istance between those sites y the motor nerev conduction velocity. Second, there is a brief turnaround time at the anterior horn cell, which has been estimated to be approximately 1ms.Third, the time it takes the F response to travel back down from the anterior horn cell to the stimulation site. Finally, the time it takes from the stimulation site to the muscle is the distant motor latency. 1,6,7
F Estimate=(2D/CV)X10 1 ms DL
Where D is the distance from stimulation site to the spinal cord, Cv is conduction velocity, DL is Distal motor latency and 10 is the conversion factor to milliseconds. The actual measured F response is shorter than F estimate. Therefore, if F response is prolonged as compared to the F estimate it is suggestive of proximal nerve segment injury.
It is a sensitive measure of polyneuropathy and radiculopathy.In Guillain Barré syndrome (GBS) even if the nerve conduction study in terms of distal motor latency and conduction velocity is normal, the F response could be the only parameter which would help in diagnosis. F wave latencies are prolonged in GBS affecting the proximal nerve segment. In amyotrophic lateral sclerosis, reduced persistence of F wave is attributed to the loss of anterior horn cell. 5,8
It checks only the nerve or nerve segments which innervates the muscle being recorded.
Could not check for radiculopathy affecting the sensory nerve root fibers, as F response measures the motor fibres.
Could measure only severe radiculopathy or plexopathy.