With Every Fiber of Their Being

With the recent Athens Olympics as our inspiration, we might all be pondering what it takes to go "faster, higher, and stronger." Whether you're a human, a hamster, or a horse, the answer, at least in part, is muscle fibers--each of which holds within it a certain potential for athletic performance.

When muscle fibers work together, they can provide the impetus for explosive forward or upward motion, steady exertion over long periods, or more commonly some combination of all of these types of movement. As we unlock the secrets of muscle structure, we begin to see how different types of fibers function, how much a horse's performance might be predetermined by the distribution of muscle fibers he's born with, and even how we might be able to influence the proportions of various muscle fibers, and their efficiency, through training.

Muscle Fiber Basics

According to Ray Geor, BVSc, PhD, Dipl. ACVIM, an associate professor in the Biomedical Sciences Department at the Ontario Veterinary College in the University of Guelph, 90% of a skeletal muscle is made up of muscle fibers (sometimes called myofibrils). These are elongated cells with tapered ends that are bundled parallel to each other. The rest of the muscle composition is largely nerves and capillaries, which keep the cells supplied with nutrients.

Myofibers are not all created equal. Some are designed to support long, slow, distance-type activities, while others are designed for short-term, maximum-intensity effort, and some are even adaptable depending on how the horse is trained. The more we delve into the physiological and biochemical differences between these fiber types, the more complex the picture grows.

A few decades ago, muscle fibers were described mostly as "red" and "white," or as slow-twitch and fast-twitch, respectively. A slow-twitch muscle fiber has an affinity for fat storage, a moderate ability to store glycogen (an energy source molecule converted from carbohydrates), and the capacity to deplete glycogen quickly with exercise. Slow-twitch muscles have the least strength of the muscle fiber types, but because they utilize oxygen well and store fat well, they can support prolonged muscular activity with minimal fatigue. Since they're capillary-rich, they appear "red" to the naked eye.

A fast-twitch muscle fiber is generally thicker, but it has about half the capillary content of a slow-twitch fiber, making it appear white on visual examination. Fast-twitch fibers have high strength, but low oxidative capacity compared with slow-twitch fibers, and fast-twitch fibers are better at supporting athletic tasks requiring explosive speed or power.

But it soon became clear to researchers that there was, in fact, more than one kind of fast-twitch fiber. Some seemed to have a better capacity for storing glycogen, others for storing fats. So, they were soon re-categorized as "fast-twitch, high oxidative" fibers (Type IIA) and "fast-twitch, low oxidative" fibers (Type IIB). Slow-twitch fibers were designated Type I fibers, a description that is still in use.

According to Michael Lindinger, PhD, a comparative zoologist and colleague of Geor's at the University of Guelph, recent research (much of it compiled by Jose Luis Rivero, BVSc, MS, of the University of Cordoba, Spain) has complicated the picture even further. "We now recognize that Type IIA and IIB are a continuum. There are intermediate fiber types, based on staining characteristics, and more and more, it looks as if it's probably incorrect to categorize them at all. But of course, we like to label things and put them in neat categories!"

Geor notes, "We're using techniques that are more sensitive than were available a few years ago, and this is revealing changes that weren't visible to us in the 1970s." He added that most researchers now classify equine muscle fibers into five different types. "Three of those are regarded as 'pure,' " he says. "The other two are hybrids, sort of a mixture."

To add to the confusion, there's more than one nomenclature for describing the types. Virtually everyone agrees that slow-twitch fibers are designated Type I, but thereafter the picture gets fuzzier. There are Type IIA/d fibers (also known as IIA/x) as well as Type IIa/D fibers (aka IIa/X), as well as Type IID (or Type IIX). The Type IID is what used to be known as Type IIB. Geor adds that there is a hybrid Type I/IIA fiber that is described as "slow to moderately fast twitch."

