Dynamic F wave from lower limbs: value and clinical application

We evaluated changes in F wave from the lower limbs after walking in normals. Twenty-two healthy subjects ranging in age from 24 to 74 years (mean 40.2 SD 15.2) were investigated. The peroneal nerve was examined in 16 subjects and the posterior tibial nerve in 22 subjects on the dominant side. F waves were evaluated at rest and 1 minute and 10 minutes after effort testing consisting in prolonged walking for 15 minutes. F wave minimum, maximum and mean latencies, chronodispersion, duration, persistence and amplitude were evaluated. Only chronodispersion showed a significant decrease for both the peroneal and tibial nerve (p < 0.05) after walking. Moreover we evaluated the F wave after effort in five patients affected by neurogenic claudication: three patients presented a further slowing of proximal conduction, while two patients exhibited a transitory conduction block. In conclusion, F wave changes observed in dynamic conditions in normals probably reflect a synchronization of motorneuron firing requiring a certain amount of descending facilitation. On the other hand, the marked F wave changes after effort observed in patients increase the sensitivity of this method in the clinical setting and may provide insights into the pathophysiologic basis of neurogenic claudication.