Strong bones are essential if a horse is to perform successfully and still remain sound. Bones that are weakened by disease, injury, or inappropriate training regimens can result in catastrophic injury, as anyone involved with racing well knows. There are a number of elements involved in the production of strong bones. The two prime elements are proper nutrition and the correct kind of exercise. We will focus in this article on the role played by exercise.

Here is a graphic depiction of the periosteal bone lifting the periosteum causing inflammation (stretched vessels and sharpey's fibers).

Illustration: Robin Peterson, DVM

The equine world is indebted to David Nunamaker, VMD, PhD, an orthopedic surgeon and head of the University of Pennsylvania’s New Bolton Center, for information on the role exercise plays in the formation of bone. Aided by grants from the United States Department of Agriculture, the New York Chapter of the Horsemen’s Benevolent and Protective Association, Grayson-Jockey Club Research Foundation, and the National Institute of Health, Nunamaker and his associates carried out in-depth research on bone development beginning in the late 1980s and continuing to the present that provided a great deal of information about the role played by exercise. Heavily involved in the early research with Nunamaker were Bill Moyer, DVM, formerly at New Bolton and today head of the Large Animal Medicine and Surgery Department at Texas A&M University, and John Fisher, DVM, a horse training veterinarian from Maryland, who was the first to try Nunamaker’s suggested approach for building strong bones in young racehorses.

The focus of their research involved bucked shins, but what they learned also provided enlightening information about exercise-induced development of bone.

The condition of bucked shins was and, continues to be, a costly and time-consuming problem in Thoroughbred racehorses. Estimates varied when Nunamaker’s research began, but it was believed that somewhere between 65-90% of all Thoroughbreds in the United States bucked their shins in early training. Why did this happen? the researchers wondered. Could it be avoided?

Nunamaker’s research revealed early on that casual, or light, exercise did not have a profound effect on growing and developing bone, but that more strenuous exercise did.

In an early segment of the study, four yearlings were purchased shortly before becoming two years of age. Two of them were turned out to pasture and never ridden. The other two were put into training that consisted primarily of two-minute gallops nearly every day, but they were never ridden at speed.

At the conclusion of that phase of the study, the bones of the four horses were examined. There was no difference in the structure of the bones. The horses which had done nothing but roam a pasture had bones as strong as the horses which had galloped two miles on an almost daily basis.

This indicated that galloping horses was not appropriate exercise to strengthen and prepare bones properly for the stress of racing.

Later research would reveal that working the young horses at speed periodically early in their careers would strengthen or remodel the bone so that it could withstand racing stress. More about that later.

First, a look at how bones grow and develop.

Start With Cartilage

Cartilage is a specialized connective tissue in which the collagen (a protein) matrix between cells is formed at positions of mechanical stress. In cartilage, the fibers are laid down along the lines of stress in long parallel arrays. This process results in a firm and flexible tissue that has great tensile strength. Cartilage binds together the bones that meet in joints, such as the knee and ankle.

Bone is a special form of cartilage in which the collagen fibers are coated with a calcium phosphate salt. Healthy bone is a substance that is strong, but not brittle.

One comparison to bone is fiberglass. Fiberglass is composed of glass fibers imbedded in epoxy glue. The individual glass fibers are rigid and strong, but they also are brittle. The epoxy component is flexible, but weak. The composite, however, is both rigid and strong. When a glass fiber breaks because of stress and a crack starts to form, the crack runs into glue before it reaches another fiber. The glue distorts and reduces the concentration of the stress, and the adjacent fibers consequently are not exposed to the same high stress. In effect, the epoxy glue acts to spread the stress over many fibers.

Bone is constructed in a similar manner. Small needle-shaped crystals of a calcium-containing mineral, hydroxyapatite, surround and impregnate the collagen fibrils (a small filament of fiber) of bone. Within bone, the collagen is laid down along the lines of stress. No crack can penetrate far into bone without encountering myriad hydroxyapatite crystals in a collagenous matrix. Bone is more rigid than collagen, just as fiberglass is more rigid than epoxy glue.

The ends and interiors of long bones are composed of an open lattice of bone called spongy bone tissue or marrow. Within this lattice framework, most of the body’s red blood cells are formed. Surrounding the spongy bone tissue at the core of bones are concentric layers of compact bone tissue in which the collagen fibrils are laid down in a pattern that is far denser than marrow. The compact bone tissue gives the bone the strength to withstand mechanical forces.

New bone is formed by cells called osteoblasts, which secrete collagen fibers on which calcium subsequently is deposited. Bone is laid down in thin concentric layers called lamellae. The lamellae are laid down as a series of tubes around narrow channels called Haversian canals, which run parallel to the length of the bone. The Haversian canals are interconnected and carry blood vessels and nerve cells. The blood vessels provide a lifeline to living bone-forming cells and the nerves control the diameter of the blood vessels and thus the flow through them.

With that rather pedantic, textbook explanation of bone as a basis, we now can look into what happens to it as a result of exercise. We return to information provided by the New Bolton research.

