Definition of Motor Unit Recruitment and Overview
Motor unit recruitment may be defined as "the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction." 
The central nervous system can increase the strength of muscle contraction by the following:
Increasing the number of active motor units (ie, spatial recruitment)
Increasing the firing rate (firing frequency) at which individual motor units fire to optimize the summated tension generated (ie, temporal recruitment)
Both mechanisms occur concurrently. The primary mechanism at lower levels of muscle contraction strength is the addition of more motor units, but the firing rate of the initially recruited motor units also increases. When nearly all motor units are recruited, increase in firing frequency becomes the predominating mechanism to increase motor strength. At this level and beyond, motor units may be driven to fire in their secondary range to rates greater than 50 Hz.
The next section of this article discusses the physiology of motor unit recruitment in detail. Subsequent sections look at ways of examining recruitment during an electromyography (EMG) study. Assessment is made at different levels of innervation—minimal muscle contraction to determine the onset and recruitment firing rates (ie, recruitment pattern); maximal voluntary contraction to provide information about the interference pattern; and moderate voluntary contraction at various levels for assessment of the turns/amplitude analysis.
Order of Recruitment
As a general rule, motor units are recruited in order of their size. When the muscle is activated initially, the first motor units to fire are small in size and weak in the degree of tension they can generate. Starting with the smallest motor units, progressively larger units are recruited with increasing strength of muscle contraction. The result is an orderly addition of sequentially larger and stronger motor units resulting in a smooth increase in muscle strength. 
This orderly recruitment of sequentially larger motor units is referred to as the "Henneman size principle", or simply "size principle." [2, 3, 4] Recording from the ventral rootlets in cats and measuring the amplitudes of motor axon spikes, Henneman et al concluded that motor axon diameter, conduction velocity and, by further inference, motor neuron cell size all increase with functional threshold. 
There are exceptions to the size-ordered activation of motor units. Motor unit recruitment patterns vary for different movement tasks, depending on many factors, including the mechanical function of the muscle, sensory feedback, and central control.  After nerve injury, the relationship between motoneuron size and the number and size of muscle fibers that the motoneuron reinnervates is initially lost.  With time, however, a size-dependent branching of axons accounts for the rematching of motor neuron size and muscle unit size, and the size-ordered organization of motor units properties is restored. 
The 3 main types of motor units, which have different physiologic and staining properties, include the following:
Type I or type S (slow) - Slow twitch, fatigue-resistant units with smallest force or twitch tension and slowest contraction; contain oxidative enzymes
Type IIa or type FR (fast, resistant) - Fast twitch, fatigue-resistant units with larger forces and faster contraction times; contain oxidative and glycolytic enzymes
Type IIb or type FF (fast, fatigable) - Fast twitch, easily fatigable units with largest force and fastest contraction; contain glycolytic enzymes
The recruitment sequence is thought to begin with type I motor units analogous to type S units, to progress to type II units that first include type FR (type IIa), and to end with units analogous to type FF (type IIb), which are active only at relatively high force output.
The needle EMG examination cannot assess anatomic size or degree of tension of a motor unit. In an EMG study, the term "size" of a motor unit usually refers to the amplitude of the motor unit action potential (MUAP). The size principle is also true to a limited extent for the EMG study. In rather general terms, the later recruited type II fibers, especially the FF type, have larger diameter muscle fibers generating higher potentials than the smaller, slow twitch type I units. Because of the small uptake area of standard EMG needle electrodes, however, the size of consecutively recruited MUAPs during an EMG study varies considerably. 
Assessment of Recruitment at Low Level of Muscle Contraction
An essential part of an EMG study is the assessment of motor unit recruitment at low levels of muscle contraction. The goal is to identify the recruitment pattern by measuring the firing rate of the first few recruited MUAPs. See the image below.
As described in the previous section, the first recruited motor units arise from the small and relatively slow-conducting type I motor units exclusively. Recruitment analysis at low levels of muscle contraction, therefore, assesses type I motor units predominantly. Type II motor units are recruited later and are not analyzed in this way.
The patient is instructed to make only a very gentle contraction of the muscle under investigation. In the normal situation, the first motor unit usually begins to fire irregularly at 2-3 Hz and then achieves a stable and fairly regular firing rate at 5-7 Hz. This is the "onset frequency." When the patient minimally increases the force of contraction, the first unit increases the rate of firing to 6-10 Hz. With further increase of muscle contraction, the second unit is recruited once the first unit achieves a firing rate of about 10 Hz.
Recruitment frequency is the firing frequency of the first motor unit when the second unit just begins to fire regularly. The term "recruitment rate" is used interchangeably.
Recruitment interval is the time difference between 2 motor unit potentials belonging to the first firing motor unit when the second unit first appears. The recruitment interval is the reciprocal of the recruitment frequency.
