Biomechanics
of the
Stallion’s
Hindquarters During Semen Collection
Master of Science Thesis
Emma L. van Beuzekom
Student nr 3381838
December 2013 – April 2014
Supervisors:
A.C. Hoogendoorn, B. Colenbrander, T.A.E. Stout, W. Back
1
Table of Contents
Introduction ............................................................................................................................... 4
Aim............................................................................................................................................. 7
Research Question .................................................................................................................... 7
Materials and Methods ......................................................................................................... 8
Animals ............................................................................................................................. 8
Experimental protocol ...................................................................................................... 8
Data collection .................................................................................................................. 8
Data processing ................................................................................................................ 8
Statistical Analysis........................................................................................................... 10
Results ..................................................................................................................................... 11
Comparison of Angles within Each Horse ....................................................................... 11
Comparison of Mounting Phase and Ejaculation Phase ................................................. 11
Relationship between Pelvis and Back ........................................................................... 12
Discussion ................................................................................................................................ 14
Biomechanics during semen collection .......................................................................... 14
Musculoskeletal problems of breeding and sports ........................................................ 15
Other interactions between breeding and sports .......................................................... 16
Conclusions.............................................................................................................................. 17
Acknowledgements ................................................................................................................. 17
References ............................................................................................................................... 18
2
Abstract
Introduction
It is often claimed that stallions used for both breeding and showjumping perform less well
in competitions during the breeding season. The demands on the back and pelvis during
semen collection may impair the range of movement required for showjumping. Therefore,
the aim of this study was to assess the biomechanics of the stallion’s hindquarters by
examining the range of motion (ROM) of the hindquarters, in particular the pelvic flexion,
during both the mounting and ejaculatory phases of semen collection.
Materials and methods
The kinematics of the hindquarters of six sportshorse stallions regularly used for semen
collection were studied at the mounting and ejaculation phases during semen collection on a
phantom. Green spherical skin markers were placed on Th5, tuber coxae, proximal femur
and tibia, and distal tibia and metatarsus. At each phase a total of 6 angles between the skin
markers was measured using a home-video camera positioned perpendicular to the
phantom. The differences in joint angles between these two phases were statistically
compared using commercially available software (SPSS) at a significance level of P<0.05.
Results
Horses showed a significantly larger variation in joint angle values during the mounting than
during the ejaculation phase. They also showed more extension in the tarsal than in the hip
and stifle joints; there was also more extension in the stifle than in the hip joint (P<0.05).
The pelvic flexion angle was significantly larger with more extension than the tarsal joint
angle (P<0.05). The position of the hindlimb in relation to the ground was significantly more
upright than that of the body (P<0.05).
Conclusions
During the mounting phase the pelvis shows a larger joint angle range than during the
ejaculation phase. During both phases the tarsal joint shows more extension than the hip
and stifles joints, the stifle shows more extension than the hip, and the pelvis shows more
extension than the tarsal joint. Meanwhile, the hindlimb is in a significantly more upright
position than the body. The extreme flexion and extension of the pelvis may impose a very
different loading on the musculoskeletal structures, especially when during mounting also
asymmetric lateroflexion and/or axial rotation are involved, possibly provoking locomotor
pain in the hindquarters. Still, it remains somewhat unclear how exactly this will interfere
with the ability to extend the pelvis during showjumping.
3
Introduction
This study evaluates the interaction between breeding and sports performance in the horse,
in particular the show jumping stallion simultaneously used for breeding purposes. The
motivation for this research is derived from the claim that stallions used for both breeding
and showjumping, tend to perform less well during the breeding season. It is not uncommon
for stallions to be used for semen collection five times a week and also perform in
competitions during the weekends. Riders seem to observe that more faults are made in the
jumping course when that particular stallion has been used for breeding. However, it is still
unclear whether there is a physical reason for that supposedly compromised performance.
There are several factors that play a role in breeding, which could have an influence on
jumping performance. There is a psychological factor for both the rider and the horse that
plays an important role. The riders seem to be convinced that stallions will perform less well
– this alone could influence their riding style. Also, the in-season mares can be very
distracting for stallions at competitions and the management of the stallion can play a role.
Two other factors that contribute, are the musculoskeletal support required for breeding
and sport, and also the suppleness of the back and pelvis that varies between individuals.
This research project will focus on the physical aspects of semen collection that may
influence sports performance.
Stallions are generally housed in relative social isolation, for safety reasons, amongst others.
This has consequences for the welfare of the stallions and influences their behaviour. The
type of housing used could influence the psychological state of stallions. McDonnell (2000)
describes the differences between reproduction in nature and in domesticated stallions. She
makes clear that in nature, there are numerous interactions between stallion and mare, with
continually repeated confrontations and retreating. This behaviour results in the stallions
being ‘warmed up’ prior to breeding. This is in stark contrast with breeding stations where
stallions usually are taken directly from their stable in the morning to collect semen on the
phantom. The semen needs to be sent off early in the day to inseminate mares on that same
day. This means that the stallions must exert themselves without warming up, which could
be quite challenging for their muscles and joints (Burger, 2012; McDonnell, 2000).
Breeding on a phantom namely requires physical exertion from the stallion. This could result
in a stallion having insufficient energy for optimal sports performance. Also, it varies per
stallion how much energy he needs for semen collection as the behaviour differs between
horses. Whilst some stallions mount and ejaculate within a few minutes, other stallions need
much more time and attempted mounts. Moreover, not all stallions are well-educated
and/or kept under control during breeding. The stallions sometimes behave wildly, rearing
up and coming down on the ground in an uncontrolled manner, which could be dangerous
to both horse and handler.
McDonnell describes alternatives for breeding with stallions that are of older age or are not
in a physical state that allows them to mount a phantom or mare. An example of this is the
‘ground semen collection’ method. Martin (2011) states that the ground semen collection
procedure is a valuable option for stallions suffering aorto-iliac thrombosis, cervical
vertebral malformation, EPM, and other conditions with hindlimb instability or weakness,
provided their libido is good to excellent. However, according to Barrier-Battut (2013) there
is no difference in the weight supported by the hind-limbs when semen is collected standing
on the ground or mounted on a phantom (McDonnell, 2000; Martin, 2011; Barrier-Battut,
2013).
A clinical examination of a showjumper horse usually indicates substantial back pain. Back
pain in horses can cause considerable compromises in performance. However, back pain in
the horse can manifest itself in many different ways, making diagnosis difficult. Clinical
4
examination of horses with chronic back problems shows reduced movement of the
hindlimbs, reduced flexion of the tarsus and sometimes dragging of the hooves. When a
horse has chronic issues in the sacroiliac joint, this can be seen as bilateral stiffness and pain
in the proximal hindlimb (Haussler, KK, Jeffcott LB 2014).
Santamaría et al. (2005) has done research into the effects on development of jumping
technique when horses are trained at a young age. It is clear that the jumping capacity, or
the ability to move the body from the ground into the air, is dependent upon the anatomical
and physiological characteristics of the muscles and skeleton. Santamaría’s research
demonstrates that horses which underwent extra jumping training as a foal, as 4-year olds
jump with their centre of gravity closer to the fence. The 4-year-olds were able to clear the
fence because they displayed a clear rotation of the trunk when the hindlimbs were above
the fence. This study showed that during suspension phase it is important that the
hindquarters undergo a retroflexion to clear the fence (Santamaria et al. 2005). This
hindlimb retroflexion over the jump gives an extreme extension of the pelvis, as shown in
Figure 1b. When the hindquarters of the horse are above the fence the lumbosacral joint
extends, lifting the hindquarters up, and the hip joint extends, elevating the lower limbs. As
can be seen in Figure 1a, this extension of the pelvis also occurs during semen collection.
(Santamaría et al. 2005; Van Weeren, 2013).
During semen collection, the stallion exerts an extreme flexion of the hindquarters in
relation to the back (Figure 2a), during both the thrusting of the pelvis and ejaculation. This
flexion of the hindquarters also occurs just prior to take-off for jumping a fence (Figure 2b).
During breeding, there are ground reaction forces exerted on the horse’s body, which are
not present during the airborne phase of a jump. This could possibly be putting an extra
demand on the back during semen collection that may impair the range of movement
required for show jumping performance later on. However, whether there is an interaction
between breeding and sports performance is not known. To establish whether or not there
is an influence, at first the biomechanics of the stallion during semen collection must be
examined.
5
Figure 1a. Extension of the pelvis during
semen collection.
Figure 1b. Extension of the pelvis
during airborne phase over jump
( www.equisearch.com).
Figure 2a. Flexion of the pelvis during
semen collection.
Figure 2b. Flexion of the pelvis just prior to takeoff for a jump (www.yourhorse.co.uk).
6
Aim
The aim of this study was to assess the biomechanics of the stallion during a semen
collection procedure on a dummy mount. Hereby the range of motion of the hindquarters
was examined, in particular the degree of pelvic flexion during both the mounting and
ejaculatory phases of semen collection. This study would serve as a first step in a larger
piece of research into the interactions between breeding and sports performance in showjumping stallions.
Research Question
How could the semen collection procedure (in particular the biomechanics) have an
influence on the locomotor performance of stallions?
