To be serious about body-building you need to know about the muscles that make up your body.
Muscles are composed of fibers which are cylindrical cells about 50-100 nanometers (nanometer is one billionth of a meter) in diameter and several centimeters long. The strength of an individual fiber increases with its diameter, the greater its diameter the stronger it is. These are arranged in parallel bundles along the length of the muscle. Individuals have the same number of fibers within a particular muscle, and it is the diameter of the muscle fibers in their muscles that determines their strength.
Muscles contract when an electrical pulse is received from a nerve cell called a motor neuron attached to the muscle fiber, which responds to this electrical pulse by contracting momentarily. This short contraction and relaxation is called a twitch and lasts for 10-100 milliseconds. A single motor neuron and the fibers it stimulates are called a motor unit. One motor neuron may control from 2-2,000 muscle fibers.
For a stronger contraction another pulse must be received from the attached nerve cell before the first twitch subsides. For a sustained contraction a rapid series of pulses must be received by the muscle fiber. As a muscle fiber contracts it is constantly contracting and relaxing in response to a steady flow of electrical impulses from the nerve cell. A typical skeletal muscle has several hundred muscle fibers each with an attached nerve cell.
Most muscles contain a combination of two types of fibers:
- Slow-twitch (Type I) muscle fibers which respond more slowly and produce weaker contractions, but over a longer duration, and also have high aerobic capacity as they have a larger number of capillaries surrounding them.
- Fast-twitch (Type II) muscle fibers which respond more quickly with stronger contractions, but with a shorter duration, and also have high anaerobic capacity. These fibers are larger the slow-twitch muscles giving them the capability of stronger contractions.
Fast-twitch fibers tend to tire more quickly than slow-twitch fibers. The Fast-twitch type are used to produce bursts of intense effort and are mainly used for intense anaerobic activity, while slow-twitch type are used for endurance and are used for aerobic activity. This means that to exercise fast-twitch fibers, high intensity training is required. Slow-twitch variety are darker in color than fast-twitch fibers.
The average person has a roughly the same number of each of these two types of fibers. However, some individuals have either higher percentage of slow-twitch fibers or a higher percentage of fast-twitch fibers. Competitive rowers and long-distance runners typically have a higher percentage (70-85%) of slow-twitch fibers that enables them to perform with endurance.
Persons with a higher percentage of fast-twitch fibers tend to do better at sports such as sprinting or power lifting that need short bursts of energy.
A person cannot cannot change the relative percentage of fast-twitch to slow-twitch fibers they have in their body by training, however, they can maximize the efficiency and strength of the muscle type that they do have.
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