The differences, of course, are subtle and based on a number of factors, including:

  1. The activity of a large muscle protein called myosin heavy-chain, or MyHC (myosin and actin being the two main proteins making up the contractile apparatus of a muscle);
  2. The ability and rate at which the muscle fiber can break down ATP (adenosine triphosphate) molecules to release energy;
  3. The ability of the fiber to break down certain aerobic enzymes (those active in the presence of oxygen);
  4. The number of capillaries that perfuse the muscle;
  5. The size of the muscle (the more oxidative the muscle is, the smaller it tends to be and the more capillaries it has); and
  6. The number of mitochondria in the fiber--a higher number of mitochondria usually indicates a higher aerobic capacity.

What's most significant is that some of the Type II fibers can shift their composition and ability in response to training.

"The ability of the muscle fiber to change is genetically coded into them," Lindinger explains, "and training signals will turn those genes on or off. That's what allows humans or horses to improve with conditioning."

As an example, in one of several studies published by Rivero in the Equine Veterinary Journal in 1999, Andalusian mares that underwent carriage training for eight months showed an increased percentage of Type I fibers in their muscling and a corresponding decrease in Type IIX and IIa/X fibers. These results were interpreted to mean that the mares had developed a "reduction in the velocity of the shortening of the muscle, but an increase in fatigue resistance."

Lindinger notes that the process is reversible, so when a horse is allowed to go from fit performance animal to pasture potato, his muscle fibers revert to their original configuration.

"We know that with endurance training, there's an increase in Type IIA and a reduction in Type IIX fibers," says Geor. "But there's still a lot we've got to learn about how training changes the proportion of the various fiber types. For example, there hasn't been a whole lot of work done on sprint training and how it influences the fibers. We suspect the changes aren't huge, but there just haven't been enough studies done comparing different types of training, so it's difficult to draw any conclusions."

Training induces other changes in muscles beyond the shift in fiber types. "There is an increase in capillary density, which gives the muscle enhanced oxygen delivery, and the number of mitochondria per unit area may increase, indicating that the fibers are adapting to the training stimulus," Geor says. "More mitochondria equals more machinery for the oxygenation of tissues, which equals an improvement in the total aerobic capacity of the muscle."

Training isn't the only thing that can alter muscle fiber types. In studies done at the University of Utrecht in the Netherlands (published in the Equine Veterinary Journal in 2002), the proportions of various muscle fiber types was seen to change as a group of Dutch Warmblood foals matured from 22 to 48 weeks of age, suggesting that muscle fiber composition is fairly fluid as horses grow and develop.

Breed Differences

Humans have known for centuries that certain horses suit certain types of activities. Arabians excel at endurance, Quarter Horses are sprinting specialists, and if you have stumps to pull, you'd better call on the power of a draft breed. But muscle biopsies are now showing us exactly what proportions of various muscle fibers are found in each breed. It's information that goes a long way toward improving our understanding of how breeds perform.

"When you look at a breed, it's an average," says Lindinger. "To get a picture of the proportion of muscle fiber types, you need to (take samples) from at least 20 elite animals, 20 poor ones, and 20 average performers. A standard procedure has been developed for this--you take about 80 mg of tissue from a punch biopsy, which is about the diameter of a pencil, from the gluteus medius muscle, which is a main locomotor muscle. We know that the gluteus medius isn't representative of the whole muscle fiber profile, but what it does show is the profile of the locomotor muscles. We're not really interested in the postural muscles, which tend to be 'slower' because there's no need for them to be fast and/or powerful."

Geor adds, "The gluteus medius is a power muscle, and it's easily accessible, which is why it's usually used (for muscle biopsies). But even within that muscle, you'll get different proportions of fiber types if you take the sample in a different part of the muscle or go to a different depth. Researchers try to use the same spot as much as possible, but there can be great variability from horse to horse.

"If you've only got a small number of animals--something that is chronic in the world of equine research--and a high degree of variability, you might not get statistically significant results," he adds. "But that doesn't necessarily mean there's no result, just that the design of that study didn't reveal it."

