Recruiting Fast Twitch Fibers

Type I (or ST fibers) CANNOT change to Type II (or FT fibers) or vice versa. You are born with a set proportion of each. No amount of training can change this. However, with training (for eg. distance running) you CAN make FT fibers develop ST properties, but will never be as good as a ST fiber.

I haven't read the article you are refering to, but I think what he might be trying to say is that after 90 mins of running, you will begin to recruit FT fibers to help in your running. Makes sense, as this was the basis for Lydiard's 100 miles a week with 800m guys.

Tuppy

The theory I've heard is like this: Your body adapts to your last distance run by storing the amount of glycogen needed to complete that run in the slow-twitch muscle fibers. During your next distance run, those slow twitch fibers do most of the work until they run out of energy(of course you burn fat, too, so you never really run out of energy, but production of it in those muscles drops after the glycogen is depleted). In order to maintain pace while going farther(or faster) then last week, you "recuit" the fast-twitch fibers do do the heavy shoving. This stresses those lazy muscles and they adapt by becoming stronger and making you a better runner, which means you've got to stack another couple on your long run next week to keep improving.

As mentioned above, when you run at a moderate/easy pace for an extended period of time (90 min. minimum, 2+hrs more ideal) you reach a point where your slow-twitch fibers become glycogen depleated and fatigued and cannot contract anymore. At that point you begin to recruit fat-twitch fibers to continue running. In this way, over-distance training will help improve basic speed.

Here is the reference to the research where this was found:

Gollnick PD. Piehl K. Saltin B. Selective glycogen depletion pattern in

human muscle fibres after exercise of varying intensity and at varying

pedalling rates. [Journal Article] Journal of Physiology. 241(1):45-57,

1974 Aug

Would it then be a good idea to run for 90 minutes and then finish with some light speed work? Maybe do 3x 400-500 metre tempo runs? Might be risky to run flat out but say running at 1500-3000 pace?

Light speed work is fine. 100m accelerations of some sort is completely appropriate. However, I would avoid longer intervals. The key purpose of the long run is to develop endurance, stimulate mitochondrial, myglobin and capilarization -- the FT recruitment is just icing on the cake.

Hey, trackhead. Was it Lydiard who said a couple of years ago that if Jim Ryun had used his training methods, he'd still have the WR in the mile?

I can't say -- I never heard that quote. I'll have to ask around for that one.

I like the idea of more mileage and less intensity for distance training and I'm trying to get into shape after roughly 5 years off from running. My runs aren't very far with my long runs currently being at about 50 minutes. I keep my heart rate below my lactic threshold but my legs usually feel tired and energy depleted afterward.

So the question is, what is it that makes 90 minutes the magic number for why ST muscle fibers become glycogen exhausted and where FT fibers start getting recruited? Is that 90 minutes a constant or does that time vary depending on what kind of shape someone's in? Is it possible that the FT's are being used even during much shorter runs if a runner is not yet in decent shape?

As far as I know, it applies universally. Certainly there are better-trained athletes who don't tire to the extent as others do after 90 minutes of running.

Does a marathoner with 50,000 lifetime miles take longer to fatigue their muscle fibers than an 800m runner with 6,000 lifetime miles? I'm not sure. Probably, but a 90 min long run seems appropriate for the latter, whereas a 3hr run seems more appropriate for the former.

A good question, though.

fly wrote:

So the question is, what is it that makes 90 minutes the magic number for why ST muscle fibers become glycogen exhausted and where FT fibers start getting recruited? Is that 90 minutes a constant or does that time vary depending on what kind of shape someone's in?

I don't think it is meant to be a "magic" number. I think it's meant to be a ballpark figure, give or take a little, depending on the runneer

There is a debate on the number of different fibers. Dr Lancaster from Britain believes there are 4 types of muscle fiber, and how each type is recruited depends on genetics. You can't train to recruite a certain type of Fiber.

so glad to see this post and find some useful information. thanks guys

Dr. Peter Snell has often referred to the value of long runs as a method of safely recruiting fast-twitch fibers. I know he is referring to Gollnick and Saltin's research and combines it with his personal experience. I have wondered a lot about the approach of depleting one's glycogen stores in slow-twitch cells and then trying to run a bit faster to deplete and thereby stimulate faster fibers. Since glycogen use is dependent upon exercise intensity, would it not be probable that faster distance running (say for an hour at slightly less than all-out) would deplete the stores and then provide and ideal situation to train faster fibers? I could see that FOG fibers (fast fibers with oxidative ability) which are recruited at half-marathon pace would be mostly depleted at the end of the fast hour of running, so speedwork or strides would be a bit difficult to do at moderate speeds thereafter due to depletion of the stores in those fibers. Perhaps the only fibers left to recruit would be FG (fast-non-oxidative) fibers used to sprint.

The research articles cited above and others referring to the same topic have NOT shown that fast twitch fibers are recruited after long durations of running.

What they have shown is that some glycogen depletion begins to occur in the fast twitch fibers. Some have assumed that glycogen depletion is concomitant with fiber recruitment. This is not the case.

**Fibers do NOT have to be recruited to be glycogen depleted**

It is much more likely that the ft fibers are dumping their glycogen in the form of lactate which is then being used for fuel by the heart and st fibers when st fiber fuel is depleted.

However, glycogen depletion IS a stimulus for aerobic (AMPK signaling pathway for mitochondrial biogenesis)adaptation whether or not the fiber is recruited.

So, in the short, long runs DO deplete ft fibers. This depletion IS a training system but is probably independent of recruitment.

Phoenix wrote:

**Fibers do NOT have to be recruited to be glycogen depleted**

It is much more likely that the ft fibers are dumping their glycogen in the form of lactate which is then being used for fuel by the heart and st fibers when st fiber fuel is depleted.

How exactly does ft fibers 'dump glycogen in the form of lactate?' Lactate is a waste product of glycolysis when no oxygen is present, so doesn't that mean that the FT fibers are working anaerobically i.e. being recruited? And whenever there's an oxygen shortage in the muscle it stimulates adaptations. Please correct me if I'm wrong. Do you have any links or studies that show your point? Thanks!

Good question:

Norephinephrine spill-over from nearby nerves or circulating epinephrine from the adrenal glands act on adrenergic receptors located on the membrane of the muscle cell. These adrenergic receptor signals through a g-protein/pka mediated pathway that leads to glycolysis without contraction of the fiber. The generated lactate is released from the muscle cell (fiber) via a lactate transporter into the blood stream where it is taken up and directly metabolized by other tissues or converted into glucose by the liver and released back into the bloodstream. This is a known phenomenon called the Cori Cycle.

Hope that helps.

So FT fibers aren't recruited. Is the adrenergic activation

at alpha or beta receptors?

Deak is anorexic, why would you listen to physiology advice from him?

Phoenix, interesting do you have a reference for that? I would like to read the original paper. Also whose lab do you work in? Wayne Willis?