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Exercise during training is associated with racing performance in Thoroughbreds
The Veterinary Journal 181 (2009) 43–47
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The Veterinary Journal journal homepage: www.elsevier.com/locate/tvjl
Exercise during training is associated with racing performance in Thoroughbreds Kristien L.P. Verheyen a,*, Joanna S. Price b, James L.N. Wood c a b c
Department of Veterinary Clinical Sciences, Royal Veterinary College, Hertfordshire AL9 7TA, UK Department of Veterinary Basic Sciences, Royal Veterinary College, London NW1 0TU, UK Department of Veterinary Medicine, University of Cambridge, Cambridge CB3 0ES, UK
a r t i c l e Keywords: Horse Thoroughbred Race performance Training Exercise Injury
i n f o
a b s t r a c t This study aimed to determine the effects of exercise on racecourse performance in horses racing on the flat. Daily exercise and race records were obtained over a 2-year period for a cohort of racehorses in training for which injury data were also available. Multivariable regression techniques were used to investigate associations between canter, training gallop and race distances accumulated in the 30 days prior to each race and the odds of winning the race, earning prize money and the amount of prize money won. Higher cumulative high-speed (gallop + race) distances were associated with increased likelihood of winning a race and earning prize money. Having raced in the previous 30 days increased the odds of winning. There was an interactive effect of distance cantered and galloped during training on amount of prize money won, which was also associated with distance raced in the previous 30 days. Taken together with findings from previous injury studies in the same study population, these results indicate that training regimens designed to reduce skeletal injuries are unlikely to adversely affect race performance. Ó 2009 Elsevier Ltd. All rights reserved.
Introduction Recent epidemiological studies have shown exercise to be related to the risk of skeletal injury in Thoroughbred racehorses in training (Parkin et al., 2004; Verheyen et al., 2005; Cogger et al., 2006; Verheyen et al., 2006a, 2006b). Results from some of these reports suggest that decreasing canter exercise in favour of highspeed exercise, preferably introduced early in the training program, may reduce the risk of fracture and dorsometacarpal disease (DMD). Although it is possible to use findings from such studies to make recommendations on training regimens that are likely to reduce the incidence of these injuries, adjusted training regimens should not compromise a horse’s potential to succeed on the racecourse. To the best of our knowledge, no scientific reports have related exercise undertaken during training to racing performance in Thoroughbreds. The aim of the study reported here was to identify factors associated with race performance in our study population and, in particular, exercise distance covered during training. We hypothesised that the odds of winning a race or earning any prize money increased with increasing high-speed exercise distance undertaken during training and that the amount of high-speed exercise was positively associated with the amount of prize money won. The objectives of this study were to investigate the associations between exercise distances performed in the 30-day period
* Corresponding author. Tel.: +44 1707 666625; fax: +44 1707 666574. E-mail address: [email protected] (K.L.P. Verheyen). 1090-0233/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tvjl.2009.03.008
preceding a race and the odds of winning, earning any prize money and the amount of prize money won. A 30-day period was chosen to allow comparison with previous studies in the same study population that reported associations between exercise in a 30-day period and risk of fracture and DMD (Verheyen et al., 2005; Verheyen et al., 2006a, 2006b). Materials and methods Data collection The study population consisted of Thoroughbred racehorses from 13 trainers for which detailed exercise data were available following their participation in a largescale epidemiological study of injuries in horses in flat-race training in England (Verheyen and Wood, 2004). Details of the design of the main study and methods of exercise data collection have been described elsewhere (Verheyen et al., 2006a). Exercise data were collected between October 1998 and October 2000, and race records for study horses were obtained for the years 1999 and 2000 from Raceform Interactive (Raceform Ltd, Newbury, UK) (flat races only). For horses that joined the study in the autumn of 1998, race records were manually obtained from the Racing Post online database1 for the period October to December 1998. Race data included date, type and class of race, finishing position and prize money won. For each race event, total exercise distance, distances exercised at canter and gallop during training and distance raced in the 30-day period preceding the ‘case race’ were calculated (distances are expressed in furlongs (f); 1 furlong is approximately 200 m), as well as the number of races run in the 30 days prior to but excluding the case race. Other variables studied were horse, age, gender and trainer.
1
See: http://www.racingpost.co.uk/horses.
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K.L.P. Verheyen et al. / The Veterinary Journal 181 (2009) 43–47
Statistical analyses Exposure variables were related to three possible measures of race performance: (1) whether the horse won the race (binary outcome); (2) whether it won any prize money (binary outcome), and (3) the amount of prize money won (continuous outcome). Exercise distances (total, canter, gallop, race, high-speed [gallop + race]) in the 30-day period prior to the ‘case race’ were initially investigated as categorical variables, the categories being dictated by quartile values. The shape of the association between exercise variables and the outcome of interest was explored and likelihood ratio tests for departure from linear trend were conducted where warranted. Variables were subsequently modelled as continuous with or without inclusion of quadratic terms in the models as appropriate. Races where prior exercise data were missing for one day or more were excluded from all analyses. Logistic regression analysis was used for binary outcome variables, including horse as a random effects term in multivariable models to account for clustering of outcome measures at the horse level. All variables were assessed for inclusion in multivariable models and plausible interaction terms were tested for. Exercise variables were modelled in continuous association with the outcome where appropriate. Linear regression was used to model the association between exposure variables and prize money. The natural logarithmic (ln) transformation of prize money (in £) was used in the analyses in order to normalise the distribution of this outcome measure, increasing the likelihood of normally distributed residuals. This approach resulted in observations where no prize money was won being excluded from analyses and, therefore, analyses were also conducted using ln (prize money + 1) as the outcome. However, analysis of residuals from these models indicated poor model fit and therefore no results for this outcome variable are presented. In order to assess the linearity of the association with the outcome and inform the modelling approach, exercise distances were initially investigated as categorical variables. Mixed effects multivariable linear regression models were constructed, including horse as a random effects term to account for correlation of outcome measures within horse. All variables were assessed for inclusion in multivariable models. Analyses were performed in STATA (Intercooled STATA 8.2 for Windows 98/95/NT, StataCorp LP) and the level of statistical significance was set at P = 0.05.
