Associations of Sedentary Behavior and Abdominal Muscle Density: The Multi-Ethnic Study of Atherosclerosis.

BACKGROUND: Sedentary behaviors (SB) may exacerbate loss of muscle mass and function, independent of physical activity levels. This study examined the associations of SB with abdominal muscle area and density, a marker of muscle quality, in adults.
METHODS: A total of 1895 participants from the Multi-Ethnic Study of Atherosclerosis completed detailed health history, physical activity and SB questionnaires, computed tomography to quantify body composition, and measurements of inflammatory markers. Analyses included linear and nonlinear regression.
RESULTS: The mean age and body mass index were 64.6 years and 28 kg·m-2, respectively, and 50% were women. On average, participants engaged in 28 metabolic equivalent hours·week-1 of SB. With adjustment for age, sex, race/ethnicity, physical activity, cardiovascular disease risk factors, and inflammation, multivariable regression modeling revealed a nonlinear (quadratic) relationship between SB and locomotor, stability, and total abdominal muscle density (P < .01) but not muscle area. The SB inflection point at which locomotor, stability, and total abdominal muscle density began to decrease was 38.2, 39.6, and 39.2 metabolic equivalent hours·week-1 of SB, respectively.
CONCLUSIONS: SB is associated with reduced muscle density when practiced as little as 5.5 metabolic equivalent hours·day-1. These findings may have important implications for SB guidelines for targeting skeletal muscle health in older adults.