In a typical F wave study, a strong electrical stimulus (supramaximal stimulation) is applied to the skin surface above the distal portion of a nerve so that the impulse travels both distally (towards the muscle fiber) and proximally (back to the motor neurons of the spinal cord). (These directions are also known as orthodromic and antidromic, respectively.) When the orthodromic stimulus reaches the muscle fiber, it elicits a strong M-response indicative of muscle contraction. When the antidromic stimulus reaches the motor neuron cell bodies, a small portion of the motor neurons backfire and orthodromic wave travels back down the nerve towards the muscle. This reflected stimulus evokes small proportion of the muscle fibers causing a small, second CMAP called the F wave.