The Athletic Horse || Conformation

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Conformation can be defined as the “formation of something by appropriate arrangement of parts or elements: an

assembling into a whole” (Webster’s dictionary, 1976) and equine conformation appraisal is traditionally based on the external appearance of the body shape, form or outline of the animal. This evaluation may be regarded as the front line for judgments when selecting horses for specific intended tasks, including breeding selection. Prepurchase recommendations and perceived animal value rest highly on this assessment. There is wide variation of conformation between and within different breeds, the significance of which requires expert understanding of optimal breed characteristics and potential effects on soundness or performance. The success of a horse in any equine discipline or industry is not dependent on “perfect” conformation, as this does not guarantee performance or soundness, and “imperfect” conformation does not necessarily exclude a horse from performing at elite levels. Other factors such as human management, environmental conditions, genetics, nutrition, temperament, training, and the health status of the horse will also have a large bearing on ultimate performance. Conformation can, therefore, only be considered an indicator for future athletic potential. Nonetheless, conformation can assist prediction of possible musculoskeletal strengths and weaknesses, possible predisposition to injury, or both, based on known etiology and pathophysiology of musculoskeletal disorders.

Despite considerable anecdotal information, there is still a considerable lack of evidencebased quantification of conformation assessment and the relationships among conformation, performance, and orthopedic health. Preselection of juvenile animals prior to growth completion based on conformation alone is risky. Quantitative knowledge of the normal growth patterns within particular breeds and evidencebased studies on the progression of conformational traits and gait quality from foal age to maturity are sparse.

The conformation or inherent anatomic structure of the horse is an integral part of the equine musculoskeletal constitution and will influence the quality of dynamic performance. The skeletal format will affect such factors as joint range of motion, limb arc and hoof flight patterns, and weight distribution in motion, with subsequent effects on coordination of movement (including limb interference), balance,

power (propulsion, impulsion, and collection), agility, and endurance. Conformation will, therefore, partially dictate the relationship between form and function, thus modifying the potential for biomechanical efficiency, superior performance, musculoskeletal durability, and perhaps even longevity (Wallin et al., 2001). As some conformational traits are dynamic and will only be apparent during ambulation, the traditional emphasis of conformational assessment as a pure description of static external appearance has been extended to include a more functional assessment of conformation during unridden and ridden gaits in some of the studies cited in this chapter.

There is a great need to clarify and standardize the descriptive terminology of joint alignments, as most conformational traits are described using multiple traditional and variable nonscientific terms, rather than by defining anatomic configuration. For example, a caudal deviation at the radiocarpal or metacarpal joint complex (knee) may be described as “back at the knee,” “calf knee,” or “carpal hyperextension,” none of which describes the precise origin of segmental misalignment. Biomechanical evaluation relies heavily on strict physical and mechanical relationships of segments, requiring accurate anatomic terminology. Yet, most studies have employed generalized or horsemanship terms in describing conformational traits. The lack of anatomic precision, documentation, or both limits the interpretation of some studies. The literature presented in this chapter will follow the terminology appearing in the research papers. Some common terms describing conformational alignments are defined anatomically in Table 151 and illustrated in Figure 151 and Figure 152.

ASSESSMENT OF CONFORMATION

All assessment of equine conformation should be conducted with the horse standing squarely (loading all limbs symmetrically) on a level surface. The stance of the horse has been identified as a major source of error in conformation assessment, as small changes in limb placement and weight distribution can introduce significant variation in segmental alignment. When assessing deviation of the limb from the vertical, Weller et al. (2006a) found measurement variations in stance within one horse to be almost as large as between horses, thus

Conformation

SECTION III BIOMECHANICS/KINEMATICS AND PERFORMANCE

BRONWYN GREGORY

CHAPTER

15

254 S E C T I O N I I I BIOMECHANICS/KINEMATICS AND PERFORMANCE

Anatomic Description of Commonly Used Conformational Terms

Common Term Anatomic Description

Base narrow Distance between the forelimbs is greater at the chest than feet, the limb sloping medially

Back at the knee/calf knee Carpal hyperextension due to a caudal displacement of the proximal row of carpal bones, the radiocarpal joint being ,180 degrees (Ross, 2003). An upright pastern is often also related to this conformation (Ducro et al., 2009a)

Forward at the knee/bucked knee/over at the knee/sprung knee

Radiocarpal joint angle .180 degrees or lack of full carpal extension causing a flexion moment

Offset knee/bench knee Traditionally described as the metacarpus laterally deviated relative to the carpus; however, the displacement is usually in the radiocarpal joint (Ross, 2003)

In at the knee/knock knee Carpal valgus

Tied in below the knee Distinct notch distal to the accessory carpal bone on the palmar aspect of the limb causing the circumference of the leg below the carpus to be less than that above the metacarpophalangeal joint (fetlock)

Upright pastern Metacarpophalangeal and proximal interphalangeal (pastern) joints have a straight appearance

Toed out feet Metacarpophalangeal valgus

Toed in feet Metacarpophalangeal varus

Uneven feet Forefeet differ in size, shape, or both, causing variable hoof–ground angles

Sickle hock/curby hock Tibiotarsal (hock) angle 53 degrees or less (Holmstrom et al., 1990)

Straight behind Tibiotarsal angle .170 degrees (Marks, 2000), usually due to a more upright tibia

Cow hocked/in at the hock Either a rotational change in the hindlimb or tarsus valgus .180 degrees

TABLE 15–1

highlighting the importance of standardized repeatable positioning of the horse.

Conformation assessment should be a systematic and organized process incorporating a general overall observation of size, symmetry, musculature, posture, balance, and demeanor, followed by a more specific evaluation of conformational traits of the body, individual limbs, and feet. Briefly, relevant body observations should include head shape and size; height at the withers and croup; body length; neck length; shoulder length (top of the withers to point of the shoulder); pelvic length (tuber coxae to tuber ischii); scapular and humeral inclination; pelvic and femoral inclination; and chest width. From these observations, an overall proportioned symmetry in lengths and heights is desirable, both left to right and fore to hind. Congruent sloping angulation of the shoulder and hip is also desirable, with a proportional length of individual limbs in relation to the height and size of the body (Figure 153). The segment lengths of specific long bones of limbs should also be noted at this time. Establishing the exact source of the alignment deviation is imperative; for example, does a laterally pointing hoof, commonly described as toed out, originate from an externally rotated limb or from a particular distal joint? Cranial, caudal, and lateral views are needed to determine limb deviations in the sagittal, coronal (frontal), and transverse planes (see Figures 151 and 152).

