Why do racehorses have an optimum trip? | Trainer Magazine | European Trainer Article Index

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Jul 16 Why do racehorses have an optimum trip?

Irrespective of where in the world Thoroughbreds are raced, individual horses are usually campaigned over a narrow range of distance and are often categorized as sprinters (5-6f/1000-1200m)), middle distance performers (7-10f/1400-2000m) or stayers (>10f/>2000m). Whereas in the USA and Australia most races are run at up to and around a mile (1600m), in Europe there is a greater range of racing distances, from 5f (1000m) to 2 miles 6f (4400m) on the flat and from 2 miles (3200m) to 4 miles 4f (7200m) over jumps. So why is it, when the Thoroughbred breed arose from such a small gene pool, that individual horses seem to be suited by different and narrow ranges of trip? Here we shall explore the principal factors behind these observations and the genetic influences that might be responsible for them. By Dr Jeremy Naylor TRAINING 36 TRAINERMAGAZINE.com ISSUE 13 WHY DO RACEHORSES HAVE AN OPTIMUM TRIP? Performance determinants and optimum distance The principal factors that determine a horse’s racing performance are its genetic potential, its training and its environment (including atmospheric conditions, nutrition, tactics, rider, shoeing, racing surface and terrain). The former factor is the responsibility of the breeder while the others are to some extent controlled by the trainer. The relative importance of these factors is still very much open to debate. However, few would question the primary role of genetics: even when conditioned by the world’s best trainers, moderately bred horses rarely perform competitively in the highest grades. A more in-depth look at what determines an individual’s performance reveals another array of essential elements. These are the physiological attributes of its muscles and cardio-respiratory system (its “engine capacity”), the biomechanics of how it converts muscle contraction into propulsion (its “transmission”), and very importantly its motivation (its “will to win”). The principal physiological components are the muscle mass and the cardiorespiratory systems that play supporting roles in carrying oxygen and other essential fuels to the muscles. The elite status of the Thoroughbred horse among the mammalian species relates to all these components, with muscle being a greater proportion of their body weight and cardiorespiratory systems that have enormously superior capacity for carrying oxygen. It is clear from studies of racehorses and humans that maximal capacity for oxygen consumption (VO2max) is an important factor in determining peak performance. Muscle Fiber Types Absolutely central to the issue of optimal trip are the relative proportions of slow (type I) and fast twitch muscle fibers (types IIa and IIb, the latter being the faster). A high proportion of fast twitch fibers conveys greater power and strength, hence speed potential, whereas more slow twitch fibers provide greater resistance to fatigue but at lower speed, hence greater stamina. Compared with other breeds, Thoroughbreds have a higher proportion of types IIa and IIb fibers, with sprinters tending to have more IIb than stayers. In the 1970s and 80s the prospect of muscle fiber typing was popularized for helping select horses for trip, but was never adopted widely. A principal reason was that the differences between individuals was not sufficiently clear-cut to be of everyday value, coupled with the difficult technical aspects of taking repeatable samples. Causes of Fatigue Causes of fatigue play a central role in dictating optimum distance and are closely allied to the concept of varying muscle fiber proportions. In high intensity exercise the main cause of fatigue is the increasing acidity within the muscle fibers brought about by the accumulation of lactic acid. This arises as a byproduct of glucose breakdown which is incomplete due to the relative lack of oxygen supply in the face of very high demand. Acid accumulation slows down muscle contraction by limiting the rate at which the essential metabolic reactions can operate. This process is considered by many to be the primary determinant of a horse’s cruising speed. A horse that can maintain a high running speed without fatiguing is more likely to be successful in races. All DNA US.qxd:Jerkins feature.qxd 7/7/09 01:13 Page 1 G horses have a critical threshold speed: below this they can continue to run for relatively prolonged periods, whereas every fraction over that speed leads to more rapidly ensuing fatigue. This is represented graphically above to compare two horses with different threshold speeds for the onset of blood lactate accumulation (or “OBLA”). Here, Horse A has a higher threshold speed for OBLA and hence a higher cruising speed. By measuring lactic acid concentration in the blood after exercise it is possible to measure this for individual horses (it spills over into the blood as its concentration increases in muscle cells) and to follow its improvement during training. A further important factor is the tolerance of individuals to the build up of acid in the muscle cells: some horses have a higher “buffering capacity” than others and so have a greater ability to withstand acid accumulation without causing fatigue. Studies in horses have illustrated this by showing different thresholds for lactate and ammonia accumulation in the blood. Biomechanics Biomechanical factors include the horse’s conformation and the relative proportions of its various limb segments. These, coupled with the muscle fiber types, will dictate the stride length and stride frequency, which are principal determinants of speed and the efficiency by which energy is converted from muscle contraction to forward propulsion. Studies of stride length and stride frequency at racing pace have provided TRAINING 38 TRAINERMAGAZINE.com ISSUE 13 some evidence that there are differences between sprinting and staying horses. This preliminary data suggests that sprinters are able to maintain higher stride frequencies whereas stayers tend to favor longer strides with relatively lower frequency. These studies are in their infancy and further confirmed results could potentially influence what trainers are looking for in breeze-up sales in matching pedigree to stride pattern. Motivation Finally, but as important as all other factors that determine performance, is motivation, “ or the horse’s attitude to racing. Every trainer knows of individuals full of talent in training that fail to reproduce it under racing conditions. No horse has a conscious incentive to win a race, and few, if any (at least in European racing) know where the finishing line is located. Without the horse being prepared to exert itself in the hurlyburly of a race and withstand a measure of the discomfort of high-intensity fatiguing exercise, it is unlikely