Results Descriptive results During the study period, 860 study horses from 13 trainers ran 5210 flat races, the majority (89%) of which were on turf. The mean number of races per horse was 6.1, the median 5, minimum 1, and maximum 37. Of these races, 25% (n = 1302) were run by 2-yearolds, 47% (n = 2473) by 3-year-olds and 28% (n = 1435) by horses older than 3 years. The majority of races (67%) were run by male horses (colts and geldings combined), 33% by females. A total of 755 races (14% of the 5210) were won by 425 horses. Prize money was won by 655 of 860 horses (76%) in 2537 events. The mean prize money won per race start was £18682 (median £0, minimum £0 and maximum £166,850). Excluding observations where no money was won, mean prize money was £3836 (median £1356, minimum £87 and maximum £166,850). Table 1 summarises descriptive statistics of exercise distances covered during training and racing in a 30-day period prior to racing. Exercise data were incomplete in 1194 observations (23%) that were therefore excluded from analyses. On 54 occasions (1%), no high-speed exercise was performed in the 30 days preceding a race.
Table 1 Descriptive statistics of total exercise distance, exercise distance at canter and gallop in training and distance raced in a 30-day period prior to a race event (in furlongs).
Total exercise distance Canter distance Gallop distance Race distance High-speed distance (gallop + race)
Mean
Median
Minimum
Maximum
216.6 186.2 21.9 8.5 30.4
207.0 173.8 21.0 7.0 30.0
30 13 0 0 0
443 443 82 100 100
due to distance raced in that period rather than galloping at home. Horses that had raced in the month preceding the case race were 1.23 times more likely to win (95% CI = 1.03, 1.48; P = 0.03) but not more likely to earn any prize money (P = 0.20). Univariable effects of trainer (fitted as a fixed effect) and horse (fitted as a random term in an intercept-only model) were highly significant (both P < 0.001). Age was not significantly associated with odds of winning (P = 0.12) but 3-year-olds were less likely to earn prize money than 2-year-olds (OR = 0.83; 95% CI = 0.73, 0.95; P = 0.008). Females were less likely than males to win (OR = 0.85; 95% CI = 0.72, 1.01; P = 0.07) or earn prize money (OR = 0.79; 95% CI = 0.71, 0.89; P < 0.001). Multivariable analyses Final multivariable models for binary outcome measures have been summarised in Table 2. Amount of high-speed exercise in a 30-day period, trainer, horse and age were all factors significantly associated with the likelihood of winning a race or earning any prize money. Trainer odds ratios for ‘winning’ varied from 0.25 (95% CI = 0.14, 0.45; P < 0.001) to 1.84 (95% CI = 1.03, 3.30; P = 0.04) compared to the baseline trainer, and from 0.20 (95% CI = 0.13, 0.31; P < 0.001) to 1.38 (95% CI = 0.76, 2.53; P = 0.29) for ‘earning any prize money’. A graphical representation of the association between high-speed exercise in the 30-day period and the binary outcomes is displayed in Fig. 1. Horses that had raced in the month preceding the case race were 1.3 times more likely to win than those that had not. When high-speed exercise was divided into distance galloped during training and distance raced, galloping during training was not significantly associated with the likelihood of winning or earning any prize money (P = 0.20 and P = 0.43, respectively). Distance raced in the 30 days was significantly associated with the odds of winning the case race (OR = 1.02/f; 95% CI = 1.00, 1.03; P = 0.007) and earning prize money (OR = 1.02/f; 95% CI = 1.01, 1.03; P < 0.001). For example, a horse that had accumulated 12 f (75th percentile) in race distance in the previous 30 days was at 1.2 times the odds (or 20% more likely) to win the case race, or at 1.3 times the odds (30% more likely) to be placed, compared to a horse that had accumulated 0 f (25th percentile) race distance in the same period. Gender was not significantly associated with the binary race performance outcomes in multivariable analysis. Linear regression analyses
Logistic regression analyses Univariable analyses Total exercise and canter distance in a 30-day period did not affect the odds of winning a race (P = 0.96 and P = 0.85, respectively) or earning any prize money (P = 0.19 and P = 0.27, respectively). Increasing amounts of high-speed exercise in 30 days was associated with increasing odds of winning a race (P = 0.06) and earning any prize money (P = 0.02). These effects were mainly
2
£1 = approx. €1.12, US$ 1.44 as at 15 February 2009.
No prize money was won in 2673 races (51% of all observations) that were therefore excluded from the analyses presented in this section. Univariable analyses Total and canter distance in a 30-day period were not associated with ln (prize money) when unadjusted for other exposure variables. The ln (prize money) increased with increasing distances galloped (P < 0.001), raced (P = 0.06) and total high-speed exercise distance (P = 0.001) in a 30-day period but decreased for horses in the highest categories (gallop P35 f, race >12 f, high-speed >42 f),
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K.L.P. Verheyen et al. / The Veterinary Journal 181 (2009) 43–47 Table 2 Results of multivariable mixed effects logistic regression analysis of variables associated with the likelihood of winning a race or earning any prize money. Outcome: won
Outcome: any prize money a
Odds ratio (95% CI ) High-speed distance in 30 days prior to case race (furlongs) Raced in previous 30 days
No Yes 2 3 >3
Age at race (years)
b
LRS P-value
1.01 (1.00, 1.02) Baseline 1.32 (1.08, 1.61) Baseline 0.76 (0.60, 0.95) 0.67 (0.50, 0.89)
0.004 0.006
Odds ratio (95% CIa)
LRSb P-value
1.02 (1.01, 1.02) Not significant
<0.001
0.01
0.02 0.76 (0.63, 0.92) 0.87 (0.67, 1.13)
Trainer (12 df) Horsec
<0.001 0.006
<0.001 <0.001
CI, confidence interval; df, degrees of freedom. a Confidence interval. b Likelihood ratio statistic. c Fitted as a random effects term; P-value derived from a likelihood ratio v2-test that there is no within-horse correlation.
variation in prize money (both P < 0.001). Horses older than 3 years earned on average £622 more than 2-year-olds (P < 0.001), but earnings did not differ between 2- and 3-year-olds (P = 0.65). Females earned on average £333 less than male horses (P < 0.001).