In practice (see image below), the patient is instructed to make only a minimal contraction of the target muscle, often by using a phrase such as "...just think about contracting the muscle..." Just 1 motor unit firing regularly should be identified (ie, MUAP A). The patient then is asked to very gradually increase the force of muscle contraction. MUAP A then may increase its firing frequency and at one point a second motor unit (MUAP B) appears. This event can be recognized by observing the screen and by listening for a change in sound associated with firing of 2 motor units. Once this event is recognized, the examiner should "freeze" the screen. The time difference between 2 sequential potentials of MUAP A is the recruitment interval.
The recruitment interval may be measured by placing 2 time markers on the 2 sequential MUAPs A. The recruitment frequency may be calculated as the reciprocal of the measured recruitment interval. This is a precise but cumbersome way of determining the recruitment frequency; in practice, most examiners use an estimate of the recruitment frequency instead.
A fast estimate of the firing rate of MUAPs is obtained by looking at the screen of the EMG machine. Assuming that the sweep speed is 10 milliseconds (ms)/cm, and the monitor of a typical EMG machine has 10 cm (10 divisions) across the entire screen, therefore, one screen represents 100 ms. A MUAP firing at 10 Hz means that it is firing 10x per 1000 ms, equivalent to 1x per 100 ms. It appears, therefore, once on the screen. As long as the sweep speed is 10 ms/cm and the screen is 10 cm across the screen, simply multiplying the number of times the MUAP is present on the screen by 10 yields an estimate of the firing frequency. A MUAP firing twice per screen, therefore, has a firing frequency of 20 Hz; if the unit is seen only once per screen but successively closer to the beginning of the trace with each new sweep, firing frequency is between 10 and 20 Hz.
Newer EMG equipment often has a 20-cm across the screen. At the same sweep speed of 10 ms/cm, therefore, the width of a single screen represents 200 ms (see images below). A MUAP firing at 10 Hz appears twice on the screen. A unit seen only once per screen is firing at 5 Hz, a unit seen 3 times is firing at 15 Hz, and so on. In this setting, therefore, the multiplication factor of 5 is used to arrive at an estimate of the firing frequency. The same multiplication factor is used for the 10-cm screen; if the sweep speed is increased to 20 ms/cm, it results in 200 ms across the entire screen.
Most extremity muscles have a recruitment interval of about 90-100 ms, corresponding to a recruitment frequency of about 10-11 Hz. Facial muscles are an exception to this rough guide. MUAPs of facial muscles have shorter recruitment intervals (around 40 ms) and higher recruitment frequencies (about 25 Hz).
In the example of an EMG screen set at 200 ms across the entire screen and recording from an extremity muscle, a single motor unit firing should not be seen more than twice. If the first recruited motor unit is seen 3 times or more before the second unit is activated, then this suggests an abnormality.
The orderly recruitment of successive motor units may be described as a rough approximation by the "rule of fives." Motor units begin firing at stable rates at 5 Hz. When the first unit to fire (MUAP A) reaches 10 Hz, the second motor unit (MUAP B) is activated and fires at 5 Hz. With further increase in muscle contraction force, MUAP A and B increase their firing frequencies, until MUAP A reaches about 15 Hz and MUAP B about 10 Hz. At this point, MUAP C is activated. Each time a motor unit is recruited, 5 Hz is serially added to the firing frequency of each MUAP already present.
The recruitment ratio is calculated from the firing frequency of the fastest firing MUAP divided by the number of different MUAPs on the screen. This ratio should be close to 5. In the example just discussed, MUAP C is activated when the firing frequency of the fastest firing MUAP (ie, MUAP A) is 15 Hz. The recruitment ratio is 5 (15/3).
If the recruitment ratio approaches 10, motor units are too few for the greatest firing frequency and force produced (ie, decreased recruitment). If it is reduced to less than 4 or 5, then motor units are too many for the highest firing rate (ie, early recruitment). Abnormal recruitment patterns such as these are discussed in the next sections.
Decreased Recruitment in Neurogenic Conditions
Damage may occur to the neural portion of a motor unit, anterior horn cell, or corresponding axon. Such injury may result in wallerian degeneration of the motor axon, and all the muscle fibers previously innervated by this axon will be denervated. As a result of such motor unit loss, fewer motor units are available for muscle activation.
Normally, when the first recruited motor unit reaches a firing frequency of 10 Hz, a second unit should begin firing with increasing muscular effort. In a neurogenic condition, this second unit is missing and an increase in force can be achieved only by increasing the firing rate of the first unit.
Successful activation of a second motor unit occurs only at a higher level of muscular effort than in the normal condition. The recruitment frequency, defined above as the firing rate of the first motor unit at the point when the second motor unit is activated, is therefore increased in a neurogenic lesion. Such an abnormally fast firing motor unit is called "rapid firing unit" (RFU). Because in such cases fewer MUAPs are active than expected, given the first motor unit firing rate, this pattern is called "decreased recruitment" or "reduced recruitment."
This pattern of decreased recruitment may occur whenever a lesion results in a reduced number of functionally intact motor neurons and axons, whether it is the result of actual motor unit loss or temporary conduction block as in neurapraxia. It is an early finding after acute nerve injury (eg, radiculopathy from disk herniation or nerve trauma) and may precede other evidence of denervation in the EMG study.
