EQUINE VETERINARY JOURNAL
Equine vet. J. (2010) 42 (Suppl. 38) 1-5
doi: 10.1111/j.2042-3306.2010.00184.x
1
Oxygen consumption and gait variables of Arabian
endurance horses measured during a field exercise test
F. COTTIN*, N. METAYER†, A. G. GOACHET‡, V. JULLIAND‡, J. SLAWINSKI, V. BILLAT and E. BARREY†
Unité de Biologie Intégrative des Adaptations à l’Exercice; †Génétique Animale et Biologie Intégrative, INRA; and ‡Agrosup, France.
Keywords: horse; endurance race; respiratory exchange ratio; running speed; respiration; heart rate
Summary
Reasons for performing study: Arabian horses have
morphological, muscular and metabolic features designed for
endurance races. Their gas exchange and gait variables were
therefore measured during a field exercise test. This study
presents original respiratory and locomotor data recorded in
endurance horses under field conditions.
Hypothesis and objectives: Respiratory gas exchange ratio
(RER) of Arabian horses at the speed required to win
endurance races (18 km/h for 120–160 km) are <1 and
running economy (RE) is also low in order to maintain
exercise intensity using aerobic metabolism for long intervals.
The purpose of this study was to measure oxygen
consumption and gait variables in Arabian endurance horses
running in the field in order to estimate RER and RE.
Materials and methods: Five Arabian horses trained for
endurance racing were test ridden at increasing speeds on the
field. Their speed was recorded and controlled by the rider
using a GPS logger. Each horse was equipped with a portable
respiratory gas analyser, which measured breath-by-breath
respiratory variables and heart rate. The gait variables were
recorded using tri-axial accelerometer data loggers and
software for gait analysis. Descriptive statistics and linear
regressions were used to analyse the speed related changes in
each variable with P<0.05 taken as significant.
Results: At a canter speed corresponding to endurance race
winning speed (18 km/h), horses presented a VO2 = 42 ⫾
9 ml/min/kg bwt, RER = 0.96 ⫾ 0.10 and RE (= VO2/speed) =
134 ⫾ 27 l/km/kg bwt. Linear relationships were observed
between speed and VO2, HR and gait variables. Significant
correlations were observed between VO2 and gait variables.
Conclusions and potential relevance: The RER of 0.96 at
winning endurance speed indicates that Arabian horses
mainly use aerobic metabolism based on lipid oxidation and
that RER may also be related to a good coordination between
running speed, respiratory and gait parameters.
Introduction
Arabian horses have morphological, muscular and metabolic
features designed for endurance races being short in height at the
withers (150–155 cm), with a high fat free mass and high
proportion of muscular fibres of type I compared to type II (LopezRivero et al. 1989). In addition, they have an energetic metabolism
particularly suited to endurance exercise. Prince et al. (2002)
compared responses to prolonged moderate exercise between
Arabian pure-breds and Thoroughbreds. Arabian horses showed a
lower respiratory exchange ratio (RER) and higher plasmatic
concentration in free fatty acids than in Thoroughbreds. Therefore,
Arabians could preferentially use lipids as energetic substrates and
have a better adaptation to endurance exercise than Thoroughbreds.
Arabian horses frequently win endurance races (90–120 km). Since
1996, 51% of the best ranking endurance races in France (between
the first and the fifth rank) are attributed to Arabian horses vs.
9.97% for saddle horses, 9.54% for pure-bred Anglo-Arabian and
6.72% for French saddle horse (SF) (Buffet et al. 2009).
Running economy (RE = VO2/speed) is defined as the amount
of oxygen consumed (ml/kg bwt/min) per distance covered (ml/kg
bwt/km). Those horses able to consume less oxygen while running
at a given velocity are said to have a better running economy.
Running economy has already been studied in human marathon
runners (Billat et al. 2001) where it has been shown that top class
marathon male runners (i.e. performance <2 h 11 min) have a better
RE compared to high level marathon male runners (i.e.
performance <2 h 16 min). To date, no data have been obtained in
horses during exercise in field conditions. However, RE can now be
measured in a standardised field exercise test with ambulatory gas
exchange (Art et al. 2006; Leprêtre et al. 2009) and gait analysers
(Barrey et al. 2001). It is therefore relevant to measure the RE of
Arabian horses in field conditions as they show good performance
in endurance races.
