*1.1.1. H-wave (H-reflex)*

The name H-wave was derived from that of Johann Hoffmann, who found the response for the first time in 1918. 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. The action potential that reaches from excitatory alpha motor neuron to skeletal muscle

© 2013 Kai and Nakabayashi; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Kai and Nakabayashi; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

is called H-wave or H-response. With soleus or flexor carpi radialis muscle the H-wave is observed at rest, but in other muscles the H-wave can only be induced with mild voluntary contraction.

reach the innervated skeletal muscle after re-firing at part of the axon hillock of anterior horn cells in the spinal cord. The action potential in question is called F-wave or F-response. The F-

Evoked EMG Makes Measurement of Muscle Tone Possible by Analysis of the H/M Ratio

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With evoked EMG, the action potential is derived from a skeletal muscle that is innervated by

In this section, we briefly introduce the measurement method of H-wave in soleus muscle in accordance with the guidelines of clinical neurophysiology test as stipulated by International Federation of Clinical Neurophysiology. Kaeser advocated that the measurement of H-wave be standardized in the study of spinal reflex (Kaeser 1973). It is not always easy to apply his method in clinical situations, but we recommend that you should examine his method carefully

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

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

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

wave is said to be evoked on any peripheral muscle.

electrical stimulation conducted to peripheral nerves.

because it is considered to be a correct way of recording the H-wave.

in the H-wave. That is how postural change can cause variability in H-wave.

**1.2. Measurement of H- and M-wave**

*1.2.1. Postural position at measurement*

position (about 20 degrees).

and amplitude can be measured.

*1.2.2. Stimulus conditions*

M-wave: Excitement is conducted to efferent. H-wave: Excitement is conducted via the monosynaptic reflex. F-wave: Excitement is conducted to antidromic.

**Figure 1.** Evoked EMG

#### *1.1.2. M-wave (M-response)*

Alpha motor neurons are activated directly by a gradual increase in the intensity of electrical stimulation, which occurs as the activity of Ia fiber is enhanced. The action potential of the skeletal muscle caused by this excitement is called M-wave or M-response.

#### *1.1.3. F-wave (F-response)*

If an alpha motor neuron receives a strong electrical stimulation, its antidromic impulse is conducted to axons of all motor neurons that have received the stimulus. The impulse is to reach the innervated skeletal muscle after re-firing at part of the axon hillock of anterior horn cells in the spinal cord. The action potential in question is called F-wave or F-response. The Fwave is said to be evoked on any peripheral muscle.
