Figure 19.

Method for H reflex study. H reflex in humans has been studied mostly for the triceps surae muscles (soleus and gastrocnemius) in a sitting position on a reclining chair supported at the ankle. Electrical pulse stimuli are given to the tibial nerve at the popliteal fossa. The tibial nerve is a mixed nerve containing Ia afferent fibers from and alpha efferent fibers to the triceps surae. The threshold for excitation of Ia fibers is lower than alpha motor fibers with stimulation with square wave electric pulses of 1–3 milliseconds duration. Thus, with gradual increases in stimulus intensity, the appearance and gradual increase of the H reflex (monosynaptic reflex from Ia afferent fibers) is followed by the appearance of an M wave, which is caused by direct activation of alpha motor fibers. Studies on reciprocal reflex connections and influences from descending motor tracts are made with the stimulus condition for the maximal amplitude of H reflex at rest.