Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
The Veterinary Journal 2002, 164, 224±234 doi:10.1053/tvjl.2001.0702, available online at http://www.idealibrary.com on Review Overview of Horse Body Composition and Muscle Architecture: Implications for Performance C. F. KEARNS*, K. H. MCKEEVER* and T. ABEy * Department of Animal Science, Rutgers the State University of New Jersey, New Brunswick, NJ, 08901, USA; y Department of Exercise and Sport Science, Tokyo Metropolitan University, 1-1 Minamiohsawa, Hachioji, Tokyo 192-0397, Japan SUMMARY Locomotion requires skeletal muscle to sustain and generate force. A muscle's force potential is proportional to its weight. Since the larger the muscle the larger its potential power output, a better understanding of the proportion of skeletal muscle a horse possesses may lead to a better understanding of horse performance. Several techniques exist to assess body composition, which include dual energy X-ray absorption, underwater (hydrostatic) weighing, derivation from total body water, bio-electric impedance, air displacement, body condition scoring, cadaver dissection and ultrasound. The relevance of each method to the equine industry will be discussed as will the practical information that the existing horse body composition studies have provided. Attention will be given to the data regarding the implications of body composition on the performance horse. The limited number of studies discussing different varieties of muscle architectures and the functional importance of these muscles will also be addressed. These body composition data may provide a better understanding of important issues in horse care that can lead to more optimal horse care techniques and a # 2002 Elsevier Science Ltd. All rights reserved. healthier and safer environment for horses. KEYWORDS: Muscle mass; fat mass; muscle architecture; performance. INTRODUCTION The ability of an animal to move and perform physical activity requires the locomotor muscles to generate and maintain a high power output (Hill, 1950; Gunn, 1987). The maximal producible muscle force is related to the muscles's physiological cross-sectional area (Hettinger & Muller, 1953), whereas the power produced by skeletal muscle is the product of the force it generates and the velocity at which it shortens (Leiber, 1992). The force and velocity potential of a muscle is in turn a function of the biochemical characteristics of the muscle fibres as well as the arrangement of the fibres with respect to the whole muscle. Moreover, a muscle's Correspondence to: Kenneth H. McKeever, Ph.D., FACSM Equine Exercise Physiology Laboratory, Department of Animal Sciences, 84 Lipman Drive, New Brunswick, NJ, 08901-8525, USA. Tel.: (732) 932-9390; Fax: (732) 932-6996; E-mail: [email protected] force potential is proportional to the number of sarcomeres in parallel and its velocity potential is proportional to the number of sarcomeres in series. Therefore, the volume of the locomotor muscles (the number of sarcomeres in parallel multiplied by the number of sarcomeres in series) and also muscle architectural arrangements have the potential to exert a profound influence on the power potential of the muscle at varying velocities of shortening. It has been shown previously in humans that successful sports performance is related both to the amount of fat-free mass (FFM) the athlete possesses (Thorland et al., 1987; Siders et al., 1993) and to the muscle's architectural properties, such as muscle fascicle length and muscle pennation angle, of the athlete's muscle (Abe et al., 2000; Kumagai et al., 2000). While much of the research on factors affecting equine performance has centred on the microscopic and biochemical properties of skeletal muscle (Gunn, 1983), limited data have been 1090-0233/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. HORSE BODY COMPOSITION AND MUSCLE ARCHITECTURE published on the gross anatomical attributes of muscle. Therefore, it is obvious to see the potential importance of assessment of total FFM, body composition and muscle architecture to our understanding of the physiology of performance. The most variable of all the body's components is body fat (Lohman, 1971). This is in contrast to FFM, which is considered more constant (Lohman, 1971). The variability in fat mass is related to the genetic make up of the individual horse and environmental factors such as nutrition or chronic exercise training. Furthermore, genetic differences have been seen in body composition and fat content between breeds within several species, such as cattle, pigs and sheep (Reid et al., 1968; Lohman, 1971). The issue of breed-related differences is a concern when attempting to analyse the previous literature regarding horses. There appears to be a breedrelated difference in fat and muscle distribution in horses. This may be related to the fact that many breeds have been bred and developed for special functions. For example, Thoroughbreds were developed for speed and endurance; whereas, Quarter Horses for speed and Percherons and the 225 other draft breeds for power without speed (Julian et al., 1956). To this end, Thoroughbreds have been reported to be one of the leanest breeds (Gunn, 1983; 1987) while the various pony breeds have been shown to deposit more fat mass than Thoroughbreds, even when fed the same diet (Gunn, 1987). Confounding this issue is the fact that several published studies do not specify the breed of horse studied (Table I). However, despite these limitations, there is still important information that can be taken from these studies, especially from the relatively few that have attempted to examine the relationship between body composition and athletic performance (Westervelt et al., 1976; Gunn, 1983; Lawrence et al., 1992; Kearns et al., 2002). It is the primary purpose of this paper to review the available literature regarding horse body composition, especially with regard to muscle mass and FFM, and to discuss the implications of each in terms of running performance. In addition, the review will cover the few published studies that have attempted to relate muscle architecture to locomotive function in the horse. However, while biochemical factors such as myosin ATPase activity Table I Review of body composition articles in the horse. Average per cent fat and the breed (where available) Author (year) Julian (1956) Robb et al. (1972) Westervelt et al. (1976) Webb & Weaver (1979) Elser et al. (1983) Lawrence et al. (1986) Gunn (1987) n 6 4 11 8 12 17 Kane et al. (1987) Webb et al. (1989) 10 10 9 5 6 6 Lawrence et al. (1992) Kearns et al. (2001) Kearns et al. (2002) 38 23 19 a Sex N/A N/A N/A g5 s2 m 10 g 10 N/A N/A N/A N/A N/A m 23 g2 s6 m 11 Breed Hot Blood Percheron Pony N/A a Thoroughbred Pony Pony