As the horses that usually win have an average canter speed
around 18 km/h (Buffet et al. 2009), the energetic metabolism
should be strictly aerobic during most of the race duration in order
to avoid rapid fatigue due to lactic acid accumulation. Of course, a
short anaerobic boost can occur for a very short time but depth of
oxygen recovery will take time and consequently decrease the
speed. Therefore, It was hypothesised that the respiratory gas
exchange ratio (RER) of Arabian horses at the speed required to
win endurance races (18 km/h for 120–160 km) is <1 and the RE is
low in order to maintain for a long duration the exercise intensity
using aerobic metabolism. The purpose of this study was to
measure the oxygen consumption and gait variables in Arabian
*Corresponding author email: [email protected]
[Paper received for publication 08.01.10; Accepted 28.05.10]
© 2010 EVJ Ltd
2
Running economy in Arabian horses
endurance horses running in field conditions to estimate the RER
and RE at the specific endurance competition winning speed.
Materials and methods
Five healthy Arabian horses (age = 8 ⫾ 2 years, weight = 408 ⫾
27 kg, height = 153 ⫾ 2 cm) trained for endurance racing were test
ridden at increasing speeds in field conditions. The protocol and
care of the horses complied with the Declaration of Helsinki. The
horses were housed in individual loose boxes during the whole
experimental session. Each horse had free access to a drinking
trough at a constant level and was fed with a 3 kg hay intake at
10.00 and 16.00 h and with condensed food at 08.00 and 17.30 h.
The amount of condensed food fed was based on the weight of the
horse at the beginning of the study (Martin-Rosset and Vermorel
1991) and adjusted on a weekly basis as necessary.
The horses were trained 4 times a week under field conditions
during a conditioning period of 3 months. The training programme
consisted of alternate phases of trotting for 13 km at 3.6 m/s and
galloping for 20 km at 4.2 m/s. At the end of the training period, all
horses were qualified for a regional level of endurance race
(60 km).
During the exercise protocol, horses were ridden by the same
experienced endurance rider. The protocol consisted of 6 steps at
increasing speeds of 3 min each: trot: 13–15.5 km/h, canter and
gallop: 18.7–25.2, 33.5–36 km/h (author to confirm change) and
then galloping at increasing speed up to 45 km/h.
The rider could adjust the speed of the gait during exercise. The
speed was displayed, recorded and controlled by the rider by a GPS
logger (Garmin12)1 with an accuracy of one knot (⫾ 1.85 km/h).
The GPS provided a speed value every 2 s. The mean speed of each
step was calculated (Table 1).
TABLE 1: Mean ⫾ s.d. running speed, oxygen uptake (VO2) and running
economy (RE) during different speed stages corresponding to different
gaits: trotting (T 1, T2), canter (C, corresponding to the speed in
endurance race), gallop (G1, G2, G3, G4, G5, G6) with G5 and G6 only for
horses 4 and 5. Data are given for each horse and for the average
Each horse was equipped with a validated portable respiratory
gas analyser, which measured breath-by-breath respiratory variables
(K4b2)2 (Art et al. 2006; Leprêtre et al. 2009) and heart rate (HR,
Polar610)3 (Holopherne et al. 1999; Kingsley et al. 2005). The
following respiratory variables were continuously measured:
respiratory frequency (RF), tidal volume (Vt), oxygen uptake (VO2)
and carbon dioxide production (VCO2). These data allowed
calculation of ventilatory flow (VE), and the RER (= VCO2/VO2).
Each horse was equipped with a homemade respiratory mask
with a seal for expiratory flow. The horse’s nostril and mouth were
entirely enclosed in the mask, which was attached to the snaffle.
The volume of the mask was closed by an elastic rubber membrane
adjusted tightly on the nose, just under the zygomatic arch, so that
the horse could move its nostrils and lips freely within the mask and
all the expired air was able to flow through the 2 turbines. Before
the experiment, the horses were accustomed to wearing the mask at
rest and exercise.
Before each exercise test the K4b2 was calibrated. According to
the manufacturer’s instructions, the O2 and CO2 analysers were
calibrated with ambient air containing 20.9% O2 and 0.03% CO2
and calibration gas containing 16.0% O2 and 5.0% CO2, before and
after each test. A pretest was used to establish the delay between the
airflow and gas signals. The turbine flow meter of the analyser was
calibrated with a 3 l syringe, using the sampling line used during
the test.
