Recruiting Fast Twitch Fibers

I'll chime in with my layman comments.

What is fascinating about this topic is that FT fibers, who's task is speed, can be adapted to contribute to *endurance* when doing high mileage training. The consensus appears to agree with that, but as to how exactly FT contributes is still a subject of debate. Does FT get recruited, or does the FT energy supply get handed off to exhausted ST, or does something different happen, etc...

High mileage leads to faster performances by allowing a runner to sustain pace for a longer duration, however it probably does very little or nothing to stimulate *raw* speed improvements. Pure speed is improved through speedwork.

This is not in fact true. No studies have shown that a person can change muscle fiber type to any extent (never shown in people, and seldom shown in rats). I am working in exercise physiology research, and working on a Master's in the same.

Ryan R. wrote:

This is not in fact true. No studies have shown that a person can change muscle fiber type to any extent (never shown in people, and seldom shown in rats). I am working in exercise physiology research, and working on a Master's in the same.

That is not absolutely true. Several studies have shown that the myosin expression profile of HUMANS changes with training. It is true that wholesale fiber type shift won't occur--you will get changes though.

Phoenix - I concur with several of the other posters - THANKS. This is very interesting, inciteful, and even USEFUL!

At the risk of broadening this topic too much and getting off onto other subjects.....aren't your thoughts and beliefs more or less a vote for "multi-tier" type training systems.....or at least some sort of multi-tier hybrid?

Funny you should ask. Yes, it is my opinion that a multi-tier/multi-pace training system is superior to any other form. The tiering and pacing should be specific to the event though. Everyone from 800m to marathon needs a base, needs threshold work, VO2max work, speed etc. They just need it in different proportions.

There was a study that showed a change in fiber type from oxidative to non oxidative in mice and rats when their hind legs are suspended for weeks at a time, making it impossible for them to use them. Muscle recruitment always follows nerve innervation. What makes a muscle fiber slow or fast twitch is not the muscle, but the nerve that innervates it. In theory, it could be possible to change the fiber type, but first you would have to change the nerve, and IF that is possible it would take years. When a nerve is injured, it can regenerate up to one inch per year. So if the muscle is to change, the nerve would have to have serious need for change (be damaged by overuse), then regenerate as either fast or slow twitch. This will not happen because a person will run out of fuel for the muscle (glycogen, or even glycerol if neccessary) before anything would ever happen to the nerve. I guess I should have explained what I meant before.

Thanks Phoenix. Through years of my own running and now coaching at the High School level, I've slowly come to that conclusion myself (although it's a constantly evolving thing). Thanks again.

Has anyone on here tried the online coaching through the lydiard website? Do you have to pay for it? How personal is it, like do yu maintain e-amil contact once a week?

Every college textbook I've read says that you can change subtypes, that some fast twitch fibers can take on the characteristics of slow twitch fibers (increased mitochondria, capillary density, ect). I think we may just be confusing similar thoughts. You can't completely change a fiber from I to II or vice versa because that would require a change in the nerve. You seemingly can however change the type II subtype so that it becomes more oxidative. How to do that have been the subject of this 60+ post thread. I think there is some confusion among those w/o the physiological background as to what we are talking about. When most regular people here about changing fiber types they think from slow to fast or vice versa and that's where we cross hairs. No study that I am aware of has shown that there is a complete change from slow to fast (in humans) but there have been many studies that show a change in the fast twitch subtypes. The theory seems to be that in order to change the subtype, to improve the oxidative ability of fast twitch fibers, you have to fatigue some of the slow twitch fibers first. One of the common and most used ways of doing this is through very long runs. During very long runs muscle fibers (both fast and slow) become "damaged" and so their ability to keep up the force needed to continue the run is reduced and that's one of the reasons why people tend to slow down during really long runs, especially if they are not used to the distance. Keeping up the same speed and thus the same rate of muscle contraction would have to involve the recruitment of other muscle fibers to help take some of the burden off the "damaged" fibers. This is done by recruiting more fast twitch fibers. If you also increase speed during the end of the run you are forced to enlist more fast twitch fibers. This is a difficult process though because these fibers aren't "used" to functioning this way so the body makes adaptations to make this "job" easier the next time around. This is one of the reasons why long runs of a certain duration become easier and that if you progress in duration over time the shorter long runs become even easier. Now whether or not this has to do with depletion of fuel or transfering of fuel or simply fiber recruitment is up for debate and probably beyond the scope of this board.

Alan

Bump

I'm really doubting whether fast twitch muscles are being recruited. If they were being recurited, then one would expect some improvement in basic speed. This is obviously not the case, people who train very long and slow tend to run very slow. It simply seems that people who do longer runs, put in more miles, etc have better trained aerobic systems, this allows them to maintain a faster overall pace and allows them to "save" more for a better kick/surge in the home stretch.

So what is the conclussion? can someone take everything which have been said and put it into simple terms, without let their idelogy get into their way.