7
Materials and Methods
Animals
Six warmblood sport horse stallions were used for this study. There were 3 KWPN horses, 2
Holsteiners and 1 Westfaler. The average age was 16.2 years, ranging from 10 years to 21
years. The average height of the horses (measured at the withers) was 1.72 m, ranging from
1.70 to 1.74 m. With the information gained from the pilot study (see Appendices), it was
determined that a population of n=4 would be required to have sufficient statistical power.
Experimental protocol
Before each data collection, the horses
were kept stabled indoors. Six spherical
green skin markers (25 mm diameter)
were attached to specific anatomic
locations on the right-hand side of the
body whilst the horse was kept in the
stable (Figure 3): the highest point of the
withers; tuber coxae; trochanter major;
lateral side of the knee on the fibula
head; lateral side of the tarsus on the
distal end of the tibia; lateral side of the
fetlock on the distal end of metatarsus III.
The horses were guided towards the
phantom, allowed facial contact with a
mare, then the stallions were allowed to
mount the dummy and semen was
collected in an artificial vagina. The
phantom was set at a fixed height and
slope for all six stallions.
Figure 3. Schematic illustration of the anatomic
locations at which markers were attached to
the horse. 1: highest point of the withers; 2:
tuber coxae; 3: proximal femur; 4: proximal
tibia; 5: distal tibia; 6: distal metatarsus III.
Data collection
A video recording (using video camera with 1080p HD 30 fps) was made of each stallion
during the semen collection procedure. The camera was positioned perpendicular to the
phantom. Three video recordings of separate semen collections were made of each stallion.
Data processing
Using commercially available video processing software (MPEG Streamclip©), the video
recordings were converted into Image Sequences, with an interval of 10 frames. The image
sequences were analysed using commercially available image-processing software ImageJ
(version 1.47). Using this software, a total of six angles were measured between the
anatomically located skin markers during the mounting and ejaculation phases (Figure 4 - 9).
8
Figure 4. Schematic illustration showing
pelvic flexion angle (between marker
1,2,3).
Figure 5. Schematic illustration showing
hip angle (between marker 2,3,4).
Figure 6. Schematic illustration showing
stifle angle (between marker 3,4,5).
Figure 7. Schematic illustration showing
tarsus angle (between marker 4,5,6).
Figure 8. Schematic illustration showing
body stance angle (between marker 1,2
and horizontal).
Figure 9. Schematic illustration showing
hindlimb stance angle (between marker
5,6 and vertical).
9
Statistical Analysis
Differences in joint angle and movement measurements during the two phases were
compared using graphical representations of the data and paired t-tests, with P<0.05 to
indicate statistically significant differences. Statistical analysis was done using commercially
available software (SPSS and Microsoft Excel).
10
Results
Comparison of Angles within Each Horse
Paired t-tests were performed (using commercially available software Microsoft Excel) to
compare the relationship of angles within horses. Pairs of angles were compared for each
horse during both phases of all three trials. Differences were considered to be significant
when p<0.05. The following results were found:
- All horses show a significantly larger angle of the tarsus (on the flexor aspect) than
the hip.
- All horses show a significantly larger angle of the tarsus (on the flexor aspect) than
the stifle.
- All horses show a significantly larger angle of the body stance than the hind limb
stance.
- All horses show a significantly larger angle of the stifle than of the hip.
- All horses show a significantly larger angle of the pelvic flexion than of the tarsus (on
the flexor aspect).
Comparison of Mounting Phase and Ejaculation Phase
For each horse and trial, a graph was made to show how the six different angles change
during the course of the mounting phase and the ejaculation phase. The graphs in Figure 10a
and 10b show the mounting phase and ejaculation phase of one trial for one horse.
Figure 10a. Graph showing the 6 measured angles during the mounting
phase.
11
Figure 10b. Graph showing the 6 measured angles during the ejaculation phase.
It can be seen that the six measured angles have the tendency to fluctuate during the
mounting phase. The angles increase and decrease with a certain degree of regularity.
During the ejaculation phase, the fluctuations are reduced and the angles become more
constant as the horse’s body movement decreases. There is clearly a difference between the
two phases.
The fluctuations of hip, stifle, tarsus and hindlimb stance are reflective of the alternately
stepping with the hindlimbs. The pelvic flexion shows extreme extension and flexion during
the mounting phase, whilst in the ejaculation phase the pelvis is relatively flexed.
Relationship between Pelvis and Back
The relationship between the pelvis and the back was also examined for each horse and
trial. The relationship between these two parameters differs during the two phases. Figure
11 shows the mounting phase, where the stallion is thrusting. When the pelvic flexion is at
smaller angle, so the pelvis is flexed, the body is more upright. When the pelvis extends, the
body stance is more horizontal.
Figure 11. Graph showing relationship between pelvic flexion and
body stance during mounting phase.
12
In contrast, during the ejaculation phase, this relationship is the other way round, as
indicated by Figure 12. The body stance is more upright when the pelvis has a larger,
extended angle. The body stance is more horizontal when the pelvis is flexed. This can be
explained by the way in which the stallions tend to sink down onto the dummy mount, in
contrast to their half standing position during the mounting phase.
Figure 12. Graph showing relationship between pelvic flexion and
body stance during ejaculation phase.
In a few cases, this pattern was not found. Instead, in the mounting phase, the body stance
becomes more upright as the pelvic flexion increases. This happens when the stallion stands
further away from the dummy mount, not coupling as closely.
13
Discussion
Biomechanics during semen collection
The paired t-tests on pairs of angles within each horse would give an idea of the relation of
angles within a horse: the tarsus angle is larger than the hip and the stifle angles; body
stance angle is larger than hindlimb stance angle. These findings are quite logical, when the
anatomy of the horse is taken into consideration. The stifle angle is significantly larger than
the hip angle, and the pelvic flexion is significantly larger than the tarsus angle. This could be
due to the lifting of the hindlimbs, whereby the tarsus changes position, causing the stifle
angle to increase as well as the hindlimb stance angle.
The results indicate a clear difference between the mounting and ejaculation phase,
whereby the mounting phase has characteristic flexing and extending of the pelvis. It is
unclear whether this extreme flexion and extension of the pelvis is a positive or a negative
aspect of the semen collection procedure. Also, it is unclear if either flexion or extension
would have more of a negative impact on the stallion’s body. It could be argued that the
extreme flexion imposes a different loading on the musculoskeletal structures involved in
pelvic rotation and this may interfere with the ability to extend the pelvic during
showjumping. On the other hand, it could be argued that it is beneficial to have this range of
movement in the pelvic region, as it indicates a certain degree of flexibility in the body.
The results from this research project may be compared with findings in a study by
Santamaría et al. (2005). In that study, several angles where measured whilst a horse
jumped over an obstacle. It was found that hindlimb retroflexion had an angle between 109
and 113 degrees (Santamaría et al. 2005). As can be seen in Figure 13, this angle is measured
in a similar way to the pelvic flexion angle in this research project. The hindlimb retroflexion
angle
found
by
Santamaría is quite a bit
smaller than the pelvic
flexion angle measured in
this research project
(ranging between 144
and 193 degrees). It may
be possible that a slight
overextension of the
hindlimbs over a jump Figure 13. Diagram indicating the measured angles in a
could be exacerbated study by Santamaria et al. (2005).
when it is followed by a
semen
collection
procedure.
14
Musculoskeletal problems of breeding and sports
The interaction between breeding and sports performance can be seen as a ‘chicken-or-theegg’ situation. The question is whether breeding has a negative influence on the body, and
that causes the performance to be reduced, or if showjumping causes an issue that is
aggravated by breeding. Breeding can be considered a natural activity for the horse;
although it should be taken into consideration that breeding on a dummy mount is of course
unnatural. For example, in nature the stallion does not have to dismount the mare, rather
the mare walks forwards and the stallion slides off, whilst with a dummy mount the stallion
must take a few steps backwards in order to dismount. Jumping over multiple obstacles with
a height of 1.6 meters and a rider on the back of the horse is not a natural activity either. As
both the intensive breeding on dummy mounts and showjumping are unnatural activities for
horses, it is difficult to determine what would cause a reduced sports performance.
It is often thought that the problem in compromised showjumping performance lies in the
stifle. As Murray explains, it is reasonably common to find injuries to the stifle joint in
showjumping horses. However, Murray also points out that back pain too is common in
showjumping horses (Murray, 2014).
In a study by Martin (1998), 25 breeding stallions were presented for either routine breeding
soundness evaluations or specific problems in breeding performance. A sore back was a very
common finding, as was lameness and degenerative joint disease. It was found that
musculoskeletal problems were often caused by the incorrect positioning of the stallion on
the dummy mount. For example, when the stallion mounts to one side of the dummy and
advances up that side, the back will be markedly curved during thrusting. This highlights the
importance of proper positioning of the stallion and correct settings of the dummy mount
regarding height and slope (Martin, 1998).
Qualitative observations where also made during the video recordings in this research
project. Some horses had the tendency to mount the dummy, but then would not ejaculate.
