Breast Muscles of the Racing Pigeon

Gordon A Chalmers, DVM

Written originally in the magazine Pigeon Sport (UK), and later reprinted in the Racing Pigeon Digest (USA), a very intriguing, well-presented article on the breast muscles of pigeons by Alan Wheeldon of Britain advanced his views on the differences between the flight muscles of sprint and distance birds. Like a welcome bolt from the blue, the article, which was based on one published in Scientific American (Sept, 2000) in reference to human athletes, was certainly stimulating, as it attempted to explain the differences between sprinting strains vs distance strains of racing pigeons.

His proposal was based on a key structure in the muscle fibres, actually a contractile protein known as myosin, which is closely involved with the function of any muscle, including the powerful major breast muscles of the racing pigeon. The actual form of myosin present in any muscle fibre determines its contraction velocity, that is, its speed of operation, usually called its twitch speed. Thus, the form of myosin in the thigh muscles of humans trained for sprint events for example, is different from the form of myosin in the thigh muscles of humans who compete in endurance events. Alan Wheeldon is certainly to be complimented for his efforts, as they appeared to explain for perhaps the first time, a fundamental, illuminating difference between sprint and distance birds.

Oh, if it were only so! How beautifully this logical information would dovetail with what we would like to hope and believe are tangible differences between sprint and distance performing pigeons. Regrettably, this forward-looking article, which is remarkable for its stimulating, thought-provoking information, is based on the muscles of human and other mammals, which are significantly different from those of the great breast muscles of the pigeon.

At this juncture, I would also make the point that, in general, sprinting as we describe it in pigeons is completely different from, and therefore, in my opinion, not at all comparable to sprinting in humans and other racing mammals. For example, if a human athlete competes in a 100 meter sprint event, what flight distance (1, 2, 5, 10, 20, 30, 60, or more miles, etc.) for pigeons is actually known to be exactly comparable?? How do we (or can we) measure and compare the two in any meaningful way? I know that some fanciers like to guess about this, but in the final analysis, that is all it is - a guess.

I would further suggest that comparing sprinting humans and other racing mammals with sprinting pigeons is very much like comparing chalk and cheese - which is no comparison at all. One group competes on solid earth, at distances up to a few hundred meters, and the other competes in the air above it, at distances up to a few hundred miles, so is it truly possible to make valid comparisons? I have serious doubts about this, but maybe someone knows the answer.

Add to these points the fact that regardless of the distances of the so-called sprint/middle distance races in which our birds are entered, the birds utilise fat as the major source of fuel during these races, as they certainly do in long distance races as well. Conversely, sprinting human and other mammalian athletes utilise primarily glycogen as fuel in their races - so once again, any comparison between human sprinters and sprinting pigeons just doesn't seem to be valid at all. On a practical level, perhaps we should simply refer to so-called sprinting strains of pigeons as short/middle-distance strains, terms I will use hereinafter.

Using one of the thigh muscles of the human as a good example of a powerful muscle, we can make some basic comparisons with the great breast muscles, known as the major pectoral muscles of the racing pigeon. Recall that the major pectorals are the largest muscles in the body of the pigeon and make up 20% to over 30% of the total weight of the bird. They are the large muscles that we feel with our fingertips as we handle a bird. At the microscopic level, we can see that the large breast muscles are made up of elongated, cigar-shaped cells that, by convention, have been called fibres whose tapered ends attach to one another to make up the entire muscle. These cells are called fibres because when delicate micro-techniques are used to tease them out, they resemble fine threads or fibres.

In the breast muscle of the pigeon, there are only two types of muscle fibres, one a narrow diameter fibre, and the other, a broad-diameter fibre. In cross section, these fibres are round or oval. They are arranged in bundles, with the broad-diameter fibres on the edge of each bundle for the most part, and the narrow-diameter fibres located more deeply within each bundle. (A very rough, comparable example would be a number of thick and thin cigars tied together by a rubber band. For the most part, the thick cigars would touch the rubber band, whereas the thin cigars would be located more deeply within the whole bundle of cigars.) Many thousands of bundles of fibres, arranged end to end and beside one another, make up the entire muscle we feel with our fingers.