N/A Thorougbred Otherc N/A N/A Arab Part Arab Standardbred Standardbred Condition Method % Fat (average) N/A Calculated from body water N/A N/A Chemical Composition Dissection Ultrasound Emaciated-thin Dissection 12.8 24.5 6.6±18.9 15.9 10.7 5.1 Poor-lean N/A N/A Lean Urea Dilution Urea Dilution Dissection N/A Fleshy-moderate Dissection Ultrasound Ultrasound Moderate Moderately fleshy Lean Ultrasound Ultrasound Ultrasound Hot-blooded horses include: Thoroughbred, Quarter, Arabian and American Saddle bred. Values of per cent fat calculated from author's data. Other breeds include: 2 Welsh Mountain, 1 Shetland, 1 Clydesdale and 1 Thoroughbred cross. s stallion, g gelding, m mare. b c 8.1b 15.4 1.1 2.1 13.0 10.1 7.4 7.8 22.3 male 6.9 female 10.3 226 THE VETERINARY JOURNAL, 164, 3 or muscle fibre typing are clearly important to equine performance, it is not the intent of this review to address that literature. Body composition-assessing techniques and studies Although body composition has been considered an important factor in the assessment of horse health, there is a paucity of published data available on horse body composition (Tables I and II), and hardly any obtained from elite equine athletes. This lack of published data is due primarily to the difficulties associated with previous methods used to assess body composition in the horse. Due to its size, many techniques are not applicable to the horse (Lawrence, 1994) and still others bring far too many technical problems with them, making those techniques impractical (Lawrence, 1994). Commonly used techniques in assessing body composition include dual energy X-ray absorption (DEXA), underwater (hydrostatic) weighing, derived from total body water (TBW), bio-electric impedance (BIA), air displacement, body condition scoring, cadaver dissection and ultrasound. The `gold standard' for human body composition assessment is hydrostatic weighing. This technique is based on Archimedes' principle that states that an object immersed in water is buoyed up by a counterforce equal to the water displaced. Bone and muscle tissue (1.2 to 3.0) are more dense than water, whereas fat tissue (0.90) is less dense (Brozek et al., 1963). At the same total body mass, an animal with greater FFM weighs more in water, has a higher body density and has a lower percentage of body fat than an animal with less FFM. Horses are too large to fit in most underwater weighing tanks and it is implausible to believe horses would subject themselves to being fully and quietly submerged long enough to facilitate an accurate measurement of water displacement. Therefore, to the best of our knowledge, this method has never been considered as an option in measuring horse body composition. The DEXA machine uses an X-ray technique to look at the density of the body and can then estimate FFM and fat mass. To date, DEXA has been used to assess body composition in small animals such as chickens (Mitchell et al., 1997), pigs (Mitchell et al., 1996; 1998; 2000), cats (Lauten et al., 2000) and dogs (Toll et al., 1994; Freeman et al., 1996; Lauten et al., 2001). Size is also an issue with DEXA. Few, if any, DEXA machines are large enough to accommodate Table II Body mass, fat-free mass, fat mass and per cent fat Author (year) Julian (1956) n Breed a 6 Hot Blood 4 Percheron Westervelt et al. (1976) 12 N/A Pre-training Post-training Webb & Weaver (1979) 17 Thoroughbred Webb et al. (1989) 6 Thoroughbred Otherb Lawrence et al. (1992) 38 Arab and Part Arab Group I Group II Group III Group IV Kearns et al. (2001) 23 Standardbred mare Kearns et al. (2002) 19 Standardbred mare Stallion & gelding Kearns et al. (unpublished) 23 Thoroughbred mare Gelding Stallion a Body weight (kg) Fat-free mass (kg) Fat mass (kg) % Fat (average) 436.5 865.8 384.2 653.7 52.3 212.1 12.8 24.5 479.4 480.3 291.0 503.0 465.0 428.1 438.2 276.2 452.2 430.6 51.3 41.8 14.8 50.8 34.4 10.7 8.7 5.1 10.1 7.4 409.5 418.8 404.9 407.7 514.3 451.3 438.9 477.0 499.0 527.0 380.4 379.7 377.2 374.5 397.8 397.8 402.4 440.2 451.5 483.0 29.1 39.1 27.7 33.2 116.5 53.5 36.5 36.8 47.8 44.0 7.1 9.3 6.9 8.2 22.3 10.3 6.9 7.5 9.6 11.0 Hot-blooded horses include: Thoroughbred, Quarter, Arabian and American Saddlebred. Other breeds include: 2 Welsh Mountain, 1 Shetland, 1 Clydesdale and 1 Thoroughbred cross. Group I non-finishers (lameness or injury); Group II non-finishers (metabolic); Group III Top Ten finishers; Group IV finished, not Top Ten. b HORSE BODY COMPOSITION AND MUSCLE ARCHITECTURE an entire horse and the imaging process takes a substantial amount of time, up to 50 min in some cases. Therefore, a horse must be fully or partly tranquilized to ensure it remains motionless for the complete duration of the imaging, making this a cost-prohibitive technique. The use of DEXA to measure bone mineral density in horses however has grown in increasing popularity in the last few years (Grier et al., 1996). Total body water (TBW) can be assessed through the disappearance of tritiated water ( Julian et al., 1956), deuterium oxide (Andrews et al., 1997), alcohol (Elser et al., 1983) or urea (Lawrence et al., 1986) and the subsequent value of TBW can then be used to predict body fat. Body composition is a derived measurement whose calculation takes in all the variation in the measurement of TBW. However, TBW is only an indirect measurement that is highly dependent on hydration status. Interestingly, one of the first studies to address the issue of horse body composition was performed by Julian and co-workers (1956). Using isotope dilution techniques, they were able to determine the compositions of the various body compartments including blood volume, total body water and body fat. Blood volume was determined by in vitro labelling of red cells while total body water was determined with tritiated water. Body fat was then calculated from the formula: %body water 0.73(100 ÿ %fat). Although this study was more concerned with total body water and blood volume, it did establish some important information about horse body composition, especially breedrelated differences. In general, `hot-blooded' breeds had slightly more water per unit of body weight than `cold-blooded' breeds like Percherons, and therefore, less per cent fat (12% versus 24%; Julian et al., 1956). Other workers have attempted to estimate total body water by the disappearance of ethanol (Elser et al., 1983) or urea dilution (Lawrence et al., 1986). One major weakness in the TBW technique was the fact that the formula used by Julian et al. (1956) was based on the assumption that lean tissue is comprised of 73% water and that adipose tissue contains no water. Unfortunately, their prediction equation was derived from data obtained in guinea pigs and rats (Robb et al., 1972) and the proportion of fat-free body consisting of water is known to show a considerable degree of variation from species to species (Reid et al., 1968). One study (Robb et al., 1972) found that 93.3% of the variation in body fat was associated with variation in the concentration of water, illustrating the danger of assuming a fixed percentage of water in any body compartment. 