They then had to dismount the dummy and start again, sometimes requiring up to 9
attempts before the semen collection was completed. One factor that could play a role here
is the temperature of the artificial vagina. The dismounting of the dummy sometimes
occurred very suddenly, whereby the horse would rapidly move backwards and to the side,
twisting its body towards the handler. Not only does this create a potentially dangerous
situation for the handler, these sudden movements could also have an influence on the
musculoskeletal structures. Furthermore, several of the horses had the tendency to mount
the dummy slightly on the left side. This means that the pelvis is rotating asymmetrically in
several planes, illustrated in Figure
14. Not just the pelvic flexion, but
also the lateral bending and axial
rotation would be unbalanced
when the horse stands with a
curved
spine.
Perhaps
this
positioning comes from the artificial
vagina being held to the left side of
the horse. It could also be from Figure 14. Diagram illustrating the three planes of
movement of the back. (A) Pelvic Flexion: rotation
nature when a stallion has to mount
around an axis perpendicular to the sagittal plane. (B)
a mare from the side, rather than Lateral bending: rotation around the dorsoventral axis.
directly from behind for his own (C) Axial rotation: rotation around the craniocaudal axis
safety. The result of this body (Van Weeren, 2009).
positioning is that the spinal column
is somewhat twisted whilst strong
15
forces are exerted to flex and extend the pelvis. Also, this body positioning would lead to
asymmetrical loading of the hindlimbs, and perhaps in combination with a strongly extended
hindlimb this might lead to unilateral irritation of the interosseus tendon.
In order to reduce musculoskeletal problems, there are a number of things that can be done.
These are: assessment of the breeding shed, comfort of the stallion during breeding, and
handling of the stallion. The breeding shed should be evaluated so that the semen collection
procedure can be made as comfortable and safe as possible for the stallion. For example, it
is found that for stallions with hindlimb, back or neck problems, a dummy mount set at a
greater height may work better (Martin, 2011; McDonnell, 2005).
Martin (2011) and McDonnell (2005) also emphasize the importance of monitoring the
stallion’s comfort during breeding. Comfort is reflected by the number, pattern and strength
of pelvic thrusts. It is stated that normal, sound stallions require seven to nine thrusts of
even strength, at approximately 1-second intervals to ejaculate. Signs of discomfort may be:
more thrusts required; irregular pattern of depth, rhythm and strength; and restless
hindlimbs. It also seen that horses with sore backs or necks do not couple as closely, and
thrusts may be jerky and shallow, which would result in a hollow back position with
extended pelvis (Martin, 2011; McDonnell, 2005).
As mentioned earlier, Martin (1998) stresses the value of correct positioning of the horse on
the dummy mount, so that the back is not curved during pelvic thrusting (Martin, 1998).
Ensuring the stallion is well behaved and can breed on the dummy mount in a controlled and
safe manner could contribute to reducing the chances of sustaining injury from breeding.
This could also be achieved by allowing the stallion to warm up his body before breeding.
Just as horses are warmed up before ridden work, it would be fair to either walk or lightly
lunge the stallion before the semen collection procedure.
Other interactions between breeding and sports
Lange et al. (1997) studied the influence of training and competition on the semen
parameters in stallions. The study suggests that the fertility of the stallions taking part in
sports competitions was unchanged because the semen quality did not decrease. In human
studies it has been shown that overtraining with elevated cortisol and decreased
testosterone can lead to reduced sperm concentration in humans (Roberts et al. 1993). As
this change in semen quality was not the case in Lange’s study, it was assumed that the
stallions’ training intensity must have been below the threshold required for negative effects
on semen quality to take place (Lange et al. 1997).
Pasing et al. (2013) investigated the influence of semen collection on salivary cortisol release
and heart rate in stallions to determine whether sexual arousal or physical exercise caused
by breeding might negatively affect the sports performance of stallions. It was argued that if
a major stress response occurred, there may be influences on the physical performance of
the stallions. It was found that there was a temporary increase in the sympathetic activity,
which was reflected by an increased heart rate due to sexual arousal and ejaculation.
However, the heart rate returned to baseline levels within 15 minutes, therefore the
sympathetic system increase was only short-lived. They suggest that the increase in
sympathetic activity is mainly the result of emotional factors and not of physical demands,
because the heart rate increased already during exposure to the teaser mare. They conclude
that semen collection is only a modest temporary stressor for sexually experienced and welltrained stallions, and that that it is unlikely to affect the physical performance of stallions in
sports performance (Pasing et al. 2013).
16
Conclusions
There appears to be a clear difference in the biomechanics of the stallion’s back and
hindquarters between the mounting and ejaculation phase at semen collection. The pelvis
shows hyperflexion and hyperextension, but the effect of this on jump locomotor
performance remains unclear.
Still, based on the findings of this study, there are three main pieces of advice to people
working at a stud farm. Firstly, the stallion should be under control and care should be taken
to safely collect semen. Secondly, it is advisable to warm up the stallion prior to semen
collection either by walking or lightly lunging. Thirdly, take into consideration the comfort of
the stallion during semen collection. Preventing factors that could influence the stallion’s
body and working to minimise these issues, could help to reduce the chance of developing
and/or activating an injury during semen collection.
This study of the biomechanics provides a basis for further research. It could be of interest
to explore whether the hyperflexion and hyperextension of the pelvis have a negative or
positive effect on showjumping ability. Also, it could be useful to investigate the stride
length of the hindlimbs as a measure for back pain in horses before and after semen
collection, and before and after showjumping. The information gained from this project
could be built upon with further research in order to further elucidate the interaction
between breeding and sports performance.
Acknowledgements
I would like to thank the members of Team Nijhof stud farm and the animal caretakers of
the equine reproduction department at the Faculty of Veterinary Medicine of Utrecht
University for their technical assistance.
17
References
Barrier-Battut, I., 2013. Collection and treatment of stallion semen: what's new? (Original
title: Collecte et traitement de la semence d'etalon: quoi de neuf?). Pratique
Veterinaire Equine 45, 35-39.
Burger, D., Wedekind, C., Wespi, B., Imboden, I., Meinecke-Tillmann, S., Sieme, H., 2012. The
Potential Effects of Social Interactions on Reproductive Efficiency of Stallions. Journal
of Equine Veterinary Science 32, 455-457.
Haussler, K.K., Jeffcott, L.B., 2014. 21 - Back and pelvis. In: Equine Sports Medicine and
Surgery (Second Edition) W.B. Saunders, pp. 419-456.
Lange, J., Matheja, S., Klug, E., Aurich, C., Aurich, J., 1997. Influence of training and
competition on the endocrine regulation of testicular function and on semen
parameters in stallions. Reproduction in Domestic Animals 32, 297-302.
Martin Jr, B.B., McDonnell, S.M., Love, C.C., 1998. Effects of musculoskeletal and neurologic
diseases on breeding performance in stallions. The Compendium on continuing
education for the practicing veterinarian (USA) .
Martin Jr., B.B., McDonnell, S.M., 2011. Chapter 127 - Lameness in Breeding Stallions and
Broodmares. In: Diagnosis and Management of Lameness in the Horse (Second
Edition) W.B. Saunders, Saint Louis, pp. 1235-1242.
McDonnell, S.M., 2000. Reproductive behavior of stallions and mares: comparison of freerunning and domestic in-hand breeding. Animal Reproduction Science 60, 211-219.
McDonnell, S.M., 2005. Techniques for Extending the Breeding Career of Aging and Disabled
Stallions. Clinical Techniques in Equine Practice 4, 269-276.
Murray, R.C., 2014. 55 - Veterinary aspects of training the show jumping horse. In: Equine
Sports Medicine and Surgery (Second Edition) W.B. Saunders, pp. 1127-1135.
Pasing, S., von Lewinski, M., Wulf, M., Erber, R., Aurich, C., 2013. Influence of semen
collection on salivary cortisol release, heart rate, and heart rate variability in stallions.
Theriogenology 80, 256-261.
Roberts, A C McClure, R D Weiner, R I Brooks,G A., 1993. Overtraining affects male
reproductive status. Fertility and sterility 60, 686-692.
Santamaría, S., Bobbert, M.F., Back, W., Barneveld, A., van Weeren, P.R., 2005. Effect of
early training on the jumping technique of horses. American Journal of Veterinary
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Van Weeren, P.R., 2013. Chapter 25: Training show jumpers. In: The Athletic Horse:
Principles and Practice of Equine Sports Medicine, 2nd editionEd. Elsevier, pp. 337337-346.
19
Appendix I: Pilot Study
Materials and Methods: Animals and Experimental Protocol
Two Dutch Warmblood stallions (Bogie Boy and Billy) aged 3 years and 23 years, belonging
to the Faculty of Veterinary Medicine of Utrecht University were used in this study.
The same experimental protocol was used as for the main research project.
Results & Data Analysis
Graphs
Figure 1a. Graph showing the 6
measured angles during the
mounting phase for Billy Trial 1.
Figure 1b. Graph showing the 6
measured
angles
during
the
ejaculation phase for Billy Trial 1.
Figure 2a. Graph showing the 6
measured angles during the mounting
phase for Billy Trial 2.
Figure 2b. Graph showing the 6
measured angles during the
ejaculation phase for Billy Trial 2.
Figure 3a. Graph showing the 6
measured
angles
during
the
mounting phase for Billy Trial 3.