Bone Responding To Stress

In addition to learning that mild exercise had no real effect on bone development, the researchers also discovered that bucked shins weren’t what many had thought the condition to be. The general consensus for years was that bucked shins were microscopic fissure fractures of the cannon bone that occurred when the young, growing bone was placed under stress, such as in race training.

Nunamaker found that bucked shins actually are the result of the bone trying to respond quickly to the strains placed upon it. The result is that the bone, when stressed, seeks immediately to form a new layer of bone at the point of stress on the cannon bone. The quickly formed bone is periosteal or fiber bone and is more porous, and thus weaker, than the dense lamellar bone that is formed slowly over a longer period. In the process of the relatively rapid formation of bone, the periosteum is lifted and becomes inflamed and the horse is afflicted with bucked shins.

"We found that a horse changes the shape of its bone in response to its training," Nunamaker said, "and, depending on what the training is like, you can just about change the bone in any direction you want. The way most conventional (race) training is conducted, a horse changes its bone in an abnormal way and not the way it should change the bone."

The process that ends up in bucked shins begins, Nunamaker said, as the young horse’s bones become fatigued during training.

"When you take a specimen of anything," he said, "and you cycle it for a long enough time, it eventually will break. This is called fatigue failure. When you take a piece of bone and cycle it, what happens first of all is that it starts to lose stiffness and bends more. As it starts to bend more, there are higher strains on the bone, then suddenly something happens.

"To accommodate the strains, the bone tries to change shape and make itself larger. There is a fourth power involved in the equation here, and it takes only a little larger bone mass for it to become very much stronger.

"Normally, the bone changes itself very slowly and lays down lamellar (strong, dense) bone, but when stresses are suddenly applied, it moves to fiber (periosteal) bone formation. What happens then is that the horse changes the kind of bone it lays down. This fiber bone lifts the periosteum and makes the horse sore.

"When you have a horse that is starting to buck its shins, you have a horse that is starting to lose stiffness in its cannon bones and the bone is trying to compensate."

When the sore-shinned horse is removed from training, the bone reconstitutes itself or remodels, the inflammation affecting the periosteum disappears, and the horse is no longer sore. Left behind, however, is a layer of periosteal bone that is prone to another common problem in race horses--saucer fractures.

Nunamaker’s research not only revealed just what constitutes bucked shins, but also the timetable when the condition will occur under conventional training programs that involve a lot of trotting and galloping. Normally, he reported, the problem will be manifested after about 50,000 cycles, with each cycle being equated with one fast stride.

However, he also found that if the horse were training on a more yielding track surface, such as a wood chip track, the number of cycles might reach 85,000 or 90,000 before bucked shins showed up.

Nunamaker then was ready to conclude that conventional training programs that concentrated on long, slow gallops with young horses were not resulting in leg bones strong enough to withstand the stress of racing at speed. The prime problem, he felt, was that the young horses were not being trained in a way that signals their bones to prepare for the concussion and fatigue that comes with running at speed.

Already armed with information resulting from the experiment with pasture horses compared to those which galloped two miles nearly every day, Nunamaker was ready to launch another study. If, as the first study revealed, long gallops did not strengthen the bones any more than roaming a pasture, there must be another way to get the job done. Nunamaker reasoned that the way to achieve stronger bones would be with short, fast workouts early in the horse’s career.

A second research project was conducted to check out his theory.

Four groups of two-year-olds were studied. One group was not involved in a training program and roamed a pasture. A second group was trained at the Fair Hill wood chip track in Maryland in the conventional manner with long, slow works. A third group was trained at Delaware Park, also in the conventional manner. A fourth group was trained at Delaware Park with a training regimen that called for short, sharp bursts of speed three days a week.

When the bones of these horses were examined, it was found that there had been very little change in the structure of the cannon bones of the group that roamed the pasture. This was exactly what the researchers expected to find.

In the group trained with frequent bursts of speed, it was found that the cannon bones of two-year-olds had, in only a matter of months, developed to the stage equivalent to most four-year-olds which had been involved in a training and racing program. There was almost no periosteal bone growth.

The cannon bones of the group trained the conventional way at Delaware Park showed heavy concentrations of periosteal bone growth.

The group trained in the conventional way on the wood chip track showed some periosteal bone growth of the cannon bone, but to a much lesser degree than that noted for horses trained in the conventional way at Delaware Park.

"The reason that speed work is so important," Nunamaker said, "is that when a horse is going slowly, the principal angle of strain is about 40 degrees out of its vertical axis. When this happens, the bone is going to remodel to the direction of the magnitude of the strains. When a horse runs at speed, the angle of strain is much greater. So, horses on long, slow works remodel their bones for training while horses that breeze more often remodel their bones for racing.

"The Standardbred doesn’t have a problem with bucked shins because it trains at the same speed at which it races. You never see a pacer do anything but pace, but a Thoroughbred will train while walking, trotting, and cantering. Rarely do Thoroughbreds (in many conventional training programs) run while in training. They only run every 10 to 14 days. Therefore, the bone remodels to what it feels—which is not racing."