When examining the conformational traits of individual limbs, a plumb line approach is useful in identifying angular or torsional deviation of segments from the vertical or horizontal at each joint level (Figure 154). In horses with “ideal” conformation, a visualized vertical plumb line dropped from the tuberosity of the scapular spine should bisect the longitudinal axis of the forelimb to the metacarpophalangeal joint

(MCPJ or fetlock) and fall 5 cm behind the heel in the lateral view. A line dropped from the cranial aspect of the greater tubercle of the humerus (point of the shoulder) should bisect the forelimb in the cranial view. In the hindlimb, a plumb line dropped from the ischial tuberosity should touch the point of the calcaneous (prominent caudally in the tarsus or hock), follow the plantar metatarsal surface to the metatarsophalangeal joint (MTPJ or fetlock), and fall 7.5 to 10 cm (Ross, 2003) caudal to the heel in the lateral view. The entire hindlimb should be bisected evenly in the caudal view (see Figure 154). When assessing foal conformation, limbs can also be viewed from above at the shoulder and hip (skyline view).

Particular attention is warranted in evaluation of distal limb alignment, hoof quality, size, and balance due to the concentration of locomotive stresses in this area. Although different breeds will have feet of different shapes and sizes, it is universally and anecdotally desirable to have balanced feet positioned symmetrically under the central limb axis with a straight hoof–pastern axis (the dorsal surface of the hoof wall lies parallel to the dorsal surface of the pastern region) (see Figure 153 and Figure 155). The constant growth of the hoof creates a dynamic relationship between the digital axis and dorsal hoof wall, which suggests that completely straight hoof–pastern axes cannot exist over time without natural wear or appropriate trimming (Moleman et al., 2006).

After assessment, overall observations can be related to desirable or “benchmark” breedspecific conformational characteristics and judgment made on the horse’s suitability to a given career. Notably, the definition and number of traits evaluated, the point scale scoring system of conformational traits, and the image of an ideal phenotype varies greatly

CHAPTER 15 Conformation 255

FORELIMBSShoulder to ground

1 - Camped-under4 - Intermediate7 - Camped-out

FORELIMBSKnees 1

1 - Bucked knees4 - Intermediate7 - Calf knees

FORELIMBSUpstandingness

1 - Base-wide4 - Intermediate7 - Base narrow

FORELIMBSCannon angle

1 - Bow-legged4 - Straight7 - Knock-kneed

FORELIMBSKnees relativeto cannon orforearm

1 - Offset cannon4 - Straight7 - Inset cannon

FORELIMBSKnees 2

1 - Tied-in knees4 - Straight knees7 - Chpped at knees

1 4 7

1 4 71 4 7

FIGURE 15-1 Illustrations of some common conformational defects of the forelimbs (see Table 151 for description). (From Mawdsley A, Kelly EP, Smith FH, Brophy PO: Linear assessment of the thoroughbred horse: an approach to conformation evaluation, Equine Vet J 28:461, 1996.)

among registries, organizations, and countries; therefore, specific classification is essential for comparative evaluations. Selection of a horse in the presence of a lessthandesirable conformation is not always considered unwise. A study on Thoroughbred racehorses highlighted that variation in horses and performance is not fully explained by a few underlying conformational components but is a result of a complex interaction of all conformational parameters (Weller et al., 2006b). However, certain conformational faults such as extreme tarsal angulation (large or small) and tarsal valgus are almost certainly predisposing to injury or lameness in racing events and are best avoided. Veterinarian conformational assessment should particularly focus on the presence of any such faults and the relationship of these faults to existing or potential pathologic conditions (Rossdale and Butterfield, 2006). In many instances, coexisting conformational anomalies will be present, at times allowing biomechanical compensation and at other times exacerbating musculoskeletal stresses during locomotion.

SUBJECTIVE ASSESSMENT OF CONFORMATIONThe evaluation of conformation has traditionally been subjective or empirical and remains the primary method of

assessment. Visual appraisal of defined criteria (the outlines and axes described above) and manual palpation of specific bony landmarks have been the basis of assessment, giving the examiner multiple threedimensional images over a period. The combinations of joint configurations and segment lengths are infinite and multifaceted, so the resulting judgment is variable and directly dependent on the individual expertise and personal “ideal” of the practitioner. Magnusson (1985) showed less variance among judges on overall impressions and type traits. However, opinions concerning segment lengths, joint angles, and skeletal inclinations were largely discrepant. This finding was supported by a study comparing radiographic and visual assessments of hoof–pastern conformation in Warmblood foals (Kroekenstoel et al., 2006).Visual assessment was only in agreement with radiologic evidence in 6 of 92 (6.5%) evaluations. Weller et al. (2006c) also suggested that variability in judgment is affected by the limited repeatability of measurement techniques due to inaccurate identification of anatomic landmarks and inconsistent positioning of the subject. Some studies and studbooks have used a system of linear scoring in an attempt to quantify the repeatability of subjective evaluation (Dolvik and Klemetsdal, 1999; Koenen et al., 1995; Mawdsley et al., 1996). This


HINDLIMBSHip to ground

1 - Stands under4 - Intermediate7 - Camped-out

HINDLIMBSHock set

1 - Straight and posty4 - Intermediate7 - Sickled

HINDLIMBSUpstandingness

1 - Base-wide4 - Intermediate7 - Base-narrow

HINDLIMBSHock set

1 - Bow-hocked4 - Straight7 - Cow-hocked

1 4 7 1 4 7

1 4 7 1 4 7

FIGURE 15-2 Illustrations of some common conformational defects of the hindlimbs (see Table 151 for description). (From Mawdsley A, Kelly EP, Smith FH, Brophy PO: Linear assessment of the thoroughbred horse: an approach to conformation evaluation, Equine Vet J 28:461, 1996.)

Shoulderlength

Rumplength

Rumpangle

Shoulderangle

Stridelength

Pasternangle

A B

C

D

E

FIGURE 15-3 Measurement of shoulder length (A), rump length (B), shoulder angle (C), and rump angle (D). The pastern angle (E) should be equal to the shoulder angle.

method of assessment employs a numeric scale to describe defined conformational traits across the entire spectrum of possible configurations, one biologic extreme to the other. Although meeting with some success, 6 of 21 traits were classified unacceptably low in repeatability (Mawdsley et al., 1996). These traits were hoof–pastern axis in both forelimbs and hindlimbs, head size, and vertical alignment of the

forelimbs and hindlimbs, all having a coefficient of variation greater than 10%. Despite these limitations, subjective evaluation can be easily and quickly performed by an experienced evaluator, expediting the assessment of large numbers of horses within a short time frame. The absence of standardized evaluation standards, lack of centralized training programs internationally, and a large source of error introduced by


A B C D

FIGURE 15-4 In horses with “ideal” conformation, a visualized vertical plumb line dropped from the tuberosity of the scapular spine should bisect the longitudinal axis of the forelimb to the metacarpophalangeal joint (fetlock) and fall 5 cm behind the heel in the lateral view. A line dropped from the cranial aspect of the greater tubercle of the humerus (point of the shoulder) should bisect the forelimb in the cranial view. In the hindlimb, a plumb line dropped from the ischial tuberosity should touch the point of the calcaneous (prominent caudally in the tarsus or hock), follow the plantar metatarsal surface to the metatarsophalangeal joint (MTPJ or fetlock) and fall 7.5 to 10 cm caudal to the heel in the lateral view. The entire hindlimb should be bisected evenly in the caudal view. (From Ross MW: Confor-mation and lameness. In Ross MW, Dyson SJ, editors: Diagnosis and management of lameness in the horse, Philadelphia, PA, 2003, WB Saunders, p 21.)