Fig. 1. Graphical representation of the association between distances exercised at high-speed (raced + galloped during training) in a 30-day period prior to a race and odds of winning a race (___) and winning any prize money (- - -). Estimated odds of winning have been adjusted for whether the horse raced in the 30-day period, trainer and horse effects (including age); estimated odds of winning any prize money have been adjusted for trainer and horse effects (including age).
suggesting inclusion of quadratic terms in the multivariable model. Horses that had raced once in the 30 days preceding a race won on average £302 more than horses that had not raced (P = 0.003), but racing four or more times in this period decreased the average prize money won. Trainer and horse (fitted as a random effects term in an intercept-only model) had a significant effect on the
Multivariable analyses The final multivariable model is presented in Table 3. The intraclass correlation coefficient (ICC), describing the correlation between observations within horse, was 0.22. This high ICC suggests that 22% of variation in prize money, unexplained by fixed variables in the model, occurred at the horse level. Distances cantered and galloped in training in the 30 days before a race were associated with prize money won, with an interaction between the two. A graphical representation of the association between galloping exercise and prize money for different levels of canter exercise is shown in Fig. 2. Higher canter distances were associated with higher average prize money, but only if galloping exercise was kept below about 24 f. Horses galloping more than 24 f in a 30-day period earned, on average, more prize money with decreasing canter distance, although this effect levelled out or decreased for the highest gallop distances. Fig. 3 shows the association between distance raced in a 30-day period and the average ln (prize money) won, adjusted for other variables in the model (Table 3). Prize money increased as cumulative race distance increased up to 40 f, after which the effect levelled out and then decreased for cumulative race distances over 50 f. When adjusted for trainer and exercise variables, horses older than 3 years earned significantly more prize money than
Table 3 Results of multivariable mixed effects linear regression analysis of variables associated with ln (prize money). Exercise variables represent distances undertaken in a 30-day period prior to racing. b-Coefficient Intercept Canter distance (furlongs) Gallop distance (furlongs) (Gallop distance)2 (furlongs2) Canter distance Gallop distance (furlongs2) Race distance (furlongs) (Race distance)2 (furlongs2) Age at race (years)
2 3 >3
6.000 0.003 0.038 2.48 10 1.12 10 0.020 2.32 10 Baseline 0.017 0.231
4 4
4
Trainer (12 df) Horsea Estimate of between-horse variation Estimate of within-horse correlation
95% CI 5.436, 6.565 0.001, 0.004 0.016, 0.059 4.67 10 4, 1.84 10 4, 0.010, 0.031 4.08 10 4, 0.164, 0.130 0.031, 0.431
P-value
0.29 10 0.39 10
4
0.57 10
4
4
<0.001 0.002 <0.001 0.03 0.003 <0.001 0.01 0.82 0.02 <0.001 <0.001
0.553 0.222
CI, confidence interval; df, degrees of freedom. a Fitted as a random effects term; P-value derived from a v2-test that there is no within-horse correlation.
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K.L.P. Verheyen et al. / The Veterinary Journal 181 (2009) 43–47 7.4 7.3
Ln(prize money)
7.2 7.1 7 6.9 6.8 6.7 6.6 6.5 6.4 0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
Distance galloped during training in 30 days (furlongs) Canter=50f
Canter=150f
Canter=100f Canter=250f
Canter=200f Canter=350f
Canter=300f
Fig. 2. Graphical representation of the association between distances cantered and galloped during training in a 30-day period prior to a race and average prize money won (natural logarithm scale). Estimates have been adjusted for race distance in the same period, trainer and horse effects (including age).
Ln(prize money)
6.8 6.7 6.6 6.5 6.4 6.3 0
5
10
15
20
25
30
35
40
45
50
Distance raced in 30 days (furlongs) Fig. 3. Graphical representation of the association between distance raced in a 30day period and average prize money won (natural logarithm scale). Estimates have been adjusted for canter and training gallop distances in the same period, trainer and horse effects (including age).
2-year-olds (£105 on average) but there was no difference in amount of prize money won between 2- and 3-year-olds. Gender was not significantly associated with amount of prize money won when adjusted for the variables shown in Table 3 (P = 0.16). Discussion Previous studies have assessed the effects of a variety of factors on race performance in Thoroughbreds, including radiographic measurements (Kane et al., 2003; Spike-Pierce and Bramlage, 2003), orthopaedic injury (Martin et al., 1988; Kannegieter and Ryan, 1991; Martin, 2000), and respiratory conditions (Strand et al., 2000; Stick et al., 2001). To our knowledge, the study reported here is the first to investigate the association between exercise undertaken prior to a race and racing performance. The amount of high-speed exercise undertaken affected the odds of a horse winning a race as well as the likelihood of it winning prize money. The odds of winning were mainly associated with racing experience in the previous 30 days, suggesting that horses with an intense racing schedule were more successful. However, the direction of any potentially causal association cannot be determined from our study: it is possible that horses were racing more because they were being successful and remained sound rather than being successful because they were accumulating higher race distances. By using the natural logarithmic transformation of prize money won to assess the effects of exercise on this outcome variable, analyses excluded observations where prize money was zero. Therefore, results of these analyses can be applied only to horses
capable of winning prize money (76% of horses included in the analyses reported here) and should not be extrapolated to the general racehorse population. The combination of canter and gallop exercise during training affected the amount of prize money won. Higher canter distances in the 30 days before a race were associated with higher average prize money, but only if galloping was kept to a minimum. Horses undertaking more than 24 f of galloping in the 30 days leading up to a race won on average more prize money if they did less cantering exercise. Further investigation of our data (not shown) revealed that horses racing in longer races accumulated higher canter distances in the previous 30 days, but no association was found between distance galloped in the previous 30 days and distance of the case race. However, longer races were associated with higher amounts of prize money being available, which may partly explain our finding that higher canter distances in training were associated with higher amounts of prize money won. The average amount of prize money won increased with accumulation of race distance in the 30-day period, reaching a plateau at around 40 f. However, the 95th percentile for race distance accumulated in 30 days was 31 f, and thus apparent associations with prize money won above this level should be interpreted with caution. The race variables in the model shown in Table 3 could be substituted for number of races and its squared term (results not shown), suggesting a beneficial effect of some but not excessive racing. As race distance and number of races were highly correlated, it is not clear whether it is the racing experience per se that affects prize money won in subsequent races or the accumulation of exercise at racing speed. The latter may have a beneficial effect on bone’s adaptive response to exercise thus protecting horses from injury while at the same time improving their race-fitness, up to certain level, beyond which more racing becomes detrimental. Race records of 860 of the 1178 horses monitored in previous epidemiological studies of fracture (Verheyen and Wood, 2004; Verheyen et al., 2006a, 2006b) were included in the analyses reported here. Further examination of race records from the Racing Post3 and the injury data available to us revealed that 1053 (89%) of the study horses ever raced, and that the probability of appearing on the racecourse was not significantly different among horses that suffered an exercise-induced fracture during the study period (n = 148 of which 130 [88%] ever raced) and those that did not (n = 1030 of which 923 [90%] ever raced]). It is therefore unlikely that our study population of horses that raced was biased toward those that did not incur a fracture, and thus we combined findings from the current study with those from previous injury studies to establish whether training regimens aimed at reducing injuries might adversely affect racecourse performance. The results from our injury studies indicated a potentially protective effect of accumulating high-speed exercise with regard to skeletal injury (Verheyen et al., 2006a), whereas higher amounts of cantering, especially within short time periods, have been associated with an increased risk of injury, in particular stress fractures (Verheyen et al., 2006b). It therefore seems reasonable to advise trainers to try to adjust training schedules in favour of more high-speed and less canter exercise, in order to both reduce the risk of injury and create a successful racehorse. However, the exercise variables studied were only a 30-day ‘snapshot’ of the horses’ training regimen and other training-related factors may be important. Further analyses should investigate associations between exercise undertaken in different time periods and measures of racecourse performance. Not surprisingly, there was a strong horse effect associated with race outcomes, indicating unmeasured variables at the horse level
3
See: http://racingpost.co.uk/horses.
