Physiology for Dummies - Muscles

[Paul H]

Muscle Fiber Types

There are two types of fibers:

Type I- also known as slow twitch (ST). These fibers are aerobic, the smallest, weakest and have the slowest contraction speed. Despite their weakness, they can contract repeatedly over long periods without fatiguing and are the fibers you would uses for a long steady ride.

Type II - also known as fast twitch (FT). has two subgroups:

Type IIa- these fibers have a faster contraction speed and are approx 5 times stronger than type I fibers. Like type I, they are aerobic & Anaerobic, resistant to fatigue and can sustain a high power output for several minutes. These fibers are crucial for road racing and are responsible, for race surges, attacks, climbing hills etc. These fibers come into play when the intensity of your work is between 75% and 85% max heart rate.

Type IIb- These fibers are anaerobic and are 10 times stronger that type I. They come into play when intensity of work is in the anaerobic range lasting up to approx 15 seconds (sprints).

Training effects are seen only in the muscle fibers that are recruited.

As intensity increases, you move from Type I fibers upwards. There is not a shift from one type to the next, rather the larger stronger Type II fibers are added to help the smaller ones.

Training cannot create new fibers bit it can cause them to grow larger. Fibres can also be transformed into another. Excessive low intensity endurance training can change Type II fibers to Type I (bad) it can also cause shrinkage of all fibers.

[Marek]

Ok, back to the high intensity stuff then.

Cheers

Marek...

[Robh]

Slow-twitch, or type I, fibers (sometimes referred to as "Red") have more mitochondria, store oxygen in myoglobin, rely on aerobic metabolism, have a greater capillary to volume ratio and are associated with endurance; these produce ATP more slowly. Marathon runners tend to have more type I fibers, generally through a combination of genetics and training.

Fast-twitch, or type II, fibers (sometimes referred to as "White") have fewer mitochondria, are capable of more powerful (but shorter) contractions, metabolize ATP more quickly, have a lower capillary to volume ratio, and are more likely to accumulate lactic acid. Weightlifters and sprinters tend to have more type II fibers. Type II fibers are distinguished by their primary sub-types, IIa, IIx, and IIb, as described below.


[img]http://i161.photobucket.com/albums/t209/sbc205/muscles1.jpg[/img]