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Started By dafortae (a, a, U.S.A.)

Started on: 12/2/2003 11:35:25 AM, viewed 5021 times
Slow and Fast Twitch Muscle Fibers

I have noticed some members of the forum have been asking questions about muscle fiber types lately. I decided to do some "deep" thinking myself, from scratch, so to speak about this topic.

We all probably know this, but there are several types of muscle fibers. The MAIN classification is Slow twitch (red) and Fast twitch (white). The slow twitch are reddish in color, because of the abundant blood supply they receive. The fast twitch are white in appearance, because of LACK of blood flow. There are actually other "intermediate" types, that are inbetween these two extremes, which people often refer to as Type IIa. The slow twitch are Type I and fast twitch are Type IIb.

Slow twitch are "slow", because they contract with little force. However, they have great endurance capabilities, because the great abundance of blood flow. Obviously the blood caries a ton of energy for our hungry muscles, so the red fibers can get to it pretty easily.

Fast twitch are "fast", because they contract with a LOT of force. But, unlike the slow twitch, they don′t receive much blood. Thus, they cannot replenish quickly, and run out of energy fast.

Muscles "make" more of their energy, by usage of internal powerhouses called Mitochondria. I like to think of Mitochondria like the alternator on a car. I think of the muscles as the batteries. The alternator on a car continually charges the battery and powers devices in the car. Without it, the car would die after the battery used up all it′s power. Carbohydrates are the prime fuel for fast twitch fibers, because these Mitochondria convert the broken down carbs to ATP energy, which can be used within the muscle. However, this is a VERY slow process.

I started to wonder how slow the actual replenishment of energy to white fibers is. Could it take days?

I then started thinking about connective tissue, and how long it takes to heal. It can take MONTHS for tendons and connective tissue to heal, because of the lack of blood flow. I′m sure they receive MUCH less blood flow then white fibers, but still, white doesn′t receive a whole lot.

I then started thinking about intermediate, or type iia fibers. How long does it take to replenish those fibers?

I then started to wonder if the time to replenish was exponential, or follows some type of formula that would be similar. I did some research on the internet, and found out that generally, red fiber have 2.6x the blood supply as white fibers. Also, red have MANY more mitochondria to work with! This means more energy is coming in AND more can be converted at the same time! It DOES appear to be exponential, or closely related!

This could mean that red fibers that are close to exhaustion may recover in a few minutes, but it may take white fibers hours, or even days, to "refill".

Now the reason you can maybe "feel" like you have enough strength to workout the next day is because SOME of the energy in the white fibers are replenished, and probably ALL the other fibers are replenished. So sure, you could probably do 1 set of intense exercise. However, you would NOT be able to generate 100% intensity in those white fibers, because they weren′t totally refilled yet.

This refill has NOTHING to do with damage repair or muscle mass increase either! I′m not sure if energy refill and repair occurs at the same time, but the repair would ALSO take exponential time for the fast twitch, because of the low blood flow! Building of mass is the same story too! So basically, those 1 to 2 days of energy refill would ALSO take another 2-3 days of repair and 2-3 days of building. The slow twitch probably could be rebuilt, refilled, etc in a day or less! But we all know those don′t have much capability to grow. You would "max out" your size VERY quickly with these fiber types, and be stuck forever at that size. This is probably why volume trainers can get some size to them, but then they are stuck FOREVER at that size.

Our goal is to hammer the type IIb fibers with super high intensity, and other advanced techniques, then allow enough time off for that growth mechanism to do it′s job. Everyone is different with how quickly they can recover, but we all work under one common thread. As we get stronger and do more damage, or even unload MORE energy, there is a bigger "hole" to repair, and not a lot of blood to work with.

Repairing slow twitch is like a giant lake, with the muscle wrapped around the whole lake and there are millions of canoes filled with workers doing the repair on the shore of the lake.

Repairing intermediate twitch is like a small pond, with the muscle wrapped around the whole pond and there are hundreds of swimmers doing the repair on the shore of the pond.

Repairing fast twitch is like a stream, with the muscle wrapped around the edge and there are 2 guys doing the repair standing in the middle of the stream.

This is how I visualize the replenish, repair, build phases. It′s pretty easy to see why days, sometimes weeks, must be taken off to all the phases to complete.


This Topic has 74 Replies: Displaying out of 74 Replies:

Analyzer (CDA, id, U.S.A.) on 12/2/2003 11:45:23 AM

Hey Darrell,

That′s very interesting, where did you find the info on the blood supply? Everything I′ve read indicates that the slow fibers are such due to the nerve that intervenes them. They have grafted a fast twich fibers nerve over to a slow twitch fiber and it turned into a fast twitch fiber later.

I′d love to read the info. you found 🙂

Take care dude!


Ferrari (Gatineau, QC, Canada) on 12/2/2003 11:57:16 AM


Interesting way of thinking. One issue though.

"Now the reason you can maybe "feel" like you have enough strength to workout the next day is because SOME of the energy in the white fibers are replenished, and probably ALL the other fibers are replenished. So sure, you could probably do 1 set of intense exercise. However, you would NOT be able to generate 100% intensity in those white fibers, because they werenŒt totally refilled yet."

Intensity is defined as a percentage of momentary capability. Even if you were weaker, if you trained as hard as possible you could still reach 100% intensity.

I would have used a different analogy. Think of a grocery store where the slow twitch are the staples such as; milk, eggs, butter, and bread. They are purchased quickly and the shelves are restocked constantly. Just like our slow twitch muscles are used for standing, walking, keeping our head up for 16 hours a day etc.

Fast twitch is like the specialty items. Used only on occasions so they are restocked slower. There is no sense of urgency (except for HIT enthusiasts).

If the grocery store runs out of Balsmic Vinegar then we pick it up the next time. If they run out of the staples, anarchy!


Analyzer (CDA, id, U.S.A.) on 12/2/2003 11:58:55 AM

Here is my favorite site for muscle cell anatomy, check this out and tell me what ya think. :-)http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Muscles.html

About 2/3s of the way down that page it get′s into type I and II fibers, plus you can click on key words and have it explain them. 🙂


dafortae (a, a, U.S.A.) on 12/2/2003 3:49:20 PM


I actually got the specifics from the following link:


It was talking about fish, but the concept of red vs white muscle fibers is universal I believe. I read all kinds of info. from other locations, but that was the page I found the 2.6x details on.

Maybe if a new nerve is attached to a muscle cell, like a white fiber, the body starts to convert it to a red fiber by growing the vascular area. That′s probably built into the DNA, so it′s an automatic thing. The brain probably doesn′t have anything to do with it.

I′ll check out your links.



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