Muscle mass is commonly estimated using whole-body assessments of body composition, which differ in the number of compartments body mass is divided into. A 4-compartment (4C) model is considered the gold-standard and divides the body into total body water, bone mineral content, fat mass, and fat-free mass. However, it requires a variety of equipment and is costly and time-consuming, so it’s not commonly used. Other methods to evaluate body composition include the 3-compartment (3C) model, which divides the body into bone mineral content, fat mass, and fat-free mass (e.g., dual-energy x-ray absorptiometry — DXA), and the 2-compartment (2C) model, which divides the body into fat mass and fat-free mass (e.g., bioelectrical impedance analysis, skinfolds). To assess growth of specific muscle groups, assessment methods such as ultrasound, computed tomography, and magnetic resonance imaging are also used.[1]