Femoral nerve block (FNB) is widely used in patients undergoing knee arthroscopy. However, the most commonly used concentration of ropivacaine (0.2% or above) may cause an unexpected decrease in the muscle strength of the quadriceps. Therefore, a lower concentration of ropivacaine (0.1%) for FNB was administered to investigate the effect on quadriceps strength and postoperative pain after knee arthroscopy.This was a double-blind, randomized, controlled trial (ChiCTR2000041404). A total of 83 patients scheduled for elective knee arthroscopy were randomized to receive 0.1% or 0.2% ropivacaine for FNB under ultrasound guidance. The primary outcomes were quadriceps strength and numerical rating scale (NRS) pain score. Quadriceps strength was measured before surgery and 6 h and 24 h after surgery, while NRS score was recorded before surgery, at the postanaesthesia care unit (PACU), and 6 h and 24 h after surgery. Multiple linear regression tests were used to compare the differences in quadriceps strength and NRS score between the two groups. Two-factor analysis of variance, using the factors group and time of measurement, was used for repeated NRS scores. Secondary outcomes included knee mobility, side effects, patient satisfaction, and length of hospital stay.The mean (SD) quadriceps strength at 6 h after surgery was 7.5 (5.7) kg for the 0.1% ropivacaine group and 3.0 (4.4) kg for the 0.2% ropivacaine group. The mean difference adjusted for baseline characteristics was - 5.2 (95% CI - 7.2 to - 3.1) kg (P < 0.001). There was no significant difference between the two groups in quadriceps strength at 24 h after surgery. The mean differences in the average NRS score and maximum NRS score in the PACU were - 0.6 (P = 0.008) and - 1.0 (P < 0.001), respectively. There was no significant difference in NRS score at 6 h or 24 h after surgery. Two-factor analysis of variance showed no significant difference in the interaction factors of time and group for average NRS score and maximum NRS score.Compared with 0.2% ropivacaine, 0.1% ropivacaine for FNB preserved quadriceps strength at 6 h after knee arthroscopy while providing similar analgesic effects.I.© 2023. The Author(s).