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Muscle Fiber Types & The Four Energy Systems!

Learn All About Type I, Type IIa & Type IIb Muscle Fibers & How The Body Supplies Them Energy To Train Longer & HARDER!

Posted by GT_turbo - March 21st, 2014

As a true Hermanite, you should know how your body works and what is happening during training. In this article I will concentrate on some basics of anatomy and energy systems during a workout. Whether you are a gym freak, an endurance athlete or just want to lose some fat, there are similar principles of training that can be applied to all. But there is not just one way to reach your goal. The human body is a very adaptable and versatile organism. We all differ in genetics, body constitution, metabolism, regeneration capabilities, lifestyle and wishes; but understanding some of the basics can drive you faster and more efficient to your goal. Regardless to all knowledge, advices and advertisements, you should always listen to your body, experiment and try to find a way which works best for you.

What Are The Types of Muscle Fibers In The Body & How Do They Relate to Training?

The first topic I would like to discuss are the three types of muscle tissue in the body.

  1. Cardiac
  2. Smooth
  3. Skeletal

Cardiac muscle tissue is the most important. As you have probably already guessed, it is the muscle tissue our heart is made of.

Smooth muscle tissue makes up all the muscles which we cannot contract by our own will. Those muscles take care of our bodily functions such as the digestion tract.

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Now onto the muscle type we are all the most interested in, skeletal!

The primary mission of our skeletal muscle tissue is to give us ability to move and support our skeleton so we can maintain our posture. They are the main consumer of energy and at the same time also a storehouse for energy and water. One could say that the secondary task of these muscles is to give shape to the body to drive our ego and self-confidence.

A very important thing you should know about skeletal muscle tissue, whether a strength or endurance athlete, are the three types. These types are slow twitch fibers, called Type I, and fast twitch fibers, called Type IIa and Type IIb. Each of the three types differ in few characteristics. The main differences are the fuel they burn, contraction speed, aerobic and anaerobic capacity, fatigue resistance, storage capabilities and growth capabilities.

The ratio of each muscle type is similar in all humans, however there are small differences here and there due to genetics. Some people have slightly more Type II muscle fibers, explosive athletes & sprinters, while others have more Type I muscle fibers such as marathon runners. It is possible to specifically target and improve your Type II or Type I muscle, but when it comes to the top professional level you will be limited by your genetics.

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In general, it is very common that the majority of your lower body muscles to be mainly made up of Type I muscle fibers since they have to be able to carry us around all day long. Consequently we have more Type II muscle fibers in our upper body muscles. For instance, calf muscles have around 75% Type I muscle fibers and that is why they are a problem area to grow for most weightlifters unless they are the lucky few who are genetically blessed with great calves. In contrast, biceps have around 55% Type II muscle fibers and triceps contain around 60%. So when your calves don't grow in comparison to your arms, blame genetics and muscle fiber types.

Let's take a closer look at the basic description of Type I and Type II muscle fibers. Type I fibers are involved in endurance activities and primarily burn fats for fuel. Keep in mind that oxygen is a key ingredient when burning fats and as such is high in supply during aerobic conditions. For this reason they are surrounded with an abundance of capillaries, which deliver oxygen to muscles, and have a high aerobic capacity and a low anaerobic capacity.

Type I muscle fibers also have a high resistance to fatigue as well as a slow contraction speed. They are also only mainly recruited in low/moderate intensities and have a lower glycogen storage capacity.

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In contrast to Type I muscle fibers, Type IIb are totally the opposite. They are activated in mainly anaerobic conditions and burn almost exclusively the glucose stored in your muscle; therefore their glycogen storage capacity is very high. Type IIb have a faster contraction speed and a higher anaerobic capacity as well. However, because they burn glycogen in anaerobic conditions, where lactic acid is a byproduct, they fatigue much quicker than Type I muscle fibers.

Type IIa is would be considered a transitional muscle fiber. They still have more Type II characteristics when compared to Type I such as a fast contraction speed and high anaerobic capacity. But the difference is that they also have a moderate/high aerobic capacity, burning glucose for fuel. This allows Type IIa muscle fibers to have a higher resistance to fatigue in both aerobic and anaerobic conditions.

It is very difficult to convert Type I muscle fibers into Type IIb and vice versa. However, the development of Type IIa muscle fibers are dependent on training stimulus and adapt to the most common load when training; strength, endurance etc.