227 Bio-electric impedance (BIA) is another technique frequently used in human research and clinical settings. In BIA, an electrical current is sent through the body to measure the resistance or impedance to that current. Each biological tissue acts as either a conductor or insulator to the current. Muscle tissue, which is comprised mostly of water (80%) has a low resistance and acts as a good conductor. Fat tissue, on the other hand, is made up of little water (ca. 16%) and acts as an insulator. Therefore, the more fat tissue a body has, the higher the impedance to the current. The impedance value can be placed in a regression equation, usually with other variables such as height, weight and gender, to predict body density and per cent body fat. This method, however, has technical problems when used to measure body composition in horses. The large volume and variability of hind-gut water, specifically the large intestinal water volume, has been shown to cause a longer and highly variable equilibrium time and plateau effect while measuring TBW using deutrium oxide dilution in the horse (Andrews et al., 1997). This would tend to over-predict TBW and therefore horses would appear to have more muscle tissue and less fat tissue than would be actual. A recent study by Forro and collegues (2000) has demonstrated that BIA can be a useful tool to predict TBW, extracellular fluid and plasma volume in horses, however, there are no published data on the reliability of this technique to accurately predict per cent body fat in horses. There are some questions as to the accuracy of BIA in humans as results are affected by the type of instrumentation used, hydration level, food intake and skin temperature. These same questions should be of concern when considering this method for use in horses. Finally, air displacement techniques, like the Bod Pod (Life Measurement, Inc.), use computerized pressure sensors to determine the amount of air displaced by the body. Air displacement must be measured in an enclosed and sealed area and no such device exists for the horse. Whether this technique will prove to be a meaningful and costefficient technique has yet to be determined. To date, the two most prevalent methods reported for the evaluation of body composition in equines have been body condition score (Henneke et al., 1983; Carroll & Huntington, 1988) and cadaver dissection (Webb & Weaver, 1979; Gunn, 1987). Evaluation of body condition is a simple visual appraisal process that identifies areas of the body where fat cover is visible and palpable. The system is easy to apply and has been demonstrated to be 228 THE VETERINARY JOURNAL, 164, 3 sensitive enough to differentiate between major classes of equine athletes (i.e. elite vs. non-elite) (Lawrence et al., 1992). However, this method only gives a qualitative condition score and does not provide any quantitative assessment of total fat mass or FFM. Cadaver dissection, on the other hand, is a long laborious process that provides quantitative data on all compartments of body. Obviously, it is not feasible for horses still competing. However, much of what is known about horse muscle architecture (Hermanson & Hurley, 1990; Hermanson et al., 1991; Hermanson & Cobb, 1992; Ryan et al., 1992; Hermanson, 1997) and gross morphological (Gunn, 1979, 1983, 1987) properties have been derived from cadaver dissection studies. Using direct analysis of the carcass, Robb and colleagues (1972) were able to determine the chemical composition and energy values in ponies. They determined that body fat ranged from 6.6% to 18.9% in these horses. Webb and Weaver (1979) used direct analysis of carcass composition and reported values that were similar but in the lower range reported by Robb and colleagues (1972). Per cent fat in the Thoroughbred horses and pony breeds used in that study had a per cent fat that ranged from less than 1% body weight to over 11%, with a mean fat content of 5.1%. Interestingly, visual appraisal demonstrated that most of the animals studied were in a lean or emaciated condition. This observation points out the fact that casually derived body condition scores are not useful for appraising functional components of body composition, i.e. %fat, FFM and fat mass. The most variable component of body weight in the study by Webb and Weaver (1979) was adipose tissue, which ranged from less than 1% to over 11%. This variability in the level of adipose is in agreement with data from other species (Lohman, 1971). Direct carcass analysis provides quantitative data regarding both muscle and fat mass. The method is labour-intensive and time-consuming. Also, it cannot be used for longitudinal studies where both the beginning and end time points must be measured. Over the past 30 years, advances in imaging technologies, such as ultrasound and magnetic resonance imaging (MRI), have allowed for the development of a more dynamic and more importantly, non-invasive techniques for the measurement of body composition. Unfortunately, there are no published papers to date that have incorporated MRI technology to measure body composition in horses. Again, the size of the horse appears to be the limiting factor that has so far precluded the use of MRI. Ultrasonic measurements, on the other hand, can be made quickly and safely on a variety of subject populations without disrupting the animal's schedule, an element that is of particular importance when using elite racingbreeds. Furthermore, measurements can be repeated over an unlimited period of time, making longitudinal studies feasible and cost-effective. Westervelt and co-workers (1976) were able to estimate per cent fat in horses using B-mode ultrasound based on methods to measure rump fat thickness. These methods were adapted from well established protocols used to measure carcass composition in meat animals (Price et al., 1960; Stouffer et al., 1961). The technique has been reported to be highly repeatable, precise, accurate and easy to perform (Westervelt et al., 1976; Kane et al., 1987; Kearns et al., 2001). Comparisons of ultrasonic measurements of rump fat thickness to direct measurements of rump fat thickness (from cadavers) and per cent body fat have yielded a relatively low coefficient of variation of 2.4% and a significant coefficient of correlation of R2 0.86 in horses (Westervelt et al., 1976). The use of the technique in the horse was further validated in a study by Kane