The gait variables (running speed and stride frequency [SF])
were recorded simultaneously using tri-axial accelerometer data
logger and gait analysis software (Equimetrix)4 (Barrey et al. 2001;
Leleu et al. 2002). The device recorded accelerations with a
sampling frequency of 100 Hz. Therefore, from the recorded VO2
and speed, the RE was calculated as VO2/Speed.
Descriptive statistics (mean ⫾ s.d.) and linear regressions were
used to analyse the speed related changes in each variable.
Results
Bioenergetics
Stages
T1
T2
C
G1
G2
G3
G4
G5
G6
Speed (km/min)
0.21
0.26
0.31
0.42
0.56
0.60
0.66
0.72
0.75
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
VO2 (ml/min/kg bwt)
0.01
0.01
0.01
0.02
0.01
0.00
0.00
0.28
0.27
28
36
42
50
60
63
71
85
86
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
RE (l/kg bwt/km)
3
5
9
3
5
5
6
31
28
132
140
134
121
108
106
107
118
114
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
19
20
27
12
10
8
9
19
18
Oxygen uptake and RE at different speeds and gaits are given in
Table 1. Speed vs. gas exchange components (VO2, VCO2, RER)
and heart rate are given in Table 2.
Ventilatory components
Speed vs. ventilatory components (Rf, Vt and VE) are given in
Table 3.
TABLE 2: Mean ⫾ s.d. running speed, heart rate and gas exchange components during different speed stages corresponding to different gaits: trotting
(T 1, T2), canter (C, corresponding to the speed in endurance race), gallop (G1, G2, G3, G4, G5, G6) with G5 and G6 only for horses 4 and 5. Data are
given for each horse and for the average
Stages
Speed (km/h)
T1
T2
C
G1
G2
G3
G4
G5
G6
12.8
15.3
18.8
25.0
33.6
36.0
39.6
43.2
45.0
© 2010 EVJ Ltd
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
0.5
0.5
0.5
1.4
0.5
0.0
0.0
16.8
16.3
HR (beats/min)
141
160
125
150
180
191
200
207
202
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
28.3
33.5
18.3
10.2
9.9
15.6
10.5
47.4
37.3
VO2 (ml/min/kg bwt)
28.2
35.9
42.0
50.2
60.2
63.3
70.8
85.1
85.8
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
3.3
5.5
8.8
3.2
5.3
4.8
5.8
31.1
27.7
VCO2 (ml/min/kg bwt)
25.4
35.0
40.2
49.6
62.8
69.7
78.2
101.0
103.8
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
4.2
7.3
7.8
8.7
9.2
9.4
13.5
41.6
39.0
RER
0.90
0.97
0.97
0.99
1.07
1.13
1.11
1.20
1.21
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
0.1
0.1
0.1
0.1
0.2
0.2
0.2
0.2
0.2
F. Cottin et al.
3
TABLE 3: Mean ⫾ s.d. running speed, heart rate and respiratory components during different speed stages corresponding to different gaits: trotting
(T 1, T2), canter (C, corresponding to the speed in endurance race), gallop (G1, G2, G3, G4, G5, G6) with G5 and G6 only for horses 4 and 5. Data are
given for each horse and for the average
Stages
Speed GPS (km/h)
T1
T2
C
G1
G2
G3
G4
G5
G6
12.8
15.3
18.8
25.0
33.6
36.0
39.6
43.2
45.0
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
0.5
0.5
0.5
1.4
0.5
0.0
0.0
16.8
16.3
RF (breaths/min)
67.6
71.8
97.3
113.6
123.5
128.7
133.6
136.1
135.7
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
23.9
25.8
18.1
3.2
3.8
6.5
3.8
35.2
33.7
100
90
VT (l)
9.5
11.0
9.9
10.7
11.7
12.3
12.9
13.4
13.2
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
VE (l/min)
2.7
3.1
1.6
0.5
0.4
0.5
0.4
3.0
2.1
584.1
722.2
929.3
1211.2
1450.6
1580.0
1720.0
1818.9
1784.7
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
⫾
114.2
140.5
94.1
63.9
72.4
88.3
69.3
662.4
567.5
120
Winning speed
Winning speed
100
80
60
50
VO2 = 1.7 (V) + 10.0
40
R = 0.95, P<0.001
30
VCO2 ml(min/kg)
VO2 ml/(min/kg)
70
80
60
40
VCO2 = 2V − 0.1
20
R = 0.92, P<0.001
20
10
0
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
0
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Running speed (V, km/h)
Fig 1: Oxygen uptake (VO2, ml/min/kg bwt) vs. Speed (km/h) in 5 Arabian
horses. VO2 increases linearly when the running speed increases (r = 0.95,
P<0.001). The slope of the linear regression corresponds to the running
economy (RE). Thus, the mean RE at all speeds and gaits is 102 ml/kg
bwt/km (= 1.7 ml h/kg bwt/km/min in the chart).
