Download Overview of Horse Body Composition and Muscle Architecture

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
gˆ5
sˆ2
m ˆ 10
g ˆ 10
N/A
N/A
N/A
N/A
N/A
m ˆ 23
gˆ2
sˆ6
m ˆ 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 ˆ stallion, g ˆ gelding, m ˆ mare.
b
c
8.1b
15.4
1.1
2.1
13.0
10.1
7.4
7.8
22.3
male ˆ 6.9
female ˆ 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 ˆ non-finishers (lameness or injury); Group II ˆ non-finishers (metabolic); Group III ˆ Top Ten finishers;
Group IV ˆ 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 ˆ 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 ˆ 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 ˆ 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)