FORELIMBSHoof–pastern axis

1 - Broken and upright4 - Straight7 - Sloping

FORELIMBSPastern angle

1 - Toe out4 - Straight7 - Toe in

FORELIMBSFoot slope

1 - Straight4 - Intermediate7 - Sloping 1 4 71 4 7

1 4 7

FIGURE 15-5 Illustrations of some common conformational defects of the hooves (see Table 151 for description). (From Mawdsley A, Kelly EP, Smith FH, Brophy PO: Linear assessment of the thoroughbred horse: an approach to conformation evaluation, Equine Vet J 28:461, 1996).

subjective assessment precludes sole use of this method to compare results between studies or substantiate the more complex relationships among conformation, performance, and soundness. For these, quantitative conformational assessment, in addition to these traditional judging methods, has been suggested to improve predictive capability (Holmstrom and Philipsson, 1993).

OBJECTIVE ASSESSMENT OF CONFORMATIONInitial attempts to provide absolute values in conformation assessment have used the tools listed in Table 152 in combination with a reference marker system. A founding study by Magnussen (1985) described the comprehensive set of landmarks listed below, and many research studies have followed this protocol or a derivative of it.


Tools of Conformation Measurement

Tool Measurements Taken

Goniometer (see Figure 15-3)

Joint anglesScapular/pelvic inclinations

Tape measure Height at withersLength of croup and backWidth of chest and mandibleCircumference of girth; neck at poll and

withers (Mawdsley et al., 1996); carpus; the third metacarpal/metatarsal; girth

Box level 1/– crossbar

Height at withers, back, and croupLength of head, body, limbsDepth of chestWidth of breast and pelvis

Calipers Width of head and third metacarpal/ metatarsal

Width of chest and pelvis

TABLE 15–2

Neck and Forelimb 1. Cranial end of the wing of atlas 2. Proximal end of the spine of the scapula 3. Caudal part of the greater tubercle 4. Transition between the proximal and the middle thirds of

the lateral collateral ligament of the elbow 5. Lateral tuberosity of the distal end of the radius 6. Space between the fourth carpal, the third metacarpal, and

the fourth metacarpal bones 7. Proximal attachment of the lateral collateral ligament of the

fetlock joint to the distal end of the third metacarpal bone 8. Dorsal edge of the coronary band

Hindlimb 1. Proximal end of the tuber coxae 2. Center of the anterior part of the greater trochanter of

the femur 3. Proximal attachment of the lateral collateral ligament of

the stifle joint to the femur 4. Attachment of the long lateral ligament of the tibiotarsal

joint to the plantar border of the calcaneus 5. Space between the fourth tarsal, the third metatarsal, and

the fourth metatarsal bones 6. Proximal attachment of the lateral collateral ligament of the

fetlock joint to the distal end of the third metatarsal bone 7. Dorsal edge of the coronary band

The major disadvantages in using these methods are the possible errors introduced by marker placement on skeletal landmarks, particularly in the proximal skeleton, the consequent reliability of findings, and the time required to perform the measurements (Weller et al., 2006a). Radiography has also been used to measure joint angles and segment lengths. However, this requires expensive equipment, has the health and safety implications of possible radiation exposure to personnel involved, and is very sensitive to subject positioning (Barr, 1994; White et al., 2008).

Advancing technology has allowed more objective, quantitative evaluation of conformation amenable to statistical

analysis and aims to find evidencebased relationships among conformation, performance, and soundness. This has resulted in verification of some traditional empirical ideals and refuting of others, though results are often conflicting. For global advancement in this area of study, it is clearly imperative to use universally comparative methodology, which is somewhat lacking. Objective conformational evaluation provides a useful adjunct to subjective assessment by quantification of some conformational traits; however, it must be remembered that not all conformational aspects can be measured objectively. Aesthetic factors such as athletic elegance, suppleness, overall balance and harmony, jumping style, and movement symmetry are necessarily subjectively based.

Photography or Videography and Digital PhotographyPhotographic analysis employing reference skin markers has been used widely to assess whole animal linear and angular conformation. The advantage of this technique is the ability to assess large numbers of horses quickly. However, several potential sources of error are introduced: distortion of measurements due to the lens angle and distance of camera placement relative to the horse; geometric error when a threedimensional object is assessed in two dimensions; and limited accuracy of repeatable manual placement and identification of markers. Intra operator repeatability for manual identification of markers on photographs in one study was below 1% for length measurements but was around 10% for segment inclinations (Weller et al., 2006a). The largest interoperator variations were found at the carpus (length) and inclinations related to the tuber sacrale, humerus, and stifle markers. In addition, whole animal assessment requires a minimum of four photographs (left and right lateral, cranial, and caudal) which is difficult to attain in uniform stance without subject movement. When using this technique, the camera should ideally be directed at the center of the horse’s thorax, and a reference frame should be included in the camera view to scale and validate the measurements taken (Figure 156).

FIGURE 15-6 Photographic analysis using a reference marker set. (Pho-tograph courtesy of Hilary Clayton, Mary Anne McPhail Equine Performance Centre, School of Veterinary Science, Michigan State University)


In contrast to whole animal assessment, digital photography was compared with radiographic imaging in a small population and found to be a precise, easily applied method for objective external assessment of equine foot conformation both in clinical and research settings (White et al., 2008). This technique is dependent on having a horse compliant enough to stand stationary on low wooden blocks for long enough to complete image acquisition. In contrast, Weller et al. (2006a) compared twodimensional digital photography with threedimensional motion analysis and showed the biggest intraoperator (hoof angles) and intrasubject (heel measurements) errors during foot conformation assessment. A possible explanation for this finding is geometric error caused by the camera angle, as the lens was directed toward the tuber olecrani rather than specifically at the hoof in the latter study.

Motion SystemsThe sources of error using photography are primarily overcome when conformational parameters are measured by using threedimensional computerized motion analysis systems, although similar experimental flaws with marker placement reliability and stance of the horse will occur. The major advantages of using motion analysis include the facility to acquire rapid, repetitive data without subject movement, the relative independence of camera angle relative to the horse, and the redundancy of postacquisition identification of markers by an operator. Studying whole animal conformation requires a minimum of four (ideally eight for bilateral data acquisition) video cameras, customwritten software, and an appropriate examination area, so current use is mostly limited to research and educational facilities.

INFLUENCES ON CONFORMATION

Before evaluating the current literature in this area, it should be mentioned that most studies either compare elite performance horses with inferior performers or evaluate only elite horses and so may not be representative of all horses. Though significant conformational differences are apparent between these groups, horses with severe conformational anomalies are unlikely to be represented by this research as they will have been rejected from studbook admission or withdrawn from selection for performance. The effect of this is to skew the populations and limit broad application of the results.

