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.
Normal recruitment pattern. (A) With minimal effort of muscle contraction, a single motor unit is seen firing at 6 Hz. The time between 2 discharges is approximately 166 milliseconds (ms), corresponding to a firing rate of 6 Hz (ie, the reciprocal). (B) Gradual increase in muscle strength results in recruitment of a second motor unit. Recruitment frequency is defined as the firing frequency of the first motor unit when a second motor unit is recruited. In this example, it is 12 Hz, the reciprocal of the recruitment interval, which is 85 ms. (C) With further increase in muscle strength, a third motor unit is recruited.
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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.
Normal recruitment pattern. (A) With minimal effort of muscle contraction, a single motor unit is seen firing at 6 Hz. The time between 2 discharges is approximately 166 milliseconds (ms), corresponding to a firing rate of 6 Hz (ie, the reciprocal). (B) Gradual increase in muscle strength results in recruitment of a second motor unit. Recruitment frequency is defined as the firing frequency of the first motor unit when a second motor unit is recruited. In this example, it is 12 Hz, the reciprocal of the recruitment interval, which is 85 ms. (C) With further increase in muscle strength, a third motor unit is recruited.
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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.
Normal recruitment pattern. (A) With minimal effort of muscle contraction, a single motor unit is seen firing at 6 Hz. The time between 2 discharges is approximately 166 milliseconds (ms), corresponding to a firing rate of 6 Hz (ie, the reciprocal). (B) Gradual increase in muscle strength results in recruitment of a second motor unit. Recruitment frequency is defined as the firing frequency of the first motor unit when a second motor unit is recruited. In this example, it is 12 Hz, the reciprocal of the recruitment interval, which is 85 ms. (C) With further increase in muscle strength, a third motor unit is recruited.
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Decreased recruitment in neurogenic conditions. This single motor unit is firing at 15 Hz. The firing rate is calculated from the presence of 3 MUAPs on a screen of 200 milliseconds. This rapid firing unit indicates a neurogenic pathology; the underlying condition in this patient is amyotrophic lateral sclerosis.
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Early recruitment in myopathic conditions. In this electromyographic (EMG) study of a patient with inclusion body myositis, many motor units are activated simultaneously at a low level of muscle contraction. Note the low amplitude and short duration of individual units.
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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.
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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.
