Type 1 fibers; these are slow oxidative (slow twitch) muscle fibers that contain large amounts of myoglobin, this gives these muscle fibers their characteristic red colour. These muscle fibers are capable of carrying out aerobic exercise for hours on end, they are highly fatigue resistant.
Type IIa fibers; these are also red. They are used for long-duration anaerobic exertion typically lasting up to 30 minutes.
Type IIx fibers; these are white and are used for short-duration anaerobic exercise, typically lasting up to 5 minutes.
Type 1 fibers
These are slow twitch (also known as slow oxidative) muscle fibres that are red in colour due to their large volume of myoglobin, they contain many mitochondria and produce low power contractions. Due to their relatively high level of mitochondria and relatively low power output, these muscle fibres are specialised for low intensity endurance exertion. They are capable of maintaining their power output for hours of exercise. This type of muscle fibres is commonly found in muscles that are used very often during the day, like our postural muscles for example. They are linked to large numbers of blood vessels which supply them with a large volume of oxygen, among many other essential molecules and nutrients. The reason that so much oxygen is needed is because type I fibres split ATP by an aerobic mechanism (hence the name slow oxidative fibre). Without an adequate oxygen supply, these fibres would be unable to produce energy. Endurance runners rely heavily on these muscle fibres as they are highly resistant to fatigue.
Type IIa fibers
These are fast twitch muscle fibers, though they are the slowest of the fast twitch fibers. They contract at around 5 times the speed of slow twitch fibers. They produce energy through oxidative processes just like the type I fibers, though they have less mitochondria and have a higher power output. They are also red due to a high myoglobin content. They contain many blood vessels to supply this oxygen need and are consequently resistant to fatigue, though not as much as the type I fibers. In many ways they can be considered to possess a mixture of both type I and type IIb muscle fiber characteristics. Their main storage fuels are glycogen and creatine phosphate. However, these muscle fibers are uncommon in humans, but through training it is possible to convert type IIb fibers into type IIa fibers. Training of any kind causes type IIa fibers to be formed in the body, due to their increased efficiency at generating energy.
Type IIx fibers
These are typically referred to as type IIb fibers as they were once indistinguishable and many people recognise them as being different. Type IIx fibers are present in humans the IIb fibers are present in other animals. The IIX muscle fibers are white due to a low myoglobin content and produce energy anaerobically, therefore they contain few blood vessels and few mitochondria. THeir main storage fuel is creatine phosphate though they also contain glycogen. They are extremely fast twitch and are also known as fast glycolytic fibers. A consequence of their high power output is that they fatigue rapidly and are also inefficient in producing energy. Whenever a person trains, these fibers are converted into IIa fibers, no matter what type of training is undergone. This is because the IIA fibers are more energy efficient and the human body favours efficiency over power. For this reason, even though these fibers are most useful in sprinters, the people with the higher type IIb fiber content in their muscles are those who do barely any exercise at all.
Myoglobin
Myoglobin is an intracellular oxygen storage protein that is found in both cardiac and skeletal muscle tissue, it is the protein that is responsible for the red colouration of these muscles. It has a capacity for one oxygen molecule (O2), equivalent to two oxygen atoms. It is rare to find myoglobin in the blood stream of humans and this is typically an indicator of muscular damage. As myoglobin has a high affinity for oxygen, it is used by muscles in times of oxygen deprivation. This is because the myoglobin binds to oxygen when there are low concentrations of oxygen and only dissociates when these concentrations become very low. This means that only when there is extremely little oxygen contained within the muscles, will the myoglobin release the oxygen bound to it. Consequently, myoglobin is very beneficial during intense exercise and breath-holding.
Haemoglobin
This is an oxygen transport protein that is primarily found in red blood cells. When it is bound to oxygen molecules it has 2 forms. The first form is a tense (T) form and the second is a relaxed (R) form. The tense form is the one which is most likely to give up its oxygen to surrounding cells. The form of haemoglobin when bound to oxygen (called oxyhaemoglobin) is dependent upon the environment in which the protein finds itself. An environment contained a high concentration of carbon dioxide, low concentration of oxygen and low pH favours the tense form and conversely a low concentration of carbon dioxide, high concentration of oxygen and high pH favours the relaxed form. The tense form of oxyhaemoglobin has a lower affinity for oxygen and so releases it much more readily. This is of great benefit for the body when red blood cells pass by respiring tissue. As the tissue is using up all of the surrounding oxygen, releasing carbon dioxide, and this carbon dioxide subsequently forms carbonic acid (which lowers the pH), it follows that the oxyhaemoglobin is much more likely to dissociate from its oxygen molecules and therefore supply the tissue with oxygen and allow it to continue exerting itself for longer periods of time.
Bohr Effect
The essence of the Bohr effect is that as pH decreases and the concentration of carbon dioxide increases, the affinity of haemoglobin for oxygen decreases, under such conditions oxyhaemoglobin dissociates from its oxygen more readily. Respiring tissue releases elevated levels of carbon dioxide which are converted by carbonic anhydrase (an enzyme present in red blood cells) into carbonic acid. This acid lowers the pH of the blood. These effects compound each other to improve the oxygenation of respiring tissue. When we breathe out too much carbon dioxide, either through rapid or too-high volume-per-minute breathing, we deplete our bodies of carbon dioxide and this means that the haemoglobin in our blood remains attached to its oxygen and system-wide hypoxia may result. This happens most often when we are stressed and is most noticeable due to its associated light-headedness.
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