This is sort of what I’m up against right now as I’m back into sprinting at age 32. I have used the search function but couldn’t really find what I wanted, so I’ll just ask. I believe I have always been more inclined toward sprinting/jumping activities since I was younger but I never really fully developed by ability. I mentioned to DanielC101, who posts here a lot, that my first and only 55m dash, with no training at all was 6.69 seconds, I had a long jump of over 22 feet and my vertical jump was 36 inches. I also did well in olympic style weightlifting.
Now, during a period after high school and early college, I stopped all explosive athletic activity and began long, slow distance running, including several 5k,10k races, which I carried on for a few years before just dropping running all together and doing nothing for a few years.
During the several years of long, slow runs, is it possible that I converted my faster fibers over to slow fibers, or at least de-activated them by causing them to function as slower twitch? If so, is it reversable, or would it be possible to ever really be able to not only get back to my previous “fast twitch levels” but to improve them? Is it just a matter of re-training them?
In the meantime, I’m doing gpp now, which is including many 100m reps; I’m also power cleaning like a madman.