K.L.P. Verheyen et al. / The Veterinary Journal 181 (2009) 43–47
that affect the likelihood of success on the racecourse. Genetic potential is likely to play a role (Hintz, 1980; Gaffney and Cunningham, 1988), as well as environmental factors not measured in this study. In our study population, 3-year-olds were less likely to win races or earn any prize money than 2-year-olds. When investigating this further, it appeared that the significance of this effect was due to one trainer who had a good success rate with 2-year-olds. When data from this trainer were excluded, the age effects became non-significant. It may therefore be inappropriate to extrapolate the effects of age on the odds of winning or earning any prize money to the general racehorse population. However, the amount of prize money won was still marginally significantly higher for horses older than 3 (compared to 2-yearolds) when data from this trainer were excluded (P = 0.09). Although gender affected measures of race performance in univariable analysis, these effects became non-significant when adjusting for other exposures, in particular exercise variables. This indicated that the univariable gender effects could partly be explained by differences in training and racing patterns between males and females, although we did not investigate these differences in further detail. A study in New Zealand Thoroughbreds found that 3- and P5-year-old male horses were more likely to win than females in the same age groups (Perkins et al., 2004), although these findings were based on simple comparisons that did not take account of potential confounders. The trainer was also associated with horses’ race performances in multivariable analyses, which suggested that factors at the trainer level beyond those included in the models were associated with racing success. Horse management, nutrition and veterinary care may be important in this respect, as well as differences in quality of horses between trainers. Consistent with this, a study of race performance in Australian Thoroughbreds also found a strong trainer-level effect on race earnings in the first year of racing (More, 1999), although this study did not take account of detailed exercise data that may partly explain trainer-level variation. Conclusions Decreasing the amount of canter exercise during training in favour of more high-speed exercise (an approach that we have previously suggested will decrease the risk of fracture and DMD) is unlikely to adversely affect race performance in on-the-flat races.
Conflict of interest statement None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
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Acknowledgements The work presented here resulted from a grant funded by the Horserace Betting Levy Board and a scholarship provided by The Horse Trust. We are grateful to all trainers and their staff who provided us with the exercise data without which this study would not have been possible. We also thank Raceform Ltd. for providing us with the relevant race-performance records. References Cogger, N., Perkins, N., Hodgson, D.R., Reid, S.W., Evans, D.L., 2006. Risk factors for musculoskeletal injuries in 2-year-old thoroughbred racehorses. Preventive Veterinary Medicine 74, 36–43. Gaffney, B., Cunningham, E.P., 1988. Estimation of genetic trend in racing performance of thoroughbred horses. Nature 332, 722–724. Hintz, R.L., 1980. Genetics of performance in the horse. Journal of Animal Science 51, 582–594. Kane, A.J., McIlwraith, C.W., Park, R.D., Rantanen, N.W., Morehead, J.P., Bramlage, L.R., 2003. Radiographic changes in thoroughbred yearlings. Part 2: associations with racing performance. Equine Veterinary Journal 35, 366–374. Kannegieter, N.J., Ryan, N., 1991. Racing performance of thoroughbred horses after arthroscopic surgery of the carpus. Australian Veterinary Journal 68, 258–260. Martin, G.S., 2000. Factors associated with racing performance of Thoroughbreds undergoing lag screw repair of condylar fractures of the third metacarpal or metatarsal bone. Journal of the American Veterinary Medical Association 217, 1870–1877. Martin, G.S., Haynes, P.F., McClure, J.R., 1988. Effect of third carpal slab fracture and repair on racing performance in thoroughbred horses: 31 cases (1977–1984). Journal of the American Veterinary Medical Association 193, 107–110. More, S.J., 1999. A longitudinal study of racing Thoroughbreds: performance during the first year of racing. Australian Veterinary Journal 77, 105–112. Parkin, T.D., Clegg, P.D., French, N.P., Proudman, C.J., Riggs, C.M., Singer, E.R., Webbon, P.M., Morgan, K.L., 2004. Horse-level risk factors for fatal distal limb fracture in racing Thoroughbreds in the UK. Equine Veterinary Journal 36, 513– 519. Perkins, N.R., Reid, S.W.J., Morris, R.S., 2004. Profiling the New Zealand thoroughbred racing industry. 1. Training, racing and general health patterns. New Zealand Veterinary Journal 53, 59–68. Spike-Pierce, D.L., Bramlage, L.R., 2003. Correlation of racing performance with radiographic changes in the proximal sesamoid bones of 487 thoroughbred yearlings. Equine Veterinary Journal 35, 350–353. Stick, J.A., Peloso, J.G., Morehead, J.P., Lloyd, J., Eberhart, S., Padungtod, P., Derksen, F.J., 2001. Endoscopic assessment of airway function as a predictor of racing performance in thoroughbred yearlings: 427 cases (1997–2000). Journal of the American Veterinary Medical Association 219, 962–967. Strand, E., Martin, G.S., Haynes, P.F., McClure, J.R., Vice, J.D., 2000. Career racing performance in Thoroughbreds treated with prosthetic laryngoplasty for laryngeal neuropathy: 52 cases (1981–1989). Journal of the American Veterinary Medical Association 217, 1689–1696. Verheyen, K.L., Henley, W.E., Price, J.S., Wood, J.L., 2005. Training-related factors associated with dorsometacarpal disease in young Thoroughbred racehorses in the UK. Equine Veterinary Journal 37, 442–448. Verheyen, K.L., Newton, J.R., Price, J.S., Wood, J.L., 2006b. A case-control study of factors associated with pelvic and tibial stress fractures in Thoroughbred racehorses in training in the UK. Preventive Veterinary Medicine 74, 21–35. Verheyen, K., Price, J., Lanyon, L., Wood, J., 2006a. Exercise distance and speed affect the risk of fracture in racehorses. Bone 39, 1322–1330. Verheyen, K.L.P., Wood, J.L.N., 2004. Descriptive epidemiology of fractures occurring in British thoroughbred racehorses in training. Equine Veterinary Journal 36, 167–173.
