Author(s): SADULA, E., JAMES, L., SKARABOT, J., FOLLAND, J., Institution: LOUGHBOROUGH UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 2512

INTRODUCTION:
In addition to the loss of muscle size, ageing is associated with an increase in intramuscular fat, which results in a reduction of the contractile tissue contained within a muscle (Pinel et al, 2021). However, it is not well understood if certain muscles are more prone to fatty infiltration with age compared to others, which could lead to an underestimation of muscle atrophy for muscles prone to fat infiltration. Therefore, the aim of the current study was to compare the fatty infiltration and functional (non-fat) volume of the four quadriceps femoris and gluteus maximus muscles between young and older adults.
METHODS:
Participants were forty recreationally active, healthy adults, 20 young (Y, 23 ± 3 years, 10 males) and 20 older (O, 67 +/- 4 years, 10 males). MRI scans (3.0 T Discovery MR750w; GE Healthcare, Chicago, IL) of the dominant thigh were obtained (axial T1-weighted and IDEAL fat and water images, contiguous 4-mm slices). T1-weighted images were segmented to determine quadriceps and gluteus maximus muscle volume. Segmentations were then transposed onto the fat fraction image to quantify the fat fraction percentage (FF%) in each slice/image for each individual muscle, in order to calculate total FF% of each muscle ([fat volume / total volume] × 100). Functional volume was calculated by subtracting fat volume from total volume.
RESULTS:
FF% showed a main effect of age (p = 0.001) and muscle (p < 0.001), as well as an age × muscle interaction (p = 0.001). Post hoc interaction contrasts revealed that the gluteus maximus had a greater difference in FF% between young and older participants (Y 7.0%, O 12.0%) compared to the rectus femoris (p = 0.004; Y 1.6%, O 2.7%), vastus medialis (p = 0.007; Y 2.3%, O 3.7%), and vastus lateralis (p = 0.021; Y 2.4%, O 3.9%). However, the difference in FF% between young and older participants was similar across all the quadriceps muscles (p = 1.000). For total and functional volume, there was a main effect age (p < 0.001), as well as a significant age × muscle interaction (p ≤ 0.003). Post hoc comparisons revealed older adults had significantly lower total volume for the three individual vastii muscles (all p<0.001), but not the rectus femoris (p = 0.123) and gluteus maximus (p = 0.486). However, when fat infiltration of the gluteus maximus was accounted for older adults had lower functional volume compared to the young group (p = 0.037).
CONCLUSION:
The gluteus maximus appears to have a relatively high level of fat infiltration irrespective of age, and showed a markedly higher fat infiltration with age than the quadriceps. These findings indicate that the pattern of intramuscular fat deposition with age is muscle specific and for muscles prone to fat infiltration (e.g. gluteus maximus) the effect of age on muscle atrophy may be underestimated/erroneously assessed by total volume without accounting for fat infiltration.

References
Pinel S et al (2021) Exp Gerontol 156.