The H-reflex is a reflectory reaction of muscles after electrical stimulation of sensory fibers in their
innervating nerves (for example, those located behind the knee). H reflex (Hoffman reflex) has a specific
indication for the study of the first sacral root sufferance. The exam consists of a recording of the motor
reflex response (H) from soleus or gastrocnemious muscle by stimulating the nerve in the popliteal fossa
with short duration and short amplitude. In this muscle the M response appear with a latency of 4-5
msec, as in other muscles when you stimulate the corresponding motor nerve. Then, a further motor
response appears after 32-38 msec, as a consequence of the muscle contraction caused by a reflex
mechanism. After an initial stimulation of sensitive axons of sciatic nerve, there is an activation of
nervous cells and its axons create the motor contingent of sciatic nerve.
The sciatic nerve in the popliteal fossa has both sensitive and motor fibres. Sensitive fibres have a
stimulation threshold lower than motor ones. Stimulating this nerve with a low intensity and gradually
increasing it, we obtain sensitive fibres only stimulation at the beginning, involving motor fibres as the
stimulus increase.
With low stimulation intensity we usually obtain H reflex only, because through the sensitive fibres and
their indirect connection with motor cells, we activate the motor axons and the connected muscle. In this
case the stimulus path is long and response latency is high: from poplite to back at lumbar level and
then again to poplite and sural muscle.
If we stimulate the motor fibres too (too high stimulus intensity), we obtain a rapid response, which is the
direct motor response.
Higher stimulations might involve alpha fibres, causing M wave to overlap H reflex, thus compromising
the study.
H-reflex is analogous to the mechanically induced spinal stretch reflex (for example, knee jerk reflex)
because in both cases muscle-spindle innervating fibres are activated. Although stretch reflex gives just
qualitative information about muscle spindles and reflex arch activity; if the purpose of the test to
compare performances from different subjects, H-reflex should be used. In that case, in fact, latencies
(ms) and amplitudes (mV) of H-wave can be compared.
H-reflex is used to assess fitness of astronauts. H-reflex was the first medical experiment completed on
the International Space Station
Electrode positioning
H-reflex cannot be recorded in every muscle. The easier place to record H-reflex is the soleus muscle.
The recording electrodes have to be positioned so that the active one is placed on the centre of the
muscle and the reference 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
The stimulation electrodes must be positioned on the nerve innervating the muscle. On the popliteal
fossa you have to find the exact stimulation point of sciatic nerve using the lower possible current
intensity. In these condition maintaining the electrode position is crucial: it is sufficient just a little shifting
to completely loose the stimulation.
Acquisition parameters
Number channels: 1
High Pass filter: 20 Hz
Low Pass filter: 2000 Hz
Base time: 100 msec
Max Signal: 25.6 mV
Gain/Div: 1 – 5 mV/div
Working modality
Begin with 0 mA stimulation and increase it until the H-reflex disappear, being overlapped by the M
wave. At this time, acquire about ten waves fixing a stimulation rate of 0.5 Hz and increasing the
stimulation intensity of 1 mA after every response. You should see the M wave increasing and covering
H-reflex.
After the acquisition ending it is important to study the variation of H reflex and M wave amplitudes
correlated to the variation of stimulation intensity.