Abstract To assess the usefulness of a theoretical postural instability discrimination index (PI th ) in amyotrophic lateral sclerosis (ALS). Prospective regression analyzes were performed to identify the biomechanical determinants of postural instability unrelated to lower limb motor deficits from gait initiation factors. PI th was constructed using a logit function of biomechanical determinants. Discriminatory performance and performance differences were tested. Backward displacement of the pression center (APA amplitude ) and active vertical braking of the mass center (Braking-index) were the biomechanical determinants of postural instability. PI th = − 0.13 × APA amplitude − 0.12 × Braking-index + 5.67, ( P < 0.0001, RSquare = 0.6119). OR (APA amplitude ) and OR (Braking-index) were 0.878 and 0.887, respectively, i.e., for a decrease of 10 mm in APA amplitude or 10% in Braking-index, the postural instability risk was 11.391 or 11.274 times higher, respectively. PI th had the highest discriminatory performance (AUC 0.953) with a decision threshold value $$\ge$$ ≥ 0.587, a sensitivity of 90.91%, and a specificity of 83.87%, significantly increasing the sensitivity by 11.11%. PI th , as objective clinical integrator of gait initiation biomechanical processes significantly involved in dynamic postural control, was a reliable and performing discrimination index of postural instability with a significant increased sensitivity, and may be useful for a personalized approach to postural instability in ALS.