GENETIC INHERITANCEHeredity influence is considered an important part of the jigsaw of conformation and the desirability of pedigreebased traits is dependent on the intended use of the athletic horse. Quantification of heredity influence is often expressed using either: 1. A heritability index (h2): a statistical expression of the rela

tive contribution of genetic factors to the total variance of a particular conformational trait within a population under specific conditions. The heritability index ranges from 0 (no involvement of genetics in conformation) to 1 (conformational traits entirely determined by genes). This index gives an impression of the extent of environmental influence on the expression of genetic properties.

2. A genetic correlation: quantifies the proportion of variance that two traits share due to genetic causes.

These objective measures provide a standardized platform for comparing studies. Although some trends do appear to be emerging, differing populations and methodology will still preclude evidencebased conclusions and agreement as to the absolute relationship between genotype and conformation.

In a study of Dutch Warmbloods, Ducro et al. (2009a) found a significant heritability estimate for height at the withers (h2 5 0.67) and a moderate heritability estimate for foot conformation traits (h2 5 0.16–0.27). These foot conformation traits (pastern angle, heel height, and hoof shape) had a moderate genetic correlation with each other. A moderate genetic correlation between bone circumference and hoof shape was also reported (a larger bony circumference was associated with a broader hoof shape). Other studies (Dolvik and Klemetsdal, 1999; Love et al., 2006) have shown the conformational traits of back at the knee and tied in below the knee to have breeddependent, strong heritability (h2 5 0.19–1.00). Toeing in, toeing out, offset knees, sickle hock, and straight behind are also documented as being highly heritable, often presenting in both the foal and dam (Love et al., 2006; Ross, 2003). Carpal conformation of yearling Thoroughbreds has been associated with both sire and dam carpal conformation, though no such association was found for the metacarpophalangeal joint (Leibsle et al., 2005; Santschi et al., 2006). The 8% occurrence of offset carpus reported by Santschi et al. (2006) may be genetically linked, although heavy birth weights are also strongly associated with this conformational trait (Leibsle et al., 2005). In Spanish Purebred horses, the maximum hoof height in the forelimb, range of stifle angle, elbow angle, and minimum angle of the carpus showed high heritability (h2 5 0.88; 0.74; 0.86; 0.86, respectively), although tarsal angles showed only medium levels of heritability ( h2 5 0.57) (Valera et al., 2008). Similarly, Molina et al. (1999) reported a medium to high heritability of seven zoometric properties in Spanish Purebred horses: (1) height at withers, (2) height at chest, (3) body length, (4) chest width, (5) girth circumference, (6) carpus circumference, and (7) third metacarpal (MCIII) circumference.

Although not a conformational trait per se, gait quality may be inherited as the locomotive pattern of a foal is very predictive for the mature gait of the individual horse (Back et al., 1995). It has also been proposed that natural “leggedness,” or laterality, may be inherited, as 8monthold trotting Standardbred colts had developed this trait before training commenced (Drevemo et al., 1987). A laterality preference may have implications for training or athletic performance and may also be useful in selection for an intended career. For example, if a horse is to be used in dressage or cutting, where balance requirements are vital, a lack of laterality bias would be preferable. It is apparent that this research arena offers a wide scope for further investigation.

The elusive relationship between genetics, conformation, and performance has been discussed in the literature, with little consensus of opinion and sparse evidence. Variable results have been reported when using the overall subjective trait of conformation grade to define this relationship. Ducro et al. (2009a) found conformation grade in Dutch Warmblood horses to have a moderate heritability (h2 5 0.30), accompanied by a high genetic correlation with dressage ranking (h2 5 0.67). Other authors have reported the genetic correlations between conformation score and competition results in dressage or showjumping to be moderate at best (Koenen et al., 1995; Wallin et al., 2003).


In evaluating more specific descriptive conformational traits, Ducro et al. (2009a) found moderate genetic correlations between height at the withers, neck length, limb quality, and dressage performance in competition (h2 5 0.32–0.36), although genetic correlations for foot conformation and dressage or jumping ranking were only low to moderate. These results are possibly caused by the increasing influence of environmental and human factors (such as training, rider, farriery) with increasing age and competition experience. Conformation, inherited racing ability (largely stemming from the sire pedigree), and reduced Thoroughbred performance were significantly related in horses with “back at the knee” conformation (Love et al., 2006). After including the sire as a cofactor in the analysis, this result became nonsignificant, and the authors suggested that other inherited traits such as size, muscle fiber type, or cardiorespiratory function may have been responsible for reduced performance. In this study, “turned in” or “turned out” feet were also associated with reduced racing performance, although these trends were only statistically significant for animals with the most severe defect score and only weakly significant for inheritance. Therefore, although pedigree was important, conformational defects had a limited effect on racing performance. These authors did caution against extrapolating these findings to all Thoroughbred racehorses, as severe conformational defects were unlikely to be present in the population studied.

AGEThe shape, structure, and biochemical composition of bone, tendon, and articular cartilage adapt in response to the direction, magnitude, and repetition of biomechanical loading with form following function (see Chapter 13). Tissue components such as subchondral bone mineral density and the proteoglycan or collagen network of articular cartilage have been shown to be influenced by normal loading and activity in foals during early development. The distal phalanx has also been shown to be undergoing rapid bony absorption and remodeling during this period (Kroekenstoel et al., 2006). Any conformational defects appearing in the young horse may cause a consistently abnormal loading pattern, which may disturb normal cellular activity and result in mechanical weakness, loss of energy absorbing properties, or both in these structures (Brama et al., 2009a; 2009b). Although foals are susceptible to adverse influences on musculoskeletal development in their early life, the regenerative capacity is high, even in tissues with limited repair capacity in the mature individual (Barneveld and van Weeren, 1999).

A link between age and conformation is frequently stated in the literature, although this is largely unsupported by research, particularly for foals. As early (preweaning and yearling) selection is common for breeding and performance horses, a substantiated knowledge of the normal growth pattern and the effect of age on developmental conformation, or the lack thereof, would be very advantageous for prediction of adult performance. Young sporting horses frequently have one or more conformational deviations, which may or may not persist into adult life, making appropriate early selection for performance more an intuitive art form than a science. This is well illustrated by a study in which subjective evaluation was used to compare carpal conformation in Thoroughbred foals from birth to 525 days (Santschi et al., 2006). Comparisons (cf) of the respective prevalence of carpal

