Normal muscle in the resting state should be electrically silent; the small tension spoken of as muscle tone has no EMG equivalent. There are, however, two closely related types of normal spontaneous activities and another that is induced by the insertion of the needle itself. One is a low-amplitude, 10to 20-μV monophasic (negative) potential of very brief (0.5 to 1 ms) duration. These represent single or synchronized miniature end plate potentials (MEPPs) because of the small number of ACh quanta that are being released all the time. They are normally sparse but are most evident when the recording needle electrode is placed near a motor endplate (“endplate noise”). Fortuitous placement of the needle electrode very close to or in contact with the endplate gives rise to a second type of normal spontaneous activity that is characterized by irregularly discharging high-frequency (50to 100-Hz) biphasic spike discharges, 100 to 300 μV in amplitude (i.e., large enough to cause an isolated muscle action potential). These potentials have been termed endplate spikes and represent discharges of single muscle fibers excited by spontaneous activity in nerve terminals. They must be distinguished from fibrillation potentials (see later). Finally, insertion of the needle electrode into the muscle injures and mechanically stimulates a number of fibers, causing a burst of potentials of short duration (300 ms). This is referred to as normal insertional activity, but the extent of this activity is greatly raised in certain pathologic states as noted below.
When a muscle is voluntarily contracted, the depolarization potentials of motor units begin to appear. One can observe a pattern of force build up by watching the progressive recruitment of MUPs; the initial ones, representing smaller motor units, firing at rates of 5 to 10 per second. With increased force of contraction, there is an increased rate of firing (40 to 50 per second as well as a recruitment of larger, previously inactive motor units; Fig. 2-17A). Because individual MUPs can no longer be distinguished during maximal voluntary contraction, this activity is referred to as a complete interference pattern (Fig. 2-17A, right). This is seen not only as a summated signal pattern but is also heard as a mixed high-frequency clicking when the electrical activity is made audible. As muscles relax, an increasing number of units drop out. If a muscle is weakened by denervation or if electrical conduction is blocked, there will be fewer MUPs, but the remaining ones will still show a rapid firing rate (reduced recruitment; see Fig. 2-17B). In contrast, with poor voluntary effort and with upper motor neuron lesions, the MUPs fire in decreased numbers, at slower rates, and often in an irregular pattern (termed poor activation).
The Abnormal Electromyogram
Clinically important deviations from the normal EMG include (1) increased or decreased activity upon insertion of the needle; (2) the occurrence of abnormal “spontaneous” activity during the relaxed state (fibrillation potentials, positive sharp waves, fasciculation potentials, cramp potentials, myotonic discharges, myokymic potentials); (3) abnormalities in the amplitude, duration, and shape of single MUPs; (4) a decrease in the number of MUPs and changes in their firing pattern such as recruitment discussed above; (5) variation in amplitude and number of phases of MUPs during voluntary contraction; and (6) the demonstration of special phenomena such as in states of continuous muscle fiber activity or electrical silence during shortening of the muscle (physiologic contracture). The underlying physiology of these changes is discussed in Chap. 45, in relation to diseases of the muscle.