Description of the Sport

The Standardbred (STB) was developed as a racing breed by registering any horse that could trot or pace 1 mile in a set time, called the standard. An early standard was 2:30 (2 minutes, 30 seconds, 0 fifths). Early STBs were diverse because they were different types with a large genetic pool. Crossing many types resulted in a strong, rugged breed that lacked refinement of the head, ears, and bone structure. Trotters were large and coarse, whereas pacers were smaller and more refined. The breed has progressed rapidly by selective breeding in the past 4 decades. Although trotters and pacers now are bred separately, the physical appearance has become more alike, and both lines have a racy appearance similar to Thoroughbreds (TBs). Early trotters were farm or buggy horses, selected for good disposition and durability, traits that are still characteristic. Early STBs were raced several 1-mile races in the same day, called heats, a part of the sport demanding stamina and gameness.

A TB stallion, Messenger, imported from England in 1788, was prominent in the formative period of the STB. Many of his offspring had a natural inclination to trot. In 1849, a descendent of Messenger sired Hambletonian 10, the foundation sire of the STB breed. The breed developed in the northeastern United States, where it was influenced by the Hackney and Morgan breeds and local mixed-breed horses.

The STB races at two different gaits: the trot, a diagonal two-beat gait in which the left forelimb (LF) and right hindlimb (RH) move together, and the pace, a lateral two-beat gait in which the LF and left hindlimb (LH) move together. High knee action and a lateral (left to right) nodding of the head are typical of a trotter (Figure 108-1, A). A trotter travels straight ahead (see Figure 108-1, A) (Figures 108-2, A, and 108-3), but a pacer sways from side to side while moving forward (see Figure 108-1, B).

Fig. 108-1 A, Muscle Hill, a champion 3-year-old trotting colt of 2009, is seen here with driver Brian Sears. This talented young trotter equaled the all-age trotting world record of 150.1 in his overwhelming victory in the Hambletonian, raced at the Meadowlands racetrack outside of New York City. B, Paul MacDonell guides Somebeachsomewhere, a champion 3-year-old pacing colt in 2008. Somebeachsomewhere equaled the all-age pacing world record with his 146.4 performance at the Red Mile in Lexington, KY.

(Courtesy United States Trotting Association, Columbus, Ohio, United States.)

Fig. 108-2 A, A young trotter is on the left shaft coming out of a turn on a small farm track. On this track a horse has to negotiate four turns to train 1 mile. B, Horses approach the start of a race at the Red Mile, Lexington, Kentucky, a track that is 1 mile long. Compared with the turn in A, the turn on this track is substantially larger, and horses must negotiate only two turns for a 1-mile race.

Fig. 108-3 This trotter is on the left line and the right shaft in the straightaway. The horse’s head is turned to the left because the driver (Rich Ringler) has to pull harder on the left rein to keep the horse going straight. The horse’s hindquarters are closer to the right shaft. This combination of left line and right shaft would be consistent with left hindlimb lameness.

Fifty years ago trainers had to spend several months determining at which gait a STB was more talented; some could not handle speed at either gait. As late as the 1970s and 1980s, it was not unusual for a horse to race one season at one gait and the next season at the other. A few horses raced at different gaits weekly, but today most STBs race at only one gait. Many foals pace or trot naturally. Although trotters trot while racing, they occasionally break into a pace when they are lame. Pacers often jog—the term used for trotters and pacers when trotting the slow or wrong way (clockwise) of the track at the trot—but train and race at the pace. The pace is faster than the trot. The current (2009) world record for the pace during a parimutuel race is 1 : 46.4 (all-age, any size track) and that for the trot is 1 : 49.3 on a track longer than 1 mile and 1 : 50.1 on a 1-mile or smaller racetrack. Many purists in the sport do not consider a world record on a racetrack longer than 1 mile to be equivalent to that obtained on a smaller racetrack because there is only one turn compared with two or more on smaller racetracks. Many pacers race near record time, but trotters do not. Trotters and pacers do not race against each other in parimutuel races. A STB racehorse must qualify for racing, meaning the horse must better or match a minimal speed before being entered into a parimutuel race. Another form of speed test is called a time trial. At certain 1-mile-long racetracks, horses compete against a prompter, usually a TB in harness, and go as fast as possible, but the prompter cannot pass the STB. The world record for the time trial is 1 : 46.1 set by a pacer. Mean race times of STBs are decreasing constantly (the horses are becoming faster) because of selective breeding, changes in track surfaces and banking, popularity of 1-mile racetracks (racing is faster because horses only negotiate two turns), refinement in race bikes (sulky), and advances in veterinary care.