Figure 3b. Graph showing the 6
measured angles during the
ejaculation phase for Billy Trial 3.
20
During the mounting phase of Billy’s first trial (Figure 1a) the angles of pelvic flexion, hip,
stifle, tarsus and hind-limb stance have the tendency to fluctuate, whilst the angle of body
stance does not show this tendency as strongly. It is clearly seen that the fluctuations of all
angles are reduced in the ejaculation phase (Figure 1b) and the angles become more
constant as the horse’s movement decreases. In Billy’s trial 1, the angle of pelvic flexion
ranges from 156.3 to 176.9 degrees in the mounting phase and from 159.0 to 163.1 degrees
in the ejaculation phase.
In Billy’s second trial the fluctuations are present during the mounting phase in all angles
(Figure 2a), with the body stance angle being the most stable. The ejaculation phase of this
trial appears slightly different to the first trial as the measured angles do not even out as
much (Figure 2b). Only the body stance angle shows more consistency during the ejaculation
phase. In Billy’s trial 2, the pelvic flexion ranges from 166.6 to 183.3 degrees in the mounting
phase and from 164.3 to 178.4 degrees in the ejaculation phase.
Billy’s third trial also shows a clear fluctuating pattern in the angles of pelvic flexion, hip,
stifle and tarsus during the mounting phase (Figure 3a), and it is also present, though less
clearly so, in the body-stance and hind-limb stance angle. In the ejaculation phase the pelvic
flexion angle tends to smooth out, whilst the other angles still have some apparent variation
(Figure 3b). In Billy’s trial 3, the angle of pelvic flexion ranges from 162.3 to 182.0 degrees in
the mounting phase and from 158.4 to 170.9 degrees in the ejaculation phase.
Figure 4a. Graph showing the 6
measured angles during the mounting
phase for Bogie Boy Trial 1.
Figure 4b. Graph showing the 6
measured angles during the
ejaculation phase for Bogie Boy
Trial 1.
Figure 5a. Graph showing the 6
measured angles during the mounting
phase for Bogie Boy Trial 2.
Figure 5b. Graph showing the 6
measured angles during the
ejaculation phase for Bogie Boy Trial
2.
21
Figure 6b. Graph showing the 6
measured angles during the
ejaculation phase for Bogie Boy Trial
3.
Figure 6a. Graph showing the 6
measured angles during the
mounting phase for Bogie Boy
Trial 3.
Bogie Boy also demonstrates a clear oscillating pattern for the angles of pelvic flexion, hip,
stifle, tarsus and body-stance during the mounting phase (Figure 4a). The hind-limb stance
angle also shows variation, but the pattern is not as regular as it is for the other angles
(Figure 4a). During the ejaculation phase, all six angles smooth out into a steady line (Figure
4b). In Bogie Boy’s trial 1, the angle of pelvic flexion ranges from 161.3 to 178.5 degrees in
the mounting phase and from 163.7 to 170.5 degrees in the ejaculation phase.
Bogie Boy’s second trial shows a similar pattern to the first trial in the mounting phase with
a fluctuating pattern of pelvic flexion, hip, stifle, tarsus and body-stance angles, and a less
consistent pattern for hind-limb stance angle (Figure 5a). Again, during the ejaculation phase
all six angles level out to a stable, smoother line (Figure 5b). In Bogie Boy’s trial 2, the angle
of pelvic flexion ranges from 163.2 to 174.3 degrees in the mounting phase and from 162.6
to 166.9 degrees in the ejaculation phase.
Both phases of Bogie Boy’s third trial (Figure 6a and 6b) show the same pattern as the first
and second trial. In Bogie Boy’s trial 3, the angle of pelvic flexion ranges from 164.0 to 181.5
degrees in the mounting phase and from 165.5 to 171.3 degrees in the ejaculation phase.
22
Boxplots & Standard Deviation
Figure 7: Boxplot of Pelvic Flexion angles for Billy
and Bogie during trial 1,2 & 3
Figure 8: Boxplot of Hip angles for Billy
and Bogie during trial 1,2 & 3
Figure 9: Boxplot of Stifle angles for
Billy and Bogie during trial 1,2 & 3
Figure 10: Boxplot of Tarsus angles
for Billy and Bogie during trial 1,2 &
3
Figure 11: Boxplot of Body stance angles
for Billy and Bogie during trial 1,2 & 3
Figure 12: Boxplot of Hind-limb stance
angles for Billy and Bogie during trial
1,2 & 3
23
Table 1. Standard deviation, mean and coefficient of variation for Pelvic Flexion angles
Pelvic Flexion
Horse
Billy
Billy
Billy
Bogie
Bogie
Bogie
Trial
1
2
3
1
2
3
StDev (degrees)
4.27
4.15
6.73
3.96
3.03
4.13
Mean (degrees)
162.61
174.76
167.82
168.72
167.16
170.05
CV (%)
2.62
2.38
4.01
2.35
1.81
2.43
Table 2. Standard deviation, mean and coefficient of variation for hip angles
Hip
Horse
Trial
StDev (degrees)
Mean (degrees)
CV (%)
Billy
1
2.84
87.16
3.25
Billy
2
2.95
87.08
3.39
Billy
3
4.36
86.96
5.02
Bogie
1
3.16
77.05
4.10
Bogie
2
4.08
90.08
4.53
Bogie
3
2.46
79.39
3.10
Table 3. Standard deviation, mean and coefficient of variation for stifle angles
Stifle
Horse
Trial
StDev (degrees)
Mean (degrees)
CV (%)
Billy
1
5.13
96.22
5.33
Billy
2
5.02
98.23
5.12
Billy
3
4.31
94.99
4.54
Bogie
1
3.58
86.24
4.15
Bogie
2
4.30
93.99
4.57
Bogie
3
3.37
84.78
3.98
Table 4. Standard deviation, mean and coefficient of variation for tarsus angles
Stifle
Horse
Billy
Billy
Billy
Bogie
Bogie
Bogie
Trial
1
2
3
1
2
3
StDev (degrees)
6.50
5.66
4.74
2.94
6.68
4.62
24
Mean (degrees)
149.74
148.24
147.27
158.01
155.98
156.75
CV (%)
4.34
3.82
3.22
1.86
4.29
2.95
Table 5. Standard deviation, mean and coefficient of variation of body stance angles
Body Stance
Horse
Trial
StDev (degrees)
Mean (degrees)
CV (%)
Billy
1
2.95
47.34
6.23
Billy
2
1.82
48.89
3.73
Billy
3
2.23
50.09
4.46
Bogie
1
3.50
48.34
7.25
Bogie
2
2.41
49.59
4.85
Bogie
3
2.64
49.71
5.31
Table 6. Standard deviation, mean and coefficient of variation for hind-limb stance
angles
Hind-limb Stance
Horse
Trial
StDev (degrees)
Mean (degrees)
CV (%)
Billy
1
6.41
13.16
48.69
Billy
2
5.27
7.23
72.94
Billy
3
6.50
13.44
48.36
Bogie
1
2.10
3.00
70.13
Bogie
2
3.97
4.49
88.39
Bogie
3
2.53
3.50
72.27
Billy’s pelvic flexion angles have median values lying further apart from each other than for
Bogie (Figure 7). Bogie’s pelvic flexion measurements were quite similar during the three
trials, as seen in the boxplots (Figure 7) and as indicated by the mean and coefficient of
variation (CV) (Table 1).
The median hip angles for Billy lie close to each other during trial 1,2 and 3 (Figure 8) and the
spread is also quite similar for the three trials as seen by the CV (Table 2). In contrast,
Bogie’s median hip angles lie far apart (Figure 8) and the mean hip angles also differ quite a
bit (Table 2).
The stifle angles for Billy are quite similar during trial 1, 2 and 3 with the medians situated
fairly close together (Figure 9). The mean values are not far apart and the CV’s reflect similar
variances across the three trials (Table 3). In contrast, Bogie’s median and mean stifle angles
are spread out in the three trials, whilst the variance of the values is not so different (Figure
9 and Table 3).
The measurements of tarsus angles were quite similar during each of the three trials for
both horses (Figure 10). The medians lie close to each other, and the mean values are also
similar for both horses (Table 4). Bogie Boy’s second trial had a bit larger spread of values
than the other two trials as seen in the CVs, whilst Billy’s CVs don’t differ as much over the
three trials.
25
The body stance angle for both Billy and Bogie had values within similar ranges for trial 2
and 3, whereas trial 1 had a larger range (Figure 11). This is especially true for Bogie. This is
also seen in the large differences in the CV values (Table 5).
Billy’s hind-limb stance had a much wider range of angles than Bogie’s hind-limb stance
(Figure 12). For both horses, the standard deviation of the three trials are quite similar per
horse, but Bogie’s median values lie much closer to each other than Billy’s median values.
This is also true for the mean values of Billy and Bogie (Table 6). The coefficients of variation
become higher for the hind-limb stance angle compared to the other five angles measured;
this is because the angles of hind-limb stance are small values. Bogie’s CVs are greater than
for Billy, despite the boxplot indicating a smaller range of angles. This can be explained by
the fact that Bogie’s hind-limb stance angles were lower (more upright hind-limb) than
Billy’s hind-limb stance angles.
