Racing speeds of Quarter Horses, Thoroughbreds and Arabians

Tracking the Impact of Weather on Equine Activity While Pastured

Keeping horses outdoors on pasture full-time with free access to shelter holds numerous advantages over housing in stalls, promoting both better mental and physical health. One reason for these benefits is the potential for increased physical activity in horses outdoors on pasture versus those confined to stalls. However, it is not guaranteed the horse will take advantage of this opportunity for greater movement. For this reason, it is important to understand the various reasons why horse activity patterns change. The objective of this study was to investigate how various weather factors - including temperature, humidity, precipitation, and wind speed - directly affect equine movement. To achieve this, horses on two similarly-managed farms were equipped with triaxial accelerometers during five independent time periods from January to August. These devices tracked number of steps, standing time, time lying down, and number of lying bouts. The movement data were then compared to the corresponding weather conditions. No strong correlations were found between the recorded movement of the horses and any of the environmental conditions. However, differences in average number of steps and average time lying down were observed between farms and across testing periods, suggesting other influences such as ground conditions and the use of blankets. Further studies are needed to determine the best management practices to encourage pasture activity and support optimal equine physical health.

Average stride length and stride rate of Thoroughbreds and Quarter Horses during racing

The main factors influencing speed in athletes are stride length (SL) and stride rate (SR). However, conflict remains whether SL or SR is the key determinant of higher speeds. Quarter Horses (QH) generally reach higher speeds in their races than do Thoroughbreds (TB). However, the influence of SL and SR on this greater speed is unclear. Therefore, the main objective of this study was to compare SL and SR in QH and TB raced in short (sprint) and long (classic) distances. We hypothesized that QH have a higher SR in comparison to TB, and SR decreases as distance increases. Two race distances were analyzed for each breed: QH races of 100.6 and 402.3 m, and TB races of 1,207.0 and 2,011.7 m. Data from 20 horses were obtained, consisting of five horses from each race distance (10 QH and 10 TB). Five individuals watched recordings of each race three times counting the number of strides taken by each winning horse. The SR was calculated using the average number of strides over a given race duration, and SL was determined by calculating the total number of strides over the distance covered. Speed was calculated by dividing the distance by the time of the winning horse. The PROC Mixed Procedure was used to identify statistical differences between breeds, and between distances within the same breed. Results showed that although the SL of the TB was longer in comparison with the QH (P < 0.001), the average SR in QH was higher than in TB (2.88 vs. 2.34 + 0.03 strides/s; P < 0.001). Furthermore, QH classic distance demonstrated a faster speed than TB at either distance (P < 0.001). In conclusion, QH achieve a higher SR in comparison to TB (between 14% and 20% more than TB), confirming the importance of SR in achieving high racing speeds.

Comparison of β-endorphin, lactate and cortisol concentrations in winning and losing racehorses

Understanding the physiological and biochemical changes in racehorses can be invaluable. Accurate information in this area could result in better understanding of needs of sport horses. The aim of this study was to prove the hypothesis that biochemical changes could influence the outcome of competitions. In this study, β-endorphin was evaluated as an indicator of analgesia, lactate as an indicator of fatigue and cortisol as an indicator of stress in the first two horses and the last ones that cross the finish line. This study was performed on 44 horses participating in the 1000-meter national championship. In Group 1, 22 winners and second place horses were included; for Group 2, 22 last and penultimate horses were included. Blood samples were obtained in the doping room after race (T0) and 20 minutes after finishing (T20). Results for beta endorphin at T0 and T20 were higher (P>0.05) for Group 1 compared to Group 2; on the contrary, lactate concentration was lower (P>0.05) for Group 1 than Group 2 at T0 and T20. However, differences (P<0.05) were obtained within groups at T0 and T20 for beta endorphin and lactate concentrations. No significant differences were found for cortisol concentration.The results of this study showed that winning horses had higher levels of β-endorphin and lower levels of lactate than losers. Further and deeper experimental studies are needed to prove the hypothesis that biochemical changes could influence the outcome of competitions.

Slab fractures of the third carpal bone involving both radial and intermediate facets and outcomes after surgical repair in racing Quarter Horses

BACKGROUND

Characteristics and outcomes after surgical repair of third carpal bone (C3) slab fractures involving both radial and intermediate facets in racing Quarter Horses are unknown.