Early trotters were raced under saddle and then with carts and wagons, with a time under 3 minutes being noteworthy. Sulkies appeared regularly in races in the 1840s, and the 2:30 barrier fell in 1845, becoming a time standard for the era. Race times fell steadily, and in 1897 a stallion named Star Pointer paced a mile in under 2 minutes. The first horse to trot the 2-minute mile was Lou Dillon in 1903.

North American STBs start a race behind a moveable starting gate, a vehicle carrying a barrier to keep the horses in line until the gate pulls away, at which time the horses accelerate to start the race evenly. In some countries, horses start from a standing start, a staggered standing start, or a moving start without a gait. In Europe, races are only for trotters, and top 3-year-old and older North American trotters often are sold to European buyers.

STBs in North America are divided into four classes. The first is the International trotter that races in the top competitions in North America and Europe. The second is the Grand Circuit or stakes horse that travels from track to track, racing against the top 2- and 3-year-old horses. These horses may earn more than 1 million dollars a year. The third class includes the greatest number of horses stabled at or near a parimutuel track. These horses race repeatedly at one or two different tracks and are referred to as overnight horses. Overnight horses are raced in condition races, grouping similar quality horses by earnings, age, or sex, or in claiming races, where horses are grouped by price. The fourth class, county fair racehorses, popular mostly in the northeast and midwest United States, race for small purses at county fairgrounds, often under adverse track conditions, but they provide thrills for fans and owners beyond those of races with richer purses.

Amateur STB driving and racing under saddle are hobby-type events that are increasing in popularity. STBs compete at horse shows in the roadster classes, hitched to a bike or wagon, and also as road horses under saddle. Former STB racehorses are not only popular in the Amish community, but they also can be found performing diverse sporting activities such as low-level dressage and jumping. The Standardbred Retirement Foundation (www.adoptahorse.org) helps to find homes for former racehorses; the Harness Racing Museum and Hall of Fame (www.harnessmuseum.com) and the United States Trotting Association (www.ustrotting.com) are worthy causes and valuable sources of information about the North American STB.

Training

A classically trained STB has about 9 months of schooling and training before its first race. Most horses start training in the fall at years of age and begin racing the next summer. Many states have racing programs for 2-, 3-, and 4-year-old horses and require a horse to be nominated early in life. Eligibility then is maintained by periodic payments, called stake payments, creating a large pool of money divided among the winners of elimination and final events. Although some stake races receive track or corporate purse assistance, owners who put up their own money support most races. Stake races are limited by age and sex. For example, 2-year-old trotting fillies race together. Stake payments must be made early in the spring, before talent and speed of individual horses can be confirmed, and owners, trainers, and veterinarians must decide which horses to stake. Emphasis on racing young STBs and the money necessary to maintain stakes eligibility place extreme pressure on trainers to race horses at 2 and 3 years of age. Horses often are pushed farther and faster than is reasonable, placing them at risk of stress-related bone injury, usually of subchondral bone. Many owners do not realize the difficulty of having a horse compete based on a calendar of scheduled events, expecting the horse to peak for the biggest purses, rather than on the horse’s training and fitness level. The process of nominating and planning for stakes races is an art and science, and often a staking service is hired to organize periodic stake payments. Stake payments are a considerable expense, and some owners spend several hundred thousand dollars a year. Horses not nominated, those dropped from stakes programs because of injury or lack of talent, or those in which payments were missed are still eligible for overnight racing. More and more late closer and entry-only stakes are becoming available for nonstaked horses.

