Objective: The aim of our study was to assess postoperative lower limbs muscle strength (MS) as a predictor of late surgical success (36 months). Methods: Body composition analyses and isokinetic dynamometry evaluation were performed before (T0: n=123), six months (T1: n=123) and 36 months (T2: n=79) after Roux-en-y gastric bypass (RYGB). Surgical success (SS) was defined as ≥ 50% excess weight loss (EWL) 36 months after surgery or ≤ 50% surgical failure (SF). Results: There was no difference between relative MS extension (Ext) and flexion (Flex) in T1 and T2. There was also no difference between relative MS Ext and Flex in T1 and T2 between patients with SS and SF. There was a difference in relative MS Ext (144.9 ± 39.8 Nm/kg x 125.5 ± 29.2 Nm/kg; p=0.04) and Flex (73.6 ± 21.8 Nm/kg x 60.4 ± 15.8 Nm/kg; p=0.02) between SS and SF patients only in T2. Patients with an increment in Ext and Flex MS ≥4 Nm/kg at T1 had approximately 76% of SS at 36 months. Conclusion: An increase of lower limbs MS ≥4 Nm/kg 6 months after RYGB predicts SS at 36 months. ClinicalTrials.gov ID:NCT04129801