deviations within a month of birth and at 525 days were combined carpal deviations 54% cf 48%; carpal valgus 94% cf 21%; outward carpal rotation 54% cf 56%; and offset carpus 8% cf 68%, respectively. The marked reduction (73%) in the incidence of carpal valgus over time indicates a natural correction of the deviation, possibly related to the distal growth physis of the radius. The 60% increase in offset carpus is not easily explainable though a true increase with age is apparent. Perhaps these two traits are biomechanically related, with a decreasing carpal valgus influencing the medial orientation of the distal carpus, thereby increasing the occurrence of offset carpus. Anderson and McIlwraith (2004) also described an increasingly offset carpus over time and a progressive change from carpal hyperextension to slightly over at the knee from weanling to 3 years. They suggested that carpal hyperextension will probably reduce with age and that selection of a yearling over at the knee is inadvisable. These authors also found an expected increase of body segment length during the same period, with an associated strong relationship between long bone length and height at the withers. This is in accordance with Mawdsley et al. (1996), who found six traits significantly linked with age which are attributable to growth and maturation of the Thoroughbred horse 2 to 3 years old. The most significant traits (p ,0.001) were height, neck circumference at the withers or manubrium, back length, and shape of the withers. In the distal limb, Kroekenstoel et al. (2006) showed a marked radiographic change in Warmblood conformation between 27 and 55 weeks of age, with a decreasing dorsal angle of the hoof and an increase in parallelism of the hoof wall to the distal phalanx. Radiographic findings demonstrated all these young horses to be broken backward during this period. Using photographic evaluation, Anderson and McIlwraith (2004) found a similar decrease in the dorsal angle of the hoof of the juvenile Thoroughbred. However, they assessed the hoof–pastern angle to be brokenforward in weanlings, straight in yearlings, and brokenback at both 2 and 3 years of age. More clarification of the developmental aspects of distal limb conformation in young horses of different breeds is necessary to draw accurate conclusions from prepurchase radiographic assessments.

Uneven feet are defined when a significant observable difference in the size and shape of the right and left forefeet is present. The proximal interphalangeal joint appears to remain aligned in both forefeet, the dorsal hoof wall angles are variant, and the center of pressure in the foot with the lower hoof angle is moved caudally (van Heel et al., 2006). A lower hoof angle creates a brokenback hoof–pastern angle, which has been shown to increase the extension moment of the distal interphalangeal joint with subsequent increased force on the navicular region and increased tension in the deep digital flexor tendon (Eliashar et al., 2004; Moleman et al., 2006). The other more upright hoof may predispose the distal limb to suspensory ligament injury, as elevated heels have been shown to increase tension in the suspensory ligament (Lawson et al., 2007) (Figure 157). The conformational trait of asymmetrical or uneven feet is initially observed and scored at studbook admission (foal age) and appears to increase in prevalence with age. Ducro et al. (2009a) studied a large population of Warmbloods over 12 years and found the incidence of uneven feet increased from less than 4.5% at year 3 to greater than 8% at year 10 of the recording. It is uncertain whether this trend is distorted by


either corrective trimming masking the defect at the original scoring or a more stringent subjective evaluation occurring with increasing age and performance requirements. Although juvenile horses with uneven feet are commonly sound, all these interrelated structures maintain digit equilibrium, and the altered biomechanical loading and stimulation of the rapidly developing musculoskeletal system may have a negative effect on tissue architecture and integrity predisposing the horse to injury or reduced performance in later life (Kroekenstoel et al., 2006). Uneven feet are not necessarily a criterion for rejection in performance horses, as the longterm clinical implications of this fault are unclear. Varying effects of uneven feet on performance have been distinguished between disciplines and levels of excellence. A marked decrease in the probability of survival and significant reduction in the median duration of competitive life was found in elite show jumpers with uneven feet (Ducro et al., 2009b). The effect on dressage performance was less compelling in this study, showing only a trend for reduced competition life in elite dressage horses. A definitive quantification of the degree of unevenness distinguishing natural variation of hoof angles from pathologic variation has yet to be determined. Early preselection in the presence of this defect has, nevertheless, been discouraged by some authors (Ducro et al., 2009a; 2009b).

Conformational traits consistent over time may have some predictive power in early selection of performance horses. Mawdsley et al. (1996) showed the Thoroughbred to have little change in head shape, neck shape, and croup length with increasing age, indicating these traits in a juvenile to be prognostic for the mature horse. Although angular joint conformation at the elbow and stifle have been shown to be significantly influenced by age in Spanish Purebred horses (Cano et al., 2002; Valera et al., 2008), the slight influence of age on other joint angles, temporal and linear variables may enable early phenotypic preselection of horses. Other studies have suggested that dynamic conformational traits unaffected by age may assist in early determination of biomechanical aptitude. For example, there have been significant correlations shown between foals and mature horses in maximum flexion of the hock during stance (Holmstrom and Drevemo, 1997), kinematics of the hindlimbs (Back et al., 1995), and jumping capacity (Bobbert et al., 2005).

The relationship among age, conformation, and performance is difficult to quantify because of the increasing

influence of environmental, experiential, and other nonconformational factors over time (Wallin et al., 2003). Performance is frequently evaluated using lifetime earnings, starting status, win percentages, best racing time, and duration of competitive life which cannot be determined until later in the sport horse’s career and do not solely reflect the performance potential of the horse (Weller et al., 2006c). Patterns of locomotion will evolve over time, particularly in the dressage and showjumping horse. More starts necessarily means an older horse, and increasing numbers of starts appears to improve all performance parameters in the Thoroughbred racehorse (Weller et al., 2006c). Further research ascertaining the complex relationship among performance, existing or developing conformational characteristics, and the effect of age, if any, is needed to validate basic selection criterion. It is prudent to note here that when studying biomechanics and the subsequent performance of young horses, any interpretation of this relationship may be confounded by such factors as the underdeveloped power, balance, and fatigability of the immature musculoskeletal system (Ducro et al., 2007). For example, when rigorous training is superimposed on the immature skeleton of the young Thoroughbred and Quarterhorse, dorsal metacarpal disease (bucked shins) may develop from fatigue failure and inadequate remodeling of the MCIII. The incidence of dorsal metacarpal disease may range between 30% and 90% (Nunamaker, 2003), which will undoubtedly affect performance results. As this condition is thought to be caused by changing inertial properties of the MCIII during growth and training, differentiation of the effect on performance of age, conformation, and orthopedic health as independent components is difficult, thus illustrating the complexities involved in this area of research.

SEXThere appears to be significant differences in conformation between mares and stallions across most documented breeds, the gelding mean values falling somewhere in between. These differences are summarized in Table 153.

BREED AND PERFORMANCE CRITERIA

The success or failure of a breed of horse in any chosen endeavor has traditionally been attributed, at least in part, to its conformation, yet most objective studies across different

A B C

FIGURE 15-7 Ideal (A), broken forward or upright (B) and brokenback (C) hoof–pastern axis.