A STB is trained in a jog cart that is stronger, longer, and about three times as heavy as the 14-kg sulky or race bike. A young pacer is soon equipped with hobbles, which are leather or plastic straps encircling the ipsilateral forelimbs and hindlimbs, to keep legs moving in unison (see Figure 108-1, B). A horse capable of sustaining a pace without hobbles is called a free-legged pacer, but few race without hobbles. Hobbles can cause areas of hair loss on the cranial aspect of the forearm and the caudal aspect of the crus called hobble burns, which are typical marks of a pacer. Trotting hobbles, which are straps encircling both forelimbs that run through a pulley underneath the horse, have become popular and are being used on more and more trotters in recent years, even those competing in top stakes races. Trotting hobbles may stabilize the trotting gait at speed. Some in the industry frown on the use of these equipment additives, claiming they may give an unfair advantage and historically have not been used.

Equipment is important in understanding gait and lameness. Head poles, boots, brace bandages, gaiting straps, and gaiting poles are used to improve gait and performance, as well as to assist in steering the horse at racing speed. A head pole keeps the horse from turning the head and neck to the opposite side. A horse with right forelimb (RF) lameness usually bears left, called bearing in or on the right line, because the driver pulls on the right line, causing the horse to turn the head to the right. This horse would wear a left head pole. Gaiting straps and poles run alongside the horse and attach to the cart beside the driver’s seat and are used to keep the horse straight. Horses with unilateral hindlimb lameness often carry the hindquarters near the contralateral shaft, called being on the shaft. A horse with RH lameness goes on the left shaft and requires a left gaiting pole or strap. Boots are applied to minimize interference injury. Some horses chronically interfere, whereas interference in others is accidental. Soft brace bandages are used for protection and to widen the hindlimb gait.

Lameness and Poor Racing Performance

Lameness is the leading cause of poor performance and a major cause of STBs making breaks. Some overnight and stakes horses are chronically lame but race weekly, although lame horses race slower, and horses drop in class as lameness progresses. The major effect of lameness is seen during the last quarter of the race, called the last quarter time. Lower-class horses usually have a fast opening quarter but slow last quarter time, whereas open and stakes horses have fast quarter times anytime during the race. Evaluation of last quarter time is useful to determine cause and severity of poor race performance. Lameness often costs a horse 1 to 2 seconds in last quarter race time, equating to 5 to 10 horse lengths. For example, a horse that normally paces the last quarter in 27.1 seconds but for the last few starts has been coming home in 28.4 to 29.2 seconds is probably lame, although concomitant respiratory and metabolic problems cannot be overlooked.

Track Surface and Lameness

STBs usually race around an oval track, to 1 mile in circumference; a 1-mile track is most common. Races are usually 1 mile, but a few are -mile sprints to 2-mile races. The track surface is a crushed rock base, covered with a packed, sandy soil and a thin sand or stone dust surface. STB racing requires a much firmer track surface than galloping horses. Sulky tires are more efficient if rolling over a firm smooth surface, and the STB gaits require a firmer and smoother surface than does the gallop. Track surfaces have become firmer because year-round racing requires a surface suitable for racing in rain and subfreezing temperatures. Hard tracks with loose surfaces require horses to wear shoes with added grabs and welded-on spots of borium to avoid slipping. Excessive slipping or overzealous use of shoes with additives predisposes horses to lameness. A soft or deep surface can predispose horses to tendonitis and suspensory desmitis. Hard tracks with a covering of loose stone dust are slippery and may predispose horses to carpal synovitis, bruised soles, and muscle soreness. Racetracks get soft and sticky with small amounts of rain and hard and unyielding with heavy rainfall that may wash the top surface into the infield. Recently, a three-dimensional dynamometric horseshoe was used to measure acceleration and other forces on the front hoof of French trotters performing at high speed on two different track surfaces, all-weather waxed and crushed sand track surfaces.^1,2 Hoof deceleration was significantly less, and there was a 50% attenuation of shock when horses performed on the waxed surface, suggesting this surface had better shock-absorbing qualities compared with a crushed sand surface.¹ The amplitude of the maximal longitudinal braking force was significantly lower and occurred 6% later in the stance phase. The magnitude of the ground reaction force at impact was lower on the waxed track compared with the crushed sand track surface.² The attenuation of loading rate, amplitude of horizontal braking, and shock at impact on waxed surfaces suggested that there is likely a reduction in stress in the distal aspect of the forelimb, results that may be important in training normal horses or rehabilitating STBs with infirmities.²

