Article

Anticipatory response before competition in Standardbred racehorses

Abstract and Figures

It is generally accepted that besides cortisol concentrations, parameters of heart rate variability (HRV) are appropriate indicators of stress in horses. The aim of this study was to determine anticipatory stress in eight Standardbred stallions participating in harness race. Cortisol and HRV responses to a mild exercise performed in training circumstances were compared to a maximal effort exercise performed in real trotting race conditions. Parameters of HRV reflecting vagal (root mean square of the successive differences, RMSSD) and sympathetic nervous system activity (ratio of the low and high frequency component, LF/HF) were recorded before warming up (baseline) and during exercise. Plasma cortisol concentrations were obtained for the following stages of the exercise: before warming up (baseline), after warming up, after the exercise has finished and after a 30-min recovery. Baseline LF/HF ratio was higher before the race compared to the pre-training values (12.0 ± 6.6 vs. 5.9 ± 4.5, P = 0.009), while RMSSD did not show such difference (34.8 ± 15.9 ms vs. 48.0 ± 30.5 ms, P = 0.96). Cortisol level was higher in the case of race for all samples compared to training (P = 0.012). There were no significant differences between plasma cortisol levels obtained for the subsequent stages of race. Horses in the present study showed anticipatory response before race as shown by differences in pre-training (97.3 ± 16.4 nmol/L) and pre-race cortisol levels (171.8 ± 18.7 nmol/L), respectively (P < 0.001). Pre-race HRV only partly confirmed this phenomenon.
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RESEARCH ARTICLE
Anticipatory response before competition in
Standardbred racehorses
Zso
´fia Boha
´k
1
, Andrea Harnos
2‡
, Kinga Joo
´
1‡
, Otto
´Szenci
1
, Levente Kova
´cs
1,3
*
1MTA-SZIE Large Animal Clinical Research Group, U
¨llő, Do
´ra major, Hungary, 2Department of
Biomathematics and Informatics, University of Veterinary Medicine, Budapest, Hungary, 3Institute of Animal
Husbandry, Faculty of Agricultural and Environmental Science, Szent Istva
´n University, Go
¨do
¨llő, Hungary
These authors contributed equally to this work.
‡ These authors also contributed equally to this work.
*kovacs.levente@mkk.szie.hu
Abstract
It is generally accepted that besides cortisol concentrations, parameters of heart rate vari-
ability (HRV) are appropriate indicators of stress in horses. The aim of this study was to
determine anticipatory stress in eight Standardbred stallions participating in harness race.
Cortisol and HRV responses to a mild exercise performed in training circumstances were
compared to a maximal effort exercise performed in real trotting race conditions. Parame-
ters of HRV reflecting vagal (root mean square of the successive differences, RMSSD) and
sympathetic nervous system activity (ratio of the low and high frequency component, LF/
HF) were recorded before warming up (baseline) and during exercise. Plasma cortisol con-
centrations were obtained for the following stages of the exercise: before warming up (base-
line), after warming up, after the exercise has finished and after a 30-min recovery. Baseline
LF/HF ratio was higher before the race compared to the pre-training values (12.0 ±6.6 vs.
5.9 ±4.5, P = 0.009), while RMSSD did not show such difference (34.8 ±15.9 ms vs. 48.0 ±
30.5 ms, P= 0.96). Cortisol level was higher in the case of race for all samples compared to
training (P= 0.012). There were no significant differences between plasma cortisol levels
obtained for the subsequent stages of race. Horses in the present study showed anticipatory
response before race as shown by differences in pre-training (97.3 ±16.4 nmol/L) and pre-
race cortisol levels (171.8 ±18.7 nmol/L), respectively (P<0.001). Pre-race HRV only partly
confirmed this phenomenon.
Introduction
The hormone cortisol is a versatile chemical regulator in every species. It plays a key role in
daily homeostasis of the organism and regulated via the hypothalamic–pituitary–adrenal
(HPA) axis. It is the primary hormone responsible for the stress response to physical and psy-
chological challenges [1] Physical exertion results in the increase of the plasma cortisol in sev-
eral species [26]. The exercise-induced response of the HPA axis is depending on the exercise
intensity, duration [7], and the fitness level of the athlete [8,9]. Environment may also
PLOS ONE | https://doi.org/10.1371/journal.pone.0201691 August 2, 2018 1 / 8
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OPEN ACCESS
Citation: Boha
´k Z, Harnos A, Joo
´K, Szenci O,
Kova
´cs L (2018) Anticipatory response before
competition in Standardbred racehorses. PLoS
ONE 13(8): e0201691. https://doi.org/10.1371/
journal.pone.0201691
Editor: Ruud van den Bos, Radboud Universiteit,
NETHERLANDS
Received: October 18, 2017
Accepted: July 22, 2018
Published: August 2, 2018
Copyright: ©2018 Boha
´k et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper.