Summary of Conformational Differences between Stallions and Mares in Different Breeds

Breed Stallions Mares

Cold blooded trotters(Dolvic and

Klemetsdal, 1999)

Higher at withersGreater cannon and carpus circumferenceWider breast

Larger girth circumference

Warmbloods Males considerably higher scores for canter, type, and total conformation

Higher performance level than mares

Shorter at withersShorter limb lengthsLonger bodiesNarrower metacarpiSmaller hock joint anglesLarger hind fetlock joints (Holmstrom

et al., 1990)

Standardbred trotters Higher and broader at withersGreater width and circumference of cannon

and carpusFlatter croupSmaller hip angleOutwardly rotated limb axes

Greater width of pelvisLonger bodyLonger distance between the last rib and the

pelvis (Magnussen, 1985)

Thoroughbred(Mawdsley et al., 1996)

Increased head and neck shapeIncreased neck circumference at the poll/larynx

and withers/manubrium of the sternumLess upright hoof pastern axis in the forelimb

Banei draft racehorses(Kashiwamura et al.,

2001)

Greater chest widthGreater cannon bone circumference

Greater hip width, croup width, and rump length

TABLE 15–3

breeds have demonstrated only a weak correlation between conformation and performance (Dolvic and Klemetsdal, 1999; Holstrom and Philipsson, 1993; Love et al., 2006). Some of these results indicate previously “undesirable” conformations may, in fact, be within normal limits or even advantageous to performance. As “ideal” conformational attributes have historically been empirically selected and bred in or out by human design, rather than by natural selection, engineered development of a “type” best suited for performance in a discipline may not have been in the horse’s best musculoskeletal interests. The demand for highspeed locomotion may have compromised the structural stability of musculoskeletal tissues, challenging the narrow safety margins for tissue failure. The variations of selected vocations within a specific breed will also affect the desirable conformation because of the sportspecific challenges on the musculoskeletal system. A thorough knowledge of the demands of each particular sport is essential to determine the compatibility of conformation and intended use, possible contraindications for selection as well as allowing for early recognition of injury manifestation. The controversial relationships among conformation, injury, and performance will be illustrated by a review of the evidencebased literature concerning the “ideal” conformation of Thoroughbred racehorses, both in flat and National Hunt disciplines. The significant animal welfare and economic issues associated with wastage in Thoroughbred racing warrant this particular attention. For more information on other breed and sportspecific conformational requirements, refer to Chapters 2027.

Height at the withers is a standard conformational selection criterion which may be used to predict the general

skeletal format of the horse. All circumference measurements and the majority of length measurements are significantly correlated with wither height in Thoroughbreds, so a taller horse may be expected to have longer lengths of neck, back, and limb segments; a larger chest circumference; and broader hooves (Weller et al., 2006b). There was no such correlation found between joint angles, inclines, or deviations in this study, so biomechanical integrity cannot be determined from wither height alone. There is some evidence that increased height improves Thoroughbred flat racing performance, as measured by lifetime earnings, starting status, and win percentages (Smith et al., 2006). Other factors may also be relevant in determining the optimal wither height for this discipline. Larger animals have an associated faster growth rate, and this may be an intrinsic factor shaping structural limb quality, perhaps weakening mechanical durability, and predisposing taller horses to conditions such as osteochondrosis (Barneveld and van Weeran, 1999; Ducro et al., 2009b). Supporting this are the findings of a radiographic study by Stock and Distl (2006), which showed increased development of osseous fragments in the fetlock and hock with increased wither height. Additionally, longer bones have an increased mass and are subject to higher moments of inertia, increasing the applied bending moments, tensile and compressive loads, and metabolic cost of locomotion. As it is generally accepted that wither height is unrelated to stride length or frequency and increased limb mass has been shown to decrease the maximal velocity of limb movement (Dellanini et al., 2003), the biomechanical advantage of a taller horse is somewhat debatable in flat racing, and the balance between performance and orthopedic health must be seriously considered. In


contrast, Thoroughbreds competing in National Hunt may benefit from extra height and longer body segments, as this has been shown to improve performance within National Hunt competitors (Weller et al., 2006c) and in the jumping performance of other breeds (Holmstrom et al., 1990).

It is commonly thought that a straight forelimb conformation is desirable for racing performance in the Thoroughbred. Recent studies appear to challenge this belief, as the average racehorse was found to have carpal hyperextension (back at the knee) and a carpal valgus deformation of around 5 degrees (Weller et al., 2006b). Although these results indicate that these deviations may be considered a normal occurrence in Thoroughbreds, there is some controversy among authors about the possible effect on performance and soundness. It is anecdotally accepted that carpal hyperextension will create high stress levels in the carpus and may contribute to fractures of the dorsal carpal bones, especially in the fatigued Thoroughbred (Johnston et al., 1999). This idea was refuted by Barr (1994), who maintained that carpal hyperextension was unlikely to play a major role in the etiopathogenesis of carpal chip fractures. Although carpal hyperextension appears to increase in a linear manner with increasing speed (Burn et al., 2006), it has not been established if there is a causal relationship between the degree of hyperextension and bony or connective tissue injury. Weller et al. (2006b; 2006c) suggest carpal valgus is detrimental to racing and jump performance, risk of injury due to an increase in loading of the superficial digital flexor tendon, or both. Interestingly, Anderson et al. (2004) showed that carpal effusion and incidence of fracture in 3yearold Thoroughbreds decreased as the carpal valgus increased, suggesting that a slight valgus may be an important protective accommodation in racehorse conformation. However, the perennial problem of lack of uniformity among research papers makes it difficult to draw definitive conclusions.

Similarly, a straight hindlimb conformation has been sought after, especially in relation to the shock absorbing tarsal joint. An “ideal” tarsal angle has yet to be established in Thoroughbreds. However, extreme joint angulation may not optimize muscular leverage and the ensuing power generation, so this may be an important consideration in the racing conformation. Small tarsal angles (sicklehocked) or large tarsal angles (straightbehind) have been implicated as risk factors in the etiology of tarsal disease, with consequent loss of performance or useful career lifetime in other breeds. Axelsson et al. (2001) used radiography to show a significant relationship between a small tarsal angle and prevalence of degenerative joint disease in the distal tarsus of Icelandic horses. In a mixed population of horses, Gnagey et al. (2006) found that a large tarsal angle absorbed less energy (concussion) compared with a small tarsal angle during the impact phase at trot, perhaps predisposing to arthritic changes. This study also identified that horses with large tarsal angles had reduced tarsal flexion and slower angular velocity during weightbearing with a trend toward less energy generation during pushoff at the end of stance. It would also be interesting to investigate whether tarsal angle has any effect on the integrity or efficiency of the reciprocal apparatus—the coupling mechanism that links the stifle and tarsus, providing a single functional lever arm in the tarsometatarsal joint.

It is suggested that toeing out of the hindlimbs is not only a common but desirable conformation for the successful

racing Thoroughbred (Anderson et al., 2004; Love et al., 2006). Whether this appearance is related to externally rotated hindlimbs or mean pastern angles greater than 180 degrees in the sagittal plane is not always clearly defined in the literature. Mawdsley et al. (1996) found the hindlimb pastern angle to cause a toed out conformation in the entire population of 120 superior Thoroughbreds between 2 and 3 years of age, concluding that this conformation is not rejected during selection for performance. Love et al. (2006) found a much lower incidence (30%) of turned out feet in yearling Thoroughbred horses; yet, in contrast, they found a tendency for lower national mean lifetime ratings with this conformation. Another study found no such hindlimb deviation in 5yearold National Hunt Thoroughbreds and proposed that this was a reflection of the age of the horse assessed (Weller et al., 2006b). The authors suggested that a straight leg may be necessary to endure the intensive demands of training and racing, so horses with inferior conformation may have already been withdrawn from the starting line.

