Skeletal Muscle Mass Is Linked to Cardiorespiratory Fitness in Youth

ABSTRACT Introduction Cardiorespiratory fitness (CRF) measured by oxygen consumption (V˙O 2 ) during exercise is an important marker of health. The traditional method of indexing V˙O 2 to total body mass is suboptimal because skeletal muscle mass (SMM), rather than fat and extracellular fluid, is the main contributor to CRF. The traditional estimating equations for peak V˙O 2 in youth do not account for this. Bioelectric impedance analysis (BIA) is a noninvasive method to accurately measure body composition. The objectives of this study were to 1) examine the relationship of body composition indices and peak V˙O 2 in healthy children, adolescents, and young adults, and 2) derive an optimized estimating equation incorporating BIA and compare its performance with traditional estimating equations. Methods A retrospective, cross-sectional, single-center study of patients <21 yr old referred for exercise testing who did not have underlying cardiovascular disease. All patients underwent BIA immediately before exercise testing. Univariable and multivariable linear regression models were constructed and tested for model performance. Results A total of 165 young healthy people (mean age 14 yr, 48% male) were studied. There was a strong and linear relationship between peak V˙O 2 and SMM ( R 2 = 0.79). The sex difference in SMM explained the most variability in CRF between boys and girls. A generalized equation using SMM (peak V˙O 2 = 302 − (23.7 × age) − (50.3 × [female = 1, male = 0]) + (81.8 × SMM)) had superior performance ( R 2 = 0.80) compared with estimating equations currently used in clinical practice ( R 2 = 0.67). Conclusions SMM is a stronger correlate of CRF than is total body mass in youth and may be a better scaling variable to estimate expected peak V˙O 2 .