Wednesday, July 12, 2006

Proper leg structure will affect a racehorse’s movement and future soundness

by Brent Kelley
by Heather Smith Thomas

CONFORMATION of feet and legs plays a large role in determining a horse’s speed, athletic ability, and whether he will stay sound. The front legs carry more weight than the hind legs and are subject to more concussion and stress. Many types of injuries are more common in front legs, where conformation faults can have more serious consequences than faults in the hind legs.
The forelegs help in pulling the horse along at all gaits, and their conformation (from shoulder to hoof) determines the length of stride. The primary function of the front legs is to support most of the horse’s weight, absorb the shock of concussion, and lift the body for the flight phase of each stride. Strongest construction consists of relatively straight legs with sturdy bone structure; big, flat knees; and deep, well-shaped fetlock joints.
Straighter is better
Ideally a horse’s forelegs should be straight with forearms directly above the cannon bones and the cannon bones at right angles to the ground when viewed from the front or side.
Both legs should bear weight equally. Toes should point to the front, and the hoofs should be the same distance apart as the distance between the forelegs where they join the chest. A line dropped from the point of the shoulder should go down the center of the front leg, bisecting the forearm, knee, cannon, fetlock joint, pastern, and hoof.
A line dropped from the front of the withers should go down the center of the front leg (side view) and barely touch the heel of the foot or be right behind it. If a line going up from the fetlock joint hits the middle of the withers, the forelegs are set too far back under the horse, who probably has an upright shoulder.
Most horses are not perfectly correct, and some deviations are less harmful than others. How conformation relates to injury will be discussed in "Veterinary Topics" in the July 2 issue of Thoroughbred Times.
Some faults can be tolerated if they are slight, meaning they do not hinder speed or soundness, and if they are symmetrical on both legs. Keep in mind that few horses have all their bones and joints facing perfectly straight forward, and that no two front legs are exactly alike.
Even on the same horse there will be slight variations. The left leg rarely matches the right one perfectly. The leg bones in one or both forelegs might be slightly offset at the knee or fetlock joint, or slightly rotated, or might leave the joint at a slightly sideways angle instead of perfectly straight. The important thing is to be able to assess the straightness (or crookedness) and determine how the leg structure will affect the horse’s speed, agility, and future soundness.
Base-wide structure
If the front legs of a horse are farther apart at the feet than they are at the shoulders and forearms, the crookedness might start anywhere from the elbows to the fetlock joint. A turned-in elbow makes the leg turn outward. Base-wide structure is also found in horses with narrow chests. Most base-wide horses are splay-footed, which causes the feet to break over to the inside and wing inward, often leading to interference (striking the opposite leg).
Most foals toe out slightly, just because they are narrow in the chest when young. Usually they become straighter as they grow and fill out, as their chests becomes wider. Splay-footed youngsters are more apt to straighten as they grow compared with pigeon-toed foals (whose toes turn inward) because filling out the muscles of the chest will not change the toe-in stance.
In a base-wide horse, the inside portion of the limb is under greater stress, especially if he is both base wide and splay-footed. Ligaments at the inside of the fetlock joints and pasterns are always under strain. Windpuffs (swellings in the fetlock joint) generally occur if the horse is worked hard. The joint capsules and tendon sheaths may be disrupted and stay filled with extra fluid. Ringbone and side bone could occur on the inside of the feet. The inside aspect of the knee joint and cannon bone suffer more stress and concussion, making the horse likely to develop splints on the inside of the leg.
Additionally, the hoof is worn excessively on the inside. Splay-footed horses generally wing their feet to the inside whether they are base wide or base narrow, picking up the foot to the inside. A few horses are base wide and pigeon-toed, which puts even more stress on the lower leg. These horses rarely stay sound.
Base narrow affects stride
A base-narrow horse, whose feet are too close together, often has large pectoral muscles and a wide breast and, as a result, the structure is generally accompanied by pigeon toes. This puts strain on joints and often leads to windpuffs and ringbone or side bone on the outside of the feet. The hoof wall is worn excessively on the outside edge because the feet break over to the outside and land harder on the outside.
Base-narrow, pigeon-toed horses generally paddle when they run, meaning they swing their feet outward. This creates a lot of wasted motion as the lower leg and foot is flung to the outside at every stride, which cuts down on speed and agility. A pigeon-toed horse will paddle whether he is base narrow or base wide. A few horses are base narrow and splay-footed, putting even greater strain on the limbs below the fetlock joint. They often go lame if used hard. The base-narrow, splay-footed horse generally will strike himself because his feet wing inward and are very closely placed to begin with.
The horse might also "plait," put one foot directly ahead of the other, leaving hoofprints in a single line rather than a double line. This creates poor balance and less stability, and if the advancing front foot strikes the other leg, the horse might stumble.
Misalignment of any of the leg bones or joints in the foreleg (elbow, knee, and fetlock) can cause the toe to point in or out. Being splay-footed or pigeon-toed is the result, not the cause, of crooked structure higher up the leg.
Strength of upper arm
The horse’s arm bone, called the humerus, goes from his elbow to shoulder. The length and angle of this bone influence the action and stride of the front leg and determine how tightly the elbow and leg joints can bend and how far forward the entire leg can extend when the horse is moving.
If the humerus is long, it gives more strength and power, resulting in more leverage action, to the attached muscles. The additional length increases the range of motion in the front leg, creating a greater arc at the lower end of the bone, the elbow. A long humerus is desirable for speed, but it should not be disproportionately long compared with the shoulder blade, or this makes for relatively short shoulder muscles that would restrict movement of the upper arm and create a short choppy stride.
