Strengthening Young Thoroughbreds' Limbs

One of the major challenges in training racehorses is keeping them sound. Not unlike a human athlete, a racehorse's ligaments, tendons, bones and joints are susceptible to injury throughout its career and, at times, it seems impossible to avoid some sort of musculoskeletal mishap.

A vast number of components can comprise any musculoskeletal injury but many believe the economics of the Thoroughbred industry - namely the preparation of young horses for 2-year-old sales and racing 2-year-olds - are the main culprits for these sorts of injuries.
Training for most race horses commences when they are 18 to 20 months old. The skeleton of a horse often does not reach full maturity until they are four years old so training at a young age might predispose horses to a multitude of career-limiting or -ending injuries.
Shin soreness or bucked shins is an extremely common condition in young racing Thoroughbreds and Quarter Horses (and occasionally Standardbreds.) It involves the front portion of the cannon or metacarpal bone and is the result of rapid bone modeling.
Before a horse begins training, its cannon bones have the same thickness all the way around. When horses start galloping, there is a considerable increase in stress on the front of the cannon bone.  To contend with the stress, the equine body responds by adding new bone to the area in duress. Ultimately, this creates stronger bones but early on this new bone is prone to microfractures similar to the stress fractures that human athletes endure during training.
The severity of bucked shins can vary greatly, but most horses will exhibit pain when the cannon bone area is massaged, will be lame while trotting, and have a short, choppy stride. Another symptom is swelling in this area of the leg.
The condition is usually diagnosed by recognizing the clinical indicators in a horse when it begins its first training and/or racing campaign. Horses suffering from shin soreness must be rested until all signs of lameness have disappeared, which can take several days or many months.
For example, New York-based trainer Barclay Tagg's then 2-year-old colt, Tale of Ekati, had sore shins and returned after a month of light training to triumph in the Grade 2, $250,000 Belmont Futurity on September 15th of last year.
"One shin was very sore, but he got over it very quickly," Tagg said. "I got two real good works into him."
While Maimonides, a 2-year-old, owned by Ahmed Zayat, exited the Grade 1 Hopeful Stakes held at Saratoga Race Course on September 3 with the same affliction, his recovery was expected to take a bit longer. Sonny Sonbol, Zayat's racing manager, said he needed "three to four weeks to get over his shins and start back training and get ready for the winter."
Estimates vary, but it is believed between 65 and 90 percent of all Thoroughbreds in the United States and more than 40 percent of all Thoroughbreds in Australia buck their shins early in training.
About only 12 percent of young English racehorses buck their shins. Unlike the United States and Australia, much less emphasis is placed on 2-year-old racing in England and English horses are trained on straight tracks, so less strain would be placed on the cannon bone.
However, the English are not immune to their young horses being injured. In a study of 314 young Thoroughbreds in Newmarket more than 50 percent experienced some period of lameness, and in roughly 20 percent of those horses, the lameness prevented them from racing.
Also, bucked shins are not exclusively relegated to 2-year-olds but to all horses which are just beginning intense training. Some horses can suffer recurrences of shin soreness after a period of stall of paddock rest. Therefore, bucked shins do not discriminate based on the age of a horse, but depend on how intense the training is and if the horse is undertaking the action for the first time.
Dr. David Nunamaker, VMD, PhD, is an orthopedic surgeon and chair of the research department at the University of Pennsylvania's New Bolton Center who had conducted extensive research on bone development from 1982 to the present. Dr. Nunamaker, Dr. William Moyer, DVM, chair of the Large Animal and Surgery Department at Texas A&M University and Dr. John Fisher, DVM, an equine veterinarian and Maryland horse trainer, analyzed their research results and established a training system created to reduce the severity of bucked shins or erase them.
"We found that a horse's bone shape alters in response to its training," Dr. Nunamaker said. "The way most conventional training is conducted, a bone changes in a way it should not and that is why you get into trouble with bucked shins. Also saucer fractures seem to occur only in horses that have previously bucked their shins. This could lead to catastrophic fracture."
Dr. Nunamaker concluded a problem will become evident after 50,000 cycles of trotting and galloping. A cycle is equal to one swift stride.
"The Standardbred doesn't have issues with bucked shins because you never see a pacer do anything but pace while Thoroughbreds train with a variety of gaits, such as walking, trotting and galloping," Dr. Nunamaker said. "Thoroughbreds do not run while they are training and when they do run it's only every 10 to 14 days. The bone remodels to what it experiences - which is not racing."
Speed work is very important because when a horse runs at speed, the angle of strain is much greater. So horses that breeze more often remodel their bones for racing.
Utilizing the research results, Dr. John Fisher adheres to a training program that stresses and stimulates the cannon gradually.
"When a horse is breezed, the bone sees it as an emergency and immediately begins laying down new bone," Dr. Fisher said. "This new bone is weak and needs to be strengthened through later remodeling, which would be triggered by further breezes spaced closer together. If remodeling is not allowed to take place and the horse is asked to do too much before he is ready, the new bone will be weak and prone to injury. The bone-strengthening is entirely based on stress and recovery to gradually increase bone density and strength."
In Dr. Fisher's program, horses transition from a one furlong work at 15 seconds to a half-mile or more in 13 seconds over a 16-week period.
If there are more than four days between short distance works, Drs. Nunamaker and Fisher have discovered the new bone will stop rebuilding and actually weaken, with no additional stress after five days.
Once the program has been finished, a horse is prepared to begin conventional training because he should have accumulated enough bone strength that he will not buck shins. However, if a horse is subjected to different track conditions or circumferences, such as a European horse racing on American dirt, the threat of shin soreness resurfaces.
Even though Dr. Fisher has modified the program throughout the years, he is still quite pleased with its performance.
"We just don't have many injuries at all," Dr Fisher said. "No more tendons, no more suspensories, no more fractures."
How much high-speed work and distance are required to signal the bone to remodel itself correctly and not form weaker bone? Research is still being conducted but Dr. Nunamaker claims the goal is to correctly change the bone at the slowest possible speed over the shortest possible distance.
 
