Everyone involved in the racing industry knows that one of the major problems in training horses is keeping them free from injury. Bones, joints, tendons, and ligaments are placed under considerable strain during training and racing, and it seems inevitable that, at one time or another, all horses will suffer some kind of musculoskeletal injury. "Wastage" is a term used in the industry to describe losses that occur during the training and racing of a horse. These losses can be measured in terms of days out of training and lost racing opportunities, as well as the financial losses that stem from these interruptions to a horse's training program (e.g., veterinary care and reduced income from race winnings). In a worst-case scenario, the horse must be retired from racing.
A number of studies have shown that musculoskeletal injury is by far the most common reason for wastage. In fact, the statistics are staggering -- for example, a study of 314 Thoroughbreds in Newmarket, England, found that lameness was the single most important reason for wastage in young horses in training. More than half of those horses experienced a period of lameness, and in about 20% of affected horses, lameness was severe enough to prevent racing during the period of investigation.
Although it is recognized that many different factors can contribute to the development of musculoskeletal injury, in Thoroughbred circles it has been suggested that many of these problems stem from the training and racing of 2-year-olds. Most racehorses begin training at 18 to 20 months of age, a year or more before the skeleton has reached full maturity. Potentially, intensive training at that young age might predispose the horses to career-limiting injuries.
Recently, practices associated with the preparation of horses for "2-year-olds in training sales" has heightened welfare concerns relating to the early training of Thoroughbreds. Because these sales are held in early spring, horses must enter training at even earlier ages to be ready to perform a racing speed workout as part of the preview process at the sale. The level of fitness required for "race readiness" necessitates an intense training program, and some of the youngsters might not stand up to such a rigorous preparation. The end result could be serious injury and the potential loss of a promising racing career. Indeed, there have been anecdotal reports of breakdown injuries in horses being prepared for 2-year-old in training sales.
These impressions do raise concerns about the animal welfare implications of the training and racing of 2-year-old Thoroughbred horses. However, before we jump to conclusions, it is necessary to carefully weigh the available evidence.
Yes, there is survey information to indicate that there is a high incidence of injuries in young horses in training. On the other hand, recent studies have demonstrated that Thoroughbreds which begin racing as 2-year-olds have longer careers compared to those which begin training and racing at an older age. As well, there is growing evidence that in order to maximize the increases in bone and tendon strength that accompany training, horses should begin training at an early age.
To more closely examine this issue, let's review: 1) the effects of training on the skeleton and supporting tissues such as tendons and ligaments; 2) one of the common injuries sustained by young horses in training; and 3) the results of epidemiological studies to understand the impact of 2-year-old racing on injury rates and career longevity.
You will see that there is no easy answer to this perplexing issue, and that there is a need for more research to determine the best training methods for young racehorses.
Conditioning Bone, Tendon, and Ligament
Conditioning, or training, results in adaptations of the body systems vital to athletic performance. For example, improvements in the ability to transfer oxygen from the lung to working muscle (adaptations of the cardiovascular system) and in the ability to consume oxygen result in an increase in aerobic capacity (or VO2max). The bulk of this increase in aerobic capacity -- an important part of the overall increase in performance associated with training -- occurs quite soon after the start of training (within a two- to three-month period). With the possible exception of old horses (more than 15-20 years), age does not seem to influence these cardiovascular adaptations to training.
On the other hand, training adaptation in bone and other important supporting structures (particularly joint cartilage, the suspensory ligaments, and the flexor tendons) occurs over a much longer time period and is the rate-limiting step in the preparation of a horse for competition exercise. There are several important, but as yet unresolved, issues. First, while there have been several studies examining the effects of training on bone, relatively little is known about how tendons and ligaments respond to the rigors of exercise training.
Second, whether the repetitive loading associated with training is beneficial or detrimental to some of these tissues is currently unclear.
Third, the jury is still out regarding the best training methods for enhancement of the strength of bone and the supporting structures.
Finally, whether these adaptations are maximized when conditioning is commenced before or after attainment of skeletal maturity is unresolved. This is the most important question in relation to the training and racing of 2-year-olds.
In human medicine, it now is recognized that conditioning during childhood and adolescence (i.e., before skeletal maturity) might maximize developmental increases in bone mass and strength. These findings might have important implications for women -- fractures associated with loss of bone mass (osteoporosis) are a devastating problem in post-menopausal women and severely malnourished younger women. Maximizing bone mass during adolescence through a combination of conditioning and diet might help reduce the incidence of these problems.
