All skeletal-muscle fibers do not have the same metabolic and mechanical properties.
These properties include:
- The maximum speed at which they shorten, fast and slow fibers
- The main pathway they use is to form energy (ATP; adenosine triphosphate). There are two types named oxidative and glycolytic fibers.
Fast and slow fibers differ in the way they split ATP molecules for energy. Fast fibers split ATP molecules much faster than slow fibers because they have different myosin enzymes. These myosin enzymes determine the fiber’s maximal shortening velocity. If the fibers have myosin with high ATPase (enzyme which breaks down ATP into ADP and one free phosphate group) activity, they are classified as fast fibers. Those fibers with myosin that have low ATPase activity are slow fibers.
Myosin and actin
The way the muscles contract and relax is through the movement of the proteins myosin and actin. The thinner actin filaments slide inward past the thicker myosin filaments. This is done by tiny bridges (myosin heads) extending from the myosin filaments to the surrounding actin filaments. The myosin and actin together form larger structures called myofibrils, these myofibrils give muscles a striped appearance.
Oxidative fibers
These fibers contain numerous mitochondria and have a high capacity for oxidative phosphorylation and are classified as oxidative fibers. ATP production by these types of fibers is dependent on blood flow to deliver oxygen and fuel molecules to the muscles. These muscles are surrounded by large amounts of small blood vessels. Oxidative fibers contain myoglobin which is an oxygen-binding protein and they increase the rate of oxygen diffusion within the fiber and provide a small store of oxygen.
A large amount of myoglobin present in oxidative fibers give them a dark-red appearance and is often referred to as red muscle fibers.
Glycolytic fibers
This type of fiber has only small amounts of mitochondria but possesses a high concentration of glycolytic enzymes and a large glycogen store. Glycolytic fibers are not oxygen dependent and are surrounded by relatively few blood vessels and also contain only small amounts of myoglobin. The lack of myoglobin in glycolytic fibers gives them a pale color and is referred to as white muscle fibers.
The three different skeletal-muscle fibers
On the basis of these two general properties, three types of skeletal-muscle fibers can be distinguished in our body and although all skeletal muscle is controlled by the somatic nervous system (voluntary/conscious control), and consists of the same structure, they do have many important differences.
Slow-oxidative skeletal muscle fibers, Type I
These fibers are also known as type I slow-twitch muscles and have many capillaries. This means that they have a high supply of oxygen and use aerobic respiration, contain a high supply of myoglobin and appear dark-red, use triglycerides as the main supply of fuel for the cell, and fatigue very slowly. They have a low contractile velocity and break down ATP slowly, have a small muscle fiber diameter, and therefore produce low contractile force. Slow oxidative muscles are used mainly in long-distance activities such as running a marathon or cycling. You can find these muscle fibers in our body in places that require a great deal of support for instance core muscles.
Fast-oxidative skeletal muscles fibers, Type IIa
Fast-oxidative fibers are known as fast-twitch muscles and use primarily aerobic cell respiration and contain a high amount of oxygen, myoglobin, and mitochondria. They differ from slow-oxidative fibers that they break down ATP quickly and therefore have a fast contractile ability. They have a medium diameter, which means that the contraction force is greater than in slow-oxidative muscles. The main fuel source is glycogen. Fast-oxidative muscles fatigue a bit more quickly than slow-oxidative muscles because they may switch to anaerobic cell respiration(glycolysis) and are used in middle-distance activities, such as running or swimming a 400-meter event.
Fast-glycolytic skeletal muscles fibers, Type IIb
Fast-glycolytic fibers are also called fast-twitch muscles and have a low supply of oxygen and therefore very little mitochondria. For this reason, they use anaerobic cell respiration. They contain very few myoglobin molecules and therefore appear white. The muscle fibers have a large diameter and therefore produce a large contractile force. They break down ATP quickly and therefore contract quickly. Since their main source of energy is glycogen, these muscles fatigue really quickly and are therefore used in weightlifting or sprinting. Glycogen is the stored form of glucose and is a multi-branched polysaccharide.
Conclusion
The amount of thick and thin filaments per unit of cross-sectional area is more or less the same in all types of skeletal muscle fibers. This means that the larger the diameter of the muscle fiber the more thick and thin filaments it will contain working together in parallel to produce force. The result will be an increase in maximum tension and thus greater strength.
The muscles of the back and legs which are required to maintain their activity for long periods of time without fatigue while they are supporting an upright position consist of large numbers of slow-oxidative and fast-oxidative fibers. On the other side of the spectrum, the muscles in the arms may be required to produce large amounts of tension over a short period of time like lifting heavy objects. For this reason, muscles in the arms have a greater proportion of fast-glycolytic fibers.
The kind of exercise that you prefer will have a direct impact on the way that you are built. When you are doing mostly weight training your fast-oxidative muscle fibers will transform into fast glycolytic muscle fibers with a larger diameter resulting in bigger muscles. In contrast, doing more endurance-type exercises like swimming, cycling or running will convert fast-glycolytic fibers into fast-oxidative fibers making you look more lean.
