Glossary of EMG Terms
Action potential
an electrical potential that moves along an axon or muscle fiber membrane.
Action potential morphology
the electrical representation of the nerve stimulation – seen as a small hill on the screen –
commonly called a waveform.
Amplitude
the maximal height of the action potential (can be measured baseline to peak or peak to
trough); expressed in millivolts (mV) or microvolts (V).
Antidromic
when the electrical impulse travels in the opposite direction of normal physiologic
conduction (e.g., conduction of a motor nerve electrical impulse away from the muscle and
toward the spine).
Axonotmesis
injury to the axon of a nerve but not the supporting connective tissue. Results in Wallerian
degeneration distal to the injury.
Compound motor action potential (CMAP)
summation of action potentials recorded over a muscle following stimulation of a motor
nerve.
Conduction block
failure of an action potential to propagate past an area of injury, generally due to focal
demyelination.
Conduction velocity
a measure of how fast the fastest part of the impulse travels (can also be referred to as a
motor conduction velocity or a sensory conduction velocity).
Electrodiagnostic studies
includes many tests, e.g., nerve conduction studies (NCS) and electromyography (EMG) –
is a physiological assessment of the electrical functioning of nerves and/or muscles.
F-wave
a compound muscle action potential evoked from antidromically stimulated motor nerve
fibers using a supramaximal electrical stimulus. Generally represents only a small
percentage of fibers and therefore much smaller than M-wave.
Fasciculation potential
spontaneous electrical potential originating in the nerve and which can have the
morphology of a motor unit action potential.
Fibrillation potential
spontaneous potential found on EMG at rest; biphasic, initially positive deflection,
originating in the muscle.
Frequency
cycles per second (frequently abbreviated Hertz or Hz).
H-reflex
a compound muscle action potential evoked through orthodromic stimulation of sensory
fibers and orthodromic activation of motor fibers. This is evoked with a submaximal
stimulation and disappears with supramaximal stimulation. It is found in normal adults only
in the gastrocnemius-soleus and flexor carpi radialis muscles. The response is thought to
be due to a mono- or oligosynaptic spinal reflex (Hoffmann reflex).
Insertional activity
the electrical activity generated as a result of disruption of the muscle membrane by a
needle.
Late response
an evoked potential with a latency longer than an M-wave; includes H-reflexes and F-waves.
Latency
time interval between the onset of a stimulus and the onset of a response.
M-wave
muscle action potential evoked by stimulating a motor nerve.
Miniature endplate potential
potential produced spontaneously by the release of one quanta of acetylcholine from the
presynaptic axon terminal.
Motor point
where the nerve enters the muscle (endplate zone).
Motor unit
includes the anterior horn cell, its axon, neuromuscular junction and all the muscle fibers
innervated by that axon.
Myokymic discharge
motor unit action potentials that fire repetitively (often referred to as sounding like marching
soldiers).
Myopathic recruitment
increased number and early recruitment of motor unit action potentials for the strength of
contraction; motor units are generally of small amplitude. Frequently seen in myopathies.
Myotonic discharge
high frequency discharges whose amplitude and frequency wax and wane (sometimes
referred to as ‘dive bombers’).
Nerve conduction studies (NCS)
assessment of functioning of nerves via electrical stimulation.
Neurapraxia
a lesion where conduction block is present. The axon remains intact.
Neurotmesis
a complete injury of a nerve (such as a transection) involving the myelin, axon and all the
supporting structures.
Orthodromic
when the electrical impulse travels in the same direction as normal physiologic conduction
(e.g., when a motor nerve electrical impulse is transmitted toward the muscle and away
from the spine).
Positive sharp wave
primarily monophasic spontaneous potential found on EMG at rest, initially positive
deflection with a characteristic ‘V’ formation.
Recruitment
the orderly addition of motor units with increasing voluntary muscle contraction.
Sensory nerve action potential (SNAP)
summation of action potentials recorded from the nerve following stimulation of a sensory
nerve.
Stimulus
an electrical depolarization of a nerve initiating an action potential. A stimulus can be
supramaximal or submaximal.
Submaximal stimulus
an electrical stimulus that results in the initiation of an action potential in some (but not all) of
the nerve fibers. Increasing the intensity of a submaximal stimulus will change the
appearance of the CMAP or SNAP.
Supramaximal stimulus
an electrical stimulus that results in the initiation of an action potential in all of the fibers of
the targeted nerve. Increasing the intensity of a supramaximal stimulus will not change the
appearance of the CMAP or SNAP (but may shorten the latency).
Temporal dispersion
long duration, low amplitude potential due to extreme variations in the conduction velocities
of individual nerve fibers contributing to the action potential.