Gas exchange measures vs. running speed: Oxygen uptake
increases linearly when running speed increases (r = 0.95, P<0.001,
Fig 1). The same was observed for VCO2 (r = 0.92, P<0.001, Fig 2)
and RER (r = 0.53, P<0.001, Fig 3). When running speed equals
18 km/h (endurance race winning speed), the linear regression
gives an RER of 0.96 (Fig 3), indicating that the horse’s
metabolism was mainly aerobic during most of the race duration.
Heart rate vs. running speed: Heart rate increased linearly when
the running speed increased (HR: r = 0.75, P<0.001).
Ventilatory components vs. running speed: Respiratory outflow
increased linearly when the running speed increased (VE: r = 0.97,
P<0.001, Fig 4). The same was observed for RF (r = 0.85, P<0.001)
and Vt (r = 0.6, P<0.001). In addition, respiratory frequency was
correlated linearly with stride frequency when galloping (r = 0.68,
P<0.01, Fig 5) whereas it was not during trotting (r = 0.18, NS,
Fig 5).
Running speed (km/h)
Fig 2: Carbon dioxide outflow (VCO2, ml/min/kg bwt) vs. Speed (km/h) in 5
Arabian horses. VCO2 increases linearly when running speed increases (r
= 0.92, P<0.001).
Coordination between speed vs. respiratory and gait variables:
Strong linear relationships were observed between speed and VO2,
HR and gait variables. Significant (P<0.05) correlations were
observed between VO2 and gait variables.
Gait variables vs. running speed: The stride frequency was
correlated linearly with the running speed (r = 0.90, P<0.001).
Discussion
The major finding of the study is that Arabian horses are winning
endurance races with an RER <1 (Fig 3), which confirms the
hypothesis that energetic metabolism is mainly aerobic based on
lipid oxidation during most of the duration of the race. This finding
is in accordance with a previous study (Prince et al. 2002), which
showed that Arabian horses have lower RER and higher plasmatic
concentrations of free fatty acids than Thoroughbred horses during
prolonged moderate exercise (90 min at 35% VO2MAX). These
results indicate that Arabians preferentially use lipids as metabolic
substrates during moderate exercise speed corresponding to
© 2010 EVJ Ltd
4
Running economy in Arabian horses
Trotting
2.0
Galloping
140
Winning speed
Respiratory frequency (Breaths/min)
Respiratory exchange ratio
120
1.0
R = 0.53, P<0.001
RER = 0.0082 V + 0.8125
RER = 0.96 at 18 km/h
R = 0.68, P = 0.008
100
80
60
R = 0.18, NS
40
20
0.0
10
14
18
22
26
30
34
38
42
46
50
Running speed km/h
0
1.4
1.6
1.8
2
2.2
Stride frequency (stride/s)
Fig 3: Respiratory exchange ratio (RER) vs. speed (km/h) in 5 Arabian
horses. RER increases linearly when the running speed increases (r = 0.53,
P<0.001). The vertical dashed line represents the mean speed required for
winning an endurance race in France (18 km/h). The horizontal dotted line
represents RER equal to 1. Full straight line represents the linear regression
of RER vs. speed. The intercept of this straight line with the vertical dashed
line, which gives the RER, is at 0.96, i.e. the energetic metabolism is strictly
aerobic during most of the race duration.
2500
Winning speed
2000
VEL/min
1500
1000
R = 0.97, P<0.001
500
0
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48
Running speed km/h
Fig 4: Respiratory outflow (VE, l/min) vs. speed (km/h) in 5 Arabian horses.
