F-wave is a motor response (linked to the muscle stimulation) and it is caused by an action potential
feedback on the nervous fibre with a mechanism similar to echo. This is not a reflex, per se, in that the
action potential travels from the site of the stimulating electrode in the limb to the spinal cord’s anterior
horn cell and back to the limb in the same nerve that was stimulated. The F-wave latency can be used to
derive the conduction velocity of nerve between the limb and spine, whereas the motor and sensory
nerve conduction studies evaluate conduction in the segment of the limb.
When stimulating the motor nerve we get two action potential series, one going from stimulation point to
the muscle and the other from the stimulation point to the nervous cell positioned in the spinal marrow.
The first one generates the M response; the second one terminates in the spinal marrow and does not
generate any evident effect. If any action potential directed to the cell succeed in to excite again the
motor axon, a new muscle stimulation is generated, having a latency time related to the time spent by
the action potential to run the path from stimulation point to the marrow and then to the muscle.
The F-wave absence is not always a pathologic status, while it is important to evaluate F-wave temporal
dispersion following successive stimulations. A temporal dispersion or a latency increase of F-wave is
the signal of MNCV slowing down. Latency of F-Wave may vary muscle to muscle and patient to patient
depending on patient height.
Conduction velocity is derived by measuring the limb length in millimeters from the stimulation site to the
corresponding spinal segment (C7 spinous process to wrist crease for median nerve). This is multiplied
by 2 as it goes to the cord and returns to the muscle (2D). 2D is divided by the latency difference
between mean F and M and 1 millisecond subtracted (F-M-1). The formula is 2D/(F-M-1).
Electrode positioning
F wave can be recorded in several muscles. The recording electrodes have to be positioned so that the
active one is placed on the centre of the muscle to study 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 take care not to short the ground to any recording/stimulating electrode.
To obtain a better 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, at most 100 µs. The intensity values are about 10-15mA 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 supra-clavear 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 – 100 msec
Max signal: 25.6 mV
Gain/Div 1 – 5 mV/div
Working modality
10 consecutive responses are usually acquired; for every response you have to detect the M and F
wave latencies. The F-wave is not always present in all the responses and it is usually 10 times lower
than M wave, so the two signals are presented with two different amplifications: one for the M wave zone
and another one for the F-wave.