Funding: Levente Kova
´cs was supported by the
New National Excellence Program of the Ministry of
Human Capacities [U
´NKP-17-4-I/SZIE-7]. The
funder had no role in study design, data collection
and analysis, decision to publish, or preparation of
the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
influence the cortisol response; besides physical challenges, psychological challenges can also
affect the cortisol response, especially when participating to races [10,11].
As a noninvasive cardiovascular marker, heart rate variability (HRV) is often used for the
assessment of stress and welfare in farm animals [12] as it reflects the changes of the sym-
patho-parasympathetic balance of the autnomic nervous system (ANS) in response to external
stimuli [13]. Frequency-domain analysis of HRV provides relevant information about the
short-term responses of the ANS in horses [14].
The relative little number of studies on equestrian training and competition focused on
dressage or jumping horses and found increased post-competition plasma [15;16] and saliva
cortisol levels [17] and decreased pre-race RMSSD [17]. In human athletes, emotional stress
caused by racing is an important factor in the HPA response; however, no results are available
on race-related cortisol secretion and HRV in racehorses. Therefore, we aimed to focus on the
emotional effects of trotting race in Standardbred racehorses. We hypothesized higher cortisol
levels and increased sympathetic tone before the race than before a usual training.
Materials and methods
Eight trained Standardbred stallions between 3 and 4 years of age were used for the study that
was specifically approved by the Ethics Committee of the University of Veterinary Medicine.
The experiment was carried out between 2 and 4 April 2016 in Kincsem Park, Hungary, where
trotting races are organized every Wednesday and Saturday. The daily routine and feeding do
not differ between training and race days. However, on race days horses can hear the noise of
transporters and the loudspeaker. The preparation of the horses occured on the same way on
both experimental days, but the driver wore more colorful clothes on the day of race. Horses
were kept in the same stable and trained by the same driver and horses were not transported
during the experiment. During the period of study there were no significant differences in
environmental temperature and humidity. The weekly workout plan of the experimental
horses is described in Table 1.
Measurements were performed in the same place under two different conditions: during
training (mild exercise, n = 8) and during real trotting race (maximal effort, n = 8). The dis-
tance covered during training and race was 8,000–9,000 and 2,300 m, respectively. All the
exercises were performed between 10:00 AM and 11:45 AM and lasted approximately for
48–55 min at all. Blood samples were obtained by venipuncture from the jugular vein
(S-Monovette 7.5-mL Z tubes; Sarstedt, Nu¨mbrecht-Rommelsdorf, Germany) according to
the same pattern for the training and racing sessions considering the diurnal rhythm of cor-
tisol secretion [18] as follows: S0; pre-race, 5 min before warming up, S1; after warming up
(17–20 min after S0), S2; immediately after the intensive stage of the exercise (15–20 min
after S1 depending on the type of work) and S3; after a 30-min recovery (30 min after the
exercise has finished). Blood was centrifuged at 2,000 gfor 10 min, 15 min within the actual
sampling has completed.
Cortisol assay was performed in duplicate by direct radioimmunoassay with a HPLC prepa-
ration of cortisol-3-corticosterone methyloxidase, coupled with 2-[
125
I] iodohistamine as
tracer for specific antibodies raised against cortisol-3-CMO-BSA [19].
Table 1. Weekly training schedule of the experimented horses.
Training schedule
Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Mild training Mild training Intensive exercise /race Rest Mild training Intensive exercise /race Rest
https://doi.org/10.1371/journal.pone.0201691.t001
Anticipatory response of Standardbred racehorses
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For the measurement of interbeat intervals (IBI), horses were equipped with a Polar Equine
RS800CX multi device and a Polar H2 sensor (Polar Ltd., New York, USA). After a 1-h habitu-
ation period to the equipment, IBIs were recorded before the exercise for one hour between
07:00 and 08:00 AM at rest, and during the exercise. Equal length of 5-min continuous IBI
samples were used for HRV analysis. The Kubios HRV software (version 2.2, Biomedical Sig-
nal Analysis Group, Department of Applied Physics, University of Kuopio, Finland) was used
for the analysis of IBI data. Type 1 errors (QRS detected prematurely when in fact a sinus-con-
ducted wave has not occurred) and Type 2 errors (failing to detect an R wave that is present)
were removed from the dataset as well as irregular sinus rhythms. Using the custom filter of
the program, every IBI that differed more than 30% from the previous one, was replaced by an
interpolated value calculated from the differences between the previous and the next accepted
IBI. Slow nonstationary trend components were removed by using the ‘smoothness priors’
based detrending approach with λ= 1000 and f
c
= 0.029 Hz. For representing vagal regulatory
activity, the RMSSD was computed, whereas LF/HF was presented to assess the sympathovagal
balance.
