**1.1. Evoked EMG - Mechanism of evoked EMG**

When we humans perform a physical activity, it is contraction of skeletal muscles that is responsible as its driving force. Active state of a skeletal muscle during motion can be viewed on an electromyogram (EMG). Because it is non-invasive and easy to handle, surface EMG has been widely used in the field of rehabilitation as in therapeutic exercise, training, motion analysis, and research.

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.