Other conformational traits appearing to have a positive effect on Thoroughbred performance in National Hunt racing include the following (Figure 158): 1. Increased intermandibular (IM) width (Cook, 1988; Delahunty

et al., 1991; Weller et al., 2006c). It is possible that an increased IM width may have a positive effect on respiratory function because of reduced airway resistance. There is also a questionable relationship between a smaller IM width and recurrent laryngeal neuropathy (Marks, 2000), which is quite prevalent in National Hunt Thoroughbreds (Dixon et al., 2001).

FIGURE 15-8 Schematic drawing of a horse with marker positions, segment lengths, joint angles, and circumference measurements (bold lines) used in the study of Weller et al. (2006c); conformational parameters that contributed significantly to the regression models for performance or musculoskeletal injury (dotted lines). (From Weller R, Pfau T, Verheyen K, et al: The effect of conformation on orthopaedic health and performance in a cohort of National Hunt race-horses: preliminary results, Equine Vet J 38:622, 2006. Reproduced with permission).


2. Increased flexor angle of the shoulder (Weller et al., 2006c). A smaller flexor angle was shown to be detrimental for performance in this study, which infers a larger flexor angle would be positive for performance. In terms of biomechanics, a more acute (smaller) scapulohumeral joint angle may structurally limit the range of forelimb protraction, thereby affecting stride length.

3. Large lateral coxal angle—the angle between ischium and ilium (Weller et al., 2006c). An increased coxal angle showed both a lesser risk of pelvic fracture and increased performance in this study. A possible explanation for these findings are that a closer approximation of the axis of the pelvis to the alignment and direction of contraction

of gluteal musculature may help in force transmission and provide a mechanical advantage for muscular contraction, while reducing the bending moment applied to the ilium.Clinical experience based on qualitative observation often

associates a relationship between faulty conformation and musculoskeletal disease, lameness, or both in Thoroughbreds, yet very few objective studies support these observations. Table 154 summarizes the conformational traits anecdotally considered detrimental to performance or predisposing to lameness, as well as the possible biomechanical and orthopedic ramifications of the misalignments. Some references are cited for researchbased studies or controversial findings.

Conformational Traits, Applied Biomechanics, and Consequent Orthopedic Risks in Thoroughbreds

Anatomical Region and Conformational Trait Implications for Biomechanics Potential Orthopedic Outcome

RADIOCARPAL (RC)/ METACARPAL (MC) COMPLEXBack at the knee Increases tendency of carpus to extend Carpal lameness

Carpal valgus Weight concentrated on medial aspect of carpus and proximal MC region

Carpal chip or slab*

Offset knee Joint compression increases distally SDF injury; Carpal lameness; Splints

Tied in behind Diameter of flexor tendons is less proximally than distally

Splints; Injury 1/– lameness in all distal limb joints; Tendonitis SDF/DDF

METACARPOPHALANGEAL (MCP) COMPLEXMCP valgus Moves the ground reaction force vector away

from the sagittal planeSDF injury; Interference; Sub solar bruising

medial heel

Short upright pastern Increased concussion; Shorter strides SDF tendonitis (Weller et al., 2006a); Interphalan-geal joint disease 1/– lameness

Long sloping pastern Increased tension in palmar structures: SDF, DDF, suspensory ligament

SDF tendonitis; Weak significance for carpal chip* (Barr, 1994; Anderson et al., 2004); Proximal ses-amoid* ; Proximal phalanges*; Osteoarthritis of the MCP joint

TIBIOTARSAL (TT)/ METATARSAL (MT) COMPLEXStraight behind Increased stifle angle and reduced metatar-

sophalangeal angleStifle lamenessUpward fixating patellaProximal/high suspensory desmitisOsteoarthritis of metatarsophalangeal joint

Sickle hock Increases the extensor moment and vertical impulse in stance; Loads distal plantar aspect of hock

Distal tarsal diseasePlantar desmitis(Gnagey et al., 2006)Collapse middle tarsal bone

Tarsal valgus Asymmetrical loading of the hindlimb in a lateromedial direction.

Increased risk of pelvic*Increased occurrence of digital tendon sheath

effusion (Weller et al., 2006c)

TRUNKLarge girth Big body mass increases peak limb forces

and energy cost of locomotion (Weller et al., 2006c)

Increased risk of limb injury in general

TABLE 15–4

*, fracture; DDF, deep digital flexor; SDF, superficial digital flexor.


Anderson TM, McIlwraith CW: Longitudinal development of equine conformation from weanling to age 3 years in the thoroughbred, Equine Vet J 36:563, 2004.

Anderson TM, McIlwraith CW, Douay P: The role of conformation in musculoskeletal problems in the racing thoroughbred, Equine Vet J 36:571, 2004.

Axelsson M, Bjornsdottir S, Eksell P, et al: Risk factors associated with hindlimb lameness and degenerative joint disease in the distal tarsus of Icelandic horses, Equine Vet J 33:84, 2001.

Back W, Schamhardt HC, Hartman W, et al: Predictive value of foal kine-matics for the locomotor performance of adult horses, Res Vet Sci 59:64, 1995.

Barneveld A, van Weeren PR: Conclusions regarding the influence of exercise on the development of the equine musculoskeletal system with special reference to osteochondrosis, Equine Vet J 31(Suppl):112, 1999.

Barr ARS: Carpal conformation in relation to carpal chip fracture, Vet Rec 134:646, 1994.

Bobbert MF, Santamaria S, van Weeren, et al: Can jumping capacity of adult show jumping horses be predicted on the basis of submaximal free jumps at foal age? A longitudinal study, Vet J 170:212, 2005.

Brama PAJ, Frith EC, van Weeren PR, et al: Influence of intensity and changes of physical activity on bone mineral density of immature equine subchondral bone, Equine Vet J 41:564, 2009.

Brama PAJ, Holopainen J, van Weeren PR, et al: Influence of exercise and joint topography on depth-related spatial distribution of proteogly-can and collagen content in immature equine articular cartilage, Equine Vet J 41:557, 2009.

Burn JF, Portus B, Brockington C: The effect of speed and gradient on hyperextension of the equine carpus, Vet J 171:169, 2006.

Cano MR, Miro F, Vivo J, Galisteo AM: Comparative biokinematic study of young and adult Andalusian horses at the trot, J Vet Med Series A 46:91, 2002.

Cook WR: Diagnosis and grading of hereditary recurrent laryngeal neu-ropathy in the horse, J Equine Vet Sci 8:432, 1988.

Delahunty D, Webb S, Kelly EP, Smith FH: Intermandibular width and cannon bone length in “winners” versus “others,” J Equine Vet Sci 11:258, 1991.