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Macronutrients- How They Energize Your Body

As this article is not diet oriented, I will focus mainly on their role in our energy system. But before describing the function and metabolism of carbohydrates, proteins and fats, let us take a look at what the actual fuel is our cells require and the different energy metabolic systems.

Cells as our body’s “engine” are producing energy with ATP (adenosine triphosphate). When ATP is “burned”, energy is released and “exhaust gases” like ADP (adenosine diphosphate) and phosphate are produced. When there is CP (creatine phosphate) in store, ADP + CP form a new ATP molecule and this can be used again for producing energy. However, regardless of the fuel’s source, it is created through a series of chemical reactions and is converted to ATP, which is needed for producing energy.

There are four different energy metabolic systems, or ways in which energy is produced, that exist in the human body.

  1. Phosphocreatine system: This is an anaerobic production of ATP from stored creatine phosphate (CP). CP stores are permanent but limited and after exhaustion need a few minutes for restoration. This means energy from this source is immediate, for very high intensity and last for short duration, up to 10 seconds. After 10 seconds the drop in your workout intensity will be significant. The best case of usage of this type of energy system is the 100m sprint.

  2. Anaerobic glycolysis: This is a process where glucose is broken down into ATP. Usually glucose is first broken down from stored glycogen, but when glycogen stores are empty, glucose from blood is taken. Since there is no oxygen involved in this process, “burning” is incomplete, less ATP is formed and lactic acid is a byproduct. Lactic acid causes muscle fatigue and when its concentration is high enough, you are unable to continue exercising. However, once the intensity is dropped, lactic acid is cleared from your muscles. This energy system can provide energy for up to 2 minutes, depending on intensity, glycogen availability and anaerobic capacity.

  3. Aerobic glycolysis: Unlike anaerobic glycolysis, this energy system has enough oxygen for the complete “burning” process. Consequently more ATP is formed from the same amount of glucose when compared to anaerobic glycolysis and lactic acid is not formed as a by-product. This system also provides energy for high intensity until enough oxygen is delivered in muscle. Capable intensity is defined with your aerobic capacity. Therefore this system can provide energy until exhaustion of glycogen stores if oxygen supply is sufficient.

  4. Oxygen/aerobic system: This system burns carbohydrates and fats as fuel and is the most efficient, but works only at low/moderate intensities. If burning fats as fuel, its duration is virtually unlimited. However, it is important to know that fats are not used immediately as fuel. Your body’s “engine” must be preheated to be able to ignite fats as fuel. Usage of fats as direct fuel can vary as much as 20-60 minutes after start of workout depending on your fitness and training level. When you train utilizing endurance activities more you train your body, muscles and nervous system to shift to an aerobic system quicker. To put this simply, if you consistently jog at a steady pace for cardio, when your body recognizes you are jogging again it says, “Ok, we can now safely shift to an aerobic fat system.” However, when playing a sport such as basketball it will not be as obvious for your body to recognize what is happening as you are not consistently moving around at the same pace.

Now that you have a clearer understanding of the body’s energy systems I would like to take a closer look at macronutrients. As mentioned earlier, we are only going to focus on energy capacities and metabolism of nutrients and not the other tasks and roles they might play in your diet. Carbohydrates and proteins provide 4 kcal and fats provide 9 kcal per 1 gram. While all three nutrients can be used as fuel, which we discussed above in the descriptions of the different energy systems, keep in mind that this fuel will be converted into glucose or fat and finally to ATP one way or another. With that being said, carbohydrates are the most reasonable choice for providing energy, followed by fats and proteins.

Carbohydrates are a staple in the diet of endurance athletes, but have a mystery prefix for strength athletes as they are claimed to be “fat-makers”. Carbohydrates are all made by the same main building block, this is glucose. Regarding to the structure and number of connected glucose molecules, carbohydrates are divided between simple and complex carbohydrates. Simple carbohydrates are monosaccharides (1 glucose molecule) and disaccharides (2 glucose molecules). Some samples of simple sugars are glucose, fructose and lactose. Since they are in simple form, they are digested fast and quickly released into the bloodstream. This means that when we consume them in large quantities they cause a rise of blood sugar resulting in insulin production. We will discuss more about that a later.