Track Size and Lameness

Track size has substantial implications in the development and expression of lameness. Tracks are 1 mile, mile, mile, and mile, and in general race times are lower (faster) on large tracks. Because most STB races are 1 mile, track size determines the number of turns during the race. During a race on a -mile track, a STB must negotiate four turns. Turns on smaller racetracks are much tighter than those on larger tracks (see Figure 108-2). There is a single racetrack larger than a 1-mile oval, Colonial Downs in Virginia, at which a short race meet is held each year; on this track, races involve negotiating a single turn. Horses racing with mild chronic lameness are less likely to sustain speeds necessary to be competitive on larger tracks, so they may race on smaller tracks; however, if lameness is worse in the turns, the horse may be less competitive on small tracks and better suited for a 1-mile track. If lameness is worse in the straightaway, the horse may be best suited for a small track. This is particularly important when lameness worsens as the race goes on because large tracks have a long straightaway at the end.

During counterclockwise racing, right-sided lameness conditions exert more influence, even though the LF and LH are on the inside. Horses with left-sided lameness can race successfully on small tracks and apparently negotiate turns well. However, those with right-sided lameness cannot trot or pace the turns well, most likely from the effect of centripetal and compressive forces. Ideally, all racehorses should be sound, but many horses can race successfully with mild infirmity. Horses with substantial lameness should be treated or rested until the condition improves.

Conformation

Forelimb and hindlimb faults lead to lameness and interference. Severity often depends on gait. A trotter should be as long from front to back as it is tall. Trotters are passing gaited if hind feet land outside the front feet, and in-line gaited if hind feet land in plane with the front feet. Wide hindlimb gait is advantageous because it allows a passing gait. Trotters can be mildly to moderately toed out in front without substantial interference problems, but they should not be toed in because winging out leads to interference and lateral suspensory branch desmitis. Pacers can be wide and mildly toed in but not toed out because this fault predisposes the horse to hitting the knees. A club foot is less well tolerated by a STB than a TB and usually results in foot lameness. Low, underslung heels and long toes are acquired by attempts to increase stride length or to decrease interference. Back at the knee is undesirable and leads directly to carpal lameness in a trotter, but it is better tolerated by a pacer. Offset (bench) knee is a major fault, and although horses may race successfully, lameness of the carpus and metacarpal region often develops. Mild carpus valgus is tolerated by pacers but can lead to lameness of the carpus and metacarpal region. Pacers that are tied in behind the knee are at risk of developing superficial digital flexor tendonitis, but surprisingly trotters are not. Short, straight pasterns and short backs are undesirable.

A serious hindlimb fault leading directly to moderate to severe lameness is sickle-hocked conformation (see Chapters 4 and 78). Mild sickle-hocked conformation typified many early pacers and is desired by some trainers. Sickle-hocked trotters are fast but develop curb and osteoarthritis (OA) of the distal hock joints, particularly severe in the dorsal aspect, and are chronically unsound. Sickle-hocked conformation predisposes all STBs to the development of slab fractures of the third and central tarsal bones. Straight hindlimb conformation is unusual but is a severe fault, leading to metatarsophalangeal and stifle joint lameness and suspensory desmitis. In horses with straight hindlimb conformation, excessive extension of the metatarsophalangeal joint occurs. Long, sloping pasterns also predispose horses to fetlock hyperextension, but a primary metatarsophalangeal joint weakness is seen in some trotters with normal pastern lengths, in which the base of the proximal sesamoid bones (PSBs) is level with the middle of the proximal phalanx. Abnormal elasticity or loss of support from the suspensory apparatus predisposes horses to run-down injury and suspensory desmitis. Run-down injury is more commonly seen in the hindlimbs. Cow-hocked conformation is prevalent in trotters but is not problematic unless severe. In-at-the-hock conformation predisposes horses to OA of the distal hock joints, curb, and medial splint bone disease. Base-wide and base-narrow conformational faults, if severe, can lead directly to interference and lameness.

Distribution of Lameness

STB racehorses have a higher percentage of hindlimb lameness than TBs. The STB trains and races using a two-beat gait: two limbs bear weight simultaneously. Load is shared almost equally between forelimbs and hindlimbs. The caudal location of the cart and driver (load) and the addition of the overcheck apparatus shift the center of balance caudally, increasing hindlimb lameness (see Figure 2-2Figure 2-3). The distribution of forelimb to hindlimb lameness is approximately 55% to 45%.

