Age-, sex-, and region-specific differences in skeletal muscle size and quality.

Ultrasonography-derived cross-sectional area (CSA) and echo intensity (EI) are increasingly utilized by investigators to study muscle size and quality, respectively. We sought to examine age, sex, and region (proximal, middle, distal) differences in vastus lateralis and rectus femoris CSA and EI, and determine whether correction for subcutaneous fat thickness influences the magnitude of EI differences. Fifteen younger men (mean age = 23 years), 15 younger women (aged 21 years), 11 older men (aged 74 years), and 15 older women (aged 70 years) participated. Clear differences were observed among age, sex, and region for vastus lateralis CSA ( p ≤ 0.013, d = 0.38-0.73), whereas rectus femoris CSA was only different between younger and older participants at the proximal region ( p = 0.017, d = 0.65). Uncorrected EI was greatest at the distal region of both muscles ( p < 0.001, d = 0.59-1.38), with only the younger men having significantly lower EI values than the other groups ( p ≤ 0.043, d = 0.37-0.63). Subcutaneous fat correction resulted in a marked increase in the magnitude of sex-specific EI differences ( p ≤ 0.032, d ≥ 0.42). Additionally, subcutaneous fat correction increased the uniformity of EI throughout the thigh. These findings highlight considerable region-specific differences in muscle size and quality among younger and older men and women and highlight the need to correct for subcutaneous fat thickness when examining EI. Novelty Rectus femoris CSA is similar between younger and older adults except at the most proximal site evaluated. Age- and sex-specific differences in uncorrected EI are nonuniform across the thigh. Correction for subcutaneous fat thickness substantially increased EI in women, resulting in greater sex differences.