The humerus is of desirable length if it is 50% to 60% the length of the shoulder blade. This puts the elbow beneath the front of the withers. The humerus is too long if it is more than 60% the length of the shoulder blade, which limits freedom of action. A short arm bone is usually more horizontal, making its angle with the shoulder less than 90û. This increases concussion to the leg, due to the choppy stride. This construction is not a detriment to forward impulsion for a sprinter, but a horse with this conformation will usually tire if he tries to maintain high speed for very long.
For the best speed and stamina, the humerus should not be too horizontal or it will cramp the movement of the elbows and the swing of the leg. The angle between it and the shoulder blade should be the same as the angle in the hindquarters between pelvis and femur. A long, well-sloped shoulder is generally accompanied by a relatively upright humerus, whereas a steep, short shoulder usually goes hand in hand with a longer, more horizontal humerus.
Long forearm desired
The horse’s forearm, located between the elbow and knee, should be long, wide, and thick with well-developed muscles. These muscles are important for speed and should be large at the top and slimmer at the bottom as they taper to tendons at the knee.
For endurance, the muscles should be smooth and long, rather than bunched up and short. For speed, the forearm should be relatively long (since it contains the muscles) and the cannon bone relatively short.
A long forearm allows for longer muscles and shorter tendons, creating better leverage action for moving the leg quickly and with less clumsiness. The longer the forearm, the greater the arc it can make, resulting in a longer stride. If the forearm is short, it must make more total movements in the same amount of time, but the strides are shorter and the horse must work harder--moving his legs faster--in order to keep up the speed.
Knees should be large, flat
The knee should be large and flat at the front, and well proportioned. A small, pinched-in knee is not desirable because it crowds the tendons and joint cartilages and hinders free action.
A knee that is too small also increases the effects of concussion. A flat front gives a smooth surface for the extensor tendons to glide over as they straighten the leg at each stride.
The knee should be directly in line with the forearm above it and cannon bone below it. If the cannon bone is set too far back, rather than centrally located under the knee, the horse is calf-kneed (back at the knees) and the front of the leg looks concave when viewed from the side. More strain is thrown onto the tendons at each stride, which can lead to injury if the horse is used hard.
This type of structure also creates more concussion, and with overextension of the joint (bent toward the rear), the horse is at risk for carpal fractures. The opposite fault is over at the knees (also called buck knees, goat knees, or sprung knees), with the leg looking slightly bent forward at the knee. This can be an inherited conformation or due to overwork with injury to the check ligament or too much stress on the structures at the back of the knee. A severely buck-kneed horse is prone to stumbling because the knees more readily give and buckle forward.
A horse that is out at the knees (bowlegged when viewed from the front, or carpal varus) often has base-narrow, pigeon-toed conformation. This puts extra strain on the outside ligament of the knee, the inside portion of the knee bones, and outside portion of the joint capsule. A horse that is in at the knees (carpal valgus, or knock-kneed) has knees too close together. The lower leg might angle outward in a splay-footed stance, putting strain on the leg and joints in the opposite direction.
Other deviations from good leg conformation, when viewed from the side, include tied in at the knee, which occurs when the flexor tendon is too close to the cannon bone just behind and below the knee and inhibits free movement. This indentation usually means the tendons are too small and not as strong as they should be. The leverage ability of the muscles above the knee is decreased, since the tendons are pulling inward against the back of the knee instead of having a straight pull down the back of the leg.
Importance of cannons
The horse’s cannon bone between the knee and fetlock joint has a smaller splint bone on each side, toward the back. The cannon bone mainly provides support, and should be perfectly straight up and down when viewed from front, side, or rear. Muscles in the forearm continue downward (below the knee) as long tendons--in front and behind the cannon bone--to flex or extend the lower leg. An athletic horse has long forearms and relatively short front cannons for best leverage. If the cannon and associated tendons are too long, there is more stress on the tendons and they are more likely to be injured. The horse’s leg muscles will also tire more quickly when doing hard work because there is less muscle (forearm) to do the work and more weight (cannon bone) to the lower leg. The cannon should be wide when viewed from the side, rather than small and round. A lower leg with good "flat" bone has a lot of depth from front to back; this term describes a combination of bone and tendon, with the tendon set well back of the cannon bone rather than right next to it.
If the tendon and bone are too close together (a condition called "round" bone), there is too much friction between the moving parts, and the leg will not hold up as well.
Fetlock joint and pastern
The joint between the horse’s cannon bone and pastern should be broad from all angles--rounded a little at the front but firm and flat on all sides. The joint should be in line with the cannon above and pastern below it. Pasterns should slope moderately, with adequate length to give and to dissipate concussion. Too much slope, however, puts too much pressure (from tendons and ligaments) on the sesamoid bones at the back of the fetlock joint and makes a weak pastern that might go clear to the ground when the horse tires during strenuous exertion. If the pastern does not slope enough, the horse will have a choppy, jarring gait, and the increased concussion will damage his feet and legs.
A too-long pastern reduces a horse’s potential for speed because it takes longer to push off at each step to get the foot off the ground. A too-short pastern is almost always too upright, creating a pile-driving, pounding effect. A good rule is that a pastern is too short if it is less than half the length of the cannon bone; this will increase the risk for concussion injury and also create a shortened stride. Many horsemen feel that a short pastern is an advantage for propulsion, especially for fast starts, but that a short pastern must slope enough to absorb concussion.
A horse usually can manage fairly well with pasterns that are short and sloping or long and steep. But the opposite combinations (short and steep, or long and sloping) generally cause trouble.
Equine Athletes, The Equine Athlete's Heart and Racing Success

