*1.2.3. Recording method (Fig. 2)*

H-wave recording is often done with surface EMG. The skin surface is wiped clean before attaching an electrode to keep electrical resistance as low as possible. To record the H-wave from soleus muscle selectively, an electrode is firstly affixed between medial head and lateral head of gastrocnemius, and another is attached 3-5 cm distal from the first. It is important that electrodes be aligned along the long axis of the muscle for accurate measurement; values of the potential vary significantly if electrodes are placed across, at right angles or otherwise, in relation to the muscle's long axis.

from soleus muscle. When the intensity of an electrical stimulus is gradually raised, group Ia fibers will be excited more. At the same time, alpha motor neuron which runs from the spinal

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

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M-wave will appear at the 5-10 ms latency. If the stimulus intensity is raised, excitement of group Ia fibers will be lost and excitement of alpha motor neuron will become greater. H-wave amplitude decreases, whereas M-wave amplitude increases. H-wave is lost before M-wave

H-wave latency is measured at the time when the value shows the first deflection from baseline. In the soleus H-wave, the first rising edge of the positive waveform is adopted as its latency because it is impossible to affix the recording electrode on the motor point. And its H-wave amplitude is defined by the difference in potential either from baseline to the top of negative waveform, or from the top of negative waveform to the top of next positive waveform.

It is important to ensure that the same measurement method is used for H-wave and M-wave amplitude. If these two amplitudes were measured differently, the H/M amplitude ratio would

Under uniform stimulus conditions, the amplitude size of an H-wave is determined by the strength of the stimulus and the excitability of the reflex arc. Thus, H-wave has been used as an indicator of the excitability of motor neurons in the anterior horn of the spinal cord.

In clinical practice, the ratio between the maximum amplitude of H-wave (Hmax) and that of M-wave (Mmax), or H/M ratio, is often adopted as a good index. The Hmax is taken to reflect the number of excited alpha motor neurons in the anterior horn of the spinal cord, when the condition is adjusted so as to maximize the input from group Ia fibers upon electrical stimu‐ lation. The Mmax, on the other hand, is thought to show the amplitude of complex muscle action potential when all the alpha motor neurons dominating the muscle (soleus muscle here) are excited synchronously. That is, of all the alpha motor neurons that dominate the targeted muscle (e.g., soleus muscle), the H/M ratio shows the percentage of excited alpha motor neurons upon electrical stimulation. In fact, correlation has been observed between the H/M ratio and the degree of spasticity. The H/M ratio shows marked increases in the elevated

**1.3. To evaluate the muscle tone of skeletal muscle by analysis of the H/M ratio**

excitability of alpha motor neurons in the spinal cord, or in patients with spasticity.

Conversely, in cases of peripheral neuropathy, this H/M ratio is decreased. Because there is a great difference between individuals in the H/M ratio, it is more useful as a therapeutic tool in the same patient rather than as a general diagnostic tool. The H/M ratio would work when you tried to make an objective judgment of therapeutic effects on immediate change in the same patient, or when you tried to make a longitudinal change in a given condition also in the

It is surmised that if you observe changes in the H-wave at the time of intervention such as muscle contraction, vibration stimulation to the tendon, or muscle stretching, then those

reaches the maximum amplitude; only the M-wave may remain (Fig. 1).

lose its reliability in the analysis of evoked EMG study.

same patient.

cord to muscle fiber also becomes excited.

Prevention of contamination from artifacts is also a major issue. Contamination of the potential induced by the latency of about 10 ms after the stimulus is highly likely specifically in terms of the inception and waveform of the H-wave. To remove artifact contamination, it is important that electrodes be securely earthed and the width and size of square wave of electrical stimulation be closely monitored.

As for H-wave amplitude, it increases as the motor neuron pool becomes more excitable due to weak voluntary muscle contraction, but H-wave latency hardly changes. Based on this characteristic, those muscle groups from which H-wave is hardly recorded at rest (e.g., tibialis anterior, extensor carpi radialis, and abductor pollicis brevis) can be induced to elicit record‐ able levels of H-wave.

#### *1.2.4. Identification of waveforms*

To elicit H-reflex from soleus muscle, a low-intensity electrical stimulation is applied on tibial nerve in the popliteal space, whereupon only those group Ia fibers with low threshold levels become selectively excited. The waveform which appears in the 20-30 ms latency is the H-wave from soleus muscle. When the intensity of an electrical stimulus is gradually raised, group Ia fibers will be excited more. At the same time, alpha motor neuron which runs from the spinal cord to muscle fiber also becomes excited.

*1.2.3. Recording method (Fig. 2)*

198 Electrodiagnosis in New Frontiers of Clinical Research

relation to the muscle's long axis.

stimulation be closely monitored.

able levels of H-wave.

**Figure 2.** Recording method

*1.2.4. Identification of waveforms*

H-wave recording is often done with surface EMG. The skin surface is wiped clean before attaching an electrode to keep electrical resistance as low as possible. To record the H-wave from soleus muscle selectively, an electrode is firstly affixed between medial head and lateral head of gastrocnemius, and another is attached 3-5 cm distal from the first. It is important that electrodes be aligned along the long axis of the muscle for accurate measurement; values of the potential vary significantly if electrodes are placed across, at right angles or otherwise, in

Prevention of contamination from artifacts is also a major issue. Contamination of the potential induced by the latency of about 10 ms after the stimulus is highly likely specifically in terms of the inception and waveform of the H-wave. To remove artifact contamination, it is important that electrodes be securely earthed and the width and size of square wave of electrical

As for H-wave amplitude, it increases as the motor neuron pool becomes more excitable due to weak voluntary muscle contraction, but H-wave latency hardly changes. Based on this characteristic, those muscle groups from which H-wave is hardly recorded at rest (e.g., tibialis anterior, extensor carpi radialis, and abductor pollicis brevis) can be induced to elicit record‐

To elicit H-reflex from soleus muscle, a low-intensity electrical stimulation is applied on tibial nerve in the popliteal space, whereupon only those group Ia fibers with low threshold levels become selectively excited. The waveform which appears in the 20-30 ms latency is the H-wave M-wave will appear at the 5-10 ms latency. If the stimulus intensity is raised, excitement of group Ia fibers will be lost and excitement of alpha motor neuron will become greater. H-wave amplitude decreases, whereas M-wave amplitude increases. H-wave is lost before M-wave reaches the maximum amplitude; only the M-wave may remain (Fig. 1).

H-wave latency is measured at the time when the value shows the first deflection from baseline. In the soleus H-wave, the first rising edge of the positive waveform is adopted as its latency because it is impossible to affix the recording electrode on the motor point. And its H-wave amplitude is defined by the difference in potential either from baseline to the top of negative waveform, or from the top of negative waveform to the top of next positive waveform.

It is important to ensure that the same measurement method is used for H-wave and M-wave amplitude. If these two amplitudes were measured differently, the H/M amplitude ratio would lose its reliability in the analysis of evoked EMG study.
