Distribution of different fiber types in human skeletal muscles: Effects of aging studied in whole muscle cross sections

TLDR

The study examined how aging affects the number, size, proportion, and distribution of type 1 and type 2 muscle fibers in cross‑sectioned autopsied vastus lateralis from two age groups. Six previously healthy males per group (mean ages 72 ± 1 and 30 ± 6 years) provided 15 μm thick whole‑muscle cross sections for analysis. Older adults had 18% smaller vastus lateralis muscles and 25% fewer fibers, with no change in mean fiber size or overall fiber‑type proportion, though type 2 fibers were preferentially reduced and depth‑wise distribution became more even, indicating that atrophy up to age 70 is mainly due to fiber loss.

Abstract

Abstract The effects of aging on the total number and size of fibers, and the proportion and distribution of type 1 (slow twitch) and type 2 (fast twitch) fibers were studied in cross sections (15 μ thick) of autopsied whole m. vastus lateralis from two age groups. Each group consisted of six, previously physically healthy males (mean age 72 ± 1 years and 30 ± 6 years, respectively). The size of the muscles of the older individuals was 18% smaller ( P < 0.01) and the total number of fibers was 25% lower ( P < 0.01) than those of the young individuals (mean number 364,000 ± 50,000 vs 478,000 ± 56,000). There was, however, no significant difference in the mean fiber size (indirectly determined) orthe proportion of the two fiber types, though a preferential reduction in type 2 fiber number in the aged individuals was seen. The relative occurrence of the fiber types at various depths in the aged muscles was found to be more even than in muscles from the young individuals. The results suggest that the aging atrophy in m. vastus lateralis , at least up to the age of 70, is primarily the result of a loss of fibers.