26
Standard Deviation
Table 7. Standard deviation of each angle per phase and trial
Pelvic Flexion
Hip
Stifle
Tarsus
Body stance
Hind-limb stance
Trial
StDev MP
StDev EP
StDev MP
StDev EP
StDev MP
StDev EP
StDev MP
StDev EP
StDev MP
StDev EP
StDev MP
StDev EP
Billy
1
5,71
1.23
3,55
1,31
6,28
3,66
8,56
3,52
2,72
0,76
8,93
3,17
Billy
2
3,89
3.84
3,11
2,65
4,61
5,02
4,87
5,86
1,83
1,50
4,23
4,62
Billy
3
6,34
3,10
4,92
3,77
4,31
4,18
4,27
5,10
2,51
1,04
4,34
4,24
Bogie
1
4,04
1,47
3,84
0,89
4,23
0,91
3,65
1,52
3,72
0,83
2,17
0,96
Bogie
2
2,89
1,33
4,72
0,87
4,93
1,01
7,58
1,82
2,65
0,92
4,13
0,88
Bogie
3
4,82
1,59
3,19
0,56
4,58
0,91
6,24
1,12
2,45
1,49
3,35
1,15
The standard deviation of all angles is larger for the mounting phase than for the ejaculation phase in all three trials for Bogie Boy, but this is not true of all
of Billy’s trials (Table 7). This is also reflected in the graphs above in Figure 1a to 6b.
27
Table 8. Standard deviation per phase and trial, data transformed with Ln function
Pelvic Flexion
Hip
Stifle
Tarsus
Body stance
Hind-limb stance
Trial
Ln(Stdev)
MP
Ln(Stdev)
EP
Ln(Stdev)
MP
Ln(Stdev)
EP
Ln(Stdev)
MP
Ln(Stdev)
EP
Ln(Stdev)
MP
Ln(Stdev)
EP
Ln(Stdev)
MP
Ln(Stdev)
EP
Ln(Stdev)
MP
Ln(Stdev)
EP
Billy
1
1,74
0,21
1,27
0,27
1,84
1,3
2,15
1,26
1
-0,27
2,19
1,15
Billy
2
1,36
1,35
1,13
0,97
1,53
1,61
1,58
1,77
0,6
0,41
1,44
1,53
Billy
3
1,85
1,13
1,59
1,33
1,46
1,43
1,45
1,63
0,92
0,04
1,47
1,44
Bogie
1
1,40
0,39
1,35
-0,12
1,44
-0,09
1,29
0,42
1,31
-0,19
0,77
-0,04
Bogie
2
1,06
0,29
1,55
-0,14
1,6
0,01
2,03
0,6
0,97
-0,08
1,42
-0,13
Bogie
3
1,57
0,56
1,16
-0,58
1,52
-0,09
1,83
0,11
0,9
0,4
1,21
0,14
In order to analyse the data using a paired t-test, the data needs to have a normal distribution and it is suspected that this is not the case for this data set.
Therefore the data is transformed using the ln function, shown in the table above (Table 8).
28
Paired T-test on Standard deviations
Using a paired t-test, it is possible to investigate the differences between pairs of the
standard deviation of the mounting phase and the ejaculation phase. The data has been
transformed using the ln function because it is suspected that the pairs are not normally
distributed.
The null hypothesis, H0, is that the mean of differences between the pairs = 0. The
alternative hypothesis, H1, is that the mean of differences between the pairs ≠ 0.
Using commercially available software, Microsoft Excel, the paired t-test is performed. The
calculated p-values are presented in Table 9. The null hypothesis is rejected when the Pvalue is <0.05.
Table 9. P-values from Paired T-tests on pairs of standard deviation for each angle
Angle of Paired Ln(StDev)
P-value
Reject H0?
Pelvic Flexion
0,0090
Yes
Hip
0,0145
Yes
Stifle
0,0451
Yes
Tarsus
0,0679
No
Body stance
0,0063
Yes
Hind-limb stance
0,0376
Yes
The paired t-test indicates that for the angles of pelvic flexion, hip, stifle, body stance and
hind-limb stance, the mean of differences between the pairs of MP and EP is not equal to 0.
This can be interpreted to mean that there are significant differences in the standard
deviation between the mounting phase and the ejaculation phase of these angles. For the
tarsus angle this is not the case. The null hypothesis is not rejected for this pair of standard
deviations, signifying that there is no significant difference in standard deviations for this
angle during the mounting phase and ejaculation phase (Table 9).
Non-parametric tests
A non-parametric test does not make any distributional assumptions about the data. This is
useful in data sets that are not optimal regarding the number of observations or the
distribution of the data. The data set for this research project is an example of such a data
set. Two tests will be carried out with the data, the related-samples sign test and the
related-samples Wilcoxon ranked sign test.
Sign test
The sign test is a non-parametric substitute for the one-sample t-test or the paired t-test.
The sign test can be used to determine whether significantly more of the values are greater
or lesser than the median of the population from which our sample is taken. The sign test
can also be used to determine if measurements of a single variable are similar in two groups
of paired observations. We look at the differences between the pairs, whereby some
differences will be positive and others will be negative.
H0: the true proportion of positive differences is 0.5.
H1: the proportion of positive differences is not equal to 0.5.
29
The test can be run using commercially available software, IBM’s SPSS. The computer output
gives the p-value, from which the decision can be made whether or not to reject H0. H0 will
be rejected when P<0.05.
Wilcoxon signed rank test
The Wilcoxon signed rank test can also be used as a substitute for the paired t-test when the
assumptions regarding the normality and constant variance cannot be made. This test is
more powerful than the sign test because the sign test only takes into account information
about the direction of differences, and not the magnitude of differences.
Table 10. Outcomes of Sign test and Wilcoxon signed rank test
Sign Test
Wilcoxon Signed
Rank Test
Pelvic
Flexion
Hip
Stifle
Tarsus
Body
stance
P-value
0.031
0.031
0.219
0.688
0.031
Hindlimb
stance
0.219
Reject H0?
Yes
Yes
No
No
Yes
No
P-value
0.028
0.028
0.075
0.116
0.028
0.075
Reject H0?
Yes
Yes
No
No
Yes
No
Non-parametric tests performed on pairs of standard deviations of MP and EP
The tests will give an idea about whether there is a significant difference in the spread of
values during the mounting phase and the ejaculation phase.
For pelvic flexion, hip, and body stance angle, there is a significant difference in the standard
deviation of angles between the two phases (Table 10). For the stifle, tarsus and hindlimb
stance angle, there is no significant difference in the standard deviation of angles between
the two phases (Table 10).
Paired t-tests within each horse
The null hypothesis, H0, is that the mean of differences between the pairs = 0. The
alternative hypothesis, H1, is that the mean of differences between the pairs ≠ 0. Using
commercially available software, Microsoft Excel, the paired t-test is performed. The null
hypothesis is rejected when the p-value is <0.05.
Tarsus angle vs. Hip angle
Table 11: P-values from paired T-test on pairs of tarsus and hip angles for each horse
Horse
Billy
Bogie
p-value
3,8566E-164
4,6654E-204
Reject H0?
Yes
Yes
Using the paired t-test, the difference between tarsus angle and hip angle for each horse
during both phases of all three trials is evaluated (Table 11). The null hypothesis is rejected
for both horses. Both horses show a significantly larger angle of the tarsus (on the flexor
aspect) than the hip. The angle of tarsus flexion for both stallions ranged from 118.31° to
166.61°, whilst the hip angle ranged from 69.00° to 100.29°.
Tarsus angle vs. Stifle angle
Table 12: P-values from paired T-test on pairs of tarsus and stifle angles for each horse
Horse
Billy
Bogie
p-value
4,1283E-169
2,3E-214
Reject H0?
Yes
Yes
30
Using the paired t-test, the difference between tarsus angle and stifle angle for each horse
during both phases of all three trials is evaluated (Table 12). The null hypothesis is rejected
for both horses. Both horses show a significantly larger angle of the tarsus (on the flexor
aspect) than the stifle. The angle of tarsus flexion for both stallions ranged from 118.31° to
166.61°, whilst the stifle angle ranged from 73.14° to 107.03°
Hind-limb stance vs. Body stance
Table 13: P-values from Paired T-test on pairs of hind-limb stance and body stance angles for each
horse
Horse
Billy
Bogie
p-value
1,0739E-123
5,0201E-224
Reject H0?
Yes
Yes
Using the paired t-test, the difference between hind-limb stance and body stance for each
horse during both phases of all three trials is evaluated (Table 13). The null hypothesis is
rejected for both horses. This can be interpreted to mean that the hind-limb is significantly
more upright than the body in both horses. The angle of the hind-limb stance was measured
as the angle between the tarsus, pastern and vertical (marker 5,6 and vertical line). The body
angle was measured as the angle between the withers, tuber coxae and horizontal (markers
1,2, horizontal line). The hind-limb stance angle varied between 0° and 32.39°, whereas the
body angle varied between 43.91° and 61.28°.
