F-wave elicited during voluntary contraction as a monitor of upper motor neuron disorder.

Since electrophysiological techniques for assessment of upper motor neuron disorders have yet to be standardized, the potential of employing F-waves to monitor this condition was examined. Subjects were 32 normal adults, 54 patients with upper motor neuron disorder and 20 patients with lower motor neuron disorder. F-waves were recorded from the abductor digiti minimi muscle at rest and under weak voluntary contraction. F-wave amplitudes expressed as a percentage of maximum M-wave were 1.98 +/- 0.83% at rest and 3.73 +/- 1.25% during voluntary contraction in normal group, compared to 3.69 +/- 1.81% and 3.62 +/- 1.68% in the upper motor neuron disorder group, 2.08 +/- 1.45% and 3.52 +/- 1.83% in the lower motor neuron disorder group, respectively. Analysing individual data, 69% of patients with upper motor neuron disorder showed an F-wave at rest within normal limits (+/- 2.5 standard deviation from normal). Voluntary muscle contraction enlarged the F-wave amplitude in normal subjects and patients with lower motor neuron disorder, but the facilitation did not occur in patients with upper motor neuron disorder. Combining F-wave amplitudes at rest and during voluntary contraction revealed differences between the upper motor neuron disorder group and the normal and lower motor neuron disorder groups. It was therefore concluded that F-waves can be used as a monitor of upper motor neuron disorder.