For hypothesis testing, a linear mixed model was fit by the Restricted Maximum Likelihood
method [20]. Cortisol and parameters of HRV were the response, while the type of work
(training or race) and the sampling occasion in interaction were the explanatory variables. The
dependence between measurements from the same horse was considered by modelling the
within-horse error variance–covariance matrix. Model effects were tested together based on
their t-values. For multiple comparisons, we tested contrasts. Tukey’s method was applied to
avoid the accumulation of Type I error. Significance was set at the level of 0.05. The analysis
was carried out using the R 2.15.1 statistical software [21].
Results
Baseline LF/HF ratio was higher (12.0 ±6.6 vs. 5.9 ±4.5, P = 0.009), while baseline RMSSD did
not show significant difference (34.8 ±15.9 ms vs. 48.0 ±30.5 ms, P = 0.96) when pre-race and
pre-training values were compared. Pre-race cortisol levels on training and race days were
97.3 ±16.4 nmol/L and 171.8 ±18.7 nmol/L (means ±SD), respectively (P<0.001, Fig 1).
According to the linear mixed model, both the type of work and the time of sampling had a
significant effect on the cortisol level (P<0.001 and P= 0.012, respectively). Cortisol level was
higher in the case of race at every sampling (Fig 2).
Discussion
In this paper, plasma cortisol levels and HRV of Standardbred racehorses were assessed before
and after training and trotting race. Since emotional effects of trotting race might influence
basal physiology of horses, higher HPA activity and increased sympathetic tone before the race
than before training were hypothesized. Our results supported the hypothesis. Elevated plasma
cortisol concentrations and increased LF/HF ratio before the race than before a usual training
suggest that horses might be exposed to anticipatory stress. However, the unchanged pre-race
RMSSD did not confirm this assumption. A recent paper examined anticipatory responses in
horses prepared before dressage and show jumping during a 3-day championship [17].
RMSSD and heart rate continuously increased during the pre-competition preparation, while
pre-competition salivary cortisol did not change compared to previously measured basal corti-
sol concentrations. The lack of the pre-competition salivary cortisol increase indicate that
these horses are less anticipated before competitions compared to racehorses used in the pres-
ent investigation. The atmosphere and the expected performance are also less different from
Anticipatory response of Standardbred racehorses
PLOS ONE | https://doi.org/10.1371/journal.pone.0201691 August 2, 2018 3 / 8
everyday workouts, therefore, decreased RMSSD may be not the result of the forthcoming race
but rather only the usual effect of saddling in the authors’ study.
The present study describes cortisol and HRV differences between training and race. It has
been shown that human intervention and the forthcoming physical exercise mean stress for
the horse [26]. Preparation probably affected baseline RMSSD in our study as well, but this
effect was not significantly stronger before the race compared to the training. However, the
higher pre-race LF/HF ratio and cortisol level suggest a more marked sympathetic overweight
and presumable extra stress before race compared to the training day. During the experimental
Fig 1. Plasma cortisol levels (median, 25 and 75 percentiles, min/max) of Standardbred racehorses (n = 8) before training and trotting race. Samples
were taken between 10:00 and 10:30 AM, 5 min before warming up.
https://doi.org/10.1371/journal.pone.0201691.g001
Anticipatory response of Standardbred racehorses
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period, horses were kept in stables near the racing track. They were exposed to the atmosphere
of the race in every Wednesday and Saturday. It cannot be excluded that these effects affect
HPA function not only in the experimental animals, but of all horses kept in these stables, even
when they have not been entered to the given race. The elevated pre-exercise sympathetic tone
before race may reflect the totality of many impacts, like the environment, the forthcoming
exercise and the associated competition stress.