OBJECTIVES

To describe the pre- and intraoperative characteristics of C3 slab fractures of both radial and intermediate facets in Quarter Horses and to report on the long-term outcomes after internal fixation.

STUDY DESIGN

Retrospective case series.

METHODS

Case records were collected from racehorses with C3 slab fractures between 2008 and 2020. Inclusion criteria required arthroscopic-guided repair of C3 slab fractures involving both radial and intermediate facets in Quarter Horses. Routine C3 slab fractures (single facet), fractures in other breeds or those repaired with other techniques were excluded. Outcomes were obtained by standardised questionnaire. Data were presented as mean ± SD or as proportions with 95% confidence interval (CI).

RESULTS

Of 22 Quarter Horses with C3 slab fractures involving both radial and intermediate facets, 91% (CI 79%-100%; n = 20) were collapsing and 91% (CI 79%-100%; n = 20) had avulsion of the medial palmar intercarpal ligament. Articular cartilage erosion and osteochondral fracture of the radial carpal bone was observed in 91% (CI 79%-100%; n = 20) and 41% (CI 20%-62%; n = 9) cases respectively. At 5.5 ± 3.9 years after surgery, 86% (CI 72%-100%; n = 19) were alive and used for breeding or retirement. Of 18 horses with follow-up >1 year, 39% (CI 16%-61%; n = 7) resumed some athletic activity.

MAIN LIMITATIONS

Cases were referred specifically for surgical repair and horses with fractures considered too severe for surgical intervention or euthanasia at owner request were not included. Questionnaire responses are susceptible to recall bias.

CONCLUSIONS

Horses with C3 slab fractures of both radial and intermediate facets that are repaired have a favourable prognosis for retirement, breeding and potentially low-level athletic activity.

Genomics in the Horse Industry: Discovering New Questions at Every Turn

The sheer diversity of heritable physiological traits, and the ingenuity of genome derived research technologies, extends the study of genetics to impact diverse scientific fields. Equine science is no exception, experiencing a number of genome-enabled discoveries that spur further research in areas like nutrition, reproduction, and exercise physiology. Yet unexpected findings, especially those that over-turn commonly held beliefs in the horse industry, can create challenges in outreach, education and communication with stakeholders. For example, studies of ancient DNA revealed that the oldest domesticated equids in the archeological record were in fact another species, the Przewalski's horse, leaving the origins of our modern horses a mystery yet to be solved. Genomic analysis of ancestry can illuminate relationships older than our prized pedigree records, and in some cases, identify unexpected inconsistencies in those pedigrees. Even our interpretation of what constitutes a genetic disease is changing, as we re-examine common disease alleles; how these alleles impact equine physiology, and how they are perceived by breeders and professionals in the industry. Effectively translating genetic tools for utilization in horse management and preparing our community for the debate surrounding ethical questions that may arise from genomic studies, may be the next great challenges we face as scientists and educators.

Biochemical differences between distal limb extensor and flexor tendons among equine breeds selected for racing and sport

Throughout the ages, humans have selected different horse breeds for their locomotor capacities. Consequently, the properties of equine locomotor tissues could have diversified because of the specific requirements of different disciplines. Therefore, this study aimed to compare biochemical properties of tendons in different equine breeds traditionally selected for racing or sports performance. We hypothesised that tendons in racing breeds would have biochemical properties that would increase strength, whereas those in sporting breeds would have more elastic properties. An ex vivo tendon tissue study comparing the common digital extensor tendon (CDET) and superficial digital flexor tendon (SDFT) of sports horses (Friesian horse, Warmblood horse) and racehorses (Thoroughbred horse; the oldest, reference standard breed) was performed. The SDFT and CDET from middle-aged Friesian (n = 12), Warmblood (n = 12) and Thoroughbred horses (n = 8) were harvested, and their biochemical properties were compared. The biochemical analysis demonstrated significantly higher water percentage, lower collagen concentrations/glycosaminoglycan content and higher crosslink concentrations in the SDFT of sports horses compared to racing breed horses (P < 0.05); DNA content was also significantly lower in sports horses than racehorses (P < 0.05). Racehorses had mainly extra fibrillar collagen support, whereas sports horses had mainly extra crosslink collagen support. From a functional perspective, the racing Thoroughbred relied on stronger tendons, while the sporting Friesians and Warmbloods relied on less stiff, more elastic tendons. In conclusion, there were significant biochemical differences in tendon properties between breeds, possibly related to their intended locomotor performance, although this requires further biomechanical and ultimately genetic confirmation.