Lameness in the Young Standardbred

The top 10 list (see page 1024) contains those conditions seen in STB racehorses of all ages when grouped together, but unraced 2- and early 3-year-olds develop a subset of lameness conditions. Osteochondrosis (see Chapters 54 and 56), splint exostoses (splints), and curbs are common. Forelimb splints are usually medial but can occur laterally. Hindlimb splints are almost always medial and are more common in trotters. Splints can cause primary lameness or be secondary to carpal lameness, especially if the exostosis is proximal. Forelimb splints develop when the limb is carried or lands abnormally. Young horses develop splints from difficulty learning the racing gait and by making breaks. While breaking, pacers often hit themselves because galloping in hobbles is difficult. Curb is common in young pacers and may be recognized while horses are jogging, before training begins (see Chapter 78).

Carpal lameness is common in young horses. The middle carpal joint is the most common site, but synovitis can occur in the antebrachiocarpal joint. Carpal lameness is caused by early subchondral stress-related bone injury of the third carpal bone and is particularly common in trotters. Paddock turn out for 30 days is recommended. Middle carpal joint lameness must be differentiated from proximal suspensory desmitis (PSD). The thought once was that the distal radial physis was a source of occult carpal region lameness, but diagnosis was never confirmed. Physeal pain is a rare diagnosis today, but some veterinarians believe such pain is a possible source of so-called colt soreness.

Bucked shins are rare but do occur in 2-year-old pacers in July and August. Most bucked shins occur in the forelimbs. Genuine bucked shins rarely occur in trotters, but interference trauma of the dorsal cortex of the third metatarsal bone (MtIII) is common.

Distal hock joint pain is a common cause of acute unilateral lameness, but if pain is bilateral, overt lameness may not be present. Horses appear stiff and sore when starting out and warm out of lameness. Gait abnormalities and repeated breaking may cause distal hock joint pain. Lameness may be difficult to abolish completely with intraarticular analgesia because pain is multifactorial, arising from periarticular soft tissues and secondary gluteal myositis and trochanteric bursitis. Horses should be rested for 1 week and given nonsteroidal antiinflammatory drugs (NSAIDs) such as phenylbutazone (2.2 to 4.4 mg/kg orally [PO] twice daily). Intraarticular injections are avoided if possible, but treatment of the tarsometatarsal (TMT) joint with hyaluronan alone, or in combination with corticosteroids, often resolves residual lameness. Using a less aggressive training schedule for 20 to 30 days is recommended. Shoeing should be evaluated, and grabs and calks should be removed or minimized.

Interference injuries are common and plague many STBs throughout the racing career. Trotters interfere primarily by striking the toe of the ipsilateral front foot to the shin, pastern, or coronary band regions of the hindlimb (see Chapter 7 and Figure 7-2). Interference from a front foot striking the medial aspect of the contralateral forelimb (cross-firing) occurs occasionally in trotters but is a major form of interference injury in pacers. Injury to second (McII) and third metacarpal (McIII) bones, PSBs, carpus, heel, and hoof wall causes bruising and hematoma formation. Contusions can lead to soreness or frank lameness and can cause an altered gait. A trotter attempting to avoid striking the LH shin shortens the cranial phase of the stride, causing what appears to be a pelvic hike, mimicking LH lameness. Deliberate hiking can alter load distribution, causing compensatory lameness in the RH and RF. Local trauma is treated, and changes in equipment and shoeing, such as applying brace bandages and boots, are performed. A brief (5 to 7 days) reduction in exercise intensity is useful to allow a young horse to gain confidence in the corrected gait.

Young horses with poor gait should be examined carefully for neurological disease. Equine protozoal myelitis (EPM) is endemic in STBs and can be a real cause of gait deficits or a catchall diagnosis (see Chapter 11).

Clinical History

Clinical history is critical for lameness diagnosis and to put lameness into context with other causes of poor racing performance. The attending veterinarian with an established relationship with the horse may know the horse’s history. A trainer often refers to mild lameness by saying, “The horse is just sore, Doc,” or “The horse is off when jogging, but throws it away when going the right way.” STBs have unusual resiliency and race rather well with chronic lameness, but lameness does not resolve with speed. Owners do not understand this concept. Signs of lameness become less visually apparent when horses go fast. Because STBs often race with numerous compensatory lameness problems, one must be open to the possibility that current lameness is not worsening of an existing problem but is an entirely new one.

