Age-related changes in muscle volume and intramuscular fat content in quadriceps femoris and hamstrings.

Whether age-related changes in muscle components differ between the quadriceps femoris and hamstrings has remained unclear. This study aimed to compare the muscle volume and echo intensity-estimated intramuscular adipose tissue content of the vastus lateralis (VL) and long head of biceps femoris (BF) muscles between young and older adults. Thirty young adults (n = 15; mean age, 21 years) and older adults (n = 15; mean age, 71 years) participated in this study. Magnetic resonance imaging was performed to determine muscle volumes of the VL and BF, and muscle volume normalized to body weight (muscle volume/weight). Mean gray-scale echo intensity was calculated as the intramuscular adipose tissue index. Muscle volume/weight and echo intensity were normalized using Z-scores in young and older adults. Muscle volume/weight was lower in older adults than in young adults, and lower in overall women than in men for VL (both p < 0.001) and BF (p < 0.01 and p < 0.05). Echo intensity was higher in older adults than in young adults for VL and BF (both p < 0.001), but did not differ between men and women. Z-score of muscle volume/weight was lower in older adults than in young adults for VL (-2.41 ± 1.22; p < 0.05), and Z-score of echo intensity was higher in older adults than in young adults for BF (2.00 ± 0.68; p < 0.05). These results suggest that muscle volume of quadriceps femoris was lower in older adults than in young adults, whereas intramuscular adipose tissue content of hamstrings was greater in older adults than in young adults.