and co-workers (1987) who reported correlation of coefficients between actual and ultrasound-measured rump fat thickness ranging from R2 0.90 to 0.96. The ultrasonic probe can directly measure the fat thickness of key anatomic regions. These values can then be used to predict total body fat and coupled with measured body weights, one can calculate total FFM. Although the equations used to determine per cent body fat only included the single region of rump fat thickness, other regions have been assessed for their fat thickness, and they include shoulder and rib fat (Westervelt et al., 1976), longissimus fat (Dobec et al., 1994), and multiple sites along the rump (Kane et al., 1987). Therefore it is possible to quantitatively look at regional fat distribution in a given horse. Ultrasound has proven sensitive enough to measure changes in body fat in horses fed limited diets (Westervelt et al., 1976), horses training for polo competition (Westervelt et al., 1976) and horses taking a known repartitioning agent (Kearns et al., 2000). Furthermore, the portability and non-invasive nature of the ultrasound machine made it ideal for measuring body compositions of Arabs and part-Arabs during veterinary stops at a 150-mile endurance race (Lawrence et al., 1992). Ultrasound has also been utilized to measure body composition HORSE BODY COMPOSITION AND MUSCLE ARCHITECTURE in elite racing Standardbreds (Kearns et al., 2002) and Thoroughbreds (unpublished data; Tables I and II) during their normal training days, without disrupting the horses' normal daily routine. Interestingly, those studies have reported the average per cent body fat of the elite athletic horse to be approximately 7±8% (Lawrence et al., 1992; Kearns et al., 2002) while the %fat measured in normal untrained horses was higher, with averages ranging from 15.9 to 22.3% (Westervelt et al., 1976; Kearns et al., 2001). These values for elite and normal untrained horses are comparable to those seen in similar elite and untrained human populations. The importance of body composition on performance A low body fat and a large amount of muscle is of benefit to horses being considered to be elite level racers, whether it be for endurance or sprint racing. Although this general body-compositional trend has been seen with human sprinters (Barnard et al., 1979; Cureton & Sparling, 1980; Sparling & Cureton, 1983; Thorland et al., 1987; Deason et al., 1991; Meckel et al., 1995; Young et al., 1995) limited data are available for elite horses (Kearns et al., 2002) and none for jumpers. Several studies of human athletes have shown a significant correlation between sprint performance and per cent body fat. In those studies, fat content was inversely related to both sprint performance time and with 12-minute run performance in both males and females (Sparling & Cureton, 1983). The reason for this inverse relationship between per cent body fat and performance has a simple explanation. Excess body fat increases the energy requirements of weightbearing work such as running by increasing the energy requirements of exercise for any given intensity of work during a maximal oxygen consumption test (Buskirk & Taylor, 1957). This may be detrimental to running performance in that the running speed that can be sustained for a given duration is reduced (Cureton, 1992), thereby increasing race time. Body composition may also have a bearing on endurance performance. Lawrence et al. (1992) looked at the influence of fat mass on endurance horses, using primarily Arabs and part Arabs. The average body fat of horses competing in a 150-mile endurance race was 7.8%. When divided by race performance, the most successful horses possessed slightly lower body fat (6.5%) than the horses that could not finish (11.0%) (Lawrence et al., 1992). The average body fat of 7.8% was similar to that seen in elite racing Standardbreds (8.8%) (Kearns et al., 229 2002) and elite racing Thoroughbreds (8.4%; unpublished data, Table II). This suggests that even though there are small breed differences, horses competing at the elite level have similar body compositions. Functionally, a lower per cent fat, or more practically a lower fat mass would theoretically decrease the amount of work needed to move the body, thereby giving the leaner horses a performance advantage. For example, a typical 500 kg racing horse with 5% body fat would have 25 kg of fat mass, whereas a horse with a similar body weight but body fat of 10% would have 50 kg of fat mass. The difference of 25 kg in a race breed, whether it is trained for endurance or sprint events, is not a negligible weight. Horses on the diuretic furosemide can lose similar amounts of weight through urine loss (Hinchcliff et al., 1996; Gross et al., 1999; Hinchcliff & McKeever, 1999). It is believed that the ergogenic effects of furosemide work through the reduction of body weight because maximal aerobic fitness remains unaltered (Gross et al., 1999). To explain the performance-enhancing effects of the diuretic furosemide, horses carried weight equal to the weight lost after furosemide administration while they exercised (Hinchcliff et al., 1996). Furosemide decreased the accumulated O2 deficit and rate of increase in blood lactate concentration of horses during brief high-intensity exertion. The benefit of furosemide was removed when furosemide-treated horses had an external weight added to them (Hinchcliff et al., 1996; Hinchcliff & McKeever, 1999). Similar findings were seen in humans, where the addition of external weight significantly reduced running performance (Cureton & Sparling, 1980). However, care must be taken when interpreting these results. A low fat mass may appear beneficial to race performance but there are not enough data available to determine what is the optimal fat mass or per cent body fat for the racing horse. Nor are there any data available to suggest how low per cent body fat can go before health and/or performance suffers. Fat is an essential component of both health and metabolism. Lowering the fat mass on horses to the point of emaciation can have severe health effects on the animal and this issue needs to be weighed against the issue of optimal performance. The two studies (Lawrence et al., 1992; Kearns et al., 2002) to date that looked at performance racing horses both reported the average per cent fat of their elite horses to be approximately 8%. But highest performers in both studies were about 5%. Whether this is the optimal safe limit or not has yet 230 THE VETERINARY JOURNAL, 164, 3 to be determined but more data are needed to shed light on these issues. The importance of muscle mass on performance Muscle mass and the distribution of that muscle may be of critical important to performance and this speculation may