Muscle Structure and Fiber Types

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Muscles may appear as simple structures from a distance to the naked eye. However, when looking deep inside them using a microscope, you can observe highly sophisticated and complex structures. As a bodybuilder, it is helpful to learn some basic anatomy of the muscles you are training on a regular basis, as it makes you better aware of the insides of your body. And, it can make you sound like the smartest person at the gym.

Either way you look at it, it's a win-win.

Structure of Muscles

Muscles are attached to bones by tendons, which are connective tissues found at the ends of the muscle belly, or actual muscle. Within the muscle belly are fascicles (bundles) of many muscle fibers (muscle cells). Surrounding each individual muscle fiber is the sarcolemma (cell membrane) and an overlying basal lamina (basement membrane).

Throughout the sarcolemma are invaginations known as transverse tubules (T tubules), which extend into the sarcoplasm (cytoplasm) of a muscle fiber. These T tubules form a network covered by a membrane known as the sarcoplasmic reticulum. There are also several mitochondria throughout the 
sarcolemma where energy in the form of adenosine-5’-triphosphate (ATP) is produced.

Within each muscle fiber are structures known as sarcomeres (muscle units). These sarcomeres are made up of thick and thin myofibrils (contractile filaments), along with cytoskeletal proteins. Every sarcomere is defined by a Z line, which acts as a skeleton from one end of a sarcomere to the next.

Within the Z lines are alternating bands of thick and thin filaments, which provide the myofibrils with a striated appearance of light and dark bands, hence why skeletal muscles are called striated muscles.

The A (dark) band is the central region of a sarcomere consisting of an area of overlapping thick and thin filaments (zone of overlap), and an area of only thick filaments (H zone). The thick filaments are made up of the proteins myosin and titin. The I (light) band is the region that extends from the A band of a sarcomere to the A band of the neighboring sarcomere. This region consists of only the thin filaments, which are made up of the proteins actin, troponin, tropomyosin, and nebulin.

Types of Muscle Fibers

There are two main types of muscle fibers found in each muscle throughout the body, at about a 50-50 ratio, in most cases. First, there's the type I (slow-twitch oxidative) fibers, which are primarily activated during aerobic activity, such as long distance running. These fibers contain plenty of mitochondria, so they are highly resistant to fatigue. However, they have a lower number of contractile proteins, therefore generating a low amount of power. The major storage fuel in type I muscle fibers are triglycerides, which consist of three fatty acids attached to a glycerol backbone.

The type II (fast-twitch) muscle fibers are further classified into two types. The type IIa (fast-twitch oxidative glycolytic) fibers are used during both aerobic and anaerobic activities such as circuit resistance training. They have an intermediate resistance to fatigue and intermediate level of power, which are due to the intermediate number of mitochondria and contractile proteins respectively. Type IIa fibers contain stored fuel mainly in the form of creatine phosphate and glycogen, which is your body's stored form of glucose.

The type IIb (fast-twitch glycolytic) fibers are primarily used during anaerobic activity such as resistance training. These fibers comprise little mitochondria, so they have a low resistance to fatigue. On the other hand, they contain a higher number of contractile proteins, thus producing a high amount of power. The main storage fuel in type IIb muscle fibers are ATP and creatine phosphate.
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