Currently, it is not known whether early training of horses can offer similar benefits, including a reduction in fractures during training and racing. We do know that bone continually undergoes what is termed remodeling -- old bone is broken down and replaced with new bone tissue. The rate of bone remodeling is greatest in areas of the skeleton under the highest loading stress -- the bones in the feet and legs receive the greatest loading stress during running and therefore can be expected to undergo substantial remodeling during exercise training.
Studies in horses have shown increases in the density of the metacarpal (cannon) and third carpal (knee) bones during training. This is an important adaptation to the stress of training because bone density is an important determinant of bone strength. However, the intensity of training has an important bearing on this response -- low intensity exercise (e.g., trotting) results in minimal changes in density of the cannon bone, whereas training at higher speeds (galloping exercise) results in a noticeable increase in bone density.
Thus, intense exercise is necessary to elicit a beneficial response in bone. However, the difficulty comes in selecting a training program that on the one hand is sufficiently intense to induce a beneficial response in bone, but on the other hand does not overload bone and the other supporting structures. Rapid increases in training load over a short period of time often will result in development of an injury (e.g., fractures and strains of ligaments and tendons).
This last point is relevant to the preparation of young horses for 2-year-olds in training sales. Compared to a progressive training program (with a gradual increase in work intensity), a "pressure-cooker" regimen that aims to bring the youngster up to racing speed in a few weeks greatly increases the risk of injury or, worse, breakdown.
Currently, an incremental training program that gradually increases the length, speed, and repetition of galloping is recommended for enhancement of bone strength. Initial gallops should be at low speeds (canter), with an increase in speed after periods of three to four weeks. The number of repetitions can be increased in a similar manner. Even then, most young racehorses will develop some degree of shin soreness ("bucked shins," more on this later) and close monitoring is required, particularly as training gallops approach maximum speed.
Even less is known about adaptations of tendons and ligaments during exercise training. However, a recent series of investigations (see Smith et al. 1999) have found that the tendons of mature horses have a limited ability to respond to training, and over time, repeated trauma to tendons likely predisposes horses to injury.
In contrast, the tendons of young horses (less than two years) strengthen in response to training. Thus, contrary to the common belief that exercise training of immature horses is detrimental, the results of these recent studies raise the possibility that early training might enhance development of the supporting structures of the limbs and perhaps reduce the incidence of injury during training and competition.
Bucked Shins
Bucked shins (also known as "shin soreness") is a condition involving the front part of the metacarpal bone (the cannon bone of the front leg). It is extremely common in young racing Thoroughbreds and Quarter Horses (and occasionally Standardbreds). In essence, bucked shins are the result of rapid bone modeling. With the onset of galloping exercise, there is a considerable increase in strain on the front portion of the cannon bone -- the equine body's response includes addition of new bone that ultimately will result in improved bone strength. However, early on this new bone appears to be prone to microfractures similar to the stress fractures that can occur in human athletes during training.
The severity of shin soreness can vary greatly, but many horses will show pain when the cannon bone area is palpated and will be lame at the trot. There can be swelling in this area of the leg. Diagnosis is most often based on recognition of these clinical signs in a horse undertaking its first training/racing campaign. X ray and/or nuclear scintigraphic examinations can be helpful in the evaluation -- the bone cortex often is thickened and small fractures might be visible. Horses with bucked shins must be rested until signs of pain and lameness have resolved.
Just how common is shin soreness, and does this injury threaten a horse's racing career? Recent studies in Australia (see papers by Bailey et al. and discussion below) have sought answers to these questions. These researchers found that shin soreness was the most common cause of "wastage" in 2-year-old Thoroughbreds in training -- there, wastage was measured in terms of alterations in a horse's training schedule (ranging from a few days off to a lengthy layup at pasture). Importantly, development of bucked shins did not prevent a horse from racing in subsequent seasons (or even the same season).
Another important point regarding bucked shins is that the condition is not exclusive to 2-year-olds. Horses which begin training and racing as 3-year-olds also are likely to develop shin soreness of varying severity. As well, some horses can suffer recurrences of shin soreness following a period of stall or paddock rest.
Therefore, development of bucked shins is not so much due to the age of the horse, but rather the consequence of a horse entering intensive training, particularly on the first occasion.
Interestingly, shin soreness is a much less common cause of lameness in young English racehorses. One explanation might be that, compared to the United States and Australia, less emphasis is placed on 2-year-old racing in England. However, a more likely explanation relates to training methods -- English horses are trained on straight tracks, a circumstance that places less strain on the cannon bone.