help explain breed differences seen in performance horses. A simple way to estimate the potential work capacity of a muscle can be obtained from its weight (Hill, 1950; Gunn, 1987). There is very little available on muscle weight of animals selected for speed primarily because the main reason for studying muscle weight in animals is derived from the economic importance of muscle as meat (Gunn, 1983). However, some significant published information exists in the scientific literature (Gunn, 1983, 1987; Hermanson & Hurley, 1990; Hermanson, 1997). Domestic livestock species used for meat carry 30±40% of their live weight as muscle (Gunn, 1987). This is in contrast to species that perform athletic activities and those animals bred for speed, like Thoroughbreds and greyhounds, which have approximately 53±57% of their live weight. Those athletic breeds have greater muscle mass when compared to other horse (42%) and dog (44%) breeds (Gunn, 1987). Furthermore, these breeds have a larger portion of their overall muscle weight invested in their important propulsive locomotor muscles of the hindlimb region (Gunn, 1987). Thoroughbreds have a greater transverse sectional area and more large fibres in their cross-sectional areas of their musculus semitendinosus muscles than other horse breeds (Gunn, 1979). The femoral and longissisimus muscles were documented to grow at a greater rate relative to live weight in thoroughbreds than other horse breeds (Gunn, 1979). Data derived from studies of horses using ultrasound have supported the observations made from research on horse cadavers. In those studies, elite Thoroughbred horses had greater muscle thickness than elite Standardbred horses when their vastus lateralis (92 mm vs. 88 mm, respectively) and extensor carpi radialis (70 mm vs. 61 mm, respectively) muscles (unpublished data) were measured using ultrasound (Kearns et al., 2002). The elite Thoroughbreds used in that study also had slightly lower per cent body fat and significantly more FFM than the elite Standardbreds tested (Kearns et al., 2002, Table II). Since a large FFM is indicative of a greater muscle mass and thus, a greater potential force development, the FFM can be viewed as an important indicator of performance. Fat-free mass was negatively related to one mile race time (running performance) in both the male and female Standardbred racers as the horses with the largest FFM had the best racing times (Kearns et al., 2002). In that study, there was no difference in average race time between genders and the female horses had both a higher fat mass and FFM than the males with comparable race times. These data suggest that FFM and its relationship to force potential is more important in regards to producing faster racing times than fat mass. Finally, while limited data are available regarding the contributions of fat mass and muscle mass to running performance, there are no data available exploring the impact of body composition on other events like three-day-eventing or steeple chase. Most jumping studies have been concerned with other important issues like impact forces on tendons or joint moment production. The implications of muscle and fat mass to show-jumping performances have yet to be explored. It may turn out that horses with larger FFM may have a performance edge over horses of similar weight. Muscle architecture While gross muscle size is important to a muscle's power output, the sarcomere arrangement is also a critical component in the regulation of that force production (Sacks & Roy, 1982). The morphological arrangement of the locomotor muscles of the horse's hind limb is mainly fusiform (Sack, 1994). In fusiform muscles, fibre length is essentially limb length. In fact, differences in maximal shortening velocity between different muscles are more dependent on muscle fibre length than biochemical differences (Sacks & Roy, 1982; Burkholder et al., 1994). Thus, the longer a muscle fibre, the faster the muscle can contract (Lieber, 1992). The practical application of these observations leads to the hypothesis that horse breeds with the longest leg for a given body length would theoretically be able to reach the highest velocities. Animals designed for speed have long legs in relation to other parts of their body while animals designed for strength have proportionately short legs (Gunn, 1983). For example, `speed' animals, such as greyhounds, cheetahs and Thoroughbreds all have much comparatively longer legs compared to `strength' animals such as bulldogs, leopards and draft horses, respectively. The importance of this relatively longer limb can be seen in the observations of Gunn (1983), who investigated the horse limb as a simple pendulum. Natural oscillatory frequency HORSE BODY COMPOSITION AND MUSCLE ARCHITECTURE (NOF) is the speed at which the limb will swing, unaided, once it starts to swing. It will depend on the length of the limb and the distance between the centre of gravity of the limb and the centre of rotation of the limb. Muscle force is required to move a limb faster or slower than NOF. Therefore, if two limbs must swing at a rate higher than NOF, then the limb with the higher NOF will require smaller additional force (i.e. less energy). The NOF is increased by moving the centre of gravity of the limb closer to the pivot (i.e. joint) (Gunn, 1983; 1987). An index for comparing the NOF is to examine the ratio of the weights of the proximal hind limb muscles to the distal hind limbs muscles. Gunn (1983) found that Thoroughbreds had a proximal to distal ratio of 5.11, while other horses had ratio of 4.66 (Gunn, 1983). Considering the fact that Thoroughbreds have longer legs and greater mass of their limbs nearer the hip, their overall muscle design favours a higher NOF and therefore speed. Similar design arrangements have been seen in the thigh muscles of human sprinters when compared to human long distance runners (Abe et al., 2000; Kumagai et al., 2000). In addition to the importance of the gross morphological properties of muscle, architectural arrangement plays a critical role in regulation of muscle function. Muscle architecture can be very complex, allowing for regional recruitment patterns that are highly task-dependent (Hermanson & Hurley, 1990). In horses, the biceps brachii, an antigravity muscle for example, is a large pinnate muscle with two distinct architectural regions (Hermanson & Hurley, 1990; Sack, 1994). The lateral head contains primarily short fascicles (5±10 mm) while the medial head has much longer fascicles (15±25 mm) (Hermanson & Hurley, 1990). It is hypothesized that the short fascicles of the lateral head function primarily in a postural role while the longer fascicles of the medial head are more important during the dynamic activities like locomotion (Hermanson & Hurley, 1990; Hermanson et al., 1991). Not surprisingly, this architectural hierarchy of function is also seen at the level of expression of the critical contractile proteins (Hermanson et al., 1991). It has been documented that the postural heads of the biceps are comprised of slow myosin heavy chain and the locomotive medial head predominantly consists of fast myosin heavy chains (Hermanson et al., 1991). Functionally this integration of architecture and biochemical properties results in a very efficient design for locomotion. 