Reader question - "Muscle type cannot be changed??"

We're startin' the week off with another thought-provoking reader question...

Hi Kim!!!

I am actually writing you to state my opinion on your recent blog about "Toning"!! I love that you are doing this, it's great! I was just a little concerned on the fact that you say: that muscle type cannot be changed in relation to the type of exercise. What do you mean by that, since I recall having physiology classes explaining the biology behind type of muscle fibers and all... (I, IIa...) I don't want to be picky here, or underestimate your knowledge.. I just was wondering and wanted to challenge you a little!!!

Hope things are great-

Much love
JENN O.

You KNOW I love a challenge, Jenn ;)

Okay... first of all, let's get everyone up to speed on the basics of the topic.




There are three different types of muscle fibers in the human body. Slow twitch (type I), and two types of Fast twitch (type IIa & IIb).

• Type I fibers are used in weaker contractions and endurance activities.
  
• Type IIa fibers are used in stronger contractions and more intense exercises. They contract quickly, but fatigue more easily.
  
• Type IIb fibers are used in quick, powerful, ballistic activities. Movements that are very intense and that require maximal contraction require the activation of these muscle fibers. These super quick fibers fatigue very quickly. Basically, you can't sprint at maximum speed for very long... right?

Now, each muscle has a mixture of all three fiber types. The mixture of fiber types is different in each muscle, depending on the typical function of that muscle. This is the result of eons of evolution. For instance, the muscles in the eye that control rapid eye movements during sleep... those have a greater percentage of fast-twitch fibers. The deep calf muscle, the soleus, which helps us to stand and walk around all day, is much more rich in those high endurance, slow-twitch fibers. And so on...

PLUS, based on genetics, each individual contains a personal mixture of fast and slow twitch fibers throughout the body. Which explains why some people are great at sprinting, and have quick reaction times, but find themselves awful at long-distance running and other endurance activities. And vice versa.

What Jenn is saying in her question...
She's heard that fiber types can change through training.


It seems as though the topic is still unresolved.

Some researchers have decided that...
Training can allow your existing fibers to better cope with whatever consistent exercise stress you create. They believe that you cannot morph one fiber type into another, but you CAN adapt to training and improve the natural abilities of your existing muscle fibers.

Other researchers have noted that...
Type IIa fibers can convert into Type IIb fibers with a well-designed program of high intensity resistance training or endurance training. AND, on the other side of the coin... Type IIa fibers can become Type I fibers in instances of deconditioning (lack of exercise, people!!) or spinal cord injuries. It goes both ways!

So, as I mentioned, the verdict is still undetermined. But we DO know that exercising - specifically the exercise that challenges your body - will always help your muscles improve.


That being said...
I must say, Jenn, that your question was a fabulous one! However, in the TONING blog post to which you are referring, I never really said that you couldn't change your muscle fiber type...

But I DID say that you can't change the SHAPE of your muscles!

You can make them bigger by adding size to your muscle fibers. You can make them smaller by failing to exercise and losing your lean muscle (and therefore, getting weaker!). But you cannot elongate your muscles, and you cannot make them short and stumpy. Unless they already are.

Thanks for the brilliant inquiry, Jenn! Hope that answers your question.

Keep 'em coming, friends!

**Remember!! Any other fitness/nutrition questions can be directed to kim@deliberatemovement.com!