Objective:To explore the value of iterative decomposition of water and fat with asymmetry and least squares estimation-quantitative fat imaging (IDEAL-IQ) in quantitative evaluation of thigh muscle fat content and its correlation with muscle strength in middle-aged and elderly volunteers.Methods:From December 2020 to April 2021, 30 volunteers aged 45 to 70 were recruited prospectively, including 15 males and 15 females with 52.5 (49.0, 56.3) years old. All subjects were scanned at 3.0 T MR, including axial T 1WI, IDEAL-IQ and coronal T 2WI of the left thigh. The region of interest of the knee extensors (quadriceps femoris) and knee flexors (hamstrings) in the left mid-thigh were delineated, and muscle cross-sectional area (CSA), skeletal muscle index (SMI), intermuscular fat fraction (FF) and intramuscular FF were obtained. In addition, isokinetic muscle strength measurement was performed on the left knee joint of all subjects at angular speeds of 60°/s and 180°/s to obtain peak torque (PT) and total work (TW) of knee flexors and extensors. Independent sample t-test, paired t-test or Mann-Whitney U test were used to compare the differences of CSA, SMI, intermuscular FF, intramuscular FF, PT and TW between different genders and muscle groups. Pearson or Spearman correlation analysis, and multiple linear regression analysis were used to analyze the correlation between CSA, SMI, intermuscular FF, intramuscular FF and PT, TW of thigh muscles. Results:The CSA, PT and TW of thighs in males were higher than those in females ( P<0.05), while the intermuscular FF in males was lower than that in females ( P=0.005). The CSA, SMI and PT of the thigh extensors were higher than those of the flexors ( P<0.001), while the intramuscular FF and intermuscular FF were lower than those of the flexors ( P<0.001). Intramuscular FF of flexors and extensors were moderately negatively correlated with PT ( r=-0.635, P<0.001; r=-0.546, P<0.001), and highly, moderately negatively correlated with TW ( r=-0.718, P<0.001; r=-0.616, P<0.001). Intermuscular FF of flexors and extensors were moderately negatively correlated with PT ( r=-0.519, P=0.003; r=-0.443, P=0.014), and negatively correlated with TW ( r=-0.363, P=0.049; r=-0.552, P=0.002). There was no significant correlation between CSA, SMI and PT, TW in flexors and extensors of thigh ( P>0.05). Multiple linear regression analysis showed that intramuscular FF was still significantly correlated with PT and TW of flexors and extensors (flexors: R 2adj=0.505, P=0.001; R 2adj=0.540, P<0.001; extensors: R 2adj=0.351, P=0.006; R 2adj=0.470, P=0.002). Conclusion:FF based on IDEAL-IQ technology can accurately quantify the intramuscular and intermuscular fat content of thighs, and there are negative correlations between intramuscular FF, intermuscular FF and isokinetic muscle strength measurements including PT and TW. Among them, intramuscular FF is more significant.