A good starting place is to determine the chief complaint from the person(s) closest to the horse, usually the groom and trainer. What differs now from the horse’s normal activity, gait, or performance? Has a change in training or racing schedule occurred? Has there been a change in track surface? A horse noticed to be stiff and sore up front when walking to the track may be foot sore the day after the horse raced on a hard track. Have any equipment or shoeing changes been made? Additions of head poles, gaiting straps, and boots point to high-speed lameness and gait abnormalities. Shoeing changes, such as adding or removing bar shoes, are important because horses used to wearing bar shoes may have sore feet if bars are removed. Metacarpophalangeal joint lameness may be worse if bars are added. Stress-related bone injury of the metatarsophalangeal joint is seen soon after aluminum shoes with toe grabs are applied. Is the horse receiving any current medication, and when was the horse last examined? What treatments, if any, were given? Although an acute injury or worsening of chronic OA can cause acute synovitis, inflammatory and infectious arthritis can develop days to weeks after intraarticular injections, particularly if corticosteroids were used, because these drugs can suppress signs of inflammation and infection for weeks. Infectious arthritis is common 14 to 21 days after intraarticular injection, but often owners and trainers are incredulous because recent injections were not performed. What have the trainer and groom done recently to solve or treat the problem? Topical therapy may reduce or worsen inflammation and mask or augment clinical signs. Skin soreness after application of paints and blisters can cause a false-positive response to deep palpation.

Is the horse a trotter or a pacer? This information is critical. The usual distribution of pacers to trotters is about 3 : 1. Interference problems are different between gaits. Trotters often develop contralateral or diagonal compensatory lameness, whereas pacers most often develop ipsilateral compensatory lameness. A pacer with a left carpal lameness often develops LH compensatory lameness, but a trotter is more likely to develop compensatory lameness in the RH. Although this distribution of compensatory lameness is common when examining horses trotting in hand, load distribution and interference issues may complicate and change visual appearance of where primary lameness originates when a STB is pulling a cart on the track and on which gait the horse is performing. The distribution of lameness differs between gaits. Fractures of the proximal phalanx and the McIII and the MtIII and superficial digital flexor tendonitis are less common in trotters. Trotters with stifle lameness, specifically OA of the medial femorotibial joint, can cope better than pacers. Race times are different. Prognosis for certain problems such as osteochondral fragments of third carpal and radial carpal bones is worse in trotters. Pacers are more likely than trotters to start a race and to have five starts before and after surgical removal of osteochondral fragments of the carpus.³ Osteochondrosis is common in certain bloodlines of trotters and pacers.

Is the condition worse at the trot or pace? Trotters and many pacers trot while jogging the wrong way but assume the intended gait when turned the right way. Most forelimb lameness conditions are less obvious at the pace, but hindlimb lameness differs. In general, STBs with stifle lameness are worse at the pace than the trot. If trainers comment that the horse is worse at the pace than at the trot, the veterinarian should suspect stifle lameness until proved otherwise. Pacers wear hobbles, but trotting hobbles are often used in trotters making breaks.

Is lameness different depending on what direction the horse moves? Problems on the right side are worse going the right way of the track (counterclockwise). Horses with medial foot and carpal lameness may show signs in the turns, especially entering the turning radius going the right way, but not when jogging.

What is the horse’s last quarter time? Although high-speed lameness can cause a 1- to 2-second decrease in last quarter time, horses with mechanical upper airway disease and rhabdomyolysis have a 2- to 4-second reduction, and those with severe lameness or atrial fibrillation have a 5- to 10-second reduction in last quarter time.