Sample size calculation for main study
The data set used for this research project is very small, n=2. This means that the statistical
power is low. Furthermore, the number of data collections (6 in total) is also small, which
means that the assumptions of normally distributed data cannot be made for statistical
testing. To overcome this obstacle, non-parametric tests must be used, which allow for a
correction of small sample sizes and non-normally distributed data. Not all of the statistical
tests used in this research project allowed for this correction, therefore they must be
interpreted with care.
Using the data from this pilot study, a sample size calculation was performed in order to
determine the required number of horses to be measured to give a follow-up study of the
biomechanics a power of 80%. The values of the angle of pelvic flexion were used for the
sample size calculation, as shown in the table below (Table 14).
Table 14. Values for mean pelvic flexion angle in MP and EP used for calculating sample size.
Horse
Trial
Mean MP
Mean EP
Difference between MP & EP
Billy
1
164.48
161.05
3.43
Billy
2
175.81
172.46
3.35
Billy
3
172.94
163.55
9.39
Bogie
1
170.67
166.01
4.66
Bogie
2
168.10
164.44
3.66
Bogie
3
172.21
167.96
4.25
Mean Difference
4.79
SD Differences
2.31
Standardised Diff
4.15
Power
0.8
Alfa (P value)
Clinically Relevant
Difference
0.05
31
4.79
Using commercially available software “Power and Sample Size Program” (Dupont, 1990),
the sample size was calculated. Data from the pilot study indicates that the difference in the
response of matched pairs is normally distributed with standard deviation 2.31. If the true
difference in the mean response of matched pairs is 4.79, the sample size required is 4 pairs
of subjects in order to reject the null hypothesis that the difference is zero with a power of
0.8 and type 1 error probability of 0.05. The 4 pairs of subjects means that at least 4 horses
must be measured during the mounting phase and the ejaculation phase.
32
Appendix II: Results and Statistics from Main Research Project
Graphs
Figure 1a. Graph showing the 6 measured
angles during the mounting phase for Horse 1
Trial 1.
Figure 1b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
1 Trial 1.
Figure 2a. Graph showing the 6 measured
angles during the mounting phase for Horse 1
Trial 2.
Figure 2b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
1 Trial 2.
Figure 3a. Graph showing the 6 measured
angles during the mounting phase for Horse 1
Trial 3.
Figure 3b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
1 Trial 3.
33
Figure 4a. Graph showing the 6 measured
angles during the mounting phase for Horse 2
Trial 1.
Figure 4b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
2 Trial 1.
Figure 5a. Graph showing the 6 measured
angles during the mounting phase for Horse 2
Trial 2.
Figure 5b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
2 Trial 2.
Figure 6a. Graph showing the 6 measured
angles during the mounting phase for Horse 2
Trial 3.
34
Figure 6b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
2 Trial 3.
Figure 7a. Graph showing the 6 measured
angles during the mounting phase for Horse 3
Trial 1.
Figure 7b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
3 Trial 1.
Figure 8a. Graph showing the 6 measured
angles during the mounting phase for Horse 3
Trial 2.
Figure 8b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
3 Trial 2..
Figure 9a. Graph showing the 6 measured
angles during the mounting phase for Horse 3
Trial 3.
Figure 9b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
3 Trial 3.
35
Figure 10a. Graph showing the 6 measured
angles during the mounting phase for Horse 4
Trial 1.
Figure 10b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
4 Trial 1.
Figure 11a. Graph showing the 6 measured
angles during the mounting phase for Horse 4
Trial 2.
Figure 11b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
4 Trial 2.
Figure 12a. Graph showing the 6 measured
angles during the mounting phase for Horse 4
Trial 3.
Figure 12b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
4 Trial 3.
36
Figure 13a. Graph showing the 6 measured
angles during the mounting phase for Horse 5
Trial 1.
Figure 13b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
5 Trial 1.
Figure 14a. Graph showing the 6 measured
angles during the mounting phase for Horse 5
Trial 2.
Figure 14b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
5 Trial 2.
Figure 15a. Graph showing the 6 measured
angles during the mounting phase for Horse 5
Trial 3.
Figure 15b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
5 Trial 3.
37
Figure 16a. Graph showing the 6 measured
angles during the mounting phase for Horse 6
Trial 1.
Figure 16b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
6 Trial 1.
Figure 17a. Graph showing the 6 measured
angles during the mounting phase for Horse
6 Trial 2.
Figure 17b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
6 Trial 2.
Figure 18a. Graph showing the 6 measured
angles during the mounting phase for Horse 6
Trial 3.
Figure 18b. Graph showing the 6 measured
angles during the ejaculation phase for Horse
6 Trial 3.
The six measured angles fluctuate during the mounting phase and become more constant
during the ejaculation phase (Figure 1a to 18b). For horse 5 and 6 trial 1, the ejaculation
phase is unsettled and shows more movement than the other trials. The angles for the
tarsus and stifle are not consistent in these two trials, which can probably be explained by
the horse alternately lifting left and right hindlimbs.
38
Boxplots and Standard Deviation
Figure 19. Boxplot of pelvic flexion angle for each horse and trial.
Table 1. Standard deviation, mean and coefficient of variation for pelvic flexion angles.
Standard Deviation
Coefficient of Variation
Horse
Trial
Mean (degrees)
(degrees)
(%)
1
1
7.31
162.64
4.50
1
2
11.94
163.68
7.29
1
3
11.79
160.48
7.34
2
1
6.87
171.09
4.02
2
2
7.17
164.06
4.37
2
3
7.49
165.93
4.52
3
1
2.96
172.79
1.71
3
2
3.84
170.13
2.26
3
3
6.99
168.95
4.13
4
1
4.05
178.37
2.27
4
2
3.69
169.62
2.17
4
3
5.04
171.77
2.94
5
1
7.76
171.48
4.52
5
2
10.15
164.55
6.17
5
3
9.74
165.21
5.89
6
1
6.01
165.46
3.64
6
2
5.04
166.61
3.02
6
3
5.31
166.23
3.19
The median values of pelvic flexion for Horse 1 and 6 lie close to each other, whilst for the
other four horses they are more spread out (Figure 19). The range and spread of values
during trial 1, 2 and 3 is quite consistent amongst the horses, though Horse 1 trial 1 appears
to have a smaller range. Horse 3 has many outliers in trial 3, because during this trial the
horse started off mounting the dummy whilst standing quite far away from it with an
extended pelvis and then moved closer. Horse 3 has a much smaller range of values than
Horse 1 and Horse 5. The CV for the pelvic flexion is quite consistent for Horse 2, 4 and 6,
whilst for Horse 1, 3 and 5 have two to three per cent differences between their three trials
(Table 1).
39
Figure 20. Box plot showing hip angle for each horse and trial.
Table 2. Standard deviation, mean and coefficient of variation for hip angles.
Standard Deviation
Coefficient of Variation
Horse
Trial
Mean (degrees)
(degrees)
(%)
1
1
6.36
90.32
7.04
1
2
8.98
85.24
10.54
1
3
10.28
88.70
11.59
2
1
4.78
81.73
5.84
2
2
6.28
86.03
7.30
2
3
5.20
80.96
6.42
3
1
3.08
87.30
3.53
3
2
3.12
85.79
3.64
3
3
4.75
91.25
5.20
4
1
3.32
75.69
4.38
4
2
2.52
76.67
3.29
4
3
3.86
81.91
4.71
5
1
6.79
85.14
7.97
5
2
7.38
93.16
7.92
5
3
9.22
83.80
11.00
6
1
4.54
86.75
5.23
6
2
4.39
84.21
5.22
6
3
6.36
88.32
7.20
The median values of the hip angle for Horse 5 are quite different, whilst the median values
are the closest together for the trials of Horse 6 (Figure 20). The range of values is quite alike
for the three trials of each horse 2,4,5 and 6. Horse 1 trial 1 has many outliers, and the range
is smaller than of Horse 2 trial 2 and 3. There are also many outliers for Horse 4. As for the
coefficient of variation, there are a few per cent differences between trials; otherwise the
values are quite comparable for the three trials of each horse (Table 2).
40
Figure 21. Box plot showing stifle angle for each horse and trial.
Table 3. Standard deviation, mean and coefficient of variation for stifle angles.
Standard Deviation
Coefficient of Variation
Horse
Trial
Mean (degrees)
(degrees)
(%)
1
1
6.52
104.82
6.22
1
2
7.02
107.11
6.55
1
3
7.51
108.46
6.93
2
1
9.33
98.06
9.52
2
2
7.44
103.63
7.17
2
3
8.83
100.50
8.79
3
1
6.54
103.39
6.32
3
2
6.73
103.75
6.49
3
3
7.55
107.45
7.02
4
1
6.79
94.97
7.15
4
2
5.86
91.86
6.38
4
3
7.12
102.47
6.95
5
1
11.18
101.71
10.99
5
2
10.37
114.04
9.09
5
3
9.07
104.76
8.66
6
1
9.70
102.02
9.50
6
2
7.92
84.21
7.63
6
3
10.02
102.30
9.79
The median values of the stifle angle lie quite close together for the three trials of horse 1, 2,
3 and 6, whilst for horse 4 and especially 5, the median values are further apart (Figure 21).
