(1) This study compares Greyhounds and Thoroughbreds - breeds
selected for high speed running - with other breeds of their species
by gross dissection, histometric and histochemical and biochemical
methods, to Identify adaptations which would favour their superior
athletic capacity. Skeletal muscle has been the primary tissue of
interest because of its power-generating nature.
(2) Carcass dissection was carried out on 44 Greyhounds from
blrthweight to 37 kg, 31 other dogs from blrthweight to 47 kg,
30 Thoroughbreds from 0*69 kg to 509 kg and 33 other horses from
2*2 to 547 kg liveweight.
(2a) Measurements on the humerus, radius and ulna, femur and
tibia and fibula Indicated that their combined lengths
were not different in Greyhounds and other dogs, but
tended to be longer in adult Thoroughbreds than in
adult other horses.
(2b) Within limb variations in bone lengths were not apparent
between breeds. However the eplpodial segment in dogs
and the propodial segment in horses grows faster.
(2c) There is no difference in fresh bone density between the
itypes of dog and horse, but dog bones tend to be more
dense than horse bones.
(2d) The proportions of muscle, bone and fat relative to
liveweight were compared between athletes and others in
adults and during growth. In adults the most functionally
significant difference is that muscle occupies a greater
proportion of liveweight in athletes. Adult Greyhounds
have less fat than other dogs while bone weight forms a
remarkably similar proportion of liveweight in all adult
dogs and horses. In athletes there is a greater growth
rate of muscle which explains the difference in adult
proportions. Growth changes in muscle distribution
explain the greater propulsive capacity of the Greyhound
spinal column and femoral region and of the Thoroughbred
hindlimb. It is also compatible with the potentially
higher stride frequency of the Greyhound hindlimb.
(2e) Athletes tend to have heavier hearts than non-athletes
at adult llveweights, despite the lower growth rate of
the heart in athletes.
(3) In all 33 Greyhounds from birth to 37 kg, 26 other
dogs from birth to 47 kg, 34 Thoroughbreds from 11 kg to 598 kg and
34 other horses from 2*3 to 560 kg liveweight were used for histometric
and biochemical assay, of samples of their m. semitendinosus,
m. diaphragms and m. pectoralis transversus. Mean fibre areas were
established in samples of all three muscles, and in m. semitendinosus
only the transverse sectional area and total number of fibres in it
were also estimated. Histochemical profiles of Individual fibres
were estimated using myosin adenosine triphosphate (myosin ATPase),
succinate dehydrogenase (SDHase), and glycogen phosphorylase (GPase)
reactions; capillaries were also demonstrated using a modification
of the myosin ATPase reaction.
(3a) Athletes have more larger fibres in m. semitendinosus
than non-athletes. The mean fibre area of m. dlaphragma
is also larger in Greyhounds and Thoroughbreds than in
their fellows but the mean fibre area of m. pectoralis
transversus is similar in the two types of animal within
each species. Although the mean fibre area of corresponding
muscles is significantly larger in horses than in dogs
the difference is not related to their liveweight difference.
(3b) The major histochemical difference between fibres is their
myosin ATPase activity, which differentiates them according
to whether they have a high or low activity. In adult
dog muscle, all fibres have a high SDHase activity and
myosin ATPase low-reacting fibres have a low activity of
GPase. In adult horse muscle all fibres have a high
activity of GPase. In m. dlaphragma and m. pectoralis
transversus all fibres also have a high SDHase activity
so that only the myosin ATPase reaction differentiates
fibres in these muscles, however fibres with a low activity
of SDHase are present in samples of m. semitendinosus.
(3c) The myosin ATPase reaction differentiates fibres at the
earliest stage of growth observed. The GPase and SDHase
activities gradually develop from an amorphous staining
pattern in the young to the appropriate adult type.
The proportional area of myosin ATPase low-reacting fibres
in the three muscles studied is related to liveweight
from birth to near adulthood. Thereafter the relationship
is less obvious in "athletes" than "non-athletes.
(3d) There is a greater proportional area of myosin ATPase
high-reacting fibres in the limb muscles of both Greyhounds
and Thoroughbreds and in m. diaphragma of Greyhounds.
In adults this feature does not appear to be due to
training as are alterations in aerobic and anaerobic
capacity. This dissimilarity (in the proportions of
muscles occupied by myosin ATPase high-reacting fibres)
suggests that there may be differences in the nervous
systems of athletes and non-athletes.
(3e) It is concluded that the proportions of fibre types in
muscles are related to the function of muscles and
its parts. Although the proportions of fibre types
in different muscles and parts of muscles and in different
types of animals resemble those of adults at the earliest
stages investigated, histochemical evidence has been
obtained which suggests transformation of the physiological
properties of fibres as a normal occurrence but to
differing extents during growth of normal athletes and
(3f) Capillary density is remarkably similar between muscles
of all groups of animals at all except very early stages
(4) The biochemical estimation of SDHase activity does not show a
within species difference is the adult but indicates an increase in
activity in both species during growth. It has also been found that
there is a greater aerobic activity in m. diaphragma than in the other
two muscles and a greater activity in the deep medial than in the
superficial lateral region of m. semitendinosus.
(5) M. longissimus is proportionally lighter in Greyhounds taken out
of training than in others. Such specimens have a greater myosin
ATPase high-reacting fibre area in their m. diaphragma and lesser
capillary density in their m. pectoralis transversus than trained
(6) The crosses of Thoroughbreds with other horses, show anatomical
properties more like Thoroughbreds than non-athletic horses.
(7) The results are discussed in relation to stride length and
frequency. It is suggested that in adult athletes enhanced stride
length is favoured by longer limbs in horses, and a greater
acceleration capacity in both species. A higher natural frequency
of the Greyhound hindlimb, and a greater intrinsic speed of sarcomere
contraction in the athletes of both species favour enhanced stride
frequency. The combination of these endowments aids a greater
maximum speed of running in both Greyhounds and Thoroughbreds when
compared with their fellows.