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Journal of Animal Science - Animal Physiology

A comparison of the physiological response to tölt and trot in the Icelandic horse1

 

This article in JAS

  1. Vol. 93 No. 8, p. 3862-3870
     
    Received: Mar 25, 2015
    Accepted: May 20, 2015
    Published: July 24, 2015


    2 Corresponding author(s): gudrunst@holar.is
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doi:10.2527/jas.2015-9141
  1. G. J. Stefánsdóttir 2*†,
  2. S. Ragnarsson*,
  3. V. Gunnarsson*,
  4. L. Roepstorff and
  5. A. Jansson*†
  1. * Department of Equine Studies, Hólar University College, 551 Sauðárkrókur, Iceland
     Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, 75007 Uppsala, Sweden
     Department of Anatomy, Physiology and Biochemistry, Swedish University of Agricultural Sciences, 75007 Uppsala, Sweden

Abstract

This study compared the effect of ridden tölt and trot at 3 speeds on physiological responses in trained adult (15.3 ± 1.6 yr) Icelandic horses. The experiment had a crossover design with 8 horses, 2 treatments (incremental exercise test in tölt and trot), and 2 riders. Each horse performed 2 tests per day (1 gait with 2 riders, minimum 4.5 h between) on 2 separate days, with 1 d of rest in between. The exercise test consisted of three 642-m phases at 3.0 m/s (Speed3), 4.0 m/s (Speed4), and 5.0 m/s (Speed5) and was performed outdoors on a 300-m oval gravel riding track in northern Iceland in May 2012. Heart rate (HR) was measured during warm-up, the exercise test, and after 5, 15, and 30 min of recovery. Blood samples were taken at rest, after warm-up, after each phase of the exercise test, and after 5, 15, and 30 min of recovery. Respiratory rate was counted for at least 15 s at rest, at the end of the exercise test, and at the end of the 30-min recovery, and rectal temperature was measured on these occasions. There were no differences in HR between tölt and trot at any time point (P > 0.05). At Speed3, hematocrit and plasma lactate concentration were greater (P < 0.05) in tölt (40% ± 1%, 1.1 ± 0.06 mmol/L) than in trot (39% ± 1%; 0.9 ± 0.06 mmol/L). There was a prolonged recovery of hematocrit and respiratory rate, a slower decrease in rectal temperature, and a tendency of a prolonged recovery of plasma lactate concentration (P = 0.0675) after tölt. In conclusion, there were only minor differences in physiological responses to tölt and trot in this selected group of experienced adult Icelandic horses and the biological and practical significance of the slightly elevated physiological responses to tölt and the slower recovery remains to be determined.



INTRODUCTION

The Icelandic horse breed is a naturally gaited riding horse (Andersson et al., 2012) and is known particularly for its tölt (Feldmann and Rostock, 1990; Björnsson and Sveinsson, 2006). The speed range in tölt can be expected to be similar to or even wider than that for trot (Robilliard et al., 2007), and according to Biknevicius et al. (2004, 2006), both tölt and trot should be classified as running gaits. Tölt is a 4-beat symmetric gait with 1 or 2 legs always on the ground, i.e., without a suspension, whereas trot is a 2-beat symmetrical diagonal gait with a suspension (Hildebrand, 1965; Feldmann and Rostock, 1990). The general view among Icelandic horse trainers is that tölt is a difficult gait for the horse and that it should be used sparingly for riding. It is also believed that tölt is a less economical gait than trot (Arnbjörnsson, 1975; Feldmann and Rostock, 1990). These beliefs are, to some extent, based on the fact that it usually takes a longer time to train Icelandic horses to tölt in balance with a rider than to train them to trot. It is also known that when Icelandic horses run free, most of them choose to trot but not to tölt. Some studies have measured the physiological responses in horses with a 4-beat gait (Prates et al., 2009; Wanderley et al., 2010; Manso Filho et al., 2012; Silva et al., 2014; Stefánsdóttir et al., 2014). However, to the best of our knowledge no previous study has compared the physiological responses to tölt and trot at the same speed in the same horses. The aim of the present study was to compare the effect of ridden tölt and trot at 3 speeds on the physiological responses in Icelandic horses. The hypothesis tested was that horses show higher heart rate (HR), hematocrit, and plasma lactate concentrations in response to tölt compared with trot at the same speed.


MATERIALS AND METHODS

The study was approved by the National Animal Research Committee of Iceland.

Horses

Eight Icelandic riding horses (5 geldings and 3 mares) aged 15.3 ± 1.6 yr (range 13 to 18 yr), with height at withers of 141 ± 2.4 cm (range 138 to 145 cm) and BW 3 h postfeeding of 376 ± 14 kg (range 358 to 391 kg; Smartscale 300, Gallagher USA, New Providence, PA), and in their summer coat were used in the study.