Dellanini L, Hawkins D, Martin RBR, et al: An investigation of the interac-tions between lower limb bone morphology, limb inertial properties, and limb dynamics, J Biomech 36:913, 2003.

Dixon PM, McGorum BC, Railton DI, et al: Laryngeal paralysis: a study of 375 cases in a mixed-breed population of horses, Equine Vet J 33:452, 2001.

Dolvik NI, Klemetsdal G: Conformational traits of Norwegian Cold-blooded trotters: heritability and the relationship with performance, Acta Agric Scand Sect A Animal Sci 49:156, 1999.

Drevemo S, Fredricson I, Hjerten G, et al: Early development of gait asymmetries in trotting Standardbred colts, Equine Vet J 19:189, 1987.

Ducro BJ, Bovenhuis H, Back W: Heritability of foot conformation and its relationship to sports performance in a Dutch Warmblood horse population, Equine Vet J 41:139, 2009.

Ducro BJ, Gorissen B, van Eldik P, Back W: Influence of foot conformation on duration of competitive life in a Dutch Warmblood horse popula-tion, Equine Vet J 41:144, 2009.

Ducro BJ, Koenen EPC, van Tartwijk JMFM, Bovenhuis H: Genetic rela-tions of movement and free-jumping traits with dressage and show-jumping performance in competition of Dutch Warmblood horses, Livest Sci 107:227, 2007.

Eliashar E, McGuigan MP, Wilson M: Relationship of foot conformation and force applied to the navicular bone of sound horses at the trot, Equine Vet J 36:431, 2004.

Gnagey L, Clayton HM, Lanovaz JL: Effect of standing tarsal angle on joint kinematics and kinetics, Equine Vet J 38:628, 2006.

Holmstrom M, Drevemo S: Effects of trot quality and collection on the angular velocity in the hindlimbs of riding horses, Equine Vet J 23(Suppl):62, 1997.

Holmstrom M, Magnusson LE, Philipsson J: Variation in conformation of Swedish Warmblood horses and conformational characteristics of elite sport horses, Equine Vet J 22:186, 1990.

Holmstrom M, Philipsson J: Relationship between conformation, per-formance and health of 4 year old Swedish riding horses, Livest Prod Sci 33:293, 1993.

Johnston C, Gottlieb-Vedi M, Drevemo S, Roepstorff L: The kinematics of loading and fatigue in the Standardbred trotter, Equine Vet J 30(Suppl):249, 1999.

Kashiwamura F, Avgaandorj A, Furumura K: Relationships among body size, conformation, and racing performance in Banei draft racehorses, J Equine Sci 12:1, 2001.

Koenen EPC, van Veldhuizen AE, Brascamp EW: Genetic parameters of linear scored conformation traits and their relation to dressage and show-jumping performance in the Dutch Warmblood Riding Horse population, Livest Prod Sci 43:85, 1995.

Kroekenstoel AM, van Heel MCV, van Weeren PR, Back W: Developmen-tal aspects of distal limb conformation in the horse: the potential consequences of uneven feet in foals, Equine Vet J 38:652, 2006.

Lawson SE, Chateau H, Pourcelot P, et al: Effect of toe and heel elevation on calculated tendon strains in the horse and the influence of the proximal interphalangeal joint, J Anat 210:583, 2007.

Love S, Wyse CA, Stirk AJ, et al: Prevalence, heritability and significance of musculoskeletal conformational traits in Thoroughbred yearlings, Equine Vet J 38:597, 2006.

Magnussen LE: Studies on the conformation and related traits of Standard-bred trotters in Sweden, PhD Thesis Skara Swedish Uni of Ag Sci 194, 1985.

Marks D: Conformation and soundness, Am Assoc Equine Pract Proceed-ings 46:39, 2000.

Mawdsley A, Kelly EP, Smith FH, Brophy PO: Linear assessment of the Thoroughbred horse: an approach to conformation evaluation, Equine Vet J 28:461, 1996.

Moleman M, van Heel MCV, van Weeran PR, Back W: Hoof growth be-tween two shoeing sessions leads to a substantial increase of the moment about the distal, but not the proximal, interphalangeal joint, Equine Vet J 38:170, 2006.

Molina A, Valera M, Dos Santos R, Rodero A: Genetic parameters of mor-phofunctional traits in Andalusian horse, Livest Prod Sci 60:295, 1999.

Nunamaker DM: The bucked shin complex. In Ross MW, Dyson SJ, editors: Diagnosis and management of lameness in the horse, Philadelphia, PA, 2003, WB Saunders, p 847.

Rossdale PD, Butterfield RM: Conformation: a step along the pathway to objective assessment, Equine Vet J 38:588, 2006.

Ross MW: Conformation and lameness. In Ross MW, Dyson SJ, editors: Diagnosis and management of lameness in the horse, Philadelphia, PA, 2003, WB Saunders, p 15.

Santschi EM, Leibsle SR, Morehead JP, et al: Carpal and fetlock confor-mation of the juvenile Thoroughbred from birth to yearling auction age, Equine Vet J 38:604, 2006.

Smith A, Staniar W, Splan R: Associations between yearling body mea-surements and career racing performance in Thoroughbred race-horses, J Equine Vet Sci 26:212, 2006.

Stock KF, Distl O: Genetic correlations between conformation traits and radiographic findings in the limbs of German Warmblood riding horses, Genet Sel Evol 38:657, 2006.

Valera M, Galisteo AM, Molina A, et al: Genetic parameters of biokine-matic variables of the trot in Spanish Purebred horses under experi-mental treadmill conditions, Vet J 178:217, 2008.

van Heel MCV, Kroekenstoel AM, van Dierendonck MC, et al: Uneven feet in a foal may develop as a consequence of lateral grazing behaviour induced by conformational traits, Equine Vet J 38:646, 2006.

Wallin L, Strandberg E, Philipsson J: Phenotypic relationship between test results of Swedish Warmblood horses as 4 year olds and longev-ity, Livest Prod Sci 68:97, 2001.

Wallin L, Strandberg E, Philipsson J: Genetic correlations between field test results of Swedish Warmblood Riding Horses as 4-year-olds and lifetime performance results in dressage and show jumping, Livest Prod Sci 82:61, 2003.

Webster’s Third New International Dictionary, 1976.Weller R, Pfau T, Babbage D, et al: Reliability of conformational measure-

ments in the horse using a three-dimensional motion analysis sys-tem, Equine Vet J 38:610, 2006.

Weller R, Pfau T, May SA, Wilson AM: Variation in conformation in a cohort of National Hunt racehorses, Equine Vet J 38:616, 2006.

Weller R, Pfau T, Verheyen K, et al: The effect of conformation on ortho-paedic health and performance in a cohort of National Hunt race-horses: preliminary results, Equine Vet J 38:622, 2006.

White JM, Mellor DJ, Duz M, et al: Diagnostic accuracy of digital photog-raphy and image analysis for the measurement of foot conformation in the horse, Equine Vet J 40:623, 2008.

REFERENCES

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The Athletic Horse || Conformation