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Now, because they twitch so quickly, white fibres also become exhausted very rapidly, and for this reason, could not be expected to handle sustained flight, but instead, they deal with sudden, even explosive emergency flight.

Although the red fibres also have very fast contraction velocities ranging from 47 to 62 milliseconds -- which means they complete one contraction or twitch in 47/1000 to 62/1000 of a second -- they are obviously not quite so fast as the white fibres, and as a result, they become exhausted much more slowly than the white fibres. Hence, their chief function is related to rapid, prolonged flight over the few to many miles of the training toss or race, a major point of importance to us as pigeon fanciers.

In the powerful thigh muscles of racing humans and other mammals such as horses and dogs, there are two basic types of fibres, consisting of Type I and Type II fibres, but the Type II fibres are further subdivided into Types IIa and IIx, for a total of three types of fibres - note this basic and important difference from pigeons, which, to repeat a pivotal point, have only two types of fibres. in the great breast muscles. Now, Type I fibres in humans and other mammals are red, and the two forms of Type II fibres are white.

Each of these three designations is based on the particular form of myosin present, and in turn, it determines the contraction velocity of each type of fibre. Thus, Type I fibres in humans have relatively slow contraction velocities, they tire slowly, and so, are known as slow-twitch fibres. These fibres dominate the thigh muscles of those athletes who compete in marathon events.

The two forms of Type II fibres in humans have very fast-contraction velocities, they tire quickly, and are known as fast-twitch fibres. They are the dominant fibre types in the thigh muscles of humans who compete in races such as a 100-meter track event. It is known that, with appropriate training, the number of fast Type IIx fibres decreases, as they become converted to Type IIa. In humans, the maximum contraction velocity of the fast Type IIx fibres is very fast indeed, and in fact, is 10 times the contraction velocity of the slower Type I fibres. The contraction velocity of the Type IIa fibres lies somewhere between these two extremes.

For interest, Dr George has listed the percentage of slow Type I and fast Type II fibres (Types IIa and IIx combined) in the thigh muscles of each of the following species: Humans - in Sprinters, the content of Type I fibres is 24%, Type II fibres - 76%; in Elite Marathon runners, Type I fibres - 79%, Type II fibres - 21%; in Middle Distance runners, Type I fibres - 62%, Type II fibres - 38%. The average human has 53% Type I and 47% Type II fibres. Horses - in Quarter horses, Type I fibres - 7%, Type II fibres - 93%; in Thoroughbreds, Type I fibres - 12%, Type II fibres - 88%; in Heavy Hunters, Type I fibres - 31%, Type II fibres- 69%. Dogs - in Greyhounds, Type I fibres - 3%, Type II fibres - 97%; in Crossbred dogs, Type I fibres - 31%, Type II fibres- 69%. Note that for sprinting animals, including humans, the percentage of Type II fibres is very high, whereas for marathon runners, the percentage of Type I fibres is very high.

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