EMG exam

The EMG exam

With an intramuscular electrode, recordings are made with the muscle at rest and working to view for evidence of neurogenic or myogenic pathology. Waveforms are observed on the oscilloscope, mediated through the EMG needle electrode and amplifier.
Insertional activity

Normal muscle will have a significant characteristic with EMG exam. Brief insertional activity will be observed on the oscilloscope when the needle is moved through the sarcolemma, referred to as injury potentials. Muscle that is fibrotic and severely atrophied from myopathy or long standing denervation will have reduced insertional activity.
Spontaneous abnormalities

A normal muscle at rest will have a characteristic silent baseline. Spontaneous abnormalities may be seen in a muscle suffering from acute denervation. Fibrillation and positive sharp waves are commonly seen after nerve injury, the origin of such is postulated to be due to a metabolic change within the muscle after denervation, or the result of increased post synaptic receptor sensitivity to acetylcholine after denervation. The fact that curare does not inhibit fibrillation in the wake of denervation argues against the latter theory. Fibrillation and positive sharp waves are the result of spontaneous discharge of a single muscle fiber.
Other forms of spontaneous abnormalities include complex repetitive discharges and fasciculation potentials. Complex repetitive discharges are the result of several muscle fibers spontaneously firing through ephaptic transmission, with one fiber serving as a pacemaker. Fasciculation potentials are the result of the spontaneous discharge of a motor unit, are commonly benign in origin, but may be seen with other EMG abnormalities in diseases such as ALS.

fibrillation, positive sharp wave activity at rest

Volitional EMG Exam

After observing the muscle at rest, EMG activity accompanying volitional effort is observed. Electrical activity accompanying motor unit discharge will be recorded with the EMG needle, amplified and viewed on the oscilloscope screen. The amplitude and configuration of the motor units is observed, as well at the recruitment order and motor unit firing frequency.
Normal motor unit recruitment will be orderly and graded, according to Henneman’s size principle. With minimal effort, small motor units are recruited, firing at a particular frequency. With increased effort, these motor units will increase firing rate, with larger motor units recruited with more demand on the muscle. Innervated motor unit discharge will be synchronous across the X-axis, and be biphasic, triphasic, or quadriphasic in shape. The amplitude of the normal motor unit observed on the oscilloscope usually is less than 5 mV, with small amplitude motor units recruited first, larger motor units usually recruited with more forceful effort.
In neurogenic pathology, the number of motor units that are available for muscle force is compromised. The remaining motor units tend to fire more rapidly in order to comply with demand on the muscle for more force. This is referred to as late, decreased, or neurogenic recruitment. There is decreased number of motor units recruited per contraction force compared to normal. The shape of the motor unit after re-innervation will tend to be polyphasic, as its discharge will no longer be synchronous, due to collateral sprouting. A motor unit with five or more phases is referred to as polyphasic. Small amplitude polyphasic motor units are observed with early or immature re-innervation. With chronic re-innervation, the amplitude of the motor unit will increase, as will its duration, owing to the greater size of the new motor unit. A motor unit greater than 5 mV is referred to as a giant motor unit, and consistent with longer standing re-innervation.
polyphasic motor unit potential
In myogenic pathology, the number of motor units available for discharge is not compromised. However, the contractile strength of each motor unit is reduced due to sick muscle fibers. With little demand upon the muscle, a greater number of motor units needs to be recruited in order supply the necessary force. This is referred to as early, increased, or myogenic recruitment. The amplitude of the motor units will be decreased, and may appear polyphasic. Some of the polyphasia observed may simply be several individual motor units abutting in time.
Incomplete motor unit activation will be observed when a patient’s effort is reduced because of pain, fear, hysteria, or malingering. In this instance, the recruitment pattern observed will be normal in number per contraction force, with normal amplitude, shape, and firing frequency.


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Ray Jurewicz
E-mail: rj@NerveStudy.com

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