The broad-diameter fibres are known as white fibres, whereas the narrow- diameter fibres are known as red fibres. Red fibres far outnumber white fibres. For every white fibre, on average, there are approximately 14 red fibres. In actual repeated counts of 100 fibres at a time, Dr John George, the dean of muscle research in pigeons at the University of Guelph in Ontario, Canada, determined that between five to 14 were white fibres, and 86 to 95 were red fibres.

Why are these facts of value to us as racing pigeon fanciers? Well, the white fibres in the major breast muscles have very fast contraction velocities (that is, the speed at which they operate or twitch), ranging from 31-37 milliseconds. A millisecond is 1/1000 of a second, which means that one complete contraction or twitch of these white fibres takes a mere 31/1000 to 37/1000 of a second! At such rapid velocities, the white fibres are utilised for extremely swift, even explosive actions, such as launching from the transport truck, sudden dodging bursts of speed during flight, and braking to land, etc. - in fact, any action that causes the wings to beat faster. As well, one can obtain a further practical appreciation of the speed of these fibres by noting the rapidly trembling wing tips of a bird in top form, or one shivering on a cold day.

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.

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.

Pigeons have only two types of fibres in the major breast muscles, and according to Dr George, with whom I discussed this subject, this arrangement does not change with training as it does in humans. This is a major difference between the thigh muscles of mammals and the great breast muscles of pigeons. Note again that there is NO third type of muscle fibre in the major breast muscles of the pigeon as there is in humans, a highly important difference between the two. In fact, the slow-twitch, red Type I fibres of human and other mammalian athletes simply do not exist in the great breast muscles of the racing pigeon.

As we have seen, in pigeons, the two types of fibres in the major breast muscles are both extremely fast (fast-twitch), but the white fibres have a much faster contraction velocity than the red fibres. As both red and white fibres have very fast contraction velocities, their designations are based on a combination of: 1) their contraction velocities, 2) whether oxygen is needed in the metabolism (means "utilisation" or "breakdown") of fuel, and 3) the use of glycogen as fuel. Hence, red fibres are designated as Fast-twitch, Oxidative, Glycolytic (FOG for short). Not indicated in this designation is the highly important fact that the FOG fibres contain an abundant supply of fat, their key fuel for flights of any distance.

The white fibres are designated as Fast- twitch Glycolytic (FG for short). The word Glycolytic in both FOG and FG means that both fibres contain glycogen that is metabolised for energy. The word Oxidative in FOG refers to the required use of oxygen by these fibres in the metabolism of fat for the generation of energy for sustained flight. Incidentally, under the same system of terminology, the slow-twitch red Type I fibres in the thigh muscles of humans, horses and dogs, etc., are designated as Slow-twitch Oxidative (SO for short) fibres.

As we have seen, both red and white fibres in the breast muscles of the pigeon contain the fuel glycogen. Glycogen consists of many units of the sugar glucose linked together in a particular chemical configuration, and is, in fact, the storage form of glucose in the body. In addition to their content of glycogen, the red fibres also contain a preponderance of fat, the chief fuel for rapid, sustained flight. Over all, studies by Dr George on the breast muscles have shown that they contain about 10-14% fat and only 3.5% glycogen.

Fat in the FOG fibres is metabolised in the presence of both glucose that is likely derived from the glycogen stores, and, importantly, with the use of oxygen (hence the designation "Oxidative" in FOG) in the production of energy, to allow the wings to beat on the average of 5.4 beats per second for the duration of the flight. The white fibres contain only glycogen as fuel (thus, "glycolytic") that is metabolised in an anaerobic (an = without; aerobic = oxygen) system for the production of the energy needed for sudden, lightning-fast, even explosive bursts of speed.