Fast twitch muscle fibers and slow twitch muscle fibers

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    Fast twitch muscle fibers and slow twitch muscle fibers


    A trainer at the gym I used to go to talked a lot about fast twitch dominant lifters and fast twitch dominant lifters. And it's good for your hypertrophy to find out which your body is more dominant in.. is this true? Also, if it is true how do you find which one you're more dominant in?

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    For muscular hypertrophy specifically, it's about volume of exercise regardless of fiber typing. However, type II fibers have a much larger capacity for growth than type I fibers. A basic way of predicting fiber composition of a muscle group is to lift the maximum weight you can for X number of reps (2,3,4, etc) then multiply that number by a percentage of 1RM. Once that's done that's your predicted 1RM, then lift 80% of that number and do as many reps you can. This will determine roughly your fiber composition.

    For example: Someone can lift 225lbs for 2 reps. Their predicted 1RM is 236.84lbs. Multiply by .80; 236.84 * .80 = 190lbs. Say that person can lift 190lbs 3 times they would be fiber composed as type II.

    Type II fibers: 1-6 reps
    50/50: 7 reps
    Type I fibers: 8-14 reps
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    The only way to really know your fiber type profile in having a muscle biopsy done and having fiber type testing done. This isn't going to happen unless you are near a university that is doing a study with muscle biopsies.
    Those repetition tests might be OK if you fall at the extreme ends of the spectrum (lots of fast twitch or lots of slow twitch), but probably aren't really good if you aren't near one of those extremes. You also have to define what fiber typing system you are talking about. A metabolic enzyme based fiber typing doesn't always match really well with a myosin heavy chain based fiber type classification. You can also dramatically effect your performance on these repetition tests based on how you train (high reps or low reps). You can also alter the metabolic enzyme profile and fatigue properties of your muscle without actually changing fiber types from fast twitch to slow twitch or vice versa.
    Ultimately, you should probably be training in a variety of repetition ranges anyway to optimize the training effect. If you do this you have all your based covered anyway, so the little nuances of your particular fiber type profile aren't really that important.
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    Quote Originally Posted by SRS2000 View Post
    The only way to really know your fiber type profile in having a muscle biopsy done and having fiber type testing done. This isn't going to happen unless you are near a university that is doing a study with muscle biopsies.
    Those repetition tests might be OK if you fall at the extreme ends of the spectrum (lots of fast twitch or lots of slow twitch), but probably aren't really good if you aren't near one of those extremes. You also have to define what fiber typing system you are talking about. A metabolic enzyme based fiber typing doesn't always match really well with a myosin heavy chain based fiber type classification. You can also dramatically effect your performance on these repetition tests based on how you train (high reps or low reps). You can also alter the metabolic enzyme profile and fatigue properties of your muscle without actually changing fiber types from fast twitch to slow twitch or vice versa.
    Ultimately, you should probably be training in a variety of repetition ranges anyway to optimize the training effect. If you do this you have all your based covered anyway, so the little nuances of your particular fiber type profile aren't really that important.
    I said this way a basic test to figure out your fiber composition, generally speaking. While a skeletal muscular biopsy is much more accurate, many people on these boards do not have access to those types of tests performed.

    It also depends on the type of lifting the individual performs. For general strength, muscular hypertrophy, and overall fitness, a variety of mixed repetitions would be beneficial. For a bodybuilder or powerlifter having a variety of mixed repetitions would not be as beneficial, especially if that individual is planning to compete. Also, it is not possible for someone with a high composition of type I fibers (even with pure strength/power training) to lift as much weight as someone with a high composition of type II fibers. Nor is it possible for an individual that is highly composed of type II fibers to perform high endurance events. While training can alter the biochemical properties of muscle fibers (typically type IIa) they cannot perform exactly like a type I or type IIx fiber.

    All fiber types (type I, type IIa, type IIx) are metabolically enzymatic. Type I fibers being more aerobically oxidative, and type II fibers with LDH, ATPase, and Creatine Kinase activity--you refer to them as 'myosin heavy'. Type II fibers have 8 different contractile proteins, myosin isn't the only one; however, actin and myosin contribute the most to muscular contraction.
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    there is also some evidence that training specificity can alter the amount/% of fast twitch to slow twitch. thus type I can become type IIa and type IIa can become IIx depending on how you train. So if you where to want more fast twitch fibers you would have to train in a manner in which would recruit more motorunits for fast twitch fibers specifically.
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    Quote Originally Posted by slacker86 View Post
    there is also some evidence that training specificity can alter the amount/% of fast twitch to slow twitch. thus type I can become type IIa and type IIa can become IIx depending on how you train. So if you where to want more fast twitch fibers you would have to train in a manner in which would recruit more motorunits for fast twitch fibers specifically.
    Type IIa fibers will take on biochemical characteristics of either type I or type IIx depending on mode of training and quickly revert back in the absence of training, but they DO NOT become a different fiber type-that is impossible to change without changing the motor neuron which innervates those fibers.
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    Quote Originally Posted by russy_russ View Post
    I said this way a basic test to figure out your fiber composition, generally speaking. While a skeletal muscular biopsy is much more accurate, many people on these boards do not have access to those types of tests performed.