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Exercise during training is associated with racing performance in Thoroughbreds Kristien L.P. Verheyen a,*, Joanna S. Price b, James L.N. Wood c a b c
Department of Veterinary Clinical Sciences, Royal Veterinary College, Hertfordshire AL9 7TA, UK Department of Veterinary Basic Sciences, Royal Veterinary College, London NW1 0TU, UK Department of Veterinary Medicine, University of Cambridge, Cambridge CB3 0ES, UK
a r t i c l e Keywords: Horse Thoroughbred Race performance Training Exercise Injury
i n f o
a b s t r a c t This study aimed to determine the effects of exercise on racecourse performance in horses racing on the flat. Daily exercise and race records were obtained over a 2-year period for a cohort of racehorses in training for which injury data were also available. Multivariable regression techniques were used to investigate associations between canter, training gallop and race distances accumulated in the 30 days prior to each race and the odds of winning the race, earning prize money and the amount of prize money won. Higher cumulative high-speed (gallop + race) distances were associated with increased likelihood of winning a race and earning prize money. Having raced in the previous 30 days increased the odds of winning. There was an interactive effect of distance cantered and galloped during training on amount of prize money won, which was also associated with distance raced in the previous 30 days. Taken together with findings from previous injury studies in the same study population, these results indicate that training regimens designed to reduce skeletal injuries are unlikely to adversely affect race performance. Ó 2009 Elsevier Ltd. All rights reserved.
Introduction Recent epidemiological studies have shown exercise to be related to the risk of skeletal injury in Thoroughbred racehorses in training (Parkin et al., 2004; Verheyen et al., 2005; Cogger et al., 2006; Verheyen et al., 2006a, 2006b). Results from some of these reports suggest that decreasing canter exercise in favour of highspeed exercise, preferably introduced early in the training program, may reduce the risk of fracture and dorsometacarpal disease (DMD). Although it is possible to use findings from such studies to make recommendations on training regimens that are likely to reduce the incidence of these injuries, adjusted training regimens should not compromise a horse’s potential to succeed on the racecourse. To the best of our knowledge, no scientific reports have related exercise undertaken during training to racing performance in Thoroughbreds. The aim of the study reported here was to identify factors associated with race performance in our study population and, in particular, exercise distance covered during training. We hypothesised that the odds of winning a race or earning any prize money increased with increasing high-speed exercise distance undertaken during training and that the amount of high-speed exercise was positively associated with the amount of prize money won. The objectives of this study were to investigate the associations between exercise distances performed in the 30-day period
* Corresponding author. Tel.: +44 1707 666625; fax: +44 1707 666574. E-mail address: [email protected] (K.L.P. Verheyen). 1090-0233/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tvjl.2009.03.008
preceding a race and the odds of winning, earning any prize money and the amount of prize money won. A 30-day period was chosen to allow comparison with previous studies in the same study population that reported associations between exercise in a 30-day period and risk of fracture and DMD (Verheyen et al., 2005; Verheyen et al., 2006a, 2006b). Materials and methods Data collection The study population consisted of Thoroughbred racehorses from 13 trainers for which detailed exercise data were available following their participation in a largescale epidemiological study of injuries in horses in flat-race training in England (Verheyen and Wood, 2004). Details of the design of the main study and methods of exercise data collection have been described elsewhere (Verheyen et al., 2006a). Exercise data were collected between October 1998 and October 2000, and race records for study horses were obtained for the years 1999 and 2000 from Raceform Interactive (Raceform Ltd, Newbury, UK) (flat races only). For horses that joined the study in the autumn of 1998, race records were manually obtained from the Racing Post online database1 for the period October to December 1998. Race data included date, type and class of race, finishing position and prize money won. For each race event, total exercise distance, distances exercised at canter and gallop during training and distance raced in the 30-day period preceding the ‘case race’ were calculated (distances are expressed in furlongs (f); 1 furlong is approximately 200 m), as well as the number of races run in the 30 days prior to but excluding the case race. Other variables studied were horse, age, gender and trainer.
1
See: http://www.racingpost.co.uk/horses.
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Statistical analyses Exposure variables were related to three possible measures of race performance: (1) whether the horse won the race (binary outcome); (2) whether it won any prize money (binary outcome), and (3) the amount of prize money won (continuous outcome). Exercise distances (total, canter, gallop, race, high-speed [gallop + race]) in the 30-day period prior to the ‘case race’ were initially investigated as categorical variables, the categories being dictated by quartile values. The shape of the association between exercise variables and the outcome of interest was explored and likelihood ratio tests for departure from linear trend were conducted where warranted. Variables were subsequently modelled as continuous with or without inclusion of quadratic terms in the models as appropriate. Races where prior exercise data were missing for one day or more were excluded from all analyses. Logistic regression analysis was used for binary outcome variables, including horse as a random effects term in multivariable models to account for clustering of outcome measures at the horse level. All variables were assessed for inclusion in multivariable models and plausible interaction terms were tested for. Exercise variables were modelled in continuous association with the outcome where appropriate. Linear regression was used to model the association between exposure variables and prize money. The natural logarithmic (ln) transformation of prize money (in £) was used in the analyses in order to normalise the distribution of this outcome measure, increasing the likelihood of normally distributed residuals. This approach resulted in observations where no prize money was won being excluded from analyses and, therefore, analyses were also conducted using ln (prize money + 1) as the outcome. However, analysis of residuals from these models indicated poor model fit and therefore no results for this outcome variable are presented. In order to assess the linearity of the association with the outcome and inform the modelling approach, exercise distances were initially investigated as categorical variables. Mixed effects multivariable linear regression models were constructed, including horse as a random effects term to account for correlation of outcome measures within horse. All variables were assessed for inclusion in multivariable models. Analyses were performed in STATA (Intercooled STATA 8.2 for Windows 98/95/NT, StataCorp LP) and the level of statistical significance was set at P = 0.05.
