I would guess related to repetitious, low-impact use and general cardiovascular health. I would expect this to be similar to other muscles in swimmers in the same age range. A runner might, but we tend to face other deterioration issues due to long-term effects of higher impact and, usually, more injuries through time.

A sister study was released same day.

http://www.bbc.com/news/health-43308729


"How exercise in old age prevents the immune system from declining.

They followed 125 long-distance cyclists, some now in their 80s, and found they had the immune systems of 20-year-olds..."

Study linked in the article. I liked it.

Not much (anything?) really new in that article. Good confirmation of previous work with a large sample size, though.

I see they made a reference to some researchers in 1992:

Coggan, A. R., Spina, R. J., King, D. S., Rogers, M. A., Brown, M., Nemeth, P. M., & Holloszy, J. O. (1992). Histochemical and enzymatic comparison of the gastrocnemius muscle of young and elderly men and women. Journal of Gerontology, 47(3), B71–B76. https://doi.org/10.1093/geronj/47.3.B71

I am actually happier that they chose to cite this paper as well:

Coggan, A. R., Abduljalil, A. M., Swanson, S. C., Earle, M. S., Farris, J. W., Mendenhall, L. A., & Robitaille, P. M. (1993). Muscle metabolism during exercise in young and older untrained and endurance-trained men. Journal of Applied Physiology, 75(5), 2125–2133.
https://www.ncbi.nlm.nih.gov/pubmed/23539308

I say that because certain prominent researchers seem to deliberately choose to ignore it, even though it was the first to use 31P MRS to address the interactive effects of aging and training on human locomotory muscle. (There was one previous 31P MRS study of aging of human forearm muscle.)

Ironically, though, they didn't cite the paper below, which has been cited the most of all by others:

Skeletal muscle adaptations to endurance training in 60-to 70-yr-old men and women
AR Coggan, RJ Spina, DS King, MA Rogers, M Brown, PM Nemeth, ...
Journal of Applied Physiology 72 (5), 1780-1786, 1992