CO-Runner wrote:

This is obviously not the case, people who train very long and slow tend to run very slow. It simply seems that people who do longer runs, put in more miles, etc have better trained aerobic systems, this allows them to maintain a faster overall pace and allows them to "save" more for a better kick/surge in the home stretch.

Those plodders probably aren't out running 60s/80s/100s/120s at 400m pace. You need to do the basic speed work to improve basic speed. Simply running high volumes will not do.

There are a couple of things that go into it: Snells interpretation of the aforementioned article that fatiguing fiber types will eventually lead to fast-twitch recruitment; Phoenix's coutnerclaim against that while still adding that the FT fibers may be affected in some capacity, and the fact that all this enurance training will allow you to stay more aerobic in a reace than your competitors and therefore allow you to change gears at the end while your competitors are suffering through anaerobia. Snell's Olympic 800m's/1500m are supreme examples of this.

A few summarizing points and thoughts:

1) Long runs do not cause fast twitch fibers to be recruited any more than a short run at the same pace.

2) Long runs do cause glycogen depletion in fast twitch fibers that are not recruited. This is likely an adaptation stimulus.

3) Neither Lydiard, nor JK, nor any other coach in their right mind has ever advocated doing only slow LSD training. They have have included workouts to maintain speed during the base phase.

4) My purpose in shooting down the long run recruitment theory is not to discredit the training methods of those hypothesizing that effect. My purpose was to shoot down a pseudophysiological myth.

5) For reasons we don't fully understand long runs work and are an integral part of many champions' training programs.

Phoenix wrote:

4) My purpose in shooting down the long run recruitment theory is not to discredit the training methods of those hypothesizing that effect. My purpose was to shoot down a pseudophysiological myth.

100% agreement. There is obviously something to putting in the miles, long runs etc. no argument there but the FT recruitment "theory" is bunk.

bump

bump

CO-Runner, any more myths/rumors,lies to dispense? You are such a character!

Depends on what you mean by recruitment and also what happens when these fibers are recruited.

http://www.pfitzinger.com/labreports/longruns.shtml

"The higher the percentage of slow twitch fibers in your muscles, the greater your likelihood of success in the marathon. That's because slow twitch muscle fibers naturally have more of the 6 adaptations we've already discussed than do fast twitch fibers. Well, for those of you not genetically endowed with a high proportion of slow twitch muscle fibers, there is good news. Long runs give your fast twitch fibers more of the characteristics of slow twitch fibers."

So why is this? Running long runs has been proven to improve the oxidative capabilities of certain fast twitch fibers thereby converting Type IIb fibers to Type IIa fibers. Unless a muscle fiber is being used it can't adapt and change. In order to undergo these changes these fast twitch fibers must be used in an oxidative fashion. If you fatigue and damage some slow twitch fibers throughout the course of a long run their ability to produce the energy needed for the run will be diminished. Somehow more fast twitch fibers are being used toward the end of a long run and thus over time their oxidative capability will be increased.

Alan

Ask Vin Lanana. He's pretty good at recruiting.

Long runs have nothing exclusive here. Weight-lifting and sprint training do the same things.

It depends on what you mean by used. Go back and re-read this post. They are not being recruited. The whole hard-wiring of the nervous system would have to change during a single run. Glycogen depleted, yes, many studies have shown that. Recruited, no, that has never been shown.

The major mechanism is fatigue although some muscle damage undoubtable occurs, but not a lot if the pace is slow. The major source of fatigue is glycogen depletion. However, slow twitch fibers are quite capable of metabolizing lactate released by epinephrine stimulated non-recruited IIb fibers, not to mention fatty acid oxidation. The damage explanation as far as a performance impediment doesn't fly. Damage repair takes days because of the protein synthesis requirements. Try this experiment your self. Go out on a long run at marathon pace and hold it until you start to hit the wall and can't any more. Slow way down, go home and eat some good carbs, then go out to the track in the evening, warm-up, and ease into marathon pace again, then drop down to 10k and then 5k pace. Yes, you can do it because you have re-fueled. Repairing muscle damage from morning to evening is not possible.

Again, it depends on what you mean by used. Recruited, no. Not a single study in all of the literature has shown a pace-independent recruitment effect of IIb fibers during a long run. Many of shown that they do become glycogen depleted. The signaling pathway has already been explained in detail in earlier portions of this post. Glycogen inhibits the enzyme AMPK. AMPK signals for mitochondrial biogenesis. Depletion of glycogen therefore is an AMPK activator and thus a signal for aerobic adaptation.

However, go back and read the literature. Intervals at ~VO2max pace recruited fast fibers and deplete them WAY more than slow very long bouts.

**The pace of the running determines which fibers are recruited and the volume of the run determines the magnitude of the adaptation stimulus sent to the recruited fibers**

Thus for a marathoner, fast long-runs are better then slow long-runs provided you are physically prepared.