Results Descriptive results During the study period, 860 study horses from 13 trainers ran 5210 flat races, the majority (89%) of which were on turf. The mean number of races per horse was 6.1, the median 5, minimum 1, and maximum 37. Of these races, 25% (n = 1302) were run by 2-yearolds, 47% (n = 2473) by 3-year-olds and 28% (n = 1435) by horses older than 3 years. The majority of races (67%) were run by male horses (colts and geldings combined), 33% by females. A total of 755 races (14% of the 5210) were won by 425 horses. Prize money was won by 655 of 860 horses (76%) in 2537 events. The mean prize money won per race start was £18682 (median £0, minimum £0 and maximum £166,850). Excluding observations where no money was won, mean prize money was £3836 (median £1356, minimum £87 and maximum £166,850). Table 1 summarises descriptive statistics of exercise distances covered during training and racing in a 30-day period prior to racing. Exercise data were incomplete in 1194 observations (23%) that were therefore excluded from analyses. On 54 occasions (1%), no high-speed exercise was performed in the 30 days preceding a race.
Table 1 Descriptive statistics of total exercise distance, exercise distance at canter and gallop in training and distance raced in a 30-day period prior to a race event (in furlongs).
Total exercise distance Canter distance Gallop distance Race distance High-speed distance (gallop + race)
Mean
Median
Minimum
Maximum
216.6 186.2 21.9 8.5 30.4
207.0 173.8 21.0 7.0 30.0
30 13 0 0 0
443 443 82 100 100
due to distance raced in that period rather than galloping at home. Horses that had raced in the month preceding the case race were 1.23 times more likely to win (95% CI = 1.03, 1.48; P = 0.03) but not more likely to earn any prize money (P = 0.20). Univariable effects of trainer (fitted as a fixed effect) and horse (fitted as a random term in an intercept-only model) were highly significant (both P < 0.001). Age was not significantly associated with odds of winning (P = 0.12) but 3-year-olds were less likely to earn prize money than 2-year-olds (OR = 0.83; 95% CI = 0.73, 0.95; P = 0.008). Females were less likely than males to win (OR = 0.85; 95% CI = 0.72, 1.01; P = 0.07) or earn prize money (OR = 0.79; 95% CI = 0.71, 0.89; P < 0.001). Multivariable analyses Final multivariable models for binary outcome measures have been summarised in Table 2. Amount of high-speed exercise in a 30-day period, trainer, horse and age were all factors significantly associated with the likelihood of winning a race or earning any prize money. Trainer odds ratios for ‘winning’ varied from 0.25 (95% CI = 0.14, 0.45; P < 0.001) to 1.84 (95% CI = 1.03, 3.30; P = 0.04) compared to the baseline trainer, and from 0.20 (95% CI = 0.13, 0.31; P < 0.001) to 1.38 (95% CI = 0.76, 2.53; P = 0.29) for ‘earning any prize money’. A graphical representation of the association between high-speed exercise in the 30-day period and the binary outcomes is displayed in Fig. 1. Horses that had raced in the month preceding the case race were 1.3 times more likely to win than those that had not. When high-speed exercise was divided into distance galloped during training and distance raced, galloping during training was not significantly associated with the likelihood of winning or earning any prize money (P = 0.20 and P = 0.43, respectively). Distance raced in the 30 days was significantly associated with the odds of winning the case race (OR = 1.02/f; 95% CI = 1.00, 1.03; P = 0.007) and earning prize money (OR = 1.02/f; 95% CI = 1.01, 1.03; P < 0.001). For example, a horse that had accumulated 12 f (75th percentile) in race distance in the previous 30 days was at 1.2 times the odds (or 20% more likely) to win the case race, or at 1.3 times the odds (30% more likely) to be placed, compared to a horse that had accumulated 0 f (25th percentile) race distance in the same period. Gender was not significantly associated with the binary race performance outcomes in multivariable analysis. Linear regression analyses
Logistic regression analyses Univariable analyses Total exercise and canter distance in a 30-day period did not affect the odds of winning a race (P = 0.96 and P = 0.85, respectively) or earning any prize money (P = 0.19 and P = 0.27, respectively). Increasing amounts of high-speed exercise in 30 days was associated with increasing odds of winning a race (P = 0.06) and earning any prize money (P = 0.02). These effects were mainly
2
£1 = approx. €1.12, US$ 1.44 as at 15 February 2009.
No prize money was won in 2673 races (51% of all observations) that were therefore excluded from the analyses presented in this section. Univariable analyses Total and canter distance in a 30-day period were not associated with ln (prize money) when unadjusted for other exposure variables. The ln (prize money) increased with increasing distances galloped (P < 0.001), raced (P = 0.06) and total high-speed exercise distance (P = 0.001) in a 30-day period but decreased for horses in the highest categories (gallop P35 f, race >12 f, high-speed >42 f),
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K.L.P. Verheyen et al. / The Veterinary Journal 181 (2009) 43–47 Table 2 Results of multivariable mixed effects logistic regression analysis of variables associated with the likelihood of winning a race or earning any prize money. Outcome: won
Outcome: any prize money a
Odds ratio (95% CI ) High-speed distance in 30 days prior to case race (furlongs) Raced in previous 30 days
No Yes 2 3 >3
Age at race (years)
b
LRS P-value
1.01 (1.00, 1.02) Baseline 1.32 (1.08, 1.61) Baseline 0.76 (0.60, 0.95) 0.67 (0.50, 0.89)
0.004 0.006
Odds ratio (95% CIa)
LRSb P-value
1.02 (1.01, 1.02) Not significant
<0.001
0.01
0.02 0.76 (0.63, 0.92) 0.87 (0.67, 1.13)
Trainer (12 df) Horsec
<0.001 0.006
<0.001 <0.001
CI, confidence interval; df, degrees of freedom. a Confidence interval. b Likelihood ratio statistic. c Fitted as a random effects term; P-value derived from a likelihood ratio v2-test that there is no within-horse correlation.
variation in prize money (both P < 0.001). Horses older than 3 years earned on average £622 more than 2-year-olds (P < 0.001), but earnings did not differ between 2- and 3-year-olds (P = 0.65). Females earned on average £333 less than male horses (P < 0.001).