ever to be a winner. Horse psychology may well have a significant influence on optimum distance. Sprinters are characteristically more extrovert while stayers are typically more laid back in their approach to running. These qualities are important for different styles of racing: stayers need to relax and conserve energy for the finish, whereas sprinters may be running at close to maximal speed for much of the race. While the principal causes of muscle fatigue have been outlined above there are also “central” factors in fatigue that originate in the central nervous system. Ultimately horses that are least affected by the inevitable discomfort of intense exercise are more likely to win. Here physiology and psychology can never be separated because the discomfort is physiological in origin and may have negative psychological consequences. A prime example of this is seen in how, not infrequently, horses incapable of winning flat races can be successful over jumps, and vice versa. Here, moderate flat horses that excel over jumps must have greater psychological acceptance of the task. The pace is slower, the distances longer and there are jumping efforts. However it cannot be solely down to physical stamina since there are many examples of talented flat stayers who could not translate that ability into success in even modest jump races. performance and selection for distance The weight of evidence would suggest that it is nature (breeding) rather than nurture (training) that dictates a horse’s optimum distance. Most trainers start by judging a horse’s likely trip from its pedigree and the winning distances of sire, dam and immediate families. However, early training of most young horses is similar: only as they start to do faster work will trainers sort them out as to their relative ability and running style. Although there is general acceptance that the basis for optimum trip is genetically determined, very little is known beyond this, despite the explosion of genetics research in recent years. Even in human sport science, there is little known of the genes responsible for performance and the mode of inheritance of athletic ability. The candidate genes for study of elite athletic performance are those involved in energy reactions within muscle cells, in determining muscle fiber types and of the cardio-respiratory systems. There is a good deal of evidence for a direct mode of inheritance of muscle fiber types in mammals. Simply stated, if a sprinter is bred to a sprinter, there is a high likelihood that the progeny will be endowed with high proportions of fast twitch IIb muscle fibers, with the converse being true for stayers. However, as with most natural phenomena there are further layers of complexity. Some recent studies in horses have cast a new light on another means of inheritance for sprinting vs staying ability. Anecdotally, horse breeders often talk about stamina being passed down maternal lines. While there has been little factual basis for this contention, it has received a good deal of attention. In the late 1800s Bruce Lowe published “Breeding Racehorses by the Figure System,” where he divided the breed into 43 female lines based on families’ success in the English Classics. Later, during the 1950s, Captain Bobinski and Count Zamoyski produced the monumental work Family Tables of Racehorses and expanded Lowe's system to a total of 74 families. Interestingly, recent human research has provided strong evidence for stamina being the cell’s nucleus. This DNA is only passed from the dam to its offspring and is responsible for producing key enzymes in muscle cell energy reactions during exercise. Dr. Harrison pursued this line of inquiry in horses and made the revealing discovery that the inheritance of mitochondrial DNA (mtDNA) in horses can have a profound effect on whether progeny will be better suited to sprinting or staying. He discovered that there are 17 different types of mtDNA (haplotypes) in Thoroughbred horses and that five of these, found in over 50% of all young horses, can endow the progeny with either enhanced speed or stamina. Harrison’s view is that when considering mating plans, it is worthwhile establishing the haplotypes of dam and sire in order to maximize the odds for successful outcomes. In an industry whose annual turnover runs into millions of any currency you might care to consider, and where many still talk of the influence of sires and dams several generations in the past, such an approach would appear to make very good sense for anyone trying to make breeding pay. This is a refreshing step forward that uses modern advances in genetics: Thoroughbred breeding has so far largely ignored this established scientific discipline whose application has successfully transformed stock breeding of other domestic species. However, like many schemes for breeding winners, the efficacy of this approach remains to be proven in practice. Conclusion The concept of horses having an optimum trip is reinforced by our understanding of the physiology of performance. Much evidence points to there being a strong genetic basis for an optimum distance, especially in regard to heritability of muscle fiber types and a maternal basis for inheritance of oxygen consumption, mediated at least in part by mtDNA haplotypes. For those trying to breed successful racehorses, it might well make good sense to consider these factors in determining mating plans. Only by selecting stock using appropriate objective criteria will it be possible to fix the genes that are relevant to performance. ■ passed down maternal lines, through measurements within family groups of physiological attributes such as oxygen consumption during exercise before and after endurance training. In all species, capacity for oxygen consumption is an important determinant of performance. It is itself determined by many factors that include the ability to use oxygen in the muscle and the ability to transport oxygen around the body. The former is dependent on the concentrations of chemical compounds (enzymes) that break down glucose in the presence of oxygen to release energy for muscle contraction. These reactions occur within mitochondria, which are microscopic cigar-shaped structures within the muscle cells. Dr. Stephen Harrison utilizes modern genetics research techniques to investigate a subject that has been discussed by horse breeders since Thoroughbred breeding began. Intriguing evidence has emerged that many of the genes that code for mitochondrial structure are found on its own DNA segment separate to the main DNA in “If a sprinter is bred to a sprinter, there is a high likelihood that the progeny will be endowed with high proportions of fast twitch IIb muscle fibers, with the converse being true for stayers”

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Dr Jeremy Naylor (16 July 2009 - Issue Number: 13)

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