Is the horse on a line? This is one of the most important pieces of information to obtain. Trainers sometimes dismiss young horses being mildly on the line because nervous and inexperienced horses may change directions suddenly, but in reality most of these abnormal movements are caused by lameness. During counterclockwise racing of a horse on the right line, the driver has to increase the tension (pull harder) on the right rein to keep the horse straight and prevent it from bearing toward the infield. Just the opposite occurs when horses are on the left line (see Figure 108-3). Because of bit pressure, when a horse is on the left line, it turns or cocks the head to the left, toward the inside rail, and vice versa. If history is unclear, the veterinarian should watch the horse on the track and observe head position. Because most horses bear away from pain, a horse on the right line is most often lame on the right side, but exceptions occur. A common finding is metacarpophalangeal effusion and a positive response to lower limb flexion. The trainer states, “The horse was on the left line when finishing the mile, but is usually on the right line especially in the turns.” Primary lameness may be in the right carpus, but chronic RF lameness has caused compensatory overloading and lameness of the left metacarpophalangeal joint, and both areas should be evaluated and potentially treated. Most horses on a line from forelimb lameness have foot or carpal lameness. However, problems located medially, such as medial sole bruising, quarter cracks, or carpal lameness, paradoxically can cause a horse to be on the contralateral line. Horses may lose power in the affected limbs and drift at speed toward the lame side. Therefore a horse with a medial LF quarter crack could be on the right line, whereas those with a right carpal lameness could be on the left line. All limbs should be examined because not all horses read the book. “If a horse is on the left line, think right hind,” one old saying goes, and rarely a horse with RH bruised or cracked heels will be on the left line.⁴

Horses on a line without overt signs of lameness are among the most difficult lameness mysteries to solve. Because compensatory lameness problems may be numerous in a horse in which primary lameness cannot be ascribed, deciding on which limb to begin diagnostic analgesia is a dilemma. Most horses need to be examined at speed after blocking, a time-consuming and frustrating process. In many horses on a line, obscure sources of pain such as subchondral bone can be difficult to identify, and horses often are referred for scintigraphic examination. Most horses chronically on a line have had many intraarticular or other injections, and trainers will state, “We’ve done his knees, hocks, stifles, and feet and nothing helped.” Often forgotten are the metatarsophalangeal joints. In many horses numerous sites of pain must be managed simultaneously rather than sequentially. Recommending numerous intraarticular and regional injections, a shoeing change, and a change in exercise intensity in a horse that is on a line is not unusual.

Is the horse on a shaft? A horse with RH lameness drifts to the left (see Figure 108-2) and positions the hindquarters closer to the left shaft of the sulky and is thus on the left shaft; the reverse is true with LH lameness (see Figure 108-3). Not all hindlimb lameness problems cause horses to be on a shaft. Horses with OA of the medial femorotibial joint may not be on a shaft, but those with distal hock joint pain will be. A horse may be falsely on a shaft if it is hard on a line and the driver pulls hard to keep the horse straight, compelling the horse to twist its entire body, positioning the hindquarters close to one shaft to maintain balance.

Is the horse worse in the turns or on the straightaway? Although nothing is pathognomonic, a few findings are consistent. Horses with front foot lameness, splints, and curbs get worse as the mile progresses and so appear worse on the final straightaway. Horses with distal hock joint pain appear to be worse when going into or coming out of a turn but usually can pace and trot the turns successfully. Horses with carpal, metatarsophalangeal, and stifle joint lameness are worse in the turns. If a horse gets rough in the first turn just after leaving the gate, a medial RF problem such as a splint or carpal lameness should be suspected.

On what size track does the horse race? The relationship of track size to expression of lameness and the differences between left- and right-sided lameness problems were discussed (see page 1017).

Is the horse making breaks? Unsound trotters often break stride and gallop but occasionally break into a pace. Lame trotters may pace when trotted in hand. Because high-speed lameness often causes pacers to gallop and trotters to pace or gallop, it seems that the gallop may be the easiest gait that a lame horse can maintain at speed. The most difficult gait to maintain at speed is the trot, followed by the pace, and lastly the gallop (MWR). Interference injury causes breaks, and current shoeing or recent changes should be evaluated. A different way to hang up a horse, meaning how to shoe and equip it, may be all that is needed.

The veterinarian must be current on all medication rules in the racing jurisdiction because changes, such as recent (2009) modifications of allowable blood levels of corticosteroids, may impact management strategy.

Lameness Examination

The lameness examination can be divided into three stages: palpation, observation while moving on a lead shank, and observation under harness. The veterinarian should avoid narrowing the examination to the area suspected by the groom and trainer but should first stand back and observe the horse and watch the horse moving around and coming out of the stall. Subtle neurological signs may be apparent. Asymmetry, such as muscle atrophy, may point toward chronic lameness or neurological disease.