The effect of muscle fiber type on increasing muscle size

[RottenWillow]

Perhaps you've found you work hard in the gym, eat enough calories, get sufficient rest, but still don't seem to grow very much. Could be your training style is wrong for your muscle fiber type.

You need to know if a given muscle group you want to make bigger is predominantly composed of fast twitch or slow twitch muscle fibers because the different types of fibers respond very differently to progressive resistance weight training.


First, what are the different muscle fiber types


Slow- twitch (Type I fibers)
Characterized by a slow contraction time and a high resistance to fatigue.

Fast-twitch (Type II fibers)
Characterized by a quick contraction time and a low resistance to fatigue.

Fast-twitch fibers are further divided into fast-twitch A and fast-twitch B

A fibers have a moderate resistance to fatigue and are used for prolonged activities with a relatively high force output, such as middle distance sprinting.

B fibers are very sensitive to fatigue and are used for short high force production activities like Olympic weight lifting.


Second, how do I know what type predominates in a given muscle group I want to train

Short of taking a muscle biopsy, you can't know. You can guess though. First you must determine your 1 rep maximum. Once you know the absolute most weight you can lift in good form over a full range of movement, reduce the weight by 80%. After a ten minute rest, do a set with that reduced weight. If you can't get an 8th rep it suggests that muscle group is composed predominantly of fast twitch B fibers. If you can get at least 8 but not more than about 12 it suggests that muscle group is composed predominantly of fast twitch A fibers. If you can get 13 or 14 reps it suggests that muscle group is composed predominantly of slow twitch fibers.


Third, so what do my results imply for my training?

If your results suggest that a given muscle is composed primarily of fast twitch fibers, your best results will come from working in a lower rep range and with longer rests between sets. Rep range about 5-8 and rest intervals of at least 2 minutes. You should keep your total number of sets low, otherwise you may find you have a great deal of difficulty putting on size. (I cannot stress this last part enough. Your muscles will not be able to handle a lot of sets. Do a lot and you'll never grow.)

If your results suggest that a muscle is made up of mostly slow twitch fibers, your best results will come from doing considerably more reps in order to encourage muscle hypertrophy. You'll also need to maintain much shorter rest periods between sets. In short, your training will be tougher. Rep range of at least 12 reps, but probably more along the lines of 15 and up. Rest periods not longer than 1 minute. Also, you'll need more sets to grow.

And if your results suggest a muscle is made up of those medium speed (type A) fast twitch fibers, you'll be working with rep ranges and rest periods of medium length.

 

[Roxy722]

Thanks for posting Willow. I found this really interesting and am going to try it out!

 

[RottenWillow]

Thanks for posting Willow. I found this really interesting and am going to try it out!

It really does make a huge difference in how you should train, Roxy.

If you're interested in investigating this make sure you test all your muscle groups because they may well not all be composed predominantly of the same fiber types. You might find your quads are slow twitch, your bis fast twitch type B, and your pecs fast twitch type A. That would mean those groups would all need different sorts of training in order to encourage muscle growth.

 

[rp112248]

Interesting, what if strength vs size is the main goal?

 

[shirlene29]

super cool info RW...ima gonna try this out tomorrow
I'd suspect for myself Im a fast twitch B (despite my running which didn't come easy at all) because I can do amazingly large weights explosively but with no real ability to get reps in

my spine has thanked me for this too

 

[RottenWillow]

Interesting, what if strength vs size is the main goal?

Shorter sets with heavier poundages will always tend to produce the best strength gains. I'm not sure that fiber type dominance would be as much of a factor there. I'm speculating now though.

This gets into the TUT principle though. Not sure if you're familiar with the Time Under Tension principle, but it is a theory based on an understanding of the energy systems and the hormonal responses that occur during sets. There's been some good threads on it in the past, so you should be able to search this forum for more in depth info.

 

[Roxy722]

It really does make a huge difference in how you should train, Roxy.

If you're interested in investigating this make sure you test all your muscle groups because they may well not all be composed predominantly of the same fiber types. You might find your quads are slow twitch, your bis fast twitch type B, and your pecs fast twitch type A. That would mean those groups would all need different sorts of training in order to encourage muscle growth.

I was thinking about that today...that you would have to try it out on all the different areas. Thanks again. I'm always so impressed with your knowledge base!

 

[RottenWillow]

I want to share the results of my own muscle fiber type testing, which were somewhat surprising. Effective this week I will be permanently changing the way I train.

I used isolation exercises to determine the fiber type predominance of all my muscle groups. This means pec deck flyes instead of bench for pecs as the latter would heavily involve tris and delts, and lateral raises instead of overhead press for delts as OHP would heavily involve tris. For every single upper body muscle I found that when I reduced my one rep max to 80% I could only do 4 reps. WOW. That is really low. This result suggests all my upper body muscles are overwhelmingly composed of fast twitch B, or very rapidly firing, very fatigue sensitive fibers.

Quads and hams results were quite different. For both leg extensions and one legged standing leg curls I was able to get 10 reps at 80% of my 1RM. This suggests that all the thigh muscles are probably a fairly even mixture of fast twitch A and slow twitch fibers. I did not test calves because they are already very beefy and I avoid doing any direct work on them.


My test results reveal why my legs have for years been so much beefier than my upper body. I've been training them right...and I've been training my upper body wrong.


The fairly even mixture of fiber types in my thighs mean I should vary my reps for leg work between about 8-9 reps for a set up to about 13-14. Rest intervals should vary as well. Sometimes only a minute or so, sometimes upwards of 3 minutes. For years now this is exactly how I've trained legs. Upper body work sets for me should never have more than 8 reps. And really I should be working in the 5-7 rep range exclusively. Dude I almost never train that low.

 

[RottenWillow]

bumping this to provide an update on my results from training according to my muscle fiber type predominance.


wow...it works.


Because all my upper body muscles tested out as being predominantly Type IIb (very fast twitch, very rapidly fatiguing), I've been working in the 5-7 rep range for all upper body movements. I've found that the key is not only working with heavier weights and lower reps, but moving the weight explosively. The pumps I've been getting are amazing. I've not added any vasodilators or creatine that would increase pumps.

The explosiveness is really recruiting those very fast twitch muscles that apparently dominate in my upper body. My range is still full and I'm not sloppy with my form, but I am definitely thinking "explode!" on the concentric phase of every rep. My concentric phase time has probably dropped from 3 seconds to no more than 1.5 seconds at most.

Legs I'm still training with lighter weights, more reps, and medium bar speed, as dictated by my mixed fiber type in the legs. Medium speed is sort of a compromise for the mixed fiber types. Also, I notice it just feels much more natural to rep more slowly for those lower body movements.

 

[RachelM]

I was just trying to figure out amounts of reps and sets for my new workout and I remembered this thread..

I did most of my new routine and the results are very similar... legs are pretty much slow twitch, biceps are fast twitch b, tri's are fast twitch a, lateral raises - fast twitch b, lying flies fast twitch a..

Very interesting!!

Are you still training accrording to this Rotten Willow? If so, how's it going?