Fig 2. Plasma cortisol levels of Standardbred racehorses (n = 8) before and after training and trotting race (means ±SEM). 0: Sample 0 = before
warming up (baseline), 1: Sample 1 = afer warming up, 2: Sample 2 = immediately after the end of exercise, 3: Sample 3 = after a 30-min recovery. The
distance covered during training and race was 8,000–9,000 and 2,300 m, respectively. The time lag between the first and the last sample ranged between 52
and 60 min.
https://doi.org/10.1371/journal.pone.0201691.g002
Anticipatory response of Standardbred racehorses
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After the relative high pre-race cortisol level, no further cortisol rise was found during the
race, while on training days, significant increase was observed from baseline. It contradicts to
the findings of von Lewinski et al. [22], who detected similar increase in cortisol level of sport
horses during training and public performance. It is a limitation of our study that difference in
distances between training and race sessions taken by athletes might have influenced HPA
responses at S2 and S3 samplings. Nevertheless, the lack of a significant cortisol increase dur-
ing the race can be explained rather with the excessively high basal values than with the shorter
distance. The high pre-race cortisol level may have caused a negative feedback to the HPA axis
and inhibited further ACTH release during the exertion. In other investigations, where also
positive cortisol response to maximal effort was found, the basal cortisol level was unaltered,
and cortisol started to increase from a much lower value compared to our study [5,6,2325].
To study performance-related increases in cortisol levels it would be important to exclude the
effects of race atmosphere. However, maximal performance outside the competition can be
achieved only with unusually powerful encouragement of the driver which also would cause
additional stress for the horse.
Our HRV data recorded during exercise were uninterpretable for several reasons. The
length of IBI data recorded during race sessions did not allow us the proper analysis of HRV
during exercise. Moreover, the displacement of the electrodes under the harness saddle was
unavoidable at this high speed (45–50 km/h). It was shown that HRV above heart rate of 120–
130/min is predominantly determined by non-neural mechanisms [26] during a mild inten-
sive exercise, therefore, failure of recording valid IBI data could not be considered as a serious
shortcoming of our study.
Conclusions
Pre-race plasma cortisol concentrations and sympathetic tone-related HRV reflected anticipa-
tory response in racehorses before trotting race. Vagal tone related HRV did not confirmed
this phenomenon. The lack of a significant cortisol increase during race might have resulted
from the excessively high basal values. The comparison of plasma cortisol levels measured dur-
ing training and race seem to have no relevance in this experiment, because the work effort
could not be precisely determined due to various environmental and emotional effects that
should be further investigated in physiological studies on equine performance.
Acknowledgments
The authors thank Constanze Bartesch, DVM, for her assistance with this work. We are grate-
ful to ‘Kincsem Park’ for providing the horses. L. Kova
´cs was supported by the New National
Excellence Program of the Ministry of Human Capacities [U
´NKP-17-4-I/SZIE-7].
Author Contributions
Conceptualization: Zso
´fia Boha
´k, Otto
´Szenci, Levente Kova
´cs.
Data curation: Andrea Harnos.
Investigation: Zso
´fia Boha
´k, Kinga Joo
´.
Methodology: Zso
´fia Boha
´k, Levente Kova
´cs.
Supervision: Levente Kova
´cs.
Writing – original draft: Zso
´fia Boha
´k.
Writing – review & editing: Otto
´Szenci, Levente Kova
´cs.
Anticipatory response of Standardbred racehorses
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... In horses, HRV analysis is frequently used for stress [14][15][16], pain [17,18], and behavioral investigations [19,20]; however, the sport physiological implications of HRV is a lesser-studied area. Most of the equine sports physiology studies examined cardiac autonomic responses during exercise [21][22][23][24][25][26][27], and the majority of the papers concluded that HRV analysis is not useful for the evaluation of ANS activity in the heavily exercising horse. ...
... (Fig 1). 16.6% of UT horses showed a decrease in HF below SWC (P�0.05) at the post-training measurements. ...
... Cardiac autonomic regulation improves during the initial phase of training, while it decreases over the weeks preceding competition [55]. This is in line with some results found in racehorses, where the average HRV was higher in the middle of the training process than before races [32], moreover, precompetition stress further reduced HRV in trotters [16]. ...