Anti-Inflammatory State in Arabian Horses Introduced to the Endurance Training

Development of an anti-inflammatory state during physical training has been postulated in both human and equine athletes, but it is not completely understood. The aim of this study was to investigate whether endurance training changes pro- and anti-inflammatory cytokine profiles within a 20-week training season in young inexperienced endurance horses. Nine Arabian horses were examined in this prospective 20-week follow-up study. Blood samples were analysed 5 times monthly, at rest and after training sessions. Routine haematological examinations were performed. Cytokine patterns including IL-1β, IL-6, TNF-α, IL-10 mRNA expression using Real Time-PCR, and serum concentrations of IL-1β, IL-2, IL-4, IL-6, IL-17, INFγ, TNF-α, and IL-10 by ELISA test were determined. During endurance training, the most significant decrease in post-exercise cytokine type 1 levels (TNFα and IL-β) occurred within 20 weeks, beginning from the 3rd month of training. IL-6 serum level decreased after the 4th month. The results suggest that endurance training can induce advanced overall anti-inflammatory response as an adaptation to increasing workload.

A 10-year study of arthroscopic surgery in racing Thoroughbreds and Quarter Horses with osteochondral fragmentation of the carpus

BACKGROUND

Osteochondral fragmentation of the carpus is a common cause of lameness in racehorses. Prognosis following arthroscopic removal of the fragments was reported in 1987, but little is known of recent success rates.

OBJECTIVE

To identify associations between the severity and location of osteochondral fragments in the carpus, and to describe the racing performance of horses pre- and post-surgery in Thoroughbreds and Quarter Horses. To identify factors associated with a horse racing post-surgery.

STUDY DESIGN

Retrospective study of clinical records (2006-2016).

METHODS

Surgical and racing records for racehorses undergoing arthroscopic surgery of the carpus were collated during the 10-year study period. Signalment, location of osteochondral fragmentation and grade of defect left after removal and debridement identified and racing performance pre- and post-surgery were described, stratified by breed. Logistic regression analysis was used to identify factors associated with unsuccessful return to racing.

RESULTS

In total, 828 horses (n = 416 Quarter Horses; n = 412 Thoroughbreds) underwent 880 carpal arthroscopies after fragments were found on radiography. Sixty-five percent (n = 289) and 27% (n = 118) of the lesions were bilateral in Quarter Horses and Thoroughbreds respectively (P<0.001). In both breeds, the most commonly affected bone was the dorsodistal radial carpal bone (n = 320/659; 48.6%). Overall, 82% (n = 686; n = 358 Quarter Horses, n = 328 Thoroughbreds) of horses raced post-surgery, with 69.5% (n = 476; n = 228 Quarter Horses, n = 248 Thoroughbreds) racing at the same or a higher level of competition. Factors associated with horses not returning to racing post-surgery were increasing horse age, female horses, and a lesion grade of 4, while racing pre-surgery was protective.

MAIN LIMITATIONS

This study does not include a control population for comparison.

CONCLUSIONS

There were significant differences between the location and severity of lesions in Quarter Horses, when compared to Thoroughbreds. The majority of horses return to racing following surgery, although performance was influenced by lesion severity. The Summary is available in Portuguese - see Supporting information.

Heart Rate Monitoring in Mangalarga Marchador Horses During a Field Marcha Test

Measurement of heart rate (HR) and velocity before, during, and after training exercises under field conditions indicates health and fitness level and intensity of imposed physical effort and allows rapid adjustments in training and nutritional protocols, improving welfare of athletic horses. The aim of this study was to monitor HR of Mangalarga Marchador horses during a marcha field test. Fifteen Mangalarga Marchador horses were evaluated during an aerobic exercise (marcha) for 40 minutes using a HR monitor system with GPS (Polar RS800CX-G3). Results were analyzed through Polar ProTrainer 5 to record maximal heart rate (HRmax), average HR (HRmed), maximal speed (Vmax), average speed (Vmed), distance and HR and V oscillations throughout the physical effort. After 30 minutes of recovery, all animals were clinically re-evaluated. Variables were analyzed for normality with Kolmogorov-Smirnov test followed by mean comparison using ANOVA and Tukey test, considering P < .05. During the 40-minute exercise, Vmax was 3.75 ± 0.39 m/s, in a distance of 7.15 ± 1.06 km. Marcha gait influenced significantly HR (P < .0001) and, after 2 minutes of exercise, all animals reached values for speed and HR that did not change until the end of exercise trial. Results also showed a predominance of aerobic activity as animals worked mainly using 60%-70% of HRmax (∼140 beats/min). Heart rate monitoring during marcha test was a useful method to evaluate physical conditioning of athletic horses and intensity of exercise, which is helpful to improve performance and guarantee welfare of athletic horses kept under tropical conditions.

