Background:Dysfunctional spinal circuit may play a role in the pathophysiology ofamyotrophic lateral sclerosis (ALS).The purpose of this study was to use F waves for assessment of segmental motoneuronal excitability following upper motor neuron (UMN) dysfunctions in ALS.Methods:We studied the F waves of 152 ulnar nerves recorded from abductor digiti minimi in 82 patients with ALS.Two groups of hands were defined based on the presence or absence of pyramidal signs in the same upper limb.The group with pyramidal signs in the upper limbs was designated as the P group,and the group without pyramidal signs in the upper limbs was designated as the NP group.Results:The mean (P ＜ 0.001),median (P ＜ 0.001) and maximum (P =0.035) F wave amplitudes,mean (P ＜ 0.001),median (P ＜ 0.001) and maximum (P =0.003) F/M amplitude ratio,index repeating neuron (P ＜ 0.001) and index repeater F waves (P ＜ 0.001) of the P group were significantly increased compared with the NP group.No significant differences were identified for F wave chronodispersion (P =0.628),mean F wave latency (P =0.151),minimum F wave latency (P =0.211),maximum F wave latency (P =0.199),F wave persistence (P =0.738),F wave duration (P =0.152),F wave conduction velocity (P =0.813) and number of giant F waves (P =0.072) between the two groups.Conclusions:In this study,increased F wave amplitude,F/M amplitude ratio and number of repeater F waves reflected enhanced segmental motoneuronal excitability following UMN dysfunctions in ALS.