Early Recruitment in Myogenic Conditions
In muscle diseases such as polymyositis or muscular dystrophies, muscle fibers are damaged. A number of motor units are unaffected but the muscle fiber content of each motor unit is reduced; therefore, the force output of each unit is diminished. The number of units required to maintain a given force increases in proportion to the inefficiency of the individual motor unit discharge. Compensation occurs by having multiple motor units begin firing simultaneously. See the image below.
In a myopathy, isolating a single firing motor unit often is impossible. Even with minimal muscular effort, typically 2 or more units may be activated. This recruitment pattern in myopathic conditions is called "early recruitment" or "increased recruitment." The recruitment frequency is decreased.
Recruitment With Maximal Volitional Effort: Interference Pattern Analysis
With increasing effort, the firing frequency of individual motor units increases and progressively more and larger units are activated. In a healthy subject providing maximal voluntary effort of the muscle under investigation, the action potentials of individual motor units no longer can be separated from each other but are mixed with the signals of other units. The recruitment pattern with maximal voluntary contraction is called "interference pattern" because of the increasing degree of superimposition of action potentials from different units. With increasing force, the EMG becomes continuously denser and the maximal peaks in the signal have a higher amplitude.
American Association of Electrodiagnostic Medicine defines the interference pattern as "electric activity recorded from the muscle with a needle electrode during maximal voluntary effort."
During a maximal voluntary muscle contraction of a healthy individual, a "full" or "complete" interference pattern is present. No individual MUAPs can be identified clearly (this is normal). The baseline is obscured completely by motor unit activity.
Incomplete interference pattern may be divided as follows:
Reduced interference pattern (ie, intermediate interference pattern): Some of the individual MUAPs may be identified, while other individual MUAPs cannot be identified because of overlap.
Discrete activity: Each of several different MUAPs can be identified. See the image below.Incomplete interference pattern. This example shows a discrete interference pattern in a patient with amyotrophic lateral sclerosis. Despite maximal voluntary effort, individual MUAPs can be identified and the baseline is partly visible.
Single unit pattern: A single motor unit fires at rapid rate during maximum voluntary effort.
An incomplete interference pattern typically signifies a decreased number of MUAPs being activated with maximal effort. This may be suggestive of a neurogenic lesion resulting in a decreased number of functional motor units. It may, however, occur with incomplete effort of muscle contraction, possibly as a result of poor cooperation or pain. In myopathic conditions, the interference pattern is typically complete, even though low-amplitude MUAPs may be noted on the recording of the interference pattern; however, in very advanced stages of muscle disorders, the interference pattern may be incomplete because of marked loss of muscle fibers.
The interference pattern is dependent on the shape of the individual motor unit potentials (eg, amplitude, duration, polyphasia), the firing rates, and the number of active units. In general, myopathies have low amplitude and short duration components, resulting in relatively large high-frequency content. In contrast, neurogenic conditions have low-frequency components due to the large amplitude and long-duration motor unit potentials. Attempts have been made to quantify the characteristics of the interference pattern.
Willison in 1964 described a technique to analyze "turns" and "amplitudes" within the EMG trace.  The EMG is converted into 2 trains of pulses that are counted to characterize the signal in terms of amplitude and turns. A turn was defined as a change in the direction of the signal of at least 100 microvolts (µV). An amplitude count is produced for a fixed voltage change, usually 100 µV, between successive turns. In neurogenic conditions, the turns count is normal or low and the amplitude high, and in myopathic lesions, the turns count is high and the amplitude low. Both the amplitudes/second and the turns/second increase with strength of muscle contraction. Thus, the force exerted by the muscle is critical for the analysis and needs to be kept constant at a defined level. This is a major limitation of this method.
In 1983, Stalberg introduced a new version of the method, making it less dependent on force, by plotting the turns/second against the mean amplitude change/turn in an XY-diagram, the so-called turns/amplitude (T/A) analysis (see images below).  The recordings are made during an epoch of steady contraction (typically 1 s, but 250-300 ms may be sufficient) at various levels of muscle contraction. Typically, 20 recordings are collected. The position of the needle should be such that it is recording relatively sharp action potentials.
For each muscle, a normal area called "cloud" exists that was defined by plotting recordings from a large number of healthy subjects at the 95% confidence level. Normal values fall into this cloud, and for a healthy individual no more than 1 data point of 20 recordings should be outside the cloud. A recording indicates a neurogenic condition if 2 or more data points are above the cloud of normal values and a myopathic condition if 2 or more data points are below the cloud.
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- Definition of Motor Unit Recruitment and Overview
- Order of Recruitment
- Assessment of Recruitment at Low Level of Muscle Contraction
- Decreased Recruitment in Neurogenic Conditions
- Early Recruitment in Myogenic Conditions
- Recruitment With Maximal Volitional Effort: Interference Pattern Analysis
- Turns/amplitude Analysis
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