To explore differential diagnosis value of dissociated lower-limb muscle impairment, we performed a retrospective analysis of clinical and electrophysiological features in 141 lower-limb involved ALS patients, 218 normal controls, 67 disease controls, and 32 lumbar spondylosis disease patients. The dissociated lower-limb muscle impairment was quantified by plantar flexion and dorsiflexion strength, compound muscle action potentials ratio of peroneal and tibial nerves (split index, SI) and semi-quantitative scoring scale of denervation potential. Clinical features: the proportion of decreased dorsiflexion was higher than decreased planter flexor strength in lower-limb involved ALS (77.2%vs 38.3%). Electrophysiological features: (1) SI in ALS was the lowest among four groups (Test statistic = 40.57, p < 0.001). (2) Percentage of positive denervation potential was higher in tibialis anterior than gastrocnemius muscle (χ2 = 87.12, p < 0.001). ROC curve: the SI cutoff was 0.52 and 0.33 respectively to differentiate ALS from lumbar spondylosis disease and peripheral neuropathy. Lower-limb involved ALS patients exhibited "split leg" phenomenon. The SI value could be used as an electrophysiological marker to differentiate ALS from lumbar spondylosis disease and peripheral neuropathy.