By understanding how our muscles are actually constructed, you can better understand why certain exercises and workouts work well and why some people are genetically predisposed to building muscle.
The human body is made up of many different muscles, but they can all be categorized into three main groups: cardiac, smooth, and striated.
Made up of myocytes (muscle cells or more commonly known as muscle fibers), muscles contain long rod-like structures called myofibrils, composed of different types of protein. These proteins are grouped into thin and thick portions called filaments. Your muscles are able to contract when these thick and thin portions slide along each other. When skeletal muscles contract they cause a movement at a joint. They are able to do so because they are attached to bones by tendons. While all skeletal muscles share these properties, they can be further categorized by muscle fiber type.
Beyond the cardiac, smooth, and striated, muscle fibers can also be divided by type: Type I, Type IIa, and Type IIb. These are divided based on differences in the amount of mitochondria (the powerhouse of the cell) they have, how quickly they contract, color, and other factors. 1
Individual muscles in the body are made of a mixture of different fiber types and their composition will vary depending on what the muscle is used for. For example, postural muscles (e.g. spinal muscles, hip flexors, calves) are predominantly made up of Type I fibers because they do not need to produce a lot of power and are very resistant to fatigue. Furthermore, when a muscle contracts, only the fibers that are needed will contract. If a weak contraction occurs, only the Type I muscle fibers will contract. If a strong contraction occurs that requires a lot of power (like in lifting a heavy weight), the Type IIa and IIb fibers will be activated along with the Type I fibers, with the Type IIa and IIb fibers activating last.
All of us are born with a set percentage of these muscle fibers. However, some theories claim that you can change the properties of your muscle fibers based on what type of exercises you do.
For instance, someone born with a certain amount of fast twitch and slow twitch fibers can have slow twitch muscle fibers that exhibit some characteristics of fast twitch muscle fibers through training such as sprinting or heavy weightlifting.
So, while you may not be blessed with the slow twitch muscle makeup of an Olympic marathon runner or the fast twitch fiber makeup of a sprinter, it is possible to improve your performance through proper training and hard work.