VE increases linearly when the running speed increases (r = 0.97, P<0.001).
endurance races. Therefore Arabians seem to be particularly well
suited for long aerobic exercise using lipid fuel. Moreover, RER
was expected to be <1 unless the horses were hyperventilating.
During canter at the winning endurance speed (18 km/h), the
horses could not hyperventilate because their respiratory frequency
© 2010 EVJ Ltd
Fig 5: Respiratory frequency (RF, breaths/min) vs. stride frequency (SF,
strides/s) in 5 Arabian horses. Left: diamonds represent data corresponding
to trotting. Right: squares represent data corresponding to galloping. BF
increases linearly when SF increases when galloping (r = 0.68, P<0.01);
this it is not the case when trotting (r = 0.18, NS).
was exactly synchronised with the stride frequency in ratio 1:1 as
illustrated in Figure 5. The respiratory frequency remained lower
than 2 cycles/s.
This study used ambulatory respiratory and gait analysis
devices in endurance horses to measure running variables in real
field conditions. The use of these devices has allowed RE to be
computed in endurance horses at different gaits in the field that are
closer to the racing condition than treadmill exercise.
Although the gas exchange device has already been validated
during exercise (Art et al. 2006; Leprêtre et al. 2009), this study
again confirmed that the respiratory mask was well accepted by the
horses that performed the exercise test on the field. The horses
could easily gallop at submaximal speed without any respiratory
distress. In this study the horse’s bit was removed and they were
driven using only the reins attached around the respiratory mask
like a nose band.
Figure 5 shows that the respiratory frequency increases linearly
when stride frequency increases when galloping (r = 0.68, P<0.01).
Thus, it seems that there is a strong locomotor-respiratory coupling
in Arabian horses when galloping. This study confirmed the results
of previous studies (Attenburrow 1982; Butler et al. 1993;
Lafortuna et al. 1996). In all these studies, a phase-locked 1:1
coupling between respiratory and stride cycles were observed when
galloping. In contrast, Figure 5 shows that the respiratory
frequency is not linearly related with the stride frequency during
slow trotting around 4 m/s (r = 0.18, NS). This study confirmed the
results of previous studies that observed no locomotor-respiratory
coupling during trotting when riding saddle horses (Hörnicke et al.
1983) or no steady locomotor-respiratory coupling (Attenburrow
1982, 1983; Bramble and Carrier 1983; Art et al. 1990; Barrey
et al. 2000). However, a few studies have reported a strong
locomotor-respiratory coupling only during fast trot at speeds
F. Cottin et al.
higher than 8.5 m/s (Barrey et al. 2000; Cotrel et al.
2006). Therefore, to our knowledge, no study has reported a
steady locomotor-respiratory coupling during slow trot, at
speeds corresponding to those observed in the present study
(3.5–4.5 m/s).
The efficient RE of Arabian horses is probably linked to an
RER <1 indicating that aerobic metabolism using lipid oxidation is
an important energetic pathway used to produce the ATP for
muscular contraction. In addition, the smaller withers height and
bodyweight of Arabians induces a body surface/mass ratio that
favours muscular calorific loss during endurance exercise
(Wilmore and Costill 1994). All these characteristics may explain
why Arabians can canter at 18 km/h very efficiently for very long
distances. In addition, if we compare with the data given in
other studies, at the speed required for winning endurance races
(18 km/h), under treadmill conditions, with the K4B2 gas-exchange
device, the extrapolated VO2 seems lesser in Arabians than in other
breeds. Indeed, 42 ml/min/kg bwt was reported in the Arabians of
this study vs. 50.1 ml/min/kg bwt for Thoroughbred cross
Warmblood horses (Leprêtre et al. 2009) and 70.8 for untrained
saddle horses (Art et al. 2006).
Conclusions
To our knowledge, this study presents original respiratory and
locomotor data recorded in endurance horses under field
conditions. The respiratory gas exchange ratio of 0.96 at winning
endurance speed indicates that Arabian horses mainly use aerobic
metabolism. Their running economy could also be related to a good
coordination between running speed, respiratory and gait
parameters. These features of Arabian horses might explain their
good performances in endurance racing.
Conflicts of interest
The authors have declared no potential conflicts.
Acknowledgements
Thanks to Dr Wendy Brand-Williams for English corrections
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