For the previous 6 mo, all 8 horses had been kept in the same stable in individual boxes (size 1.95 × 3.40 m) on a permanent sawdust bed at the Equine Science Department, Holar University College, Saudarkrokur, Iceland. Grass forage was the main feed, and all horses were fed the same feeds according to individual requirements (NRC, 2007) and a BCS target of 3.0 (scale 1.0 to 5.0) according to Stefánsdóttir and Björnsdóttir (2001). Body condition score was 3.1 ± 0.1 (range 3.0 to 3.5). A mineral vitamin supplement (Racing Mineral, Trouw, Denmark) was fed (70 to 90 g/d), and the horses had free access to a salt block (99% salt). Water was offered from automatic water bowls.

Training and Preparation of the Horses

The horses had all been used by university students in the same teaching program for 9 mo before the study. In the last 3 wk before the experiment the horses were prepared for the experimental exercise test (e.g., adjusting the speed on each gait on the track) by the 2 professional riders who later rode them in the test. The horses were all kept at rest in their boxes, except for spending 1 to 2 h in a paddock, for at least 2 d before taking part in the experiment.

The shoes and boots of all horses complied with the regulations for international gait competitions with Icelandic horses (International Federation of Icelandic Horse Associations [FEIF], 2014). Most of the horses (n = 7) were on 8-mm shoes in both front and back, and 1 had 10-mm shoes on the front hooves and 8-mm shoes on the back hooves. Each horse had the same shoes and boots (≤170 g) when performing both gaits.

Experimental Design

The experiment was arranged as a crossover design with the 8 horses, 2 treatments (incremental exercise test in tölt and trot), and 2 riders. Each horse performed 2 tests per day (1 gait with 2 riders, minimum 4.5 h in between), and all treatments were performed on 2 separate days, with 1 d of rest in between (Table 1). The exercise tests were performed outdoors at Hólar University College in Iceland at the end of May 2012 on a 300-m oval gravel riding track accepted for gæðinga competitions (FEIF, 2012). We chose to do this study on a track (not on a treadmill) because we wanted to compare the physiological response to the 2 gaits on a competition track. The riders rode on the outer circumference of the track, which corresponded to a distance of 321 m. The incremental exercise tests were preceded by a warm-up (1.2 ± 0.1 km), consisting of 5 min of walking on flat gravel ground (1.3 to 1.8 m/s) and 5 min of slow speed (2.0 to 3.2 m/s) in tölt and trot in circles of different sizes in both directions (clockwise and counterclockwise) in the middle of the experimental track area. The riders were instructed to perform the warm-up in a standardized manner, and there was no difference in lactate and hematocrit at the end of warm-up or in HR during the last min of warm-up between the tölt and trot tests (P > 0.05). The exercise test started within 1 to 3 min after the end of warm-up and comprised an incremental exercise consisting of three 642-m phases at ∼3.0 m/s (Speed3), ∼4.0 m/s (Speed4), and ∼5.0 m/s (Speed5). The speeds chosen reflect a speed range that is commonly used in both tölt and trot in leisure riding and also in the breed evaluation field test of this breed. The time between phases was 1 to 2 min for taking blood samples. For standardization of the speed, 2 horses performed the exercise test at the same time, 1 in each gait. The horses were mounted (with rider and tack) during warm-up, exercise test, and ∼10 min of the recovery period, but from then until 30 min of recovery, they carried only the riding tack. The riders were asked to sit fully in the saddle, use minimal aids, and ride the horses as passively as possible on both gaits, just to keep the horse in balance on the gait with a clear beat and at the intended speed. The BW of the rider plus saddle was 87.5 and 119.0 kg, respectively, for the 2 riders, and the ratio of the BW of the rider plus saddle to the BW of the horse was 23.3% ± 0.8% and 31.7% ± 1.1%, respectively.


View Full Table | Close Full ViewTable 1.

Experimental design of the gait test1

 
Tölt
Trot
Day Horse Rider Horse Rider
Day 1
Test 1 Horses 1 to 4 Rider 1 Horses 5 to 8 Rider 2
Test 2 Horses 1 to 4 Rider 2 Horses 5 to 8 Rider 1
Day 3
Test 1 Horses 5 to 8 Rider 2 Horses 1 to 4 Rider 1
Test 2 Horses 5 to 8 Rider 1 Horses 1 to 4 Rider 2
1Tests 1 and 2 were performed with at least 4.5 h in between

Blood Samples and Blood Analysis

The horses were fitted with a catheter (2.0 × 105 mm, Intranule, Vygon Sweden AB, Skellefteå, Sweden) in the jugular vein under local anesthesia (Xylocaine, 20 mg/mL, AstraZeneca, Södertälje, Sweden) at least 1 h before the start of the exercise tests in the morning, and it was removed the same day after the 30-min recovery period. Blood samples were taken at rest, after warm-up, after Speed3, Speed4, and Speed5, and after 5-, 15-, and 30-min recovery, using chilled lithium heparinized tubes (9 mL, Vacuette, Greine-Bio-one, Kremsmünster, Austria). Within 30 min, hematocrit was analyzed after centrifugation in capillary tubes (6 min, 21,913 × g, room temperature; Cellokrit 2, Stockholm, Sweden). Duplicate analyses were performed, and a mean value was used for statistical analyses. The plasma was separated by centrifugation (15 min, 520 × g; Hettich, Tuttlingen, Germany) at room temperature and then stored at −18°C until analysis of lactate concentration. Plasma lactate concentration was analyzed using an enzymatic (l-lactate dehydrogenase and glutamate-pyruvate transaminase) and spectrophotometric method (Boehringer Mannheim/R-Biopharm, Darmstadt, Germany) with a CV of 2.3% according to the manufacturer. All plasma samples were analyzed in duplicate, and the CV of the plasma lactate analysis of duplicate samples was 12.7% (16.7% for values ≤1.5 mmol/L [137 samples] and CV = 7.4% for values >1.5 mmol/L [101 samples]).

