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

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 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 same patient.

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 changes have in fact been brought on by the intervention, and not due to alterations in the physical condition such as electrode attachment and the like.

group Ia fibers (Kouzaki 2000). It is believed that this is one of the reasons why a decrease is

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

http://dx.doi.org/10.5772/55783

201

The equipment that is used to apply vibration stimulation may vary from researcher to researcher, but the effect seems to be uniform as long as stimulation conditions are controlled in the same way. Stimulation conditions inevitably differ according to the research objective, but they are generally set at a frequency of 50-100 Hz and amplitude of 1-2 mm. The equipment

Effects that vibration stimulation has on the body depend on the duration of stimulation. The H/M ratio decreases immediately after the application of stimulation, and the inhibitory effect on spasticity has been confirmed in 5 minutes of stimulation (Noma 2012). Muscle output is reduced by a sustained stimulation of at least 20 minutes (Bongiovanni 1989, Kouzaki 2000,

Vibration stimulation on a skeletal muscle of healthy adults is known to bring on effects such as reflexivity contraction of an agonist muscle by muscle spindle Ia afferent nerve excitability, and inhibition of the antagonist muscle, called "tonic vibration reflex". Vibration stimulation also causes inhibition of monosynaptic reflexes such as the tendon reflex and stretch reflex while stimulation with vibration continues. These effects vary depending on stimulus

**Clarified issue 1: Anterior horn cells in the spinal cord are suppressed by vibration stimu‐**

As a result of vibration stimulation on the triceps surae of healthy adults, the H/M ratio continues to decrease from 1 to 3 minutes after stimulus onset. After 4 minutes into stimulation, though the ratio continues to decrease somewhat, no significant difference was found between

From these findings, it has been clarified that excitability of alpha motor neurons gets sup‐ pressed immediately after the intervention with vibration stimulation. Also clarified was that the decline of the H/M ratio reaches a steady state after 3 minutes of stimulation from its onset.

If vibration stimulation is to be adopted in clinical settings, it is essential that an appropriate set of stimulus conditions should be considered for each of the clinically different cases. A site of stimulation is certainly one of the factors for the treating therapist to take into account. Based on the results of our study, the H/M ratio was significantly lower after the onset of vibration stimulation on both muscle belly and the tendon (p<0.01). In addition, when stimulus to the belly was compared with stimulus to the tendon, the H/M ratio was significantly lower in the latter (p<0.05). In other words, to suppress muscle tone using a vibration stimulus, it is

**Clarified issue 2: It is recommended that vibration stimulation be applied on tendon.**

is contacted at 20-30N on the body or tendon of the targeted skeletal muscle.

observed in the excitability of motor neurons in the spinal cord.

**lation from its onset to about 3 minutes into stimulation.**

recommended that a stimulus be applied on the tendon.

values at after 4 minutes and the value at 3 minutes of stimulation.

*2.1.4. Conditions for vibration stimulation*

Stephen 2003, Konishi 2009).

**2.2. Our previous study 1**

conditions.
