Motor NCS are performed by electrical stimulation of a peripheral nerve and recording from a muscle
supplied by this nerve. The time it takes for the electrical impulse to travel from the stimulation to the
recording site is measured. This value is called the latency and is measured in milliseconds (ms). The
size of the response – called the amplitude – is also measured. Motor amplitudes are measured in
millivolts (mV).
The response latency depends on two factors: the action potential propagation on the nerve and the
signal diffusion from nerve to the muscle. The first one depends to the conduction velocity and to the
nerve length, the second one is constant for every nerve-muscle pair. This second delay is about of 3-4
msec depending on the subject, it is therefore impossible to measure the motor speed conduction
velocity by stimulating only in one point. Calculations are thus performed using the distance between the
different stimulating electrodes and the difference in latencies.
To get the basic motor speed conduction estimation, the nerve must be stimulated in two points. The
distance to be measured is between the two stimulation points , while the propagation time is obtained
by the latency time difference of the two motor response. The distance-propagation time ratio gives
MNCV between two stimulation points in the nerve considered. This operation can be made in further
points through the nerve, in a way the entire nervous path can be explored from the spinal marrow to the
periphery.
Motor response amplitude and duration can give some indication about the condition of the motor nerve
local branches, of the neuromuscular plate, and of the muscle.
MNCV can be calculated recording with needle or cup electrodes. Using surface electrodes, muscular
response appears having high voltage and simple morphology (bi or tri-phasic). By using needle
electrodes, voltage is lower and morphology is more complex.
Electrode positioning
Motor response can be recorded in several muscles. The recording electrodes have to be positioned in
a way the active one is placed on the centre of the muscle and the reference one on a remote area, far
from the muscle.
It is better to place the ground electrode close to the stimulating point, between the stimulating and the
recording area, but there can’t be a direct connection between this one and the other electrodes.
To obtain an high signal quality the electrode impedance should be lower than 5 KOhm and balanced
between the different electrodes.
Stimulation
Electrodes have to be positioned on the motor nerve with the negative one placed close to the recording
point.
The stimulation intensity has to be supra-maximal (just over the response stabilization level), and the
duration has to be short, 100 µs maximum. The intensity values are about 10-15 mA for the wrist nerves
(median, ulnar), 40-50 mA for the same nerves on the elbow and on the armpit, 60-70 mA for the nerves
on the supraclavear and 20-40 mA for the nerves on the foot and on the poplite (sciatic, internal and
external poplite).
Acquisition parameters
Number channels: 1
High Pass filter: 20 Hz
Low Pass filter: 2000 Hz
Base time: 50 msec
Signal max: 25.6 mV
Gain/Div: 1-5 mV/div
Average: you need a single shock
Working modality
The patient must stay in a relaxed and comfortable position and should be advised that he will not feel a
painful shock. Begin stimulating into the closest place to the stimulation point, with a stimulation intensity
generating a supra-maximal motor response. You recording a single response (due to the high motor
response signal amplitude, it is not necessary to average more responses), then you continue recording
into the next point.
The following parameters are calculated for every response:
• Latency: calculated on the deflection point of the base line,
• Amplitude: measured peak to peak, or from positive peak to base line,
• Response area,
• Duration from the initial deflection point to the point of coming back to the base line,
• Morphology, that can be polyphasic in some pathological situations.
In the simplest situation, stimulating the nerve in two different points (wrist and elbow) we will obtain two
different latency times. Both values have two components: the conduction time of the signal from the
stimulation point to the end of the nerve, and neuromuscular transmission time. The difference between
the value of the latency closest to the recording point (proximal) and the latency value of the more
distant one (distal) is the time spent by the stimulus to run the nerve path between the two stimulation
points. By measuring the distance between the two stimulation points and by dividing it by the latancy
time difference, we will find the maximal signal conduction velocity in the analysed path.