The range of each horse’s three trials is quite similar for all horses except Horse 2. The
outliers of the stifle angle are mostly small values, which indicates a flexed stifle. This is
probably due to the horses alternately lifting their hind-limbs. There are small differences in
coefficients of variation between the trials, around 2% difference; overall, the CV per horse
is quite stable (Table 3).
41
Figure 22. Box plot showing tarsus angle for each horse and
trial.
Table 4. Standard deviation, mean and coefficient of variation for tarsus angles.
Standard Deviation
Coefficient of Variation
Horse
Trial
Mean (degrees)
(degrees)
(%)
1
1
7.93
145.14
5.47
1
2
6.94
153.11
4.54
1
3
7.34
150.23
4.88
2
1
10.47
150.71
6.95
2
2
10.54
153.46
6.87
2
3
13.36
148.47
9.00
3
1
8.51
147.81
5.76
3
2
8.67
155.91
5.56
3
3
10.27
157.20
6.54
4
1
7.45
144.65
5.15
4
2
6.24
146.41
4.26
4
3
7.45
148.48
5.02
5
1
10.96
141.64
7.73
5
2
9.14
148.67
6.15
5
3
10.29
145.63
7.07
6
1
9.24
148.05
6.24
6
2
9.91
149.13
6.64
6
3
8.85
145.17
6.10
The median values of the tarsus angles lie quite close together for Horse 2, 4 and 6, whilst
they are more spread apart for Horse 1, 3 and 5 (Figure 22). The ranges are quite similar
within each horse, though this is not true for Horse 2 and 5. Both these horses have a much
smaller range for their second trial than for the other two trials. Between the horses, the
range of values is smaller for Horse 3 and Horse 4 in comparison to the rest. The outliers are
all small values – similar to with the stifle, these are values where the tarsus is flexed,
presumably due to the horse lifting its legs. The coefficient of variations for all horses except
horse 2 are within approximately 1% of each other, indicating that the spread of values were
quite similar (Table 4).
42
Figure 23. Box plot showing body stance angle for each horse and trial.
Table 5. Standard deviation, mean and coefficient of variation for body stance angles.
Standard Deviation
Coefficient of Variation
Horse
Trial
Mean (degrees)
(degrees)
(%)
1
1
3.23
42.46
7.61
1
2
3.6
39.02
9.22
1
3
2.96
38.56
7.68
2
1
3.85
47.21
8.16
2
2
3.31
43.22
7.66
2
3
2.8
43.64
6.42
3
1
2.71
40.57
6.67
3
2
3.04
41.46
7.34
3
3
2.61
40.21
6.49
4
1
5.97
53.21
11.22
4
2
5.95
51.97
11.45
4
3
4.99
48.2
10.34
5
1
4.05
48.4
8.37
5
2
3.86
44.58
8.66
5
3
4.17
40.26
10.35
6
1
3.93
50.57
7.77
6
2
4.36
52.06
8.37
6
3
6.14
49.47
12.4
The median values of body stance angle of Horse 3 and 6 are close together throughout the
three trials, slightly less so for Horse 1, 2 and 4, and for Horse 5 the median values are
further apart (Figure 23). The range of Horse 1, 2 and 3 are all quite similar, indicating little
inter-animal difference. This is also true for Horse 4, 5 and 6, though these three horses have
larger ranges than the other three. Within horses, the ranges appear quite consistent. The
outliers are mostly high values – this is where the body has a steep angle in relation to the
horizon. This occurs when the horse does not lean with its weight on the dummy mount, but
rather stands more on its hind limbs. The CV for horse 1 – 5 differ only a few per cent, whilst
horse 6 has a much larger CV for the third trial (Table 5).
43
Figure 24. Box plot showing hind-limb stance angle for each horse and trial.
Table 6. Standard deviation, mean and coefficient of variation for hind-limb stance angles.
Hind-limb Stance
Standard Deviation
Coefficient of Variation
Horse
Trial
Mean (degrees)
(degrees)
(%)
1
1
6.34
19.19
33.03
1
2
5.88
14.6
40.25
1
3
4.3
17.62
24.4
2
1
5.74
11.74
48.88
2
2
7.61
13.6
55.95
2
3
8.31
18.78
44.26
3
1
6.09
6.36
95.76
3
2
5.42
5.82
93.17
3
3
5.4
5.92
91.2
4
1
6.18
19.47
31.76
4
2
4.93
21.13
23.34
4
3
6.37
18.52
34.37
5
1
9.42
21.85
43.13
5
2
5.99
22.24
26.94
5
3
9.62
20.39
47.2
6
1
6.97
22.32
31.24
6
2
6.76
25.95
26.06
6
3
7.49
22.05
33.96
The median values of hind-limb stance angles for Horse 3 lie close to each other, for Horse 1
and 2 they lie far apart, and for Horse 4, 5, and 6 they are less far apart (Figure 24). Horse 3
has very small angles for the hind-limb stance, due to this horse’s tendency to stand very
upright against the dummy mount. In contrast, Horse 6 has much larger angles because he
stands with the hind-limbs nearly under the dummy mount. The range of values within
horses differs, but not as much as the range of values differ between horses. Perhaps this is
due to an individual preference for a position. The outliers are mostly large angles, occurring
when horses lift their hind limbs. The CVs have much higher values than for the other angles
measured. This is because the hind-limb angle is much smaller than the other angles, and
thus the mean is lower. The CV values are not very consistent amongst the horses, indicating
a lot of variation in the spread of values for the hind-limb stance (Table 6).
44
Standard Deviation
Table 7. Standard deviation of each angle per phase and trial.
PF
Hip
Horse
Trial
StDev MP
StDev EP
StDev MP
StDev EP
1
1
7.72
1.44
7.59
1.23
1
2
12.71
1.36
10.19
0.89
1
3
12.14
3.47
11.25
3.13
2
1
7.11
2.65
5.15
1.47
2
2
6.97
1.21
7.04
0.83
2
3
7.14
3.03
6.54
0.79
3
1
2.83
1.43
2.97
1.35
3
2
4.17
1.60
3.37
1.03
3
3
7.86
1.69
5.40
1.27
4
1
3.98
2.97
3.50
2.41
4
2
3.32
3.30
2.76
1.27
4
3
4.87
4.46
4.25
1.66
5
1
7.68
2.23
6.91
2.88
5
2
10.40
2.54
7.87
1.37
5
3
9.74
2.67
9.61
0.90
6
1
5.70
5.58
5.20
1.83
6
2
5.45
2.32
4.59
2.35
6
3
5.51
3.00
6.59
0.83
Stifle
StDev MP
StDev EP
7.19
2.68
7.79
2.82
8.49
2.09
10.03
2.64
8.15
0.71
10.33
1.44
6.79
2.69
7.40
1.83
7.93
3.33
7.27
4.18
5.99
3.24
6.95
7.24
10.98
11.60
11.06
4.16
9.49
4.78
10.01
7.21
8.08
6.71
10.25
2.32
Tarsus
StDev MP
StDev EP
9.37
2.73
7.53
1.49
8.22
1.64
10.88
2.53
11.09
0.99
14.39
1.74
9.12
1.73
8.76
1.09
10.94
2.21
8.02
4.50
6.81
1.53
7.37
7.83
10.66
11.75
9.68
3.96
10.21
3.37
9.16
9.30
9.89
9.47
9.25
3.60
Body Stance
StDev MP
StDev EP
3.77
1.35
3.35
0.74
2.72
1.48
4.09
1.56
3.66
1.29
2.82
2.61
2.31
1.55
3.32
1.47
2.61
1.42
6.43
2.81
5.76
2.92
5.31
3.78
3.55
5.64
3.90
2.60
4.02
2.56
3.97
3.07
4.74
1.33
5.85
3.33
Hind-limb Stance
StDev MP
StDev EP
7.29
1.93
6.70
0.95
4.88
1.07
6.36
1.96
8.39
1.44
10.45
2.28
6.41
2.23
5.73
1.18
5.90
1.98
6.29
5.78
5.07
4.42
6.04
4.08
9.04
7.17
6.27
3.50
9.30
3.09
6.97
6.25
7.08
5.09
6.75
2.88
For pelvic flexion, hip and hind-limb stance angles, the standard deviation of the MP is larger than that of the EP for all horses and trials. This is also true for
the stifle angle, except Horse 4 trial 3 and Horse 5 trial 1. For the tarsus the standard deviation of the MP is larger than that of the EP in all horses and trials
except for Horse 4 trial 3, Horse 5 trial 1 and Horse 6 trial 1. The standard deviation of the hind-limb stance angle is larger during the mounting phase than
the ejaculation phase for all horses and trials except Horse 5 trial 1. (Table 7).
45
46
Table 8. Standard deviation per phase and trial, data transformed with Ln function.