A retrospective cohort study of racing performance in Quarter Horses undergoing prosthetic laryngoplasty for treatment of recurrent laryngeal neuropathy

OBJECTIVE To determine effects of prosthetic laryngoplasty on return to racing, performance index, and career longevity in racing Quarter Horses with recurrent laryngeal neuropathy (RLN) and to evaluate performance variables for horses with RLN undergoing prosthetic laryngoplasty, compared with a control horse population. DESIGN Multicenter, retrospective cohort study. ANIMALS 162 racing Quarter Horses with RLN treated with prosthetic laryngoplasty (case horses) and 324 racing Quarter Horse without RLN (control horses). PROCEDURES Medical and race records of case and control horses examined at 5 referral centers between January 2000 and December 2015 were reviewed retrospectively. Two control horses were matched with each case horse. Return to racing, earnings, number of racing starts, performance index, and career longevity were evaluated. RESULTS The odds of returning to racing did not differ significantly between case and control horses but decreased with increasing age. Neither racing starts nor career longevity were affected by prosthetic laryngoplasty or by RLN grade. In fact, horses undergoing laryngoplasty for treatment of RLN and horses with the lowest RLN grade before surgery had higher performance indices after the surgery, compared with indices for control horses. CONCLUSIONS AND CLINICAL RELEVANCE The faster speeds and shorter distances raced with Quarter Horses could alter how RLN impacts respiratory variables and performance in Quarter Horses, compared with other racehorse breeds. Further study is needed to understand the impacts of RLN and surgical treatments for RLN in racing Quarter Horses.

Exercise-induced modification of the skeletal muscle transcriptome in Arabian horses

It has been found that Arabian and Thoroughbred horses differ in muscle fiber structure and thus in physiological changes occurring in muscles during exercise. The aim of the present study was to identify the global gene expression modifications that occur in skeletal muscle following a training regime to prepare for flat racing. Whole transcriptomes of muscle (gluteus medius) were compared between three time points of tissue collection: T0 (untrained horses), T1 (horses after intense gallop phase), and T2 (horses at the end of racing season), 23 samples in total. The numerous groups of exercise-regulated differentially expressed genes (DEGs) were related to muscle cell structure and signaling and included insulin-like growth factor 1 receptor ( IGF1R), insulin receptor ( INSR), transforming growth factor beta receptors 1 and 2 ( TGFBR1, TGFBR2), vascular endothelial growth factor B ( VEGFB); epidermal growth factor ( EGF), hepatocyte growth factor ( HGF), and vascular endothelial growth factor D ( FIGF). In Arabian horses, exercise modified the expression of genes belonging to the PPAR signaling pathway (e.g., PPARA, PPARD, and PLIN2), calcium signaling pathway, and pathways associated with metabolic processes (e.g., oxidative phosphorylation, fatty acid metabolism, glycolysis/gluconeogenesis, and citrate cycle). According to detected gene expression modifications, our results suggested that in Arabian horses, exercise switches energy generation toward fatty acid utilization and enhances glycogen transport and calcium signaling. The use of the RNA-Seq approach in analyzing the skeletal muscle transcriptome allowed for the proposal of a panel of new candidate genes potentially related to body homeostasis maintenance and racing performance in Arabian horses.

Exercise-Induced Cardiac Remodeling: Lessons from Humans, Horses, and Dogs

Physical activity is dependent upon the cardiovascular system adequately delivering blood to meet the metabolic and thermoregulatory demands of exercise. Animals who regularly exercise therefore require a well-adapted heart to support this delivery. The purpose of this review is to examine cardiac structure, and the potential for exercise-induced cardiac remodeling, in animals that regularly engage in strenuous activity. Specifically, we draw upon the literature that has studied the "athlete's heart" in humans, horses, and dogs, to enable the reader to compare and contrast cardiac remodeling in these three athletic species. The available literature provides compelling evidence for exercise-induced cardiac remodeling in all three species. However, more work is required to understand the influence of species/breed specific genetics and exercise-related hemodynamics, in order to fully understand the impact of exercise on cardiac structure.