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... 21 It is possible that Crescendo, mullen mouth regulator and unjointed happy mouth bits are chosen for horses that are unresponsive to light rein signals for various reasons, including an evasion or flight response due to painful stimulus or anticipation of pain, 22,23 habituation to bit pressure due to inconsistent training not reinforcing the desired reaction to bit pressure 12,16 and multiple stressors present in the competition environment such as transportation, unfamiliar horses and novel situations. 24,25 Inability to respond to a light rein signal due to erroneous learning combined with high arousal (be that excitement, anxiety or fear) may increase the rein tension needed and predispose these horses to oral trauma. As there might be complicated interactions between factors such as horse behaviour and performance, and bit type and rein tension, it would be interesting to follow the same horses driven with different bits and different rein tensions. ...
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Background: Bit-related lesions in competition horses have been documented, but little evidence exists concerning their potential risk factors. Objectives: To explore potential risk factors for oral lesions in Finnish trotters. Study design: Cross-sectional study. Methods: The rostral part of the mouth of 261 horses (151 Standardbreds, 78 Finnhorses and 32 ponies) was examined after a harness race. Information on bit type, equipment and race performance was collected. Results: A multivariable logistic regression model of Standardbreds and Finnhorses showed a higher risk of moderate or severe oral lesion status associated with horses wearing a Crescendo bit (n = 38, OR 3.6, CI 1.4-8.9), a mullen mouth regulator bit (n = 25, OR 9.9, CI 2.2-45) or a straight plastic bit (n = 14, OR 13.7, CI 1.75-110) compared with horses wearing a snaffle trotting bit (n = 98, P = 0.002). Bar lesions (67 horses) were more common in horses wearing unjointed bits than in horses wearing jointed bits (Fisher's exact test P < 0.001). Lesions in the buccal area and the inner lip commissures were not associated with bit type. Using a tongue-tie or an overcheck, galloping, placement in the top three or money earned in the race were not associated with lesion risk. Main limitations: The sample size for certain bit types was insufficient for statistical analysis. Conclusions: Moderate and severe oral lesion status was more common in horses wearing a Crescendo bit, a mullen mouth regulator bit or a straight plastic bit than in horses wearing a single-jointed snaffle trotting bit. However, lesions were observed regardless of bit type. Further studies on rein tension, the interaction between bit type and rein tension, and prevention of mouth lesions in trotters are warranted.
... As a noninvasive cardiovascular marker, heart rate variability reflects the changes of the sympathoparasympathetic balance of the autonomic nervous system (ANS) in response to external stimuli, as an indicator of stress and animal welfare [11]. PR variability was largely investigated as relevant information of ANS's short-term response in horses and, in lesser extent, in dogs [7,12]; moreover, a meta-analytic study performed in human sports medicine has reported a significant increase in cognitive and somatic anxiety in the time leading up to competition. Cortisol concentrations during moderate intensive sports, such as activity performed by working dogs, lead to positive effects on the reaction time and inhibition of aversive stimuli [10]. ...
... As a noninvasive cardiovascular marker, heart rate variability reflects the changes of the sympatho-parasympathetic balance of the autonomic nervous system (ANS) in response to external stimuli, as an indicator of stress and animal welfare [11]. PR variability was largely investigated as relevant information of ANS's short-term response in horses and, in lesser extent, in dogs [7,12]; moreover, a meta-analytic study performed in human sports medicine has reported a significant increase in cognitive and somatic anxiety in the time leading up to competition. Cortisol concentrations during moderate intensive sports, such as activity performed by working dogs, lead to positive effects on the reaction time and inhibition of aversive stimuli [10]. ...
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... It is well known that in human athletes during sporting events, cortisol production increases as a result of heightened anxiety or anticipation of the event, to the benefit of their performance [36]. Similarly, cortisol level in horses is higher during competition [37], compared to training [38], but no difference in equine plasma cortisol was found between race winners and losers [39]. Thus, a multitude of complex factors including anxiety, anticipation or other psychological factors could have an effect on an individual horse's race performance. ...