231 Other forelimb muscles in the horse have demonstrated a similar structure±function relationship (Hermanson & Cobb, 1992; Ryan et al., 1992; Hermanson, 1997). A study (Hermanson & Cobb, 1992) of the elbow extensor muscles of the horse illustrated the distinct morphological and architectural difference in four different forelimb muscles that function during quiet stance and the stance phase of locomotion. The flexor carpi radialis and flexor carpi ulnaris are considered to have simple roles while the superficial digital flexor and deep digital flexor are more complex in their functional roles and architecture (Hermanson & Cobb, 1992). The superficial and deep digital flexors have similar check ligaments, which are responsible for carrying the animal's weight to a non-tiring ligamentous system (Hermanson & Cobb, 1992) but biomechanical models suggest that the superficial digital flexor receives twice as much stress as the deep digital flexor (Ker et al., 1988). The superficial digital flexor tendon has a smaller cross-sectional area than the deep digital flexor tendon and this may predispose the muscle to a higher frequency of tendon breakdown (Ker et al., 1988). From an architectural standpoint, those muscles have very different properties and the differences in architecture may help explain the aetiology of tendon breakdown. The superficial digital flexor has a simpler morphology, being composed of a single muscle head with short (approximately 3 mm) pennate muscle fibres (Hermanson & Cobb, 1992). The deep digital flexor has a more complex architecture and is composed of three heads: ulnar, radial and humeral (Hermanson & Cobb, 1992). This complex architecture allows for a greater degree of motor control. It has been theorized that the superficial digital flexor has important elastic storage properties (Biewener & Rizzon, 1989). The simple morphology of the superficial digital flexor coupled with its small tendon cross-sectional area and high percentage of slower contracting type I fibres, which may be unable to respond to the rapid stressors that can occur during training or exercise (Hermanson & Cobb, 1992), might provide a mechanical basis for its high prevalence of injury. Another example of a muscle with pennate muscle architecture is the triceps brachii, an elbow extensor that has two distinct morphological regions (Ryan et al., 1992). The long and lateral head comprise much of the muscle's weight, approximately 96%, and therefore much of its function. These heads are composed of predominantly fast-contracting fibres with longer fibre bundles (Ryan et al., 1992) and are best suited for dynamic 232 THE VETERINARY JOURNAL, 164, 3 locomotor activity. The smaller medial head of the triceps, along with the anconeus, with their smaller fibre bundles and slower fibre population are important in passive stance (Ryan et al., 1992). Another extensor muscle, the extensor carpi radialis muscle is fusiform which appears to have uniform morphology. Recent data (Hermanson, 1997) suggest that the extensor carpi radialis may be comprised of two or three compartments. A proximal compartment with its long fibres is designed to undergo significant excursion related to elbow flexion and extension while a distal compartment with its shorter fatigue-resistant fibres has less potential for excursion (Hermanson, 1997). Research in the field of horse architecture, while limited to only a few studies, has contributed a large amount of important functional data (Hermanson & Hurley, 1990; Hermanson et al., 1991; Hermanson & Cobb, 1992; Ryan et al., 1992; Hermanson, 1997). By examining morphological compartments, a finer assessment of specific motor functions, both at rest or during locomotion, can be obtained. However, there are no published data regarding the importance of hindlimb muscle architecture, especially the important locomotor muscles (vastus lateralis, gastrocnemius etc.). And, due to the small number of horses used in these studies, those studies did not provide any information about age or gender of their subjects. It is very possible that there are age and/or gender differences in many of the architectural characteristics described above. Furthermore, traininginduced changes in muscle architecture may occur in horses as it does in humans (Kawakami et al., 1995; Kearns et al., 1998, 2000). Clearly more studies need to be conducted and other techniques need to be developed so that in vivo changes in muscle architecture can be studied. SUMMARY AND CONCLUSION For years, researchers and trainers have attempted to determine which physiological and anatomical characteristics are predictive of speed and competitive sprint performance capacity in the horse. In general, the equine athletes that will run the fastest and therefore perform the best will usually have a large overall skeletal muscle mass, longer muscle fascicles, a high percentage of fast twitch muscle fibres and a low percentage of body fat (Kearns et al., 2002). Muscle mass and muscle architecture have been shown to play critical roles in human sprint performance (Abe et al., 2000; Kumagai et al., 2000) but no published studies have investigated the importance of the variable fascicle length or pennation angle to performance in the race breeds, like Thoroughbreds or Standardbreds. These studies would be of great benefit to the racing industry and might also be important to comparative physiologists and biomechanists. Future research is needed to investigate the regulation of muscle architecture through growth and development in the young horse as well as the possible changes that occur in the older horse in the years after racing. Several studies have attempted to examine horse body composition through a variety of methodologies for a variety of reasons: to understand the energy content of the horse's carcass (Robb et al., 1972); to measure the energy requirements of the horses (Webb et al., 1989); to better calculate pharmacokinetics based on the size of organs and tissues, at both species and individual levels (Webb & Weaver, 1979); or to investigate the horses' response to environmental treatments of exercise and nutrition (Elser et al., 1983), and performance (Lawrence et al., 1992; Kearns et