H-reflex and F-wave


The H reflex is the EMG counterpart of the ankle jerk reflex. It is a EMG test used only to evaluate patients with suspected S1 nerve root pathology. By stimulating the tibial nerve at the popliteal fossa, an afferent impulse is generated which travels toward the spinal cord in the Ia sensory fibers. Coincidentally, an impulse also travels orthodromically down the tibial nerve to stimulate the soleus muscle from which are recording. This generates the early response, the CMAP. The proximal traveling impulse travels to the spinal cord and activates the alpha motor neuron of the S1 reflex arc. A action potential is generated which travels down the tibial nerve to the soleus muscle, at which point a late action potential is generated, the H response.

H reflex+

The F-wave

The F wave study tests the conduction of motor axons proximal to the stimulation site. When motor nerves are stimulated for routine NCV studies, the stimulation creates action potentials which travel not only orthodromically toward the muscle, but also antidromically toward the motor neuron. Depolarization is conducted to the axon hillock, which is now repolarized, sending a new action potential back to the muscle, causing action potentials in muscle fibers. This late response is the F wave. The recording electrodes for the F wave study are in the same location as for the motor NCV study, but, the stimulating electrodes are flipped so that the cathode is directed towards the spine. 

F wave

Comparison of H-reflex and F-wave

Comparison of F wave and H reflexComparison of F wave and H reflex2


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