Putative regulation mechanism for the MSTN gene by a CpG island generated by the SINE marker Ins227bp

Background

A single nucleotide polymorphism (SNP) in the first intron of the myostatin gene (MSTN) is associated with aptness of elite Thoroughbreds to race over sprint, middle or long distances. This intronic marker (g.66493737 T ≻ C), a short interspersed nuclear element (SINE) of 227 bp (Ins227bp) insertion polymorphism in the MSTN promoter, and the adjacent SNP BIEC2-417495 have not been studied for their association with racing aptness of the average Thoroughbreds raced in countries with lower status of the racing industry. This study investigated these markers regarding their prevalence and association with performance in common race horses. Markers were genotyped by amplification refractory mutation system-quantitative PCR (ARMS-qPCR) or amplicon melting. Furthermore, we asked whether the Ins227bp marker might theoretically regulate the expression of myostatin by generating a novel target for DNA methylation or by changing binding sites for transcription factors. Putative sites for DNA methylation or binding of transcription factors were predicted by MethPrimer and by the softwares JASPAR, MatInspector and UniPROBE, respectively.

Results

Pairwise linkage disequilibrium between g.66493737 T ≻ C and Ins227bp was high (r² = 0.93). A lower linkage was determined for g.66493737 T ≻ C and BIEC2-417495 (r² = 0.69) as well as for BIEC2-417495 and Ins227bp (r² = 0.76). The estimated frequencies for the presence of Ins227bp (I) indel and the C alleles at g.66493737 T ≻ C and BIEC2-417495 were 0.46, 0.47 and 0.43, respectively. Heterozygotes represented the most abundant genotype at each locus. The best racing distance (BRD) was significantly different between the homozygotes of each SNP (p = 0.01 to 0.03). C allele homozygotes at BIEC2-417495 or g.66493737 T ≻ C, as well as Ins227bp homozygotes earned most money on a mean distance ranging from 1211 to 1230 m. Heterozygotes earned most money on races over 1690 to 1709 m. The BRD for the T/T carriers at both SNP loci and for the SINE-free genotype was 1812 to 1854 m. Other performance parameters were not significantly different between the genotypes, except of the relative success score (RSS). The RSS was significantly slightly better on a distance of ≤1300 m for all carriers of the C allele and the Ins227bp compared to homozygous T genotypes and SINE-negative horses (p = 0.037 to 0.046). For distances of more than 1300 m the RSS was not significantly different between genotypes.

In silico assessment indicated that the Ins227bp promoter insertion might have generated a CpG island and a few novel putative binding sites for transcription factors.

Conclusions

All three target polymorphisms (Ins227bp, g.66493737 T ≻ C, BIEC2-417495) are suitable markers to assess the ability of non-elite Thoroughbreds to race at short or longer distances. The CpG island generated by Ins227bp may cause training-induced silencing of MSTN expression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0428-3) contains supplementary material, which is available to authorized users.

The role of catastrophic injury or sudden death of the horse in race-day jockey falls and injuries in California, 2007-2012

REASONS FOR PERFORMING STUDY

If equine conditions with high likelihood of jockey injury can be determined and subsequently prevented, jockey safety can be enhanced.

OBJECTIVES

To identify racehorse injuries or conditions with greatest risk for jockey falls and injuries.

STUDY DESIGN

Retrospective correlation of race-day jockey fall and injury data with racehorse fatality data.

METHODS

Thoroughbred (TB) and Quarter Horse (QH) racehorse cause of death and jockey fall and injury data for California flat races were reviewed for a 6-year period. Race and jockey race ride population data were used to determine jockey fall and injury incidence rates relative to cause of racehorse death, and were assessed using Poisson regression. Differences in proportions were assessed using Fisher's exact, Pearson's χ(2), and Cochran-Mantel-Haenszel tests.

RESULTS

707 racehorses experienced race-related catastrophic injury or sudden death. 199 jockeys had 601 falls with 325 injuries. Jockeys were 162 times more likely to fall (95% confidence interval 137-192; P<0.001) and 171 times more likely to be injured (95% confidence interval 140-208; P<0.001) when they rode a horse that died in a race. We infer that jockeys were more likely to fall or be injured when riding in QH races than in TB races because of a higher incidence of horse fatalities in QH races. Jockey falls occurred with 24% of TB and 36% of QH race-related horse fatalities, and jockey injury occurred in 64% of falls. Jockey falls were most common with TB fetlock injuries and QH carpal, metacarpal and fetlock injuries; and with axial, bilateral and multiple injuries compared with appendicular, unilateral and singular injuries, respectively.