Fig. 1. Graphical representation of the association between distances exercised at high-speed (raced + galloped during training) in a 30-day period prior to a race and odds of winning a race (___) and winning any prize money (- - -). Estimated odds of winning have been adjusted for whether the horse raced in the 30-day period, trainer and horse effects (including age); estimated odds of winning any prize money have been adjusted for trainer and horse effects (including age).
suggesting inclusion of quadratic terms in the multivariable model. Horses that had raced once in the 30 days preceding a race won on average £302 more than horses that had not raced (P = 0.003), but racing four or more times in this period decreased the average prize money won. Trainer and horse (fitted as a random effects term in an intercept-only model) had a significant effect on the
Multivariable analyses The final multivariable model is presented in Table 3. The intraclass correlation coefficient (ICC), describing the correlation between observations within horse, was 0.22. This high ICC suggests that 22% of variation in prize money, unexplained by fixed variables in the model, occurred at the horse level. Distances cantered and galloped in training in the 30 days before a race were associated with prize money won, with an interaction between the two. A graphical representation of the association between galloping exercise and prize money for different levels of canter exercise is shown in Fig. 2. Higher canter distances were associated with higher average prize money, but only if galloping exercise was kept below about 24 f. Horses galloping more than 24 f in a 30-day period earned, on average, more prize money with decreasing canter distance, although this effect levelled out or decreased for the highest gallop distances. Fig. 3 shows the association between distance raced in a 30-day period and the average ln (prize money) won, adjusted for other variables in the model (Table 3). Prize money increased as cumulative race distance increased up to 40 f, after which the effect levelled out and then decreased for cumulative race distances over 50 f. When adjusted for trainer and exercise variables, horses older than 3 years earned significantly more prize money than
Table 3 Results of multivariable mixed effects linear regression analysis of variables associated with ln (prize money). Exercise variables represent distances undertaken in a 30-day period prior to racing. b-Coefficient Intercept Canter distance (furlongs) Gallop distance (furlongs) (Gallop distance)2 (furlongs2) Canter distance Gallop distance (furlongs2) Race distance (furlongs) (Race distance)2 (furlongs2) Age at race (years)
2 3 >3
6.000 0.003 0.038 2.48 10 1.12 10 0.020 2.32 10 Baseline 0.017 0.231
4 4
4
Trainer (12 df) Horsea Estimate of between-horse variation Estimate of within-horse correlation
95% CI 5.436, 6.565 0.001, 0.004 0.016, 0.059 4.67 10 4, 1.84 10 4, 0.010, 0.031 4.08 10 4, 0.164, 0.130 0.031, 0.431
P-value
0.29 10 0.39 10
4
0.57 10
4
4
<0.001 0.002 <0.001 0.03 0.003 <0.001 0.01 0.82 0.02 <0.001 <0.001
0.553 0.222
CI, confidence interval; df, degrees of freedom. a Fitted as a random effects term; P-value derived from a v2-test that there is no within-horse correlation.
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K.L.P. Verheyen et al. / The Veterinary Journal 181 (2009) 43–47 7.4 7.3
Ln(prize money)
7.2 7.1 7 6.9 6.8 6.7 6.6 6.5 6.4 0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
Distance galloped during training in 30 days (furlongs) Canter=50f
Canter=150f
Canter=100f Canter=250f
Canter=200f Canter=350f
Canter=300f
Fig. 2. Graphical representation of the association between distances cantered and galloped during training in a 30-day period prior to a race and average prize money won (natural logarithm scale). Estimates have been adjusted for race distance in the same period, trainer and horse effects (including age).
Ln(prize money)
6.8 6.7 6.6 6.5 6.4 6.3 0
5
10
15
20
25
30
35
40
45
50
Distance raced in 30 days (furlongs) Fig. 3. Graphical representation of the association between distance raced in a 30day period and average prize money won (natural logarithm scale). Estimates have been adjusted for canter and training gallop distances in the same period, trainer and horse effects (including age).
2-year-olds (£105 on average) but there was no difference in amount of prize money won between 2- and 3-year-olds. Gender was not significantly associated with amount of prize money won when adjusted for the variables shown in Table 3 (P = 0.16). Discussion Previous studies have assessed the effects of a variety of factors on race performance in Thoroughbreds, including radiographic measurements (Kane et al., 2003; Spike-Pierce and Bramlage, 2003), orthopaedic injury (Martin et al., 1988; Kannegieter and Ryan, 1991; Martin, 2000), and respiratory conditions (Strand et al., 2000; Stick et al., 2001). To our knowledge, the study reported here is the first to investigate the association between exercise undertaken prior to a race and racing performance. The amount of high-speed exercise undertaken affected the odds of a horse winning a race as well as the likelihood of it winning prize money. The odds of winning were mainly associated with racing experience in the previous 30 days, suggesting that horses with an intense racing schedule were more successful. However, the direction of any potentially causal association cannot be determined from our study: it is possible that horses were racing more because they were being successful and remained sound rather than being successful because they were accumulating higher race distances. By using the natural logarithmic transformation of prize money won to assess the effects of exercise on this outcome variable, analyses excluded observations where prize money was zero. Therefore, results of these analyses can be applied only to horses
capable of winning prize money (76% of horses included in the analyses reported here) and should not be extrapolated to the general racehorse population. The combination of canter and gallop exercise during training affected the amount of prize money won. Higher canter distances in the 30 days before a race were associated with higher average prize money, but only if galloping was kept to a minimum. Horses undertaking more than 24 f of galloping in the 30 days leading up to a race won on average more prize money if they did less cantering exercise. Further investigation of our data (not shown) revealed that horses racing in longer races accumulated higher canter distances in the previous 30 days, but no association was found between distance galloped in the previous 30 days and distance of the case race. However, longer races were associated with higher amounts of prize money being available, which may partly explain our finding that higher canter distances in training were associated with higher amounts of prize money won. The average amount of prize money won increased with accumulation of race distance in the 30-day period, reaching a plateau at around 40 f. However, the 95th percentile for race distance accumulated in 30 days was 31 f, and thus apparent associations with prize money won above this level should be interpreted with caution. The race variables in the model shown in Table 3 could be substituted for number of races and its squared term (results not shown), suggesting a beneficial effect of some but not excessive racing. As race distance and number of races were highly correlated, it is not clear whether it is the racing experience per se that affects prize money won in subsequent races or the accumulation of exercise at racing speed. The latter may have a beneficial effect on bone’s adaptive response to exercise thus protecting horses from injury while at the same time improving their race-fitness, up to certain level, beyond which more racing becomes detrimental. Race records of 860 of the 1178 horses monitored in previous epidemiological studies of fracture (Verheyen and Wood, 2004; Verheyen et al., 2006a, 2006b) were included in the analyses reported here. Further examination of race records from the Racing Post3 and the injury data available to us revealed that 1053 (89%) of the study horses ever raced, and that the probability of appearing on the racecourse was not significantly different among horses that suffered an exercise-induced fracture during the study period (n = 148 of which 130 [88%] ever raced) and those that did not (n = 1030 of which 923 [90%] ever raced]). It is therefore unlikely that our study population of horses that raced was biased toward those that did not incur a fracture, and thus we combined findings from the current study with those from previous injury studies to establish whether training regimens aimed at reducing injuries might adversely affect racecourse performance. The results from our injury studies indicated a potentially protective effect of accumulating high-speed exercise with regard to skeletal injury (Verheyen et al., 2006a), whereas higher amounts of cantering, especially within short time periods, have been associated with an increased risk of injury, in particular stress fractures (Verheyen et al., 2006b). It therefore seems reasonable to advise trainers to try to adjust training schedules in favour of more high-speed and less canter exercise, in order to both reduce the risk of injury and create a successful racehorse. However, the exercise variables studied were only a 30-day ‘snapshot’ of the horses’ training regimen and other training-related factors may be important. Further analyses should investigate associations between exercise undertaken in different time periods and measures of racecourse performance. Not surprisingly, there was a strong horse effect associated with race outcomes, indicating unmeasured variables at the horse level
3
See: http://racingpost.co.uk/horses.
