Since F responses involve conduction in motor fibers to and from the spinal cord as well as discharge of anterior horn cells, F waves are dependent on the state of both the peripheral and central nervous system. The ratios of F to M amplitudes, determined by dividing the mean amplitude of 20 F waves by the amplitude of maximum evoked compound muscle action potential recording from the abductor pollicis brevis and abductor hallucis, were compared from age matched control subjects and patients with polyneuropathies or spastic hyperreflexia.

Compared with controls, F/M ratios were increased at statistically significant levels in all patients with either peripheral or central lesions.

Even if the mechanisms differ, the results indicate a similar physiological response in situations where muscle tension might be abnormal whether the injury is peripheral or central.