Relationship between muscle mass and fraction of intramuscular adipose tissue of the quadriceps in older inpatients

BACKGROUND: In 2021, the International Society of Physical and Rehabilitation Medicine (ISPRM) special interest group on sarcopenia included the quadriceps thickness assessed with ultrasound image as an indicator of muscle mass in the diagnosis criteria of sarcopenia. If quadriceps echo intensity of older inpatients is to be a strong predictor of quadriceps thickness, muscle quality of the quadriceps may be estimated by the muscle mass when diagnosing sarcopenia using the criteria of ISPRM. OBJECTIVE: This study aimed to examine the association between muscle mass and fraction of intramuscular adipose tissue of the quadriceps in older inpatients. METHODS: This cross-sectional study included 399 inpatients aged ≥ 65 years. Primary outcomes were muscle mass and fraction of intramuscular adipose tissue of the quadriceps. Images were acquired using a B-mode ultrasound. Muscle mass and fraction of intramuscular adipose tissue of the quadriceps were assessed based on the muscle thickness and echo intensity, respectively. A multiple regression analysis (forced entry method) was performed to confirm whether quadriceps echo intensity was related to quadriceps thickness even after adjusting for other factors. RESULTS: In the multiple regression analyses for both male and female models, quadriceps echo intensity (male: β = − 0.537, p < 0.001; female: β = − 0.438, p < 0.001), Geriatric Nutritional Risk Index (male: β = 0.236, p < 0.001; female: β = 0.213, p < 0.001), and subcutaneous fat thickness of the thigh (male: β = 0.197, p < 0.001; female: β = 0.248, p < 0.001) were independently and significantly associated with quadriceps thickness. CONCLUSIONS: Our results show that there is a negative and significant association between muscle mass and fraction of intramuscular adipose tissue in older inpatients. Muscle quality of the quadriceps in older inpatients may be estimated to some extent by the muscle mass.