Objective:To investigate the association between split foot and electrophysiology in patients with amyotrophic lateral sclerosis (ALS).Methods:The clinically definite or clinically probable ALS patients hospitalized in the Department of Neurology, the First Medical Center of Chinese People′s Liberation Army General Hospital from April 2021 to December 2022 were prospectively collected. From April 2021 to December 2022, patients who visited the Chinese People′s Liberation Army General Hospital for other reasons without abnormal electrophysiological examination were collected as the control group. The incidence of split leg [the limb whose modified Medical Research Council Muscle Strength Scale (mMRC) score of ankle dorsiflexors was lower than that of ankle plantarflexors] in ALS patients was calculated, and the incidence of split foot (the limb whose mMRC score of hallux dorsiflexors was lower than that of hallux plantarflexors) in ALS patients was calculated. The amplitude of compound muscle action potential (CMAP) of common peroneal nerve and tibial nerve was detected to observe the involvement of motor neurons innervating ankle dorsiflexors and ankle plantarflexors. The characteristics of split leg and split foot in ALS patients were analyzed from the perspective of muscle strength, and the characteristics of split foot in ALS patients were analyzed from the perspective of electrophysiology. Receiver operating characteristic (ROC) curve was used to analyze the sensitivity and specificity of peroneal nerve/tibial nerve CMAP amplitude ratio in distinguishing ALS patients from controls.Results:A total of 101 ALS patients with lower limb involvement and 110 controls with normal lower limb muscle strength were collected. Among the 101 ALS patients with lower limb involvement, strength of ankle plantarflexors was greater than that of ankle dorsiflexors in 35.64% (36/101) patients, strength of ankle dorsiflexors was greater than that of ankle plantarflexors in 5.94% (6/101) patients, and strength of ankle plantarflexors and ankle dorsiflexors was equal in 58.42% (59/101) patients. Strength of hallux dorsiflexors was lower than that of hallux plantarflexors in 53.47% (54/101) patients, strength of hallux dorsiflexors was greater than that of hallux plantarflexors in 1.98% (2/101) patients, and the strength of hallux dorsiflexors and hallux plantarflexors was equal in 44.55% (45/101) patients. The incidence of split leg was negatively correlated with age ( OR=0.25, 95% CI 0.16-0.40, P<0.05), course of disease ( OR=0.52, 95% CI 0.38-0.80 P<0.05) and ALS functional revised scores ( OR=0.29, 95% CI 0.12-0.67, P<0.05). The incidence of split foot was negatively correlated with the onset time of lower limb symptoms ( OR=0.96, 95% CI 0.93-0.99, P<0.05). Compared with the control group, the differences of the decrease of CMAP amplitude in the common peroneal nerve and tibial nerve [the common peroneal nerve (6.45±2.56) mV vs (3.63±1.83) mV, tibial nerve (12.87±4.72) mV vs (9.18±6.22) mV] were statistically significant ( t=-4.65, t=-3.44, both P<0.001) and the differences of the peroneal nerve/tibial nerve CMAP amplitude ratio (0.54±0.24 vs 0.36±0.18) decrease was statistically significant ( t=-4.31, P<0.001) in patients with split foot. ROC curve analysis showed that the area under the ROC curve of CMAP amplitude ratio of common peroneal nerve/tibial nerve in ALS patients with split foot was 0.70, indicating that the accuracy of CMAP amplitude of common peroneal nerve/tibial nerve in distinguishing ALS patients from controls was low. Conclusions:In ALS patients with lower limb involvement, strength of ankle dorsiflexors is weaker than that of ankle plantarflexors, and strength of hallux dorsiflexors is weaker than that of hallux plantarflexors. At the diagnostic level, the CMAP amplitude ratio of common peroneal nerve/tibial nerve in ALS patients with split foot has a lower accuracy in the diagnosis of ALS.