Feb. 2, 2017

Slow Twitch, Preferred Twitch: A case for type I muscle fiber
I may be the only Triathlon Coach who hears athletes hold up the idea of being predominately type II (fast twitch) muscle fiber as a badge of honor, but I doubt it. Throughout my experience in sport, slow twitch muscle fiber has carried a negative connotation. After all, fast twitch muscle fiber is sexy. Right? Truth is, triathletes are heavily reliant on, and predominantly slow twitch in muscle fiber type. Let me point out a few basic yet powerful reasons why triathletes should embrace being slow twitch muscle fiber athletes. This is, and can be a highly complex topic. We’ll take a more basic approach for this article. If you are a science nerd like me, I’ll attach some scientific literature for your reading. 

First, let’s just look at the relative nature of the word “slow” in this context. We need to be careful of such words as they must be interpreted in relation to a standard. For instance; a Porsche 911 is slow in relation to a drag car, a drag car is slow in relation to a jet, a jet is slow in relation to a discharged bullet and so on. If I asked you, “Should we consider time to peak tension within 100 milliseconds slow?” What would you say? Type I muscle fiber reaches peak contraction tension in about 90 milliseconds. That’s quick if you ask me. In relation to 40 milliseconds (fast twitch), it can be called slow. 

Next, let’s consider that with endurance training you can create an adaptation in your type II fibers to promote characteristics of type I. Basically, with training, your type II fibers will become aerobically functioning fibers. This process does not work in reverse. This means you cannot train type I fibers to function as type II. So, if you have been training for triathlon consistently for 6 weeks you have started this process of transforming your type II fibers. 

Now let’s look at 4 physiological benefits of type I fiber type for triathletes. 
1. Slow twitch fibers are extremely fatigue resistant. Fast twitch fibers fatigue quickly.
2. Slow twitch fibers have the highest capability to use fat as a substrate for energy and lowest necessity for glycogen as a substrate.
3. The diffusion distance across tissues is smaller in type I fibers. This simply means that necessary nutrients can travel into the cells of slow twitch fibers more easily. 
4. Slow twitch fibers have the highest capillary density. This is an advantage for transporting blood and necessary nutrients

Some may still be thinking, “I’m fast twitch baby!” Well here are some charts and graphs from the literature. 
These are nationally ranked swimmers who have qualified for at least 1 World Games or Olympic trials. LDS are the long-distance swimmers. Long distance in this case is 400 meters. MDS is middle distance, and SDS is short distance. You can see the trend for our purposes.
 Efficiency is measured as work accomplished per energy expended. The direct relationship between type I fibers and efficiency is shown in this graph. Triathletes strive for efficiency, especially in the cycling leg. 
These are World Class 5k runners. That means they run faster than anyone in Triathlon.
You are extreme.
Embrace your “slow” twitch muscle fibers, you need them to be “fast” triathletes.

Costill, D.L. et al. Muscle Fiber Composition and Enzyme Activities of Elite Distance Runners. Medicine and Science in Sports. 1976. 
Gerard, E.S. et al. Skeletal muscle profiles among elite long, middle, and short distance swimmers. The American Journal of Sports Medicine. 1986.
Coyle, E.F. et al. Cycling Efficiency is Related to the Percentage of Type I Muscle Fibers. Medicine and Science in Sports and Exercise. 1991.
Advanced Exercise Physiology. 2nd ed. American College of Sports Medicine. 2012. Ch.4-6.




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