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exercise physiology

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Muscle Fibers

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Muscle Fibers

Muscle fibers are made up of three different types. The types are classified according to their cellular make up, contractile properties, and their biochemical make up. The fibers are divided into fast twitch fibers and slow twitch fibers. The majority of muscle groups in the body are composed of an equal mixture of slow and fast fibers 50%/50%. However, in the athlete these ratios change. In a tailback that is fast and explosive, we may see a mixture of 70% fast twitch fibers and 30 percent slow twitch fibers. In a cross-country runner we may see the opposite because they are slow and not explosive, we may see a 75% slow to 25% mixture. The mixture of fibers will greatly depict the ability of the athlete. This mixture of muscle fibers is part of an abundance of factors that make up talent. If you flipped the roles of the two above examples, the tailback would not have the muscle fiber ratio adequate to be as successful as a cross-country runner, and on the football field the cross country runner would not survive.

It is important to mention the contractile properties of the muscle fiber types. Each has three main performance characteristics. They are as follows: maximum force production, speed of muscle contraction, and muscle fiber efficiency. Maximum force production is defined as the amount of force produced per cross sectional area or size of the muscle fiber. Speed of contraction is the ability of the muscle to contract at either a high speed or slow speed. Finally, a muscle fiber must be able to make use of its energy efficiently. This knowledge of muscle fibers will help you understand why athletes are shaped and built the way they are.

The classic story of the race between the tortise and the hare is an excellent way to think of muscle fibers. The hare is made up of fast twitch muscle fibers and can run fast. The turtle is made up of slow twitch muscle fibers and runs very slow. What the hare did not know, was that the tortes had muscles that where built for endurance. He had the right force production, speed of contraction, and efficient use of his energy systems to make him endure the race. The hare did not have these endurance properties. He simply was not built for endurance; he was built for short burst of speed. Thus, towards the end of the race when the hare became tired, the tortise passed him up and won the race. This is a great way to think of muscle fibers and their influence on performance. Depending on your sport, it is good to be the hare or the tortise.

Slow Twitch Fibers
Slow twitch fibers are what are known as type I fibers, slow twitch fibers, or slow oxidative fibers. Slow oxidative refers to the biochemical make up of the fiber. Slow fibers have more mitochondria and are more equipped to handle the third energy system (oxidative phosphoralation). This energy system is slower at producing ATP than the others, however it can sustain energy production for longer periods of time. The slow twitch fibers are also smaller fibers. That is, the diameter of the fiber is smaller than the fast twitch fiber. The smaller fiber size is one reason why you see that endurance athletes are generally small and thin. Therefore, you can see why an endurance athlete is more apt to have a higher ratio of slow twitch fibers. These are small which makes the athlete light weight, and they have the proper biochemical and cellular make up to sustain endurance activities.

Fast Twitch Fibers
There are two types of fast twitch fibers. The two types are type IIa and type IIb. Type IIb fibers are considered to be fast glycolytic. These fibers are prone to use glycolysis and the phosphagen system for their primary energy source. The fast twitch fibers are the largest in diameter. They are primarily used for power movements. The second type of fiber is the type IIa fiber and is considered the intermediate fiber. This muscle fiber is in between a slow oxidative and a fast glycolytic fiber. It carries both types of biochemical make up. In general, the fast twitch fibers are considered to be anaerobic fibers, the slow fibers are considered to be aerobic.

Fiber Adaptability
Muscle fibers have the ability to adapt. With different types of training, it has been found that the biochemical make up of the fiber changes. Endurance training has been found to produce more mitochondria in the type I and type IIa fibers. If you remember, the mitochondria are the parts of the cell were aerobic metabolism happens. Thus endurance training which is an aerobic type training, can make the type I and type IIa fibers more efficient at endurance exercise. In contrast, a tailback in football, can through strength and power training, cause his stores of PC (phosphocreatine) in the muscle fiber to increase. This helps to make more efficient use of the phosphagen system within the type IIb and type IIa fibers.

If you train and it induces a change in the fiber, what happens when you don’t train? When an athlete does not train, it is called detraining. Detraining is very important in athletics. It is a huge aspect of injury rehabilitation. A lot of times athletes are at the pinnacle of being in shape and then they get injured. Soon after the injury, if they do not continue their training, they start to lose these biochemical efficiencies within the muscle. Then once the injury is healed they have to spend even more time trying to get back in shape.

Now, you may be able to see why athletes all look different depending on the sport in which they are playing. Tailbacks are powerful and strong. They are usually lean and have a high percentage of fast twitch fibers. Cross-country runners are usually small, and thin and are composed of the smaller more oxidative fibers.

Muscle Fibers

Muscle Physiology

Neuromuscular

Properties of Skeletal Muscle

BIORES Muscle Fibers