Objective:To analyze the correlation between the muscle thickness (MT) and echo intensity (EI) of the medial head of gastrocnemius and muscle mass, strength and functional parameters, and to evaluate the diagnostic efficacy of MT and EI for decreased muscle mass.Methods:Fifty-two subjects who visited the Department of Ultrasound Medicine of the Second Affiliated Hospital of Fujian Medical University from March to December 2021 were collected.MT of the medial head of gastrocnemius was measured by two-dimensional ultrasound, and EI values in the region of interest were analyzed using Image J software. Twenty of the subjects were selected to assess the intra-observer and inter-observer agreements in measuring MT and EI values using the intraclass correlation coefficient (ICC) and Altman-Bland analysis was performed. The subjects were divided into healthy control group (30 cases) and decreased muscle mass group (22 cases) according to 2019 Asian Sarcopenia Working Group (AWGS) diagnostic criteria, the differences in MT and EI values between the two groups were compared, and the correlation between MT, EI values and muscle mass, strength, function were analyzed using Pearson correlation analysis. The diagnostic cutoff values of MT and EI values for decreased muscle mass were obtained using ROC curves, and the diagnostic efficacy was compared.Results:①The ICC of MT and EI in medial head of gastrocnemius detected by the same physician and different physicians were greater than 0.97, and the retest reliability was good. ②Compared with the healthy control group, the MT value in medial head of gastrocnemius decreased and the EI value increased in the decreased muscle mass group ( t=4.759, -3.789; all P<0.001). ③The MT value in medial head of gastrocnemius was positively correlated with appendicular skeletal muscle mass (ASM), appendicular skeletal muscle mass index (ASMI), lower limb skeletal muscle mass (LLSM), grip strength, and gait speed ( r=0.579, 0.568, 0.585, 0.495, 0.415; all P<0.001), of which, the correlation with LLSM was the best; ④EI in medial head of gastrocnemius was negatively correlated with ASM and ASMI, LLSM, grip strength, and gait speed, ( r=-0.583, -0.591, -0.590, -0.537, -0.455; all P<0.001), and among which the correlation with ASMI was the best. ⑤The medial head of gastrocnemius MT and EI were 14.33 mm and 58.85 for predicting decreased muscle mass, respectively, and there was no significant difference in the diagnostic performance of the two for decreased muscle mass ( Z=0.734 3, P>0.05). Conclusions:The MT and EI values of the medial head of gastrocnemius can accurately assess muscle mass and can be used in the evaluation of patients with decreased muscle mass.