In order to help you understand why High Intensity Training works, I’ve decided to focus on how the human body builds muscle. This will be the first part of The Science Behind High Intensity Training article. I read Body by Science, by Doug McGuff and John Little, and also used it as my main resource for this article. Although this is a science based article, I tried to keep it readable by using as little jargon as possible. If you like this article, I highly recommend buying Body by Science as it will expand on what I’ve written here. Now let’s begin!
Type I and Type II Muscle Fibers
If you’ve ever taken a biology class, you have learned that our muscles are built up of tiny fibers. These fibers can be broken down into two main classes, slow-twitch (Type I) and fast-twitch (Type II) muscle fibers. The names of the classes have nothing to do with the speed or velocity at which the muscles fibers are activated. Instead the muscle fibers are separated and named based on their endurance capacities.
The slow-twitch (Type I) muscle fibers are known as the “endurance fibers.” These muscle fibers are almost always the first to be recruited when your body starts engaging in a physical activity. This happens because these muscle fibers require little energy to function, thus the body (which is constantly trying to conserve energy as part of its evolutionary survival mechanism) recruits them first.
Type I fibers are the dominant muscle fibers used by people who engage in long aerobic activities. Some examples of aerobic activities that rely primarily on the slow-twitch muscle fibers include marathon running, long distance cycling, and swimming. Although slow-twitch muscle fibers work great for endurance based activities, they aren’t capable of generating much force – that’s where their counterparts, the fast-twitch (Type II) muscle fibers, come in.
Fast-twitch (Type II) muscle fibers are called into action when the body needs to generate force – a force that’s greater than what the Type I muscle fibers can provide. The Type II class is made up of three distinct categories – Type IIA, Type IIAB, and Type IIB.
The Type IIA fibers are the first muscle fibers to be called into action when the central nervous system determines that the slow-twitch (Type I) fibers cannot generate the force needed for a specific action. They are more powerful than the Type I fibers, but at the same time have lower endurance capabilities than the Type I fibers. In other words, Type IIA are more powerful, but also fatigue faster than the Type I muscle fibers.
If the force generated by both the Type I and Type IIA muscle fibers is still not enough, the central nervous system recruits the Type IIAB muscle fibers. These muscle fibers are more powerful and can generate even greater force than the Type IIA muscle fibers. There is a drawback though, what they gain in power they lose in endurance. Think of Type IIAB muscle fibers as the intermediates of the Type II muscle fibers.
The third Type II muscle fibers are the Type IIB muscle fibers and are the last to be recruited. They are the strongest of all the muscle fibers, but they also fatigue the fastest, which makes their endurance capability the worse out of all muscle fiber types.