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that affect the likelihood of success on the racecourse. Genetic potential is likely to play a role (Hintz, 1980; Gaffney and Cunningham, 1988), as well as environmental factors not measured in this study. In our study population, 3-year-olds were less likely to win races or earn any prize money than 2-year-olds. When investigating this further, it appeared that the significance of this effect was due to one trainer who had a good success rate with 2-year-olds. When data from this trainer were excluded, the age effects became non-significant. It may therefore be inappropriate to extrapolate the effects of age on the odds of winning or earning any prize money to the general racehorse population. However, the amount of prize money won was still marginally significantly higher for horses older than 3 (compared to 2-yearolds) when data from this trainer were excluded (P = 0.09). Although gender affected measures of race performance in univariable analysis, these effects became non-significant when adjusting for other exposures, in particular exercise variables. This indicated that the univariable gender effects could partly be explained by differences in training and racing patterns between males and females, although we did not investigate these differences in further detail. A study in New Zealand Thoroughbreds found that 3- and P5-year-old male horses were more likely to win than females in the same age groups (Perkins et al., 2004), although these findings were based on simple comparisons that did not take account of potential confounders. The trainer was also associated with horses’ race performances in multivariable analyses, which suggested that factors at the trainer level beyond those included in the models were associated with racing success. Horse management, nutrition and veterinary care may be important in this respect, as well as differences in quality of horses between trainers. Consistent with this, a study of race performance in Australian Thoroughbreds also found a strong trainer-level effect on race earnings in the first year of racing (More, 1999), although this study did not take account of detailed exercise data that may partly explain trainer-level variation. Conclusions Decreasing the amount of canter exercise during training in favour of more high-speed exercise (an approach that we have previously suggested will decrease the risk of fracture and DMD) is unlikely to adversely affect race performance in on-the-flat races.
Conflict of interest statement None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
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Acknowledgements The work presented here resulted from a grant funded by the Horserace Betting Levy Board and a scholarship provided by The Horse Trust. We are grateful to all trainers and their staff who provided us with the exercise data without which this study would not have been possible. We also thank Raceform Ltd. for providing us with the relevant race-performance records. References Cogger, N., Perkins, N., Hodgson, D.R., Reid, S.W., Evans, D.L., 2006. Risk factors for musculoskeletal injuries in 2-year-old thoroughbred racehorses. Preventive Veterinary Medicine 74, 36–43. Gaffney, B., Cunningham, E.P., 1988. Estimation of genetic trend in racing performance of thoroughbred horses. Nature 332, 722–724. Hintz, R.L., 1980. Genetics of performance in the horse. Journal of Animal Science 51, 582–594. Kane, A.J., McIlwraith, C.W., Park, R.D., Rantanen, N.W., Morehead, J.P., Bramlage, L.R., 2003. Radiographic changes in thoroughbred yearlings. Part 2: associations with racing performance. Equine Veterinary Journal 35, 366–374. Kannegieter, N.J., Ryan, N., 1991. Racing performance of thoroughbred horses after arthroscopic surgery of the carpus. Australian Veterinary Journal 68, 258–260. Martin, G.S., 2000. Factors associated with racing performance of Thoroughbreds undergoing lag screw repair of condylar fractures of the third metacarpal or metatarsal bone. Journal of the American Veterinary Medical Association 217, 1870–1877. Martin, G.S., Haynes, P.F., McClure, J.R., 1988. Effect of third carpal slab fracture and repair on racing performance in thoroughbred horses: 31 cases (1977–1984). Journal of the American Veterinary Medical Association 193, 107–110. More, S.J., 1999. A longitudinal study of racing Thoroughbreds: performance during the first year of racing. Australian Veterinary Journal 77, 105–112. Parkin, T.D., Clegg, P.D., French, N.P., Proudman, C.J., Riggs, C.M., Singer, E.R., Webbon, P.M., Morgan, K.L., 2004. Horse-level risk factors for fatal distal limb fracture in racing Thoroughbreds in the UK. Equine Veterinary Journal 36, 513– 519. Perkins, N.R., Reid, S.W.J., Morris, R.S., 2004. Profiling the New Zealand thoroughbred racing industry. 1. Training, racing and general health patterns. New Zealand Veterinary Journal 53, 59–68. Spike-Pierce, D.L., Bramlage, L.R., 2003. Correlation of racing performance with radiographic changes in the proximal sesamoid bones of 487 thoroughbred yearlings. Equine Veterinary Journal 35, 350–353. Stick, J.A., Peloso, J.G., Morehead, J.P., Lloyd, J., Eberhart, S., Padungtod, P., Derksen, F.J., 2001. Endoscopic assessment of airway function as a predictor of racing performance in thoroughbred yearlings: 427 cases (1997–2000). Journal of the American Veterinary Medical Association 219, 962–967. Strand, E., Martin, G.S., Haynes, P.F., McClure, J.R., Vice, J.D., 2000. Career racing performance in Thoroughbreds treated with prosthetic laryngoplasty for laryngeal neuropathy: 52 cases (1981–1989). Journal of the American Veterinary Medical Association 217, 1689–1696. Verheyen, K.L., Henley, W.E., Price, J.S., Wood, J.L., 2005. Training-related factors associated with dorsometacarpal disease in young Thoroughbred racehorses in the UK. Equine Veterinary Journal 37, 442–448. Verheyen, K.L., Newton, J.R., Price, J.S., Wood, J.L., 2006b. A case-control study of factors associated with pelvic and tibial stress fractures in Thoroughbred racehorses in training in the UK. Preventive Veterinary Medicine 74, 21–35. Verheyen, K., Price, J., Lanyon, L., Wood, J., 2006a. Exercise distance and speed affect the risk of fracture in racehorses. Bone 39, 1322–1330. Verheyen, K.L.P., Wood, J.L.N., 2004. Descriptive epidemiology of fractures occurring in British thoroughbred racehorses in training. Equine Veterinary Journal 36, 167–173.