Currently, there is no one training method best suited to preventing or minimizing bucked shin problems. However, traditional training programs that begin with a two- to three-month period of slow work followed by a gradual introduction of galloping exercise might not be the answer. Research conducted at the University of Pennsylvania suggests that young horses should be given short, fast workouts early in their training program (e.g., one- to two-furlong gallops). These short periods of hard exercise might provide the appropriate modeling/remodeling stimulus without creating undue stress on joints, ligaments, and tendons. Much more research is needed in this area, particularly in the development of monitoring tools that allow early detection of shin soreness and allow training programs to be modified to reduce its severity.
Epidemiological Studies
The economics of Thoroughbred racing are such that most owners and trainers aim to have their horses ready for racing as 2-year-olds and, depending on ability and potential for use as breeding stock, for that horse to race for as long as possible. On the other hand, we know that lameness problems are the most important reason for wastage in Thoroughbred racehorses, and some perceive that these injuries are due, in large part, to the training and racing of horses too early in life.
Is this fact or fiction? The short answer is that we don't know. However, recent epidemiological studies by Craig Bailey and colleagues from the University of Sydney aimed to shed further light on causes of wastage, at least under Australian training and racing conditions. As in the United States, considerable emphasis is placed on 2-year-old racing in Australia.
In one project, researchers studied the racing careers of 553 Thoroughbred horses catalogued for a yearling sale in 1991. Of the 553 horses, 279 (50.5%) had their first start as two-year-olds, 162 (29.3%) as three-year-olds, 20 (3.7%) as 4- or 5-year-olds, 81 horses (14.6%) did not make it to the races, while the remaining 11 horses were lost to follow-up.
Surprisingly, horses having their first race during the 2-year-old season had longer racing careers than horses first racing at three years or older. The reasons for this difference could not be determined. However, it is likely that many of the horses not raced until they were three years old or older did enter training as 2-year-olds, and injury (or possibly a lack of athletic ability) might have delayed their racing debut. These factors also could contribute to a shorter racing career.
Bailey and his colleagues also examined injury rates in a group of 169 2- and 3-year-old racehorses. As mentioned, the most common injury in 2-year-olds was shin soreness -- which was present in 42% of 160 horses which began training as 2-year-olds. Many of those horses developed shin soreness a second or third time as 2- or 3-year-olds.
In 3-year-olds, knee problems were the most common cause of lameness, while fetlock injuries were present in both age groups. Injuries to tendons and ligaments were much less common, as were very serious breakdown injuries. Overall, days lost to training because of injury or illness were higher in 2-year-olds (3.1%) than in 3-year-olds (2.2%), probably reflecting the greater impact of shin soreness in the younger horses.
What have we learned from these studies? First, they confirm that injuries to the musculoskeletal system are the most common reason for lost training and racing opportunities. Second, clearly shin soreness is a problem in young Thoroughbreds entering their first training campaign. It also is possible that shin soreness and other nig-gling injuries prevent a large number of horses from racing as 2-year-olds.
The biggest unanswered question relates to the issue of 2-year-old racing. Yes, it is true that injury rates in this age group are high. However, this is also the case for older racehorses. And we now have evidence that horses beginning their careers during their two-year-old seasons actually have longer racing careers than horses which first race in later seasons. As well, on-going studies of the effects of training on the body at a very young age indicate that we should be training horses before the skeleton is fully mature.
Thus, on balance, there is a sound rationale for racing horses at a young age. That said, there is a real need for research that provides greater understanding of the causes of lameness problems in young horses and the effects of training methods and surfaces on the incidence of these problems.
FURTHER READING
Smith, R.K.; Birch, H.; Patterson-Kane, J.; et al. Should equine athletes commence training during skeletal development?: changes in tendon matrix associated with development, aging, function, and exercise. Equine Veterinary Journal 1999; Supplement 30: 201-209.
Bailey, C.J.; Rose, R.J.; Hodgson, D.R. Wastage in the Australian Thoroughbred racing industry: a survey of Sydney trainers. Australian Veterinary Journal 1997; 75: 64-66.
Bailey, C.J.; Reid, S.W.; Hodgson, D.R.; Rose, R.J. Factors associated with time until first race and career duration for Thoroughbred racehorses. American Journal of Veterinary Research 1999; 60: 1196-1200.
Bailey, C.J.; Reid, S.W.; Hodgson, D.R.; Rose, R.J. Impact of injuries and disease on a cohort of 2- and 3-year-old Thoroughbreds in training. Veterinary Record 1999; 145: 487-493.
Rossdale, P.D.; Hopes, R.; Digby, N.J. Epidemiological study of wastage among racehorses: 1982 and 1983. Veterinary Record 1985; 116: 66-69
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.