al., 2002). These studies have established important information about the relationship of fat and lean mass in horses but there are still vast gaps in our understanding of muscle function. Several practical applications of horse body composition need to be explored. For example, a horse that has been on pasture due to injury may have increased its fat mass. Body composition can be used as a method to judge when a horse may return to training, or when the horse has regained its original pre-injury body composition. Also, body composition, and more specifically body fat, may be an important issue for mares during pregnancy. Dynamic techniques like B-mode ultrasound can give quantitative body compositional data during the pregnancy. These data would lead to a better understanding of important issues in horse care. Ultimately, more optimal horse care techniques can then be devised that would lead to a healthier and safer environment for horses. REFERENCES ABE, T., BRECHUE, W. F., FUJITA, S. & BROWN, J. B. (1998). Gender differences in FFM accumulation and architectural characteristics of muscle. Medicine and Science in Sports and Exercise 30, 1066±70. ABE, T., KUMAGAI, K. & BRECHUE, W. F. (2000). Fascicle length of leg muscles is greater in sprinters than distance runners. Medicine and Science in Sports and Exercise 32, 1125±9. HORSE BODY COMPOSITION AND MUSCLE ARCHITECTURE ANDREWS, F. M., NADEAU, J. A., SASBYE, L. & SAXTON, A. M. (1997). Measurement of total body water content in horses, using deuterium oxide dilution. American Journal of Veterinary Research 58, 1060±4. BARNARD, R. J., GRIMDITCH, G. K. & WILMORE, J. H. (1979). Physiological characteristics of sprint and endurance Masters runners. Medicine in Science and Sports 11, 167±71. BIEWENER, A. & A. RIZZO, N. (1989). Elastic energy storage in the horse. American Zoology 29, 182a. BROZEK, J., GRANDE, F., ANDERSON, T. & KEYS, A. (1963). Densitometric analysis of body composition: revision of some quantitative assumptions. Annals of the New York Academy of Sciences 110, 113±40. BURKHOLDER, T. J., FINGADO, B., BARON, S. & LIEBER, R. L. (1994). Relationship between muscle fiber types and sizes and muscle architecture properties in the mouse hindlimb. Journal of Morphology 221, 177±90. BUSKIRK, E. R. & TAYLOR, H. L. (1957). Maximal oxygen intake and its relation to body composition with special reference to chronic physical activity and obesity. Journal of Applied Physiology 11, 72±8. CARROLL, C. L. & HUNTINGTON, P. J. (1988). Body condition scoring and weight estimation of horses. Equine Veterinary Journal 20, 41±5. CURETON, K. J. (1992). Effects of experimental alterations in excess weight on physiological responses to exercise and physical performance. Body Composition and Physical Performance, pp. 71±88, Washington, DC: National Academy Press. CURETON, K. J. & SPARLING, P. B. (1980). Distance running performance and metabolic responses to running in men and women with excess weight experimentally equated. Medicine and Science in Sports and Exercise 12, 288±94. DEASON, J., POWERS, S. K., LAWLER, J., AYERS, J. & STUART, K. (1991). Physiological correlates to 800 meter running performance. Journal of Sports Medicine and Physical Fitness 31, 499±504. DOBEC, R. L., BORGER, M. L. & NEAL, S. M. (1994). Correlation of real-time ultrasonic measure of fat and longissimus muscle area in standardbred horses with lifetime racing records and lifetime winnings. Journal of Animal Science 72(suppl 2), 45. ELSER, A. H., JACKSON, S. G., LEW, J. P. & BAKER, J. P. (1983). Comparison of estimated total body in the equine from ethanol dilution and from carcass analysis. In Proceedings of the 8th Equine Nutrition and Physiology Symposium, Lexington, KY, pp. 61±6. FORRO, M., CIESLAR, S., ECKER, G. L., WALZAK, A., HAHN, J. & LINDINGER, M. I. (2000). Total body water and ECFV measured using bioelectrical impedance analysis and indicator dilution in horses. Journal of Applied Physiology 89, 663±71. FREEMAN, L. M., KEHAYIAS, J. J. & ROUBENOFF, R. (1996). Use of dual-energy X-ray absorptiometry (DEXA) to measure lean body mass, body fat, and bone mineral content (BMC) in dogs and cats. Journal of Veterinary Internal Medicine 10, 99±100. GRIER, S. J., TURNER, A. S. & ALVIS, M. R. (1996). The use of dual-energy X-ray absorptiometry in animals. Investigative Radiology 31, 50±62. GROSS, D. K., MORLEY, P. S., HINCHCLIFF, K. W. & WITTUM, T. E. (1999). Effect of furosemide on 233 performance of Thoroughbreds racing in the United States and Canada. Journal of the American Veterinary Medicine Association 215, 670±5. GUNN, H. M. (1979). Total fibre number in cross sections of the semitendinosus in athletic and non-athletic horses and dogs. Journal of Anatomy 128, 821±8. GUNN, H. M. (1983). Morphological attributes associated with speed of running in horses. In Equine Exercise Physiology Proceedings of the First International Conference. pp. 271±4. eds D. H. Snow, S. G. B. Persson and R. J. Rose. Cambridge: Burlington Press Ltd. GUNN, H. M. (1987). Muscle, bone and fat proportions and the muscle distribution of Thoroughbreds and other horses. In Equine Exercise Physiology 2 Proceedings of the Second International Conference on Equine Exercise Physiology, eds J. R. Gillespie and N. E. Robinson. Davis, CA: ICEEP, pp. 253±64. HENNEKE, D. R., POTTER, G. D., KREIDER, J. L. & YEATES, B. F. (1983). Relationship between condition score, physical measurements and body fat percentage in mares. Equine Veterinary Journal 15, 371±2. HERMANSON, J. W. (1997). Architecture and the division of labor in the extensor carpi radialis muscle of horses. Acta Anatomica 159, 127±35. HERMANSON, J. W. & HURLEY, K. J. (1990). Architectural and histochemical analysis of the biceps brachii muscle of the horse. Acta Anatomica 137, 146±56. HERMANSON, J. W. & COBB, M. A. (1992). Four forearm flexor muscles of the horse, Equus caballus: Anatomy and histochemistry. Journal of Morphology 212, 269±80. HERMANSON, J. W., HEGEMANN-MONACHELLI, M. T. & LAFRAMBOISE, W. A. (1991). Correlation of myosin isoforms with anatomical divisions in equine musculus biceps brachii. Acta Anatomica 141, 369±76. HETTINGER, T. H. & MULLER, E. A. (1953). Muskelleistung und musketrainug. Arbeitsphysiologie 15, 111±26. HILL, A. V. (1950). The dimensions of animals and their dynamics. Proceedings of the Royal Institute of Great Britain 34, 450. HINCHCLIFF, K. W. & MCKEEVER, K. H. (1999). Frusemide and weight carriage alter the acid : base responses of horses to incremental and to brief intense exertion. Equine Veterinary Journal Supplement 30, 375±9. HINCHCLIFF, K. W., MCKEEVER, K. H., MUIR,W. W. & SAMS, R. A. (1996). Furosemide reduces accumulated oxygen deficit in horses during brief intense exertion. Journal of Applied Physiology 