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Each conditioning programme examined consisted of eleven repetitions of exercise of 5, 15 or 25 minutes´duration at a velocity at which, mathematically, horses had a blood lactate concentration of 2.5 or 4 mmol/l. During the multiple step test the mean cortisol concentration in plasma showed a tendency to increase (p = 0.07). The mean peak of the cortisol concentration in plasma was measured 10 minutes after the test. Thereafter mean cortisol concentration continuously decreased and the day after the test values were similar to those before the test. After the first run of the two-speed test mean plasma cortisol concentration tended to increase (p = 0.08), while significant changes were registered after the second run (p < 0.01): It decreased up to the fifth minute after exercise (p < 0.05) and increased threreafter reaching a plateau between the 15th and 45th minute after exercise (p < 0.05). The morning after the test mean plasma cortisol concentration had returned to the values measured the morning before the test. None of the conditioning programmes had an effect on the mean plasma concentration of cortisol in the horses at rest. The mean coefficient of variation of plasma cortisol concentration in five horses before exercise was 34.1% and 36.4% after exercise. The large individual variability of plasma cortisol concentrations before and after exercise does not allow a good repeatability of results, and enforces the need for multiple blood sampling during exercise. The multiple step exercise test as well as the two-speed exercise test induced increases of the plasma cortisol concentration after exercise. The changes measured supply a basis for studies on treadmills on the value of monitoring the plasma cortisol concentration of sport horses for performance diagnosis. Konzentration von Plasmakortisol bei Pferden während und nach Laufband-Belastungstests und Einfluß von Training auf den Plasmakortisolgehalt bei Pferden unter Ruhebedingungen Die Wirkung von zwei verschiedenen Belastungstests auf die Plasmakortisolkonzentration von gesunden Vollblutpferden wurde untersucht, um Referenzwerte für die Leistungsdiagnostik zu bekommen. Außerdem wurde geprüft, ob sechs verschiedene Trainingsprogramme einen Einfluß auf die Plasmakortisolkonzentration von Vollblutpferden haben. Außerdem erfolgte die Berechnung des Variationskoeffizienten für die Messung der Kortisolgehalte im Plasma von Pferden vor und nach Belastung. Alle Belastungen fanden auf einem Laufband statt. Der Mehrstufen-Belastungstest bestand aus fünf Stufen. Jede Stufe dauerte 5 Minuten. In der ersten Stufe liefen die Pferde bei einer Geschwindigkeit von 6 m/ s. Die Geschwindigkeit wurde von Stufe zu Stufe um 0,5 m/s erhöht. Der Zwei-Geschwindigkeiten-Belastungstest bestand aus zwei Läufen über je 1.200 m. Den ersten Lauf absolvierten die Pferde bei einer konstanten Geschwindigkeit von 10, 10,5 oder 11 m/s. Danach wurden die Pferde 30 Minuten lang auf dem Laufband Schritt geführt. Der zweite Lauf erfolgte bei Geschwindigkeiten von 13, 13,5 oder 14 m/s. Jedes untersuchte Trainingsprogramm bestand aus 11 gleichen Belastungen, die in einem Zeitraum von 3 Wochen von den Pferden gelaufen wurden. Die Belastungen dauerten 5, 15 oder 25 Minuten. Die Laufgeschwindigkeit entsprach der Geschwindigkeit, die in einem Mehrstufen-Belastungstest vor dem jeweiligen Trainingsprogramm bei den Pferden eine Laktatkonzentration im Blut von 2,5 oder 4 mmol/l hervorgerufen hatte. Während des Mehrstufentests nahm die mittlere Plasmakortisolkonzentration tendentiell zu (p = 0,07). Der höchste mittlere Kortisolgehalt im Plasma wurde 10 Minuten nach dem Test gemessen. Danach fiel die Konzentration kontinuierlich ab und war am Tag danach auf dem gleichen Niveau wie vor dem Test. Nach dem ersten Lauf des Zwei-Geschwindigkeitentests nahm die mittlere Kortisolkonzentration im Plasma tendentiell zu (p = 0,08), nach dem zweiten Lauf sogar signifikant (p < 0,01). Allerdings fiel der mittlere Kortisolgehalt im Plasma bis zur fünften Minute nach Belastung zuerst ab (p < 0,05), stieg danach an und bildete ein Plateau zwischen der 15ten und 45ten Minute nach Belastung (p < 0,05). Am Morgen nach dem Belastungstest waren die mittleren Kortisolgehalte wieder auf dem Stand von vor dem Test. Keines der angewendeten Trainingsprogramme zeigte Wirkung auf die mittlere Kortisolkonzentration der Pferde unter Ruhebedingungen. Der mittlere Variationskoeffizient der Plasmakortisolkonzentration vor und nach Belastung betrug 34,1% und 36,4%. Der festgestellte Variationskoeffizient ist hoch. Dies bedeutet eine schlechte Wiederholbarkeit für die Kortisolmessungen und betont die Bedeutung der mehrfachen Blutentnahme bei Belastung. Die Veränderungen der Kortisolkonzentration im Blut bei den Belastungstests dienen als Referenz für weitere Studien, um den Nutzen der Messung dieser Variablen für die Leistungsdiagnostik zu prüfen.
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