Slow and Fast twitch

Muscle Fibre typesTopic 4

Types of Muscle Fibers:

A. Slow Twitch: (type 1)

*smaller in diameter

*reddish color

*use aerobic respiration for ATP supply

*contain more mitochondria

*fire slowly, but take long to fatigue.

Slow Twitch

• The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time.

• They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.

Fast Twitch: used for short explosive

movements, stop and go sports.

Type IIA:

*large diameter

*white in color

*less mitochondria

*uses both anaerobic and aerobic energy transfer

Type IIB:

*same physical characteristics as Type IIA, but strictly uses the glycolytic anaerobic system.

Fast Twitch: used for short explosive

movements, stop and go sports.

• Fast twitch fibers are responsible for the speed of muscular contraction, and a fast twitch response is the ability of a muscle to rapidly contract to a specific distance over a short period of time.

• Therefore, any training program that conditions your muscles to go from a state of complete relaxation to an immediate state of contraction is a speed training program.

Fast Twitch: used for short explosive

movements, stop and go sports.

• Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly.

• Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly.

• Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.

Mitochondria

• Mitochondria are energy-producing cells, which create adenosine triphosphate or ATP for short.

• ATP is your body's essential chemical fuel and is necessary for all energetic reactions. Slow-twitch fibers have a high number of mitochondria, which provide them with an almost limitless amount of energy.

• Combined with their high blood supply, slow-twitch muscle fibers are ideally suited to long, endurance-type activities.

• On the other hand, fast-twitch fibers have considerably fewer mitochondria and, as a result, fatigue far quicker than their slow-twitch counterparts.

Blood Supply- Slow Twitch

• Slow-twitch fibers have a very good blood supply -- so much so that they are often referred to as red fibers.

• This plentiful supply of blood ensures that slow-twitch fibers receive a large amount of oxygen, which allows them to work for a long time before becoming fatigued

Blood Supply- Fast Twitch

• In contrast, fast-twitch fibers have a relatively poor blood supply and are subsequently referred to as being white in color.

• The lack of blood results in relative oxygen restriction, so fast-twitch fibers tend to fatigue much faster than the better oxygenated slow-twitch fibers.

Slow Twitch- Diameter and Force

Production

• Slow-twitch fibers are much smaller in diameter and are far less powerful.

• While they cannot generate large amounts of force, they can generate low amounts of force for long periods of time.

• They have very little potential for hypertrophy, and are best suited to endurance activities such as walking and cycling long distances.

Fast Twitch- Diameter and Force

Production

• Fast-twitch muscle fibers have a larger diameter than slow-twitch fibers. They also have the greatest hypertrophy (growth potential).

• The larger the cross-sectional size of a muscle fiber, the greater the amount of force it will be able to produce.

• Fast-twitch fibers are best suited to high-intensity, but short-duration activities such as lifting heavy weights or sprinting.

Slow and Fast Twitch

Preferred Fuel Source

• Slow-twitch muscle fibers rely on oxygen as their main energy source.

• Fast-twitch muscle fibers rely on ATP and glycogen as their main energy source.

• As ATP sources can be rapidly depleted, lactic acid is a by product of the breakdown of glycogen, fast-twitch fibers can only be active for a short period of time.