81, 1550±4. JULIAN, L. M., LAWRENCE, J. H. & BERLIN, N. I. (1956). Blood volume, body water and body fat of the horse. Journal of Applied Physiology 8, 651±3. KANE, R. A., FISHER, M., PARRETT, D. & LAWRENCE, L. M. (1987). Estimating fatness in horses. In Proceedings of the 10th Equine Nutrition and Physiology Symposium, Fort Collins, CO, pp. 127±31. KAWAKAMI, Y., ABE, T., KUNO, S. & FUKUNAGA, T. (1995). Training-induced changes in muscle architecture and specific tension. European Journal of Applied Physiology 72, 37±43. KEARNS, C. F., BRECHUE, W. F. & ABE, T. (1998). Traininginduced changes in fascicle length: a brief review. Advances in Exercise and Sports Physiology 4, 77±81. KEARNS, C. F., ABE, T. & BRECHUE, W. F. (2000). Muscle enlargement in sumo wrestlers includes increased 234 THE VETERINARY JOURNAL, 164, 3 muscle fascicle length. European Journal of Applied Physiology 83, 289±96. KEARNS, C. F., MCKEEVER, K. H., MALINOWSKI, K., STRUCK, M. B. & ABE, T. (2001). Chronic administration of therapeutic levels of clenbuterol acts as a repartitioning agent. Journal of Applied Physiology 91, 2064±70. KEARNS, C. F., MCKEEVER, K. H., KUMAGAI, K. & ABE, T. (2002). Fat-free mass is related to one mile race performance in elite Standardbred horses. The Veterinary Journal 163, 1±7. KER, R. F., ALEXANDER, R. M. & BENNETT, M. B. (1988). Why are mammalian tendons so thick? Journal of Zoology London 216, 309±24. KUMAGAI, K., ABE, T., BRECHUE, W. F., RYUSHI, T., TAKANO, S. & MIZUNO, M. (2000). Sprint performance is related to muscle fascicle length in male 100-m sprinters. Journal of Applied Physiology 88, 811±6. LAUTEN, S. D., COX, N. R., BAKER, G. H., PAINTER, D. J., MORRISON, N. E. & BAKER, H. J. (2000). Body composition of growing and adult cats as measured by use of dual energy X-ray absorptiometry. Comparative Medicine 50, 175±83. LAUTEN, S. D., COX, N. R., BRAWNER, W. R. JR & BAKER, H. J. (2001). Use of dual energy X-ray absorptiometry for noninvasive body composition measurements in clinically normal dogs. American Journal of Veterinary Research 62, 1295±301. LAWRENCE, L. M. (1994). Nutrition and the Athletic Horse. In The Athletic Horse. eds D. R. Hodgson and R. J. Rose. pp. 205±30, Philadelphia, PA: W.B. Sanders Company. LAWRENCE, L. M., KANE, R. A., MILLER, P. A., REECE, A. & HARTMAN, C. (1986). Urea space determination and body composition in horses. Journal of Animal Science 63 (suppl 1), 233. LAWRENCE, L. M., JACKSON, S., KLINE, K., MOSER, L. & BIEL, M. (1992). Observations on body weight and condition of horses in a 150-mile endurance ride. Equine Veterinary Science 12, 320±4. LIEBER, R. L. (1992). Skeletal muscle anatomy. In Skeletal Muscle Structure and Function. pp. 1±43, Baltimore: Williams and Wilkins. LOHMAN, T. G. (1971). Biological variation in body composition. Journal of Animal Science 32, 647±53. MECKEL, Y., ATTERBOM, H., GRODJINOVSKY, A., BEN-SIRA, D. & ROTSTEIN, A. (1995). Physiological characteristics of female 100 metre sprinters of different performance levels. Journal of Sports Medicine and Physical Fitness 35, 169±75. MITCHELL, A. D., CONWAY, J. M. & SCHOLZ, A. M. (1996). Incremental changes in total and regional body composition of growing pigs measured by dual-energy X-ray absorptiometry. Growth and Development in Aging 60, 95±105. MITCHELL, A. D., ROSEBROUGH, R. W. & CONWAY, J. M. (1997). Body composition analysis of chickens by dual energy X-ray absorptiometry. Poultry Science 76, 1746±52. MITCHELL, A. D., SCHOLZ, A. M. & CONWAY, J. M. (1998). Body composition analysis of small pigs by dual-energy X-ray absorptiometry. Journal of Animal Science 76, 2392±8. MITCHELL, A. D., SCHOLZ, A. M. & PURSEL, V. G. (2000). Dual-energy X-ray absorptiometry measurements of the body composition of pigs of 90- to 130-kilograms body weight. Annals of the New York Academy of Sciences 904, 85±93. PRICE, J. F., PEARSON, A. M., PFOST, H. B. & DEANS, R. J. (1960). Application of ultrasonic reflection techniques in evalutating fatness and leanness in pigs. Journal of Animal Science 19, 381±5. REID, J. T., BENSADOUN, A., BULL, L. S., BURTON, J. H., GLEESON, P. A., HAN, I. K., TYRRELL, H. F., VAN NIEKERK, B. D. H. & WELLINGTON, G. W. (1968). Some peculiarities in the body composition of animals. In Body Composition in Animals and Man. p. 19, Washington, D.C.: National Academy of Science: 19. ROBB, J., HARPER, R. B., HINTZ, H. F., REID, J. T., LOWE, J. E., SCHRYVER, H. F. & RHEE, M. S. S. (1972). Chemical composition and energy value of the body, fatty acid composition of adipose tissue, and liver and kidney in the horse. Animal Production 14, 25±34. RYAN, J. M., COBB, M. A. & HERMANSON, J. W. (1992). Elbow extensor muscle of the horse: Postural and dynamic implications. Acta Anatomical 144, 71±9. SACK, W. O. (1994). Rooney's Guide to the Dissection of the Horse. Ithaca, NY, Veterinary Textbooks. SACKS, R. D. & ROY, R. R. (1982). Architecture of the hind limb of muscle of cats: functional significance. Journal of Morphology 173, 185±95. SIDERS, W. A., LUKASKI, H. C. & BOLONCHUK, W. W. (1993). Relationships among swimming performance, body composition and somatotype in competitive collegiate swimmers. Journal of Sports Medicine and Physical Fitness 33, 166±71. SPARLING, P. B. & CURETON, K. J. (1983). Biological determinants of the sex difference in 12-min run performance. Medicine and Science in Sports and Exercise 15, 218±23. STOUFFER, J. R., VALLENTINE, M. V., WELLINGTON, G. H. & DIEKMANN, A. (1961). Development and application of ultrasonic methods for measuring fat thickness and ribeye area in cattle and hogs. Journal of Animal Science 2, 759±64. THORLAND, W. G., JOHNSON, G. O., CISAR, C. J., HOUSH, T. J. & THARP, G. D. (1987). Strength and anaerobic responses of elite young female sprint and distance runners. Medicine and Science in Sports and Exercise 19, 56±61. TOLL, P. W., GROSS, K. L., BERRYHILL, S. A. & JEWELL, D. E. (1994). Usefulness of dual energy X-ray absorptiometry for body composition measurement in adult dogs. Journal of Nutrition 124(12Suppl), 2601S±3S. WEBB, A. I. & WEAVER, M. Q. (1979). Body composition of the horse. Equine Veterinary Journal 11, 39±47. WEBB, S. P., POTTER, G. D., EVANS, J. W. & WEBB, G. W. (1989). Influence of body fat content on digestible energy requirements of exercising horses in temperate and hot environments. Proceedings of the 11th Equine Nutrition Physiology Symposium, Stillwater, OK. WESTERVELT, R. G., STOUFFER, J. R., HINTZ, H. F. & SCHRYVER, H. F. (1976). Estimating fatness in horses and ponies. Journal of Animal Science 43, 781±5. YOUNG, W., MCLEAN, B. & ARDAGNA, J. (1995). Relationship between strength qualities and sprinting performance. Journal of Sports Medicine and Physical Fitness 35, 13±19. (Accepted for publication 22 December 2001)