    It also depends on the type of lifting the individual performs. For general strength, muscular hypertrophy, and overall fitness, a variety of mixed repetitions would be beneficial. For a bodybuilder or powerlifter having a variety of mixed repetitions would not be as beneficial, especially if that individual is planning to compete. Also, it is not possible for someone with a high composition of type I fibers (even with pure strength/power training) to lift as much weight as someone with a high composition of type II fibers. Nor is it possible for an individual that is highly composed of type II fibers to perform high endurance events. While training can alter the biochemical properties of muscle fibers (typically type IIa) they cannot perform exactly like a type I or type IIx fiber.

    All fiber types (type I, type IIa, type IIx) are metabolically enzymatic. Type I fibers being more aerobically oxidative, and type II fibers with LDH, ATPase, and Creatine Kinase activity--you refer to them as 'myosin heavy'. Type II fibers have 8 different contractile proteins, myosin isn't the only one; however, actin and myosin contribute the most to muscular contraction.
    I realize it's just a basic test, but in my opinion there are too many other variables that factor into performance during this test to put too much stock in it. As I mentioned earlier, it is likely to only identify those in the extreme ends of the range. In reading much of the research in this area, I haven't come across anything that validates this test comparing it to a biochemical fiber typing method. If it does exist, I would be interested in seeing it. Some may disagree and find this test very useful and that's fine.
    I mentioned myosin heavy chain because it's a very commonly used classification method for muscle fiber typing. When classifying fibers as type I, IIa, IIx it's using a direct or indirect way of identifying myosin heavy chain composition. I know there are many other proteins involved in muscle contraction, but they aren't typically used in muscle fiber typing.
    You're correct in that muscle fiber type doesn't typically change with training, but there are certainly changes in the fast subtypes (IIa, IIx). It should also be noted that the vast, vast majority of the available research suggests that any kind of activity results in increases in IIa fiber type % and decreases in IIx %.
    I think we're largely on the same page, just maybe a difference of opinion on the utility of the rep tests for fiber type estimates.
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    Quote Originally Posted by SRS2000 View Post
    I realize it's just a basic test, but in my opinion there are too many other variables that factor into performance during this test to put too much stock in it. As I mentioned earlier, it is likely to only identify those in the extreme ends of the range. In reading much of the research in this area, I haven't come across anything that validates this test comparing it to a biochemical fiber typing method. If it does exist, I would be interested in seeing it. Some may disagree and find this test very useful and that's fine.
    I mentioned myosin heavy chain because it's a very commonly used classification method for muscle fiber typing. When classifying fibers as type I, IIa, IIx it's using a direct or indirect way of identifying myosin heavy chain composition. I know there are many other proteins involved in muscle contraction, but they aren't typically used in muscle fiber typing.
    You're correct in that muscle fiber type doesn't typically change with training, but there are certainly changes in the fast subtypes (IIa, IIx). It should also be noted that the vast, vast majority of the available research suggests that any kind of activity results in increases in IIa fiber type % and decreases in IIx %.
    I think we're largely on the same page, just maybe a difference of opinion on the utility of the rep tests for fiber type estimates.
    For basic purposes that test is sufficient, and should not be taken as 100% accurate. You're right about the increase in type IIa characteristics and a decrease in type IIx with resistance training. Anyways, I can tell you're educated in this field, any degrees? I'm trying to finish my M.S and start Ph.D in Ex Phys.
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    Quote Originally Posted by russy_russ View Post
    For basic purposes that test is sufficient, and should not be taken as 100% accurate. You're right about the increase in type IIa characteristics and a decrease in type IIx with resistance training. Anyways, I can tell you're educated in this field, any degrees? I'm trying to finish my M.S and start Ph.D in Ex Phys.
    I have an MS in Exercise Science and I'm currently working on my PhD in exercise phys/muscle biology. Hopefully I can get all my research and writing wrapped up in the next 1-1.5 years and finish.
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    Quote Originally Posted by SRS2000 View Post
    I have an MS in Exercise Science and I'm currently working on my PhD in exercise phys/muscle biology. Hopefully I can get all my research and writing wrapped up in the next 1-1.5 years and finish.
    That's great! I'll have about another year for M.S and ~4 for Ph.D
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    ideally you should use periodization in your training anyway... ex. 2 wks endurance 12-18 reps at 50-65% max, 2 wks hypertrophy 8-12 reps at 65-80% max, 2 wks strength/power 3-6 reps at 80-95% max, 1 wk rest, ratchet up the weight and repeat cycle.

    its not quite as cut and dry as that but periodization has countless benefits. definately something to look into
  

  
 

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