The amount of speed work required to stimulate proper bone remodeling can vary on a horse-to-horse basis, and there often is a fine line between too much and too little.

"The problem with recommending high speed work," Nunamaker said, "is that if you tell someone they should do high speed work and they go out and do it for a half mile three times a week, they’re going to break that horse down. We know there is a fine line in the critical time frame as to what is too much and what is not enough."

Armed with the information from the New Bolton research, veterinarian-trainer Fisher set out to establish a specific training regimen that would stimulate the bones to strengthen appropriately.

Two years later, he was ready to report that he had come up with a plan that worked for young horses in his stable. Today, he still is using the same basic approach, although it has been somewhat refined. The program is applied to horses which come to his stable in January and are to be ready for their track debut some time in June.

Fisher Training

Once the young horses are comfortable doing 11/4 miles per day, the speed work begins and continues during what amounts to four months of preparation for launching a racing career.

He begins the speed work by asking the young horse to travel at a two-minute lick (one furlong in 15 seconds) for one-eighth of a mile. The young horse will be asked to do this twice per week.

When the horse is ready, it is next asked to travel one-fourth mile at speed—one furlong in 15 seconds. Next comes three-eighths of a mile at the same speed, and by the end of three months, the horse is doing one-half mile in a two-minute lick. At this point, however, Fisher works the horse at speed only once every five days instead of twice per week.

During the final month of preparation, the young horse will be breezing (running at speed) a quarter of a mile twice per week.

In between speed works, the horse is galloped or rested.

The theory is that after a speed work stimulates the bone, it must be given time to remodel before being subjected to speed again. When a bone remodels, large cells called osteoclasts remove the stressed bone. The bone forming cells (osteoblasts) fill in the area with new material.

Fisher also discovered that track surface figured into the remodeling equation. If his horses were to race on dirt—which they were—they had to be trained on dirt if they were to avoid bone injury. While the wood chip track at Fair Hill is more forgiving, it does not trigger the bone to remodel effectively for the stress and strain of running on dirt.

Fisher said his approach works. Where shin problems in his trainees once were commonplace, they now are rare. And, he adds, he has not had an increase in ankle or knee injuries as a result of the speed work.

Once the bone has responded correctly to the stimulus and has changed its shape by adding more density at the points of stress for maximum strength, it will remain that way. This point generally is reached when a horse is four years old.

"At this point," Nunamaker said, "after the bone has changed shape, you could take the horse out of its training program and put it in any training or racing program you want, because its bone won’t change back again.

"The interesting thing was that when we looked at the timing of the change and shape of the bone and then we looked at the timing of the injuries that occurred in horses that have shin injuries, we found that when the horse reached four years old, it no longer had shin injuries. It is in the first two years of its training program, if it starts at two years of age, that it is going to have shin injury problems."

However, if a four-year-old never has been put through a training regimen that stimulates changes in its bone, it could find itself in the same position as the two-year-old as far as the potential for bucked shins is concerned.

The untrained four-year-old also must go through the same bone transformation process as its younger counterpart.

Newest Bone Study

The most recent research headed up by Nunamaker—funded by Grayson-Jockey Club Research Foundation—involved five commercial training stables. One of them was Fisher’s. Involved, said Nunamaker, were 226 young horses. A paper on that study is to be published in the near future. The finding, in general, was that mere galloping of horses was detrimental to proper bone development, especially as it related to bucked shins, and that short bursts of speed were beneficial.

One of the tools employed by Nunamaker is a strain gauge that is attached to the cannon bone in a surgical process. The gauge measures the degree of give or bend to the bone (strain) at various speeds.

It was found that the strain on the bones of young horses in training was much greater than on those which had gone through the bone remodeling process and had reached four years of age.

Nunamaker also reported that he and his colleagues were surprised to learn, through the strain gauge, that strain levels on the cannon bones were extremely high in young horses when compared to other mammals.

"The maximum strain you will see in most animals is 3,000 micro-strain," he said, "and with the horse, it went over 6,000. Yet, when a horse that has been in training reaches four years of age, its strain level drops back to become comparable with other mammals."

Another question surfaces when discussing leg bone development in young horses. What happens to that bone when the horse is sidelined from training for another reason, such as a respiratory problem?

Nunamaker said that when a young horse in training is sidelined, the bone begins a resorption process because little strength is needed when it is standing in a stall. When that occurs, the horse must be started over in the bone conditioning process. If it is brought back too quickly, injury could be the result.

Questions Remain

The New Bolton research produced a wealth of information, but questions still remain regarding correct training regimens and the rate at which the injured young horse is brought back.

The most important finding for all young horses is that the bone will remodel in the way in which it is stimulated. Thus, the racehorse needs bursts of speed during the training process to stimulate appropriate bone development.

As with other aspects of training, common sense is a most important ingredient. It was found during the New Bolton research, for example, that frequent speed works for two-year-olds produced serious mental trauma in some cases. This is why training programs and exercise regimens to develop strong bones can have a general approach, but should be specific on a horse-by-horse basis.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at or by calling 800/582-5604.

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