"Maybe two furlongs, maybe one furlong," Dr. Nunamaker said. "Maybe it won't even have to be that far. We just don't know but there is a fine line during a crucial time period as to what is too much and what is not enough."
Once the bone has attained maximum strength by becoming thicker at its stress points, it should stay that way.
"When 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," Dr. Nunamaker said. "It may develop injuries to other parts of its body, but not to the shins. 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. After that no more shin injuries."
It is important to note the bones are the slowest part of the body to train. In most cases, the cardiovascular system and soft tissues are prepared for the stress of racing before the bones.
Study results presented at the 2005 Australian Veterinary Association depict shin soreness or bucked shins can be avoided. Certain training techniques place horses at risk for this condition.
The most significant factor was how far the horse trained and how quickly he went. If a horse trained at a speed greater than 33 mph during its first ten weeks of training, he tended to have some shin pain.
"A gradual increase in the weekly distances at these speeds is the key to reducing the number of cases," Dr. David Evans, BVSc, PhD and associate professor of veterinary science at the University of Sydney and one of the researchers on the project, said.
The study also revealed that using short gallops of 200-300 meters at 33 mph or greater can decrease shin soreness; training horses to induce shin soreness will not reduce the risk of contracting the condition during subsequent training; and shin pain occurred much less often in horses that began training at an average age of 30 months.
Dr. Evans acknowledged that much more research was necessary before any authoritative program could be implemented.
K.L.P. Verheyen, DVM, MSc, PhD, MRCVS, of the Royal Veterinary College (RVC) in London, agrees with Drs. Nunamaker and Evans that training methods are associated with injury risk.
"Stress injuries are repetitive loading injuries," Dr. Verheyen said. "Compare it to a paper clip and if you keep bending it, it will break. Interval training (alternate periods of hard exertion and rest) is a better option because high-speed exercise is as not bad as previously thought. It actually stimulates bone to respond, because bone is a living tissue and is constantly remodeling. If the same exercise is repeated again and again, the bone will stop responding, which is what we think is happening with the low-speed exercise and stress fractures."
While more research must be conducted to provide greater insight into how equine bones adapt and grow, even less is known about how tendons and ligaments respond to training. In a series of recent studies, Allan Goodrich, a professor at the Royal Veterinary College and the University College of London, discovered that the tendons of young horses (less than two years) strengthen in response to training. These results raise the possibility that early training enhances the development of the limb's support structures and could diminish injuries during training and racing.
After reviewing training methods and treatments, it is obvious much more research must be completed before any sound strength management program can be introduced.
"We just don't have all the answers yet," Dr. Nunamaker said.

Kimberly French 
(14 February 2008 - Issue 7)