Correct Response: C. The body's muscle mass steadily increases from birth through adolescence and typically plateaus between ages 16 and 20 for females and ages 18 and 25 for males. The muscular strength of both males and females peaks anywhere from 20 to 30 years old. Because muscle force is proportional to cross-sectional area, an individual's potential for maximum physical strength occurs after about age 20. Metabolic rate (A) is a measure of energy expenditure by the body. Free radicals (B) are a byproduct of metabolic functions and can damage body cells, rather than contribute to the growth and strength of muscle fibers. In the central nervous system, myelin is the insulating covering that forms around nerve fibers. Myelination (D) occurs rapidly in childhood and although it aids in skilled motor performance by enabling nerve impulses to move more quickly, it is less relevant to attaining peak strength than the size of cross-sectional areas in muscles.
