*1.2.1. Postural position at measurement*

Because H-wave is susceptible to the effects of posture, it is necessary to maintain the same posture during the measurement. A change in posture gives rise to changes in the length of muscles, which in turn bring on a change in the activity of muscle spindle receptors involved in the H-wave. That is how postural change can cause variability in H-wave.

When you try to evoke the H-wave of soleus muscle, it is important to prevent soleus muscle, and gastrocnemius muscle, from stretching. For the preventive purpose, you instruct the subject to mildly flex the knee joint (about 30 degrees). And because the H-wave of soleus muscle is inhibited by ankle dorsiflexion, the ankle joint is fixed at a mild plantar flexion position (about 20 degrees).

#### *1.2.2. Stimulus conditions*

The optimal duration of stimulation to elicit the H-wave is 0.5 ms or 1.0 ms; the choice is due to the difference in the strength-duration curves between axons of motor neurons and afferent group Ia fibers. Optimal conditions to excite group Ia fibers are 1) stimulation at a low enough intensity not to excite the axons of motor neurons, and 2) placement of the cathode of the stimulating electrode on the nerve, with the anode placed distal from the cathode on the run of the nerve. Because it is a reflex through a synapse, H-wave is easy to cause "habituation". That means that shorter stimulation intervals would tend to inhibit H-wave due to the decreased synaptic connection in the spinal cord. It is therefore important that the interval of stimulations be long enough, but you must also remember that the subjects would sometimes be poised when the stimulation interval is more than 5 seconds. For removal of the inhibitive influence of habituation, it is desirable to set the interval for stimuli at 10 seconds or more.

Clinically, it is often convenient, and optimal, to start with a rate of 1 Hz for H-wave meas‐ urement; the rate of stimulation is then decreased step by step (by 0.2-0.3 Hz) so that latency and amplitude can be measured.
