient force for a wide number of activities such as maintenance of posture and walking. The faster motor units fatigue more rapidly and so would not be able to sustain force production over a prolonged period of time. These type of motor units are thus reserved for infrequent, high intensity tasks such as jumping, where they can provide high forces for a short period of time.
Second, as faster motor units are composed of a greater number of muscle fibres they are able to produce greater force than slower motor units (Milner-Brown et al. 1973). Orderly recruitment, therefore, facilitates a smooth force increment as it leads to a force increase that is roughly proportional to the level of force at which the motor unit was recruited (Henneman and Olson 1965; Zajac and Faden 1985). This so-called size principle permits a high precision in muscle force generation since small muscle forces are produced exclusively by small motor units. Larger motor units are activated only if the total muscle force has already reached certain critical levels.
Myoelectric activity in both the soleus and plantaris muscles was initially characterised by greater relative low frequency content, followed by a relative increase in the high frequency component. It can therefore be interpreted, in accordance with the predictions of the size principle, that motor units were recruited in an orderly fashion from the slowest through to the fastest.
Identifying the recruitment strategies and determining their functional significance was challenging and despite initial formulation almost half a century ago, Henneman's 'size principle' remains a provocative concept. The reliability for the orderly recruitment of motor units, which is the cornerstone of movement control, has been substantiated over the last 40 years in animals and humans. Thus, the size principle of motor unit recruitment is perhaps the most supported principle in neurophysiology.