Echogenicity (EG) measured via ultrasound (US) represents a practical strategy to assess skeletal muscle composition. This study examined the extent to which US can detect differences between young and older adults in skeletal muscle EG, and how imaging site/anatomical location impacts this comparison. US images of the quadriceps muscle from young (26±4yr, n=8M, 8F) and older (70±7yr, n=7M, 5F) adults were captured using B-mode ultrasound (Terason 3300). From each participant, five images were collected from anatomical sites along the anterior and lateral plane of the right leg (in supine position) corresponding to 59%, 39%, and 22% of femur length. EG analyses (Image J, range: 0=Black, 255=White) was performed on anterior images for the rectus femoris (RF), and vastus intermedius (AVI). Lateral images were taken for the vastus intermedius (LVI), and vastus lateralis (VL). Collapsed across all imaging sites and muscles, older adults had higher (P\u3c0.05) average EG compared to young (53.7±11.1 vs. 39.6±18.3). For the individual muscles, older adults had higher average EG (P\u3c0.05) for both AVI (58.1±10.9 vs. 40.0±11.0) and LVI (54.4±14.1 vs. 37.1±20.5), however, no differences were observed for EG of the RF or VL (P\u3e0.05). Specific to each imaging site, differences (P\u3c0.05) between young and older adults were found at 0/2 imaging sites for the RF, 4/5 sites for the VI, and 1/3 sites for the VL. These data indicate that US is able to detect differences in composition between muscles of young and older adults, however, differences were not homogenous among the quadriceps muscles