PF
Hip
Stifle
LnStDev LnStDev LnStDev LnStDev LnStDev LnStDev
Horse Trial
MP
EP
MP
EP
MP
EP
1
1
2.04
0.36
2.03
0.21
1.97
0.99
1
2
2.54
0.31
2.32
-0.12
2.05
1.04
1
3
2.50
1.24
2.42
1.14
2.14
0.74
2
1
1.96
0.97
1.64
0.39
2.31
0.97
2
2
1.94
0.19
1.95
-0.19
2.10
-0.34
2
3
1.97
1.11
1.88
-0.24
2.34
0.36
3
1
1.04
0.36
1.09
0.30
1.92
0.99
3
2
1.43
0.47
1.21
0.03
2.00
0.60
3
3
2.06
0.52
1.69
0.24
2.07
1.20
4
1
1.38
1.09
1.25
0.88
1.98
1.43
4
2
1.20
1.19
1.02
0.24
1.79
1.18
4
3
1.58
1.50
1.45
0.51
1.94
1.98
5
1
2.04
0.80
1.93
1.06
2.40
2.45
5
2
2.34
0.93
2.06
0.31
2.40
1.43
5
3
2.28
0.98
2.26
-0.11
2.25
1.56
6
1
1.74
1.72
1.65
0.60
2.30
1.98
6
2
1.70
0.84
1.52
0.85
2.09
1.90
6
3
1.71
1.10
1.89
-0.19
2.33
0.84
Tarsus
LnStDev LnStDev
MP
EP
2.24
1.00
2.02
0.40
2.11
0.49
2.39
0.93
2.41
-0.01
2.67
0.55
2.21
0.55
2.17
0.09
2.39
0.79
2.08
1.50
1.92
0.43
2.00
2.06
2.37
2.46
2.27
1.38
2.32
1.21
2.21
2.23
2.29
2.25
2.22
1.28
Body Stance
LnStDev LnStDev
MP
EP
1.33
0.30
1.21
-0.30
1.00
0.39
1.41
0.44
1.30
0.25
1.04
0.96
0.84
0.44
1.20
0.39
0.96
0.35
1.86
1.03
1.75
1.07
1.67
1.33
1.27
1.73
1.36
0.96
1.39
0.94
1.38
1.12
1.56
0.29
1.77
1.20
Hind-limb Stance
LnStDev LnStDev
MP
EP
1.99
0.66
1.90
-0.05
1.59
0.07
1.85
0.67
2.13
0.36
2.35
0.82
1.86
0.80
1.75
0.17
1.77
0.68
1.84
1.75
1.62
1.49
1.80
1.41
2.20
1.97
1.84
1.25
2.23
1.13
1.94
1.83
1.96
1.63
1.91
1.06
In order to analyse the data using a paired t-test, the data needs to have a normal distribution. As the data set is quite small, this assumption cannot be
made. Therefore the data is transformed using the ln function, as shown in the table above (Table 8).
47
Paired T-test on Standard deviations
Using a paired t-test, it is possible to investigate the differences between pairs of the
standard deviation of the mounting phase and the ejaculation phase. The data has been
transformed using the ln function because it is suspected that the pairs are not normally
distributed.
The null hypothesis, H0, is that the mean of differences between the pairs = 0. The
alternative hypothesis, H1, is that the mean of differences between the pairs ≠ 0.
Using commercially available software, Microsoft Excel, the paired t-test is performed. The
calculated p-values are presented in table 29. The null hypothesis is rejected when the pvalue is <0.05.
Table 9. P-values from Paired T-test on pairs of ln(standard deviation) for each angle.
Angle
P-value
Reject H0?
Pelvic Flexion
4.30132E-06
Yes
Hip
3.93781E-08
Yes
Stifle
1.21164E-05
Yes
Tarsus
1.00101E-05
Yes
Body Stance
1.84394E-05
Yes
Hind-limb Stance
6.11266E-06
Yes
The paired t-test indicates that for all six angles measured, the mean of differences between
the pairs of MP and EP is not equal to 0. This can be interpreted to mean that there are
statistically significant differences between the standard deviation of the mounting phase
and of the ejaculation phase for all six angles measured (Table 9).
Non-parametric tests
A non-parametric test does not make any distributional assumptions about the data. This is
useful in data sets that are not optimal regarding the number of observations or the
distribution of the data. The data set for this research project is an example of such a data
set. Two tests will be carried out with the data, the related-samples sign test and the
related-samples Wilcoxon ranked sign test.
Sign test
The sign test is a non-parametric substitute for the one-sample t-test or the paired t-test.
The sign test can be used to determine whether significantly more of the values are greater
or lesser than the median of the population from which our sample is taken. The sign test
can also be used to determine if measurements of a single variable are similar in two groups
of paired observations. We look at the differences between the pairs, whereby some
differences will be positive and others will be negative.
H0: the true proportion of positive differences is 0.5.
H1: the proportion of positive differences is not equal to 0.5.
The test can be run using commercially available software, IBM’s SPSS. The computer output
gives the p-value, from which the decision can be made whether or not to reject H0. H0 will
be rejected when p<0.05.
Wilcoxon signed rank test
The Wilcoxon signed rank test can also be used as a substitute for the paired t-test when the
assumptions regarding the normality and constant variance cannot be made. This test is
more powerful than the sign test because the sign test only takes into account information
about the direction of differences, and not the magnitude of differences.
Non-parametric tests were performed on pairs of standard deviations of MP and EP.
The tests will give an idea about whether there is a significant difference in the spread of
values during the mounting phase and the ejaculation phase.
Table 10. Outcomes of Sign Test and Wilcoxon Signed Rank Test.
Sign Test
Wilcoxon
Rank Test
Pelvic
Flexion
Hip
Stifle
Tarsus
Body
Stance
Hind-limb
Stance
P-value
0.000
0.000
0.001
0.008
0.000
0.000
Reject H0?
Yes
Yes
Yes
Yes
Yes
Yes
P-value
0.000
0.000
0.000
0.001
0.001
0.000
Reject H0?
Yes
Yes
Yes
Yes
Yes
Yes
For all six angles, in both tests the null hypothesis is rejected (Table 10). For all six angles,
there are significant differences in the standard deviation of the angles between the
mounting phase and the ejaculation phase.
Paired t-tests within each horse
The null hypothesis, H0, is that the mean of differences between the pairs = 0. The
alternative hypothesis, H1, is that the mean of differences between the pairs ≠ 0. Using
commercially available software, Microsoft Excel, the paired t-test is performed. The null
hypothesis is rejected when the p-value is <0.05.
Tarsus angle vs. Hip angle
Table 11. P-values from Paired T-test on pairs of tarsus and hip angles for each horse.
Tarsus vs. Hip
Horse
P value
Reject H0?
1
4.74E-161
Yes
2
5.54E-180
Yes
3
3.17E-212
Yes
4
2.36E-207
Yes
5
8.58E-284
Yes
6
3.86E-252
Yes
Using the paired t-test, the difference between tarsus angle and hip angle for each horse
during both phases of all three trials is evaluated. The null hypothesis is rejected for all six
horses. All horses show a significantly larger angle of the tarsus (on the flexor aspect) than
the hip (Table 11).
49
Tarsus angle vs. Stifle angle
Table 12. P-values from Paired T-test on pairs of tarsus and stifle angles for each horse.
Tarsus vs. Stifle
Horse
P value
Reject H0?
1
9.54E-189
Yes
2
3.66E-187
Yes
3
2.67E-227
Yes
4
1.67E+178
Yes
5
2.34E-237
Yes
6
5.97E-251
Yes
Using the paired t-test, the difference between tarsus angle and stifle angle for each horse
during both phases of all three trials is evaluated. The null hypothesis is rejected for all six
horses. All horses show a significantly larger angle of the tarsus (on the flexor aspect) than
the stifle (Table 12).
Hind-limb stance vs. Body stance
Table 13. P-values from Paired T-test on pairs of hind-limb stance and body stance angles for each
horse.
Hind-limb Stance vs. Body stance
Horse
P value
Reject H0?
1
6.67E-140
Yes
2
6.41E-125
Yes
3
1.01E-199
Yes
4
8.32E-128
Yes
5
1.65E-149
Yes
6
3.88E-175
Yes
Using the paired t-test, the difference between hind-limb stance and body stance for each
horse during both phases of all three trials is evaluated. The null hypothesis is rejected for all
six horses. All horses show a significantly larger angle of the body stance than the hind-limb
stance (Table 13).
Hip angle vs. Stifle angle
Table 14: P-values from Paired T-test on pairs of hip and stifle angles for each horse
Hip vs. Stifle
Horse
P value
Reject H0?
1
7.93E-65
Yes
2
9.30E-73
Yes
3
2.40E-103
Yes
4
2.74E-84
Yes
5
7.26E-85
Yes
6
3.44E-90
Yes
Using the paired t-test, the difference between tarsus angle and hip angle for each horse
during both phases of all three trials is evaluated. The null hypothesis is rejected for all six
horses. All horses show a significantly larger angle of the stifle than of the hip (Table 14).
50
Pelvic flexion angle vs. Tarsus angle
Table 15. P-values from Paired T-test on pairs of pelvic flexion and tarsus angles for each horse.
Pelvic Flexion vs. Tarsus
Horse
P value
Reject H0?
1
5.24E-32
Yes
2
1.81E-36
Yes
3
2.13E-61
Yes
4
1.92E-98
Yes
5
3.10E-93
Yes
6
6.25E-92
Yes
Using the paired t-test, the difference between tarsus angle and hip angle for each horse
during both phases of all three trials is evaluated. The null hypothesis is rejected for all six
horses. All horses show a significantly larger angle of pelvic flexion than of the tarsus (on the
flexor aspect) (Table 15).
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