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

When a peripheral nerve is subjected to percutaneous electrical stimulation, action potentials are induced in the innervated skeletal muscle. The induced action potential is recorded by evoked EMG, which includes the H-wave, the M-wave, and the F-wave (Fig. 1). The H-wave is a good indicator of the strength and distribution of the stimulus input from muscle spindle to the motor neuron pool, which lies at the site of the anterior horn of the spinal cord. The Hwave is commonly used, therefore, in the diagnosis of peripheral neuropathy (Kaeser 1973). The H-wave is also used to examine the state of muscle tone and spasticity, or other movement disorders of the central nervous system.