Even with all these challenges, researchers have managed to generate charts that compare the proportions of muscle fiber types in various breeds of horses. Lindinger and colleague Gayle Ecker, MSc, recently did a survey of these comparisons in veterinary literature going back 24 years, using the old system of muscle fiber classification.

What these data tell us is that there are, indeed, significant differences in the muscle fiber profiles for various breeds, and that in turn can tell us (to some extent) about the type of performance we can expect from that breed. Arabians, for example, tend to have more Type I and Type IIA fibers, which predispose them to excelling at endurance sports. But could we take data from an individual horse and use it to predict what level of performance we can expect from that animal?

Geor says, "I don't know that we can get performance predictions from muscle biopsies yet. We may be able to identify which training techniques are most successfully altering the profile of the various muscle fibers, but even that will be subtle. We're not at the point yet where a trainer could take information about muscle fibers and put it to good use in selecting a yearling at a sale, or even a 2-year-old in training. It's very much an evolving science."

There is potential for the future. "Trainers would like to know (the muscle fiber profiles of the horses they're training)," says Lindinger. "It's possible that we'll have a less invasive test for generating that profile in the future because a punch muscle biopsy is not something you want to perform routinely on a performance horse. Near-infrared spectroscopy, for example, could measure the oxygenation within muscle tissue in a resting horse, which combined with other clues could probably give us a workable test within five to 10 years."

Notes Lindinger, "The big area right now is looking at the effect of training on muscle fiber adaptation. It's definitely significant in terms of the percentages. As we start to be able to track the changes as horses are trained, it will tell us to what degree we can change a horse's genetic potential, and what methods we should be using to reach that genetic maximum.

"It takes a combination of genetic potential and training to be the best, and that's true across all the breeds of horses."

ATP UTILIZATION: Energy Pathways for Muscle Performance

The sole source of energy used to produce muscular movement is adenosine triphosphate, usually shortened to ATP. The horse's body converts feed into "storage" forms of energy (glycogen, glucose, and free fatty acids), which when called on by an athletic effort, are converted to ATP (which releases energy when converted to ADP) when the fuel muscle fibers need to fire.

If the exertion is less than maximal, the horse's body will prefer to operate aerobically, a system in which oxygen (drawn in from the lungs) is added to the chemical reaction to create large amounts of ATP from glucose, glycogen, or fatty acids (or to a minor extent, protein), and the harmless by-products of the reaction--water and carbon dioxide--are excreted through sweat and exhalation. The main drawback of aerobic metabolism is that it is a slow process, and it can't fuel 100% effort. For that, the horse must use the anaerobic energy pathway, which can produce energy extremely rapidly, but only in very small amounts.

The anaerobic system operates without oxygen, but produces toxic by-products (such as lactic acid) that induce muscle fatigue. Furthermore, the anaerobic system is not as versatile an energy pathway because it can only draw from glucose and glycogen stores, not from fatty acids or protein. When glycogen and glucose become depleted, or when lactic acid buildup in the muscles starts to inhibit their utilization, the pathway shuts down, so anaerobic metabolism only supports a few minutes of intense exertion. The greater the effort, the faster muscle fatigue (or complete exhaustion) sets in.

Most forms of prolonged physical activity at less than top speeds are fueled primarily by the aerobic pathway. As the degree of exertion goes up, so does the rate of ATP utilization, until eventually it exceeds the rate at which aerobic activity can resynthesize more ATP. At that point, the anaerobic system takes over so that the horse can continue to perform. As a rule, this "anaerobic threshold" is thought to be in the 140-150 heartbeats per minute range (although it varies somewhat from horse to horse).

In racing, for example, a horse can only run at top speed for about one minute (during which he is likely to cover about 1,000 meters, or five-eighths of a mile). After this first minute, his speed decreases because anaerobic energy production can't keep up, and the aerobic system isn't fast enough to provide energy for running at 100%.

In the real world, no form of exercise is fueled exclusively by one pathway or the other; there is always an element of both in play.--Karen Briggs

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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