Doogie Howser, M.D. wrote:
There does exist proof of this. It involves intermediate muscle fibers, capillarization, and vertical integration of training, among other things.
Not all muscle cells can be categorized conveniently into "Type I" or "Type IIa" or Type IIx." Instead, there seems to be a continuum of muscle fiber types which display many intermediate properties. Training can change some, but not all, of these attributes.
Fundamentally, the limiting factor in a muscle cell's ability to produce forceful contractions is the size of its connecting neuron. There is no known technique by which you can change the size of a motor neuron. There is an invasive technique by which you can re-wire a large neuron to a "slow twitch" fiber and create a fiber with predominantly "fast twitch" properties, but this cannot be accomplished through any form of training.
Training at extremely low intensity will not lower a "fast twitch" (large) motor neuron's synaptic threshold; that muscle fiber will still be able to respond to higher-intensity training and regain the ability to contract forcefully even after months of slow running. The muscle cell will take on some characteristics of intermediate fibers with training, which will spur the need for higher-intensity work to return to top sprint speed, but "innate speed" will remain with the individual until aging factors begin to come into play.
Type II (fast twitch) fibers can take on some characteristics of Type I (slow twitch) fibers, but this seems to be accomplished more readily with high-intensity repetition running rather than with slower running or jogging. Basically, the Type II fibers are not recruited at low intensities, so it is difficult to effect maximum transformation in those fibers by just jogging. However, they can be depleted or partially depleted of substrate by running slowly for long distances, which will result in an increase of oxidative properties within the cells.
Type I fibers can also assume some qualities of Type II fibers with training, but this result is much harder to accomplish, in part because the Type I fibers are heavily involved with maintaining stability and posture; thus, they will always be recruited for these purposes even during high-intensity exercise, and they will retain most of their innate characteristics.
In young individuals, Type II fibers begin responding very quickly (often within one week) to strength training or pure speed training, so as long as the individual is not exhibiting traits of aging, it will be no problem to regain any speed or strength which may seem to have been "lost" as a result of endurance training.