Complex carbohydrates are further divided into two main groups: oligosaccharides (3-20 glucose molecules) and polysaccharides (20+ glucose molecules). Polysaccharides are also known as starch carbohydrates. As a more complex form than simple carbohydrates, complex carbohydrates need more time to be digested, broken down and released into the bloodstream. However, because they take longer to break down, they do not cause a rise in blood sugar and consequently do not trigger insulin production. They also provide a long term energy source due to that they are released into the bloodstream at a slower rate.

The speed of which glucose is released into the bloodstream is measured by the glycemic index (GI). The higher it is, the faster it will be released. The highest number on the index is 100 and represents 50g of pure glucose. Complex carbohydrates are lower on the GI and resulting in glucose being released much slower into the blood and thus have less of an impact on blood sugar levels. Since it is very rare that we would consume pure carbohydrates, the “glycemic load” was also introduced. It is also measured on grade of 0 - 100 and again the lower the value the slower the release into the blood.

Glycemic load depends on GI of nutrient, quantity of consumed carbohydrates and other consumed nutrients. Proteins and fats will lower the glycemic load as you consume carbohydrates. This means that if you consume a high GI carbohydrate along with some proteins and/or fats, the glycemic load will be lower, glucose released into the blood will be slower and the impact on your blood sugar level will be smaller when compared to consuming the pure carbohydrate by itself.

When a carbohydrate is digested and released as glucose into the bloodstream, it represents blood sugar and flows around the body as an available free energy source for muscles or the nervous system. When blood sugar levels are normal and more glucose is released in blood, this surplus must be either used or stored. If not used, the glucose is stored in the liver and muscles where it is then converted to glycogen. Glycogen is complex carbohydrate with 20+ glucose molecules. The liver can store 70-100g of glycogen and this reserve is mainly intended for providing energy to the vital organs when blood sugar levels happen to drop.

Muscles are our biggest storehouse for glycogen and their storage capacity depends on your current fitness level and of course the amount of muscle you have. Generally speaking 375 - 500g of glycogen, or 1.500 to 2.000 kcal, can be stored in the muscles. With that in mind, you should note that every gram of glycogen bonds 2 - 3 grams of water. This is why your weight can vary a lot day-by-day; because of glycogen storage.

When glycogen storages are full and glucose is still released in blood, the excess is converted in fats – triglycerides – and stored in fat cells in the muscles or as under skin fat. Remember, fats are burned as direct fuel in the oxygen/aerobic system, which is when your intensity level is low/moderate.

How Proteins Are Used For Providing Energy

Proteins are meant as a last choice / backup system for providing energy. Proteins are made of amino acids and every time you consume proteins, digestion dismantles the protein into basic amino acids. Those amino acids are stored in the so called “amino acid pool” where they wait to be used.

Since they need about three times more energy to be digested in comparison to carbohydrates, they are not the best solution for providing energy to the body. Proteins, or more precisely some amino acids, are used for energy when glycogen stores are empty. If this happens your body will search for free amino acids and if there are none, they are taken from your muscles. Meaning that your muscles are dismantled and muscle mass is decreased.

Necessary amino acids are transported to liver, where they are converted to glucose and this glucose is released into bloodstream for providing energy. A disadvantage of proteins being used as an energy source is that they are not stored like glycogen or fats. They are used only when there are no more other energy sources, but this means you are more than likely already in a bad place as if you are eating correctly you should not run into this problem.

One more thing about protein metabolism is very important. As they are the only macronutrient with nitrogen in their chemical structure, which is during metabolic processes released in blood flow, it has to be flushed from our bloodstream when protein intake is high. This is because nitrogen in high concentration can be toxic. It is flushed with urine, but high amount of water is necessary to flush it through the entire bloodstream. If you are not hydrating enough, especially when on a high protein diet, you will become dehydrated and you performance will be greatly reduced.

This article was a basic insight to “how it works”. Some things are as simple as they were written, where some subjects are much more complicated and therefore beyond limits of this article. When you define your sport goals, your needs and your diet, keep these things in mind and apply them for a much easier, efficient and more rational achievement of your desired goals.

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Herbs & Fitness!

Hello again, Hermanites! It's Matt again, and here we'll to go over how to integrate herbs into our fitness diets. Earlier we...


awesome article


Damn you genetics for ruining my calevs!


YEA! Interesting read Gregor. Just went through it. Accurate information right there.