Life with only slow twitch fibers

[Sasha Pachev]

Suppose we have somebody with only slow twitch fibers. Not one faster twitch fiber, no midgrade or anything like that. Every single fiber as slow twitch as they get, at least in the legs. Would he get lead legs at the end of an all out 800 meter run? Consider two possibilities - optimally trained, and completely untrained.

My intuition tells me that even the optimally trained would still get it, but I cannot explain why.

[James Winzenz]

Sasha, I think you are dealing with a theoretical absolute that is just not possible.  Aside from that, slow twitch fibers only know how to function aerobically.  My personal understanding is that the burn we feel from running that fast is actually from the lactic acid accumulation due to the more abundant use of fast twitch fibers.  If there were truly a possibility of someone having only Type I slow twitch fibers, I do not believe they would experience the "burn", because their speed would be limited by the capacity of their muscle fibers to function aerobically.  I think you would simply see differences in time between the three due to differences in aerobic capacity.

[Sasha Pachev]

An article to add some food for thought:

http://www.pponline.co.uk/encyc/endurance-muscles.html

if you scroll to the bottom it says that fast-twitch can be converted into slow twitch with training, and the default of a muscle is fast-twitch. But then what about all those untrained guys that cannot sprint, what is their problem?

[James Winzenz]

I think we are getting into the realm of muscle fiber recruitment there, which is even mentioned in the article.  I would also point out that it has been mentioned on one of the other topics, as well as in that article, that the Type IIa fibers are the ones that can assume the characteristics of Type I fibers.  Even the most highly trained aerobic athletes in the study that was mentioned still  had approximately 25% Type IIb (fast twitch) fibers.

[adam]

Sasha, while it is true that the muscle fibers can be converted (at least to a porportionate degree to the number of convertible fibers) to fast twitch, it is also true that they can go the opposite direction (from fast to slow).

A runner with predominately fast twitch fibers can lose some of their fast-twitch capability through slow-twitch training or inactivity.

Additionally, individuals with predominately fast twitch fibers who practice fast-twitch training (or training that optimally recruits fast twitch fibers, such as lifting and sprinting), can increase in number (from the convertible fibers) and prolong the maintence of the fibers he already has.

For this reason, it might be important to make a distinction between "untrained" and "sedentary". A sedentary individual, one who does not practice any form of exercise at all, may have issues sprinting because whatever fast-twitch recruitment capabilities he once may of had are severely diminished. I believe this is what you mean by untrained.

The definition of an "untrained" individual is dependent on your belief of what is "trained". You might consider a basketball player who decides to run the 400m "untrained" by running training standards, and yet he can run 50s in basketball shoes. Along the same lines, a sprint coach asking a marathon trained individual to run a 200m in 25s may find him "untrained" for the task, though he is probably highly competent in running.

I think one of the greatest mistakes we make in running is assuming that we are predominately slow-twitch initially because we do not run a fast sprint on some random training day. We then tend to work on getting in slow distance (emphasis on slow), more miles, and over the years, what we did have as far as fast-twitch fibers are concerned may have dimished to make room for the slow-twitch capabilities we have created. Had more effort been taken to improve on the fast-twitch work and then move up to the marathon with sufficient speed, we would probably see more results like that of the elite runners, who often spent quality time on their 1mile-5k speed before moving up. If this was the case, many runners may find that they are better suited for middle-distance work than 10k, halfs, and marathons.

By no means am I discouraging the need for mileage in the running training question. The difference is that a 90 minute long run for a faster trained individual with good mileage base would incoroperate at least 15 miles easy. Bernard Lagat even mentioned in this month's running times that he feels anything slower than 5:15 a mile is too slow! That is 1:08 Half Marathon, 2:17 marathon pace on a normal run. Based on this, he could easily be in contention at higher distances due to his optimal faster training, but he choses to run middle-distance because that is what he is best at.

Sorry for the length of the post. Its been a slow work day.