The Science Behind High Intensity Training continued…

Motor Units

Now each group of muscle fibers work in association with one nerve and together they make up a motor unit. The motor unit can be a Type I or a Type II motor unit. The type of motor unit formed depends on the muscle fibers that make it up. So a Type IIAB motor unit would be made up of Type IIAB muscle fibers.

The main difference between the motor units is that the Type I motor unit has only 100 muscle fibers connected to it, while the Type II motor unit has around 10,000 muscle fibers connected to it. Since they are more compact, the Type I motor units are more abundant than the Type II motor units. In fact, the Type I motor units make up the majority of any given muscle.

Regardless of the type, the motor units are the ones that are recruited when you want to perform an activity. The two types can be recruited at the same time or sequentially. The same time scenario is not what we are looking for. It leaves most of the Type I and quite a few of the Type II muscle fibers unstimulated. How?

Imagine yourself going to the gym and picking a weight that is too heavy for you to lift. You try lifting it and can only perform a single repetition – you fail when attempting a second one. If you read article one, you’ll remember that your central nervous system quickly assesses which type of fibers it needs to recruit to help your body perform an action. Due to the heavy weight you are attempting to lift, your central nervous system recruits all the available motor units at once. So Type I, Type IIA, Type IIAB, and Type IIB are all called upon in the same instant.

This is where the problem comes in. As I stated in article one, the Type IIB muscle fibers are the ones with the most power, but they are also the ones that fatigue the fastest. So even though they will be worked thoroughly and fatigued, the majority of your muscle fibers – the Type I, some of the Type IIA and a few of the Type IIAB – will not have had a chance to be thoroughly fatigued because they fatigue slower than the Type IIB muscle fibers. So how do you fatigue all the different types of muscle fibers in order to get them all to grow?

Sequential Recruitment

Sequential recruitment solves the problem by fatiguing the muscles in a progressive order, starting with the Type I muscle fibers and working to the Type IIB muscle fibers. This is done by performing one exercise under a moderate load over a continued period of time. Instead of picking the heaviest weight and then trying do to one rep, you pick a medium weight and proceed to do anywhere from 5-9 reps under a constant load. This guarantees that the Type I muscle fibers will go into action first. Once they fatigue, the Type IIA muscle fibers kick in to help you continue, after they fail, the Type IIAB, and then finally the Type IIB. Sequential recruitment is part of the reason each rep is performed with a 10 second cadence (5 on the positive and 5 on the negative side of the exercise).

Ideally, you are aiming for a time frame of 50-80 seconds to do one set of one exercise. This gives you plenty of time to progressively fatigue both the Type I and Type II muscle fibers, while at the same time keeping the time interval short enough so that Type I muscle fibers don’t recover. By keeping the time frame short, you keep the Type I muscle fibers from jumping back into the contraction. You don’t want them to recover because that will keep you from reaching the higher-order muscle fibers.

continued…

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.

continued…