F waves are produced when a supramaximal stimulus is applied to a motor nerve, causing antidromic impulses that travel to the spinal cord. Some motor neurons then backfire, producing the F wave. F wave latencies provide information about motor nerve conduction velocity and can help detect neuropathies. The H-reflex is produced when a weak stimulus excites muscle spindle Ia afferents, causing antidromic impulses in the spinal cord that excite alpha motor neurons. F wave and H-reflex latencies are used to evaluate spinal and peripheral nerve conduction.

1. LATE R E S P O N S E

2. LATE R E S P O N S E
F wave H reflex

3. F-WAVE
 F waves were named in reference to “foot”
 A supramaximal stimulus applied at any
point along the course of a motor nerve
elicits an F wave in a distal muscle that
follows the CMAP (M response).
 The impulse travels antidromically to the
spinal cord and the F wave is produced by
backfiring of motor neurons. An average of
5–10% of the motor neurons available in the
motor neuron pool backfire after each
stimulus.

5. CHARACTER
They are typically
variable in
latency,
amplitude, and
morphology..

7. LATENCY ; MINIMAL F WAVE LATENCY IS
THE MOST RELIABLE AND USE FUL .
• It represents conduction of the largest
and fastest motor fibers.
• Depend on the length of motor axons
which correlates with the height and
limb length
• The most sensitive criterion of
abnormality in a unilateral disorder is a
latency difference between the two sides,
or between two similar nerves in the
same limb.

8. PARAMETERS
AVERAGE LATENCY
The average minimal F- wave latencies tend to
be 25-32 msec. in the upper extremities
and 45-56 msec in the lower extremities, height
dependent
Persistence >50%
Configuration :Usually polyphasic& varies
with each stimulation

9. Amplitude 1%-5% CMAP -the gain
should be increased 200 µv cant be
elicited in low amplitude of M response
This varies between individuals and is
inhibited by muscle activity while it is
enhanced by relaxation or the use of
Jendrassik maneuver.

10. F -wave chronodispersion
is the degree of scatter among consecutive F
waves and is determined by the difference
between the minimal and maximal F wave
latencies
Chronodispersion <4 ms (median/ulnar)
&<6 ms (peroneal/tibial)

11. SIGNIFICANT OF F WAVE
 1-- EARLY NEUROPATHY
 If the neuropathy is in its early stage, the delayed F
wave may be the first indication for the diagnosis.
 Guillain-Barre Syndrome can affect any segment of
the nerve, but it will commonly affect the most
proximal segment first
 As the patient starts to recover the F waves that
were absent will start to return, or the latencies that
were prolongated will return to a more normal
value.
 2-Plexipathy

12. 3- SPINAL SHO CK
This is due to the altered state of
excitability of the motor neurons. In
acute paraparesis, the waves below the
injury or lesion in the spinal cord will
be absent or prolonged very soon after
injury and may not return for days or
weeks following

13. F WAVES. LIMITATIONS IN RADICULOPATHY
Slowing at the root level can be obscured
(“diluted”) by the normal conduction along the
long axon.
Most muscles are innervated by two or more roots.
In U L commonly used are F median & ulnar ns
which demonstrates C8& T1 roots ( uncommon
sites of radiculopathy)
If rediculopathy affects sensory nerve root
F wave affected when most of nerve fibers
involved

14. CE N T R A L LATENCY
is the conduction time from the stimulus to
the spinal cord and back down.
There is a one msec delay turn around
time at the anterior horn cell that is
subtracted and then divided by two.
Formula (F-M-1) / 2 ( Kimura, 2001).

15. F- WAVE VELOCITY
The approximate nerve length divided
by the conduction time from the spinal
cord makes up the F- wave velocity
Formula, FWCV= (2D) / (F-M-1)

16. AXILLARY F-LOOP LATENCY
 AFLL= F-wave wrist +distal motor
latency -2(axillary motor latency).
Normal values for median & ulnar
nerves were reborted as 14:16 ms

17. H –REFLEX
The name H-wave was derived from that of
Johann Hoffmann, who found the response
for the first
Weak electrical stimulation can excite group
Ia fibers from muscle spindle, and the
antidromic impulse gets conducted to the
spinal cord. Afferent fibers connect, via a
synapse, with the alpha motor neuron in the
spinal cord. Excitatory postsynaptic potential
(EPSP) causes the excitement of the alpha
motor neuron of the anterior horn cells in the
spinal cord.

20. 
H)

Normal: 28–30 milliseconds
Side to side difference: greater than 0.5–1.0 ms is significant
Above 60 years: adds 1.8 milliseconds

Soleus muscle: tibial nerve: S1 pathway
Flexor carpi radialis: median nerve: C7 pathway
Vastus medialis : femoral n : L4 pathw