Dr Lesley E. Young,
Animal Health Trust,

Although the horse is often considered to be the premier athlete amongst mammals, VO2 relative to body mass of elite racehorses (~200mls/kg/min) pales into insignificance when compared to that of the diminutive Etruscan shrew (400mls/kg/min), or the Pronghorn antelope (300mls/kg/min). Nevertheless the cardiovascular system of the Thoroughbred racehorse has evolved to allow it consume more oxygen per kilogram than most other large mammals. The superiority of the Thoroughbred cardiovascular system rests in a proportionately larger heart and spleen per unit body mass than other large mammals. The equine cardiovascular system is hugely compliant with a heart rate range from 20 – 240 beats per minute and a splenic red cell reserve able to double packed cell volume and oxygen delivery during maximal exercise. Whilst heart rate is important in determining maximal cardiac output, stroke volume will be determined principally by heart size. In the Thoroughbred industry, it has long been believed that large hearts were associated with racing success. This is a topical issue and has lead to increasing use of echocardiographic and electrocardiographic methods to assess heart size at prestigious Thoroughbred yearling sales across the world. Anecdotes and the historical post-mortem records of elite racehorses encouraged the practice. Eclipse was unbeaten in 26 races and like Pharlap, the winner of 57 races, his heart after death weighed over 6 kg. This is 20% larger than that of an average racehorse. Secretariat, a record-breaking American racehorse was alleged to have had a heart that weighed over 10kg, and based on this heart size, it was suggested that his maximal cardiac output would have exceeded 500 L/min and his VO2max 240 ml/kg/min!

Our recent data have demonstrated a significant linear relationship between British Horseracing Board Official rating or Timeformârating and heart size measured by echocardiography in 400 horses engaged in National Hunt racing. Interestingly no such relationships are found when horses employed in flat racing are examined, suggesting that, as might be expected, VO2max and heart size are more important predictors of performance for equine athletes running longer distances. These data are corroborated by the discovery of a much stronger relationship between left ventricular mass and other measurements of cardiac size and VO2max in 18 Thoroughbred racehorses exercising on a high speed treadmill.

Selective breeding of performance horses for superior athletic ability has resulted in a mammal with a huge heart and improved aerobic capacity. It seems increasingly likely that these features are also responsible for many of the problems of today’s performance horse. Exercise-induced pulmonary haemorrhage occurs in all racehorses and visible epistaxis occurs not uncommonly both during training and racing. High intraluminal pressures and very negative alveolar pressure generated during peak exercise combine to produce extreme transmural pressures and stress failure of the pulmonary capillaries. The prevalence of audible murmurs of atrioventricular valve regurgitation varies between 54 (tricuspid valve) and 21% (mitral valve) in mature National Hunt steeplechasers, and atrial fibrillation is the commonest cardiovascular cause of poor performance in the Thoroughbred. The sustained form the arrhythmia is present in 1% of National Hunt horses in training, while paroxysmal fibrillation during exercise occurs in racehorses of all types. The prevalence of paroxysmal atrial fibrillation during racing, training and competition is difficult to assess, since riders are often unaware of the condition, unless the horse’s performance is affected. In the majority of horses affected with sustained and paroxysmal atrial fibrillation, there is no evidence of significant underlying cardiac disease. It seems likely that large atrial mass and changing autonomic influence during exercise provide the anatomical and physiological substrates for the re-entrant rhythm to develop and be sustained.

The Animal Health Trust and a Research grant of the Horserace betting Levy Board supported our epidemiological studies.