Fast twitch muscle fibers and slow twitch muscle fibers
- 06-05-2009, 09:14 PM
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?
- 06-05-2009, 10:45 PM
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
06-06-2009, 11:49 AM
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.
06-06-2009, 02:25 PM
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.
06-06-2009, 03:32 PM
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.
06-06-2009, 03:55 PM
06-06-2009, 05:55 PM
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.
06-06-2009, 07:02 PM
06-06-2009, 07:54 PM
06-06-2009, 10:24 PM
06-08-2009, 03:29 PM
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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