F-wave characteristics following acute and chronic upper motor neuron lesions.

F-waves were analyzed in 40 patients shortly following the onset of moderate to severe flaccid hemiparesis due to hemispheric strokes. The test was repeated in 25 patients three or more months later. F-waves were recorded following trains of 20 stimuli from the ulnar and common peroneal nerves bilaterally. The persistence, minimal, maximal and average latencies, durations, amplitudes and areas of these waves were determined. Acutely, several F-wave parameters (persistence, maximal and average duration, amplitude and area) were significantly reduced on the paretic side. Changes were slightly more pronounced in patients with more severe weakness and flaccidity. In the chronic phase, the average F-wave amplitude and area values on the paretic side increased, becoming similar to or larger than on the normal side. However, the correlation with muscle strength and tone changes was poor.