Chapter 108The North American Standardbred
Description of the Sport
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).
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 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.
Conformation
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).
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.
Clinical History
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.
Lameness Examination
Palpation
Careful palpation is the art of diagnosis in the racehorse but often is sacrificed because palpation is time consuming. Palpation is critical in STBs because of numerous compensatory lameness problems. The veterinarian needs to be able to “read” the horse to make a diagnosis. A successful lameness detective respects what the horse is trying to say. The veterinarian should move over the entire horse with a light touch to see whether a withdrawal response is elicited. A light touch along the neck and gluteal regions may elicit pain from secondary muscle soreness or painful previous intramuscular injection sites. Horses often exhibit a withdrawal response when a limb is first picked up, indicating a problem even before the specific region is manipulated. Horses may exhibit false-positive responses in areas that have been recently painted, blistered, or freeze fired. Once horses have been freeze fired for splints and curbs, many trainers assume the problem has been solved and are incredulous if the veterinarian suggests the area is still the source of pain. Cryotherapy is not a panacea and may have to be repeated, or another form of management may be needed (see Chapters 78 and 89).