Site-specific skeletal muscle echo intensity and thickness differences in subcutaneous adipose tissue matched older and younger adults

Background Age-related deterioration of muscle mass does not occur uniformly across the body. However, there is limited knowledge on the uniformity of age-related muscle composition changes across the body. Objective Our primary objective was to evaluate muscle composition differences between younger and older adults across multiple muscle groups. Methods We re-analysed data from a previously published cohort to evaluate differences in ultrasound muscle composition (echo intensity) between younger (<45 years) and older (>60 years) adults, when matched for adipose tissue mass at the anterior upper arm, anterior upper leg and abdominal muscles. Analysis of echo intensity is confounded by subcutaneous adipose tissue (SAT) thickness overlaying the muscle; we accounted for these effects by matching older and younger adults (1:1), stratified by sex, for absolute SAT thickness at each landmark. Results From 96 adults (n = females), 58 (n = 34) were SAT matched at the anterior upper arm, 52 (n = 30) at the anterior upper leg and 60 (n = 30) at the abdominal region; thus, there were no age group differences in SAT thickness at each landmark. In comparison with younger adults, older adults presented with greater echo intensity at the anterior upper leg (females:40.3 +/- 6.8 vs. 52.4 +/- 7.6; males:35.7 +/- 8.0 vs. 54.3 +/- 9.8, p < .01) and abdominal (females:38.7 +/- 27.6 vs. 73.4 +/- 31.0; males:18.7 +/- 15.2 vs. 60.9 +/- 23.4, p < .01) muscles, but not anterior upper arm muscles (females:47.0 +/- 6.5 vs. 53.2 +/- 13.1; males:43.4 +/- 8.9 vs. 48.9 +/- 10.1, p = .18). Conclusions Distinct age-related differences in trunk and lower limb muscle composition were evident compared to upper limb muscles; highlighting the importance of quantifying specific muscle groups when evaluating age-associated muscle characteristics.