CONCLUSIONS

Prevention of the most common catastrophic injuries and conditions of the racehorse, e.g. fetlock injuries, may be most effective at decreasing rates of falls and injuries to horseracing jockeys during racing.

Jockey Falls, Injuries, and Fatalities Associated With Thoroughbred and Quarter Horse Racing in California, 2007-2011

BACKGROUND

Despite the popularity of the horse racing industry in the United States and the wide recognition that horse racing is one of the most hazardous occupations, little focused research into the prevention of falls by and injuries to jockeys has been conducted.

PURPOSE

To describe the incidence rates and characteristics of falls and injuries to Thoroughbred and Quarter Horse racing jockeys in the state of California.

STUDY DESIGN

Descriptive epidemiology study.

METHODS

Data on race-day falls and injuries were extracted from jockey accident reports submitted to the California Horse Racing Board from January 2007 to December 2011. Denominator data, number of jockey race rides, were obtained from commercial and industry databases. Jockey fall, injury, and fatality incidence rates and ratios in Thoroughbred and Quarter Horse flat races were estimated using Poisson regression. Characteristics of falls and injuries are described and compared.

RESULTS

In Thoroughbred races, 184 jockey injuries occurred from 360 reported jockey falls, 180,646 race rides, 23,500 races, and 3350 race meetings. In Quarter Horse races, 85 jockey injuries occurred from 145 jockey falls, 46,106 race rides, 6320 races, and 1053 race meetings. Jockey falls occurred at a rate of 1.99 falls per 1000 rides in Thoroughbred races, with 51% of falls resulting in jockey injury, and 3.14 falls per 1000 rides in Quarter Horse races, with 59% of falls resulting in jockey injury. The majority of falls occurred during a race, with catastrophic injury or sudden death of the horse reported as the most common cause in both Thoroughbred (29%) and Quarter Horse (44%) races. During the period studied, 1 jockey fatality resulted from a fall. Jockey fall rates were lower but injury rates were comparable to those reported internationally.

CONCLUSION

On average, a licensed jockey in California can expect to have a fall every 502 rides in Thoroughbred races and every 318 rides in Quarter Horse races. While jockey fall rates were lower, injury rates were similar to those in other racing jurisdictions. The high proportion of jockey falls caused by horse fatalities should be further investigated.

A comparison of the moment arms of pelvic limb muscles in horses bred for acceleration (Quarter Horse) and endurance (Arab)

Selective breeding for performance has resulted in distinct breeds of horse, such as the Quarter Horse (bred for acceleration) and the Arab (bred for endurance). Rapid acceleration, seen during Quarter Horse racing, requires fast powerful muscular contraction and the generation of large joint torques, particularly by the hind limb muscles. This study compared hind limb moment arm lengths in the Quarter Horse and Arab. We hypothesized that Quarter Horse hind limb extensor muscles would have longer moment arms when compared to the Arab, conferring a greater potential for torque generation at the hip, stifle and tarsus during limb extension. Six Quarter Horse and six Arab hind limbs were dissected to determine muscle moment arm lengths for the following muscles: gluteus medius, biceps femoris, semitendinosus, vastus lateralis, gastrocnemius (medialis and lateralis) and tibialis cranialis. The moment arms of biceps femoris (acting at the hip) and gastrocnemius lateralis (acting at the stifle) were significantly longer in the Quarter Horse, although the length of the remaining muscle moment arms were similar in both breeds of horse. All the Quarter Horse muscles were capable of generating greater muscle moments owing to their greater physiological cross-sectional area (PCSA) and therefore greater isometric force potential, which suggests that PCSA is a better determinant of muscle torque than moment arm length in these two breeds of horse. With the exception of gastrocnemius and tibialis cranialis, the observed muscle fascicle length to moment arm ratio (MFL : MA ratio) was greater for the Arab horse muscles. It appears that the Arab muscles have the potential to operate at slower velocities of contraction and hence generate greater force outputs when compared to the Quarter Horse muscles working over a similar range of joint motion; this would indicate that Arab hind limb muscles are optimized to function at maximum economy rather than maximum power output.