Performing late response such as the H-reflex can provide valuable information regarding the proximal nerve/nerve root involvement. The H-reflex is both a sensitive and specific marker for involvement of the S1 root. F waves, also used to detect abnormalities in the proximal portion of nerves, are too nonspecific to be clinically useful in the setting of radiculopathy.
Electrodiagnostic testing is usually not necessary in a clear-cut radiculopathy or in patients with isolated mechanical low back symptoms. Furthermore, these studies do not assess the smaller myelinated and unmyelinated nerve fibers, which are typically responsible for pain transmission.
Test Preparation and Performance: There is usually no special preparation required. The patient may lie down or sit during the test. During the test one or more needle electrodes are inserted through the skin into a specific muscle or muscle group. Then the patient may be asked to contract the muscle (e.g. bending the knee). The muscle contraction causes a waveform to be traced and detected by an oscilloscope. The oscilloscope displays a visual representation of the electrical variations on a CRT screen. The muscle at rest is displayed as inactive (e.g. small wavy line). When the muscle is contracted the waveform spikes denoting electrical activity in the muscle.