she asked the other day,

fast twich/slow twich
If the 20something starts endurance training, will s/he increase the proportion of slow twitch and if so is it reversible?

An interesting note: "Certain patterns of muscle fiber distribution appear readily in comparisons among highly proficient athletes. For example, successful endurance athletes have predominately slow-twitch fibers in the major muscles activated in their specific sport ... weight lifters, ice hockey players, and sprinters have more fast-twitch fibers and relatively lower aerobic capacities" (386).

That said, the glycogen you burn during exercise is from the muscle fibers you're using - even after 12 hours or so of endurance event (think Ironman), there is still a fair amount of muscle glycogen left in the fast twitch fibers.

Anyway - here's the important part: "The relatively clear-cut distinction between exercise performance and muscle fiber composition pertain mainly to elite athletes with prominence in a sport category. Even among this group, muscle fiber composition does not solely determine performance success. This seems reasonable because successful performance reflects blending of many physiological, biochemical, neurologic, and biomechanical 'support systems,' not simply the single factor of muscle type" (387). This means that, even if you are not born with a really high percentage of slowtwitch fibers, you may still make a good endurance athlete.

Typical metabolic and physiologic values for healthy, endurance trained and untrained men (476)
Untrained: 50% fast twitch, 50% slowtwitch
Trained: 20-30% fast twitch, 60% slowtwitch (********I do not know why the text doesn't give values that add up to 100%*******)

So the percentage difference in those is that endurance training will decrease fast twitch muscles by 50% but gain 20% slowtwitch fibers. This answers your question: endurance training increases the % of slowtwitch fibers.

Because textbooks sometimes contradict themselves, the book goes on to say:
"Aerobic training elicits metabolic adaptations in each type of muscle fiber. The basic muscle fiber does not "change" to any great extent; rather, all fibers maximize their already-existing aerobic potential. Selective hypertrophy [that means certain muscles fibers, getting bigger] occurs in the different muscle fiber types with specific overload training. Highly trained endurance athletes have larger slowtwitch fibers than fasttwitch fibers in the same muscle. Conversely, the fast twitch fibers of athletes trained in anaerobic power activities occupy a greater portion of the muscle's cross sectional area" (479).

Other places I looked in the book: the chapter on anaerobic training, to see if it held an hints.
-one table says that the response to *resistance* training on muscle fiber type is unknown (542)
-"Metabolic characteristics of specific fibers and fiber subdivisions undergo modification within 4 to 8 weeks of resistance training. This occurs despite the lack of dramatic changes in muscle fiber type. A decrease in the percentage of Type IIb and corresponding increase in Type IIa" (543) occurs - that means that training changes the fast twitch fibers, but only in a sub-category.

A final point... "Muscle Cell Remodeling: Current Thinking"
(muscle fiber = muscle cell)
-"Skeletal muscle represents dynamic tissue whose cells do not remain as fixed populations throughout life. Rather, muscle fibers undergo regeneration and remodeling in response to diverse functional demands" (544)
-Satellite cells are what change muscle type: A fiber's nuclei has a specific set of genes, but physical activity forces satellite cell proliferation, and these cells fuse with the existing fiber. The new muscle nuclei alter gene expression in the adapting muscle" (544) which means that fiber types can "change" if you have the original genetic code for a fast twitch fiber, but you endurance train and the satellite cells change the expression to make it a slowtwtich fiber.
-In one study the text cites, four athletes trained anaerobically for 11 weeks, followed by 18 weeks of endurance training. Anaerobic training increased the percentage of type IIc fibers and decreased the percentage of type I fibers; the opposite occurred in the aerobic training phase. (544-5)
-"Specific training... may convert type I to type II fibers (and vice versa) (545)

Another question is, are new muscle fibers made? If you train anaerobically, do you build new muscle fibers (type II) and THAT is why the percentage increase - or is it type I morphing to type II? We are not sure - one theory of hyperplasia (increase in number of muscle cells) occurs longitudinally; that is, a cell can only have a certain surface area:volume ratio, and when it gets "too big" it splits longitudinally. But it's inconclusive...

Your percentage of different types of muscle fibers, however, is mostly genetic. You may cause some changes but what likely happens is that your endurance ability is increased through training mostly from developing your already existing slowtwitch fibers - and maybe a little bit by recruiting more, but mostly you are causing physiological changes in the slowtwitch fibers you already have.

Citations are all from Mcardle, Katch, and Katch. Exercise Physiology: Energy, Nutrition, & Human Performance. Sixth Edition. Lippincott Williams & Wilkins: Philadelphia 2007.