Data Collection and Handling

During warm-up, exercise test, and recovery for 30 min, HR (Polar HR Monitor RS800CX, Polar Electro Oy, Kempele, Finland) was recorded. The HR monitors were set to record in 1-s mode. All HR recordings were input to the software Polar Pro Trainer 5 Equine Edition (Polar Electro, Kempele, Finland). Data on HR during the last 60 s of the warm-up, during the 3 phases of the exercise test, and after 5, 15, and 30 min of recovery were collected. Heart rate data using the last 15 or 30 s were also evaluated, but the results were not different from those for 60 s (P > 0.05). The HR recordings were occasionally of low quality or lacking (lost connectivity between the electrodes and the skin, even though the coat was clipped and a conductivity gel was used), but all horses contributed data to mean HR for all 3 speeds at both gaits, except at trot at the highest speed, for which 1 horse (A) lacked data (minimum 21 observations/speed). Rectal temperature was measured at rest, at the end of the exercise test, and at the end of the 30-min recovery period using a digital thermometer (Omron, Hoofddorp, Netherlands). Respiratory rate was counted for at least 15 s at rest, at the end of the exercise test, and at the end of the 30-min recovery period. Data from blood samples (plasma lactate concentration and hematocrit), rectal temperature, and respiratory rate were missing for 1 test for 1 horse (because it replaced another horse that was considered too excited to participate in the study, i.e., failed in the first test) and data from horse A was excluded after Speed4 at trot because it did not stay at trot at Speed5 with both riders.

The riding track had clear markings (bars) every 50 m, and the speed during the exercise test was measured in 3 ways: 1) live by an observer using a stopwatch, 2) by observation of video recordings of the tests and the use of a stopwatch, and 3) by using the GPS in the HR recorder (Polar G3 GPS sensor, Polar Electro, Kempele, Finland). The speed did not differ (P > 0.05) between gaits within phases for any of the methods used. There was no difference in Speed3 between the stopwatch speed and the GPS speed, but for the higher speeds the GPS values were lower than the stopwatch values (P < 0.01; data not shown). The GPS speed was mainly (88%) within ±0.2 m/s of the stopwatch speed, but the greatest deviations (>0.2 m/s) were found at the fastest speed, indicating that the GPS lost accuracy during such conditions. The speed estimated from the video recordings was therefore used for all calculations (because they were possible to reanalyze), except for 1 test for 2 horses, where the stopwatch speed measured live was used (video recording was missing because of technical problems). The videos (in slow motion, 50 frames/s) were also used to assess if horses performed the correct gait and to time and count number of strides per horse.

Information on the weather was obtained from the nearest national weather station 30 km away at Saudarkrokur airport. The daytime (0900 to 2200 h) weather on d 1 and 2 was, respectively, ambient temperatures of 14.6°C ± 0.8°C and 13.1°C ± 1.4°C, relative humidity of 49% ± 7% and 65% ± 8%, average wind speed of 5.9 ± 3.5 and 11.2 ± 2.5 m/s, and peak wind speed of 6.9 ± 3.1 and 12.0 ± 2.8 m/s. The wind was from the south on both days.

Statistical Analysis and Calculations

Statistical analyses were performed using SAS (version 9.2, SAS Inst. Inc., Cary, NC). Normal distribution of the data was tested with residual plots. PROC MIXED model 1, Yijkln = μ + αi + βj + εk + γl + an + eijkln, was used to analyze differences in speed, physiological responses, stride characteristics, and calculated parameters between the gaits, where Yijkln is the observation/parameter, µ is the mean value, αi is the fixed effect of gait, βj is the fixed effect of rider, εk is the fixed effect of day, γl is the fixed effect of test (within day), an is the random effect of horse, and eijkln is the residuals: eijkln ∼ (0, δ2). Differences in physiological responses and speed between repeated samples within each gait were analyzed using model 2, Yijkl = μ + αi + βj + εk + γl + eijkl, where Yijkl is the observation/parameter, µ is the mean value, αi is the fixed effect of rider, βj is the fixed effect of day, εk is the fixed effect of test (within day), γl is the fixed effect of sample, and eijkl is the residuals. The residuals had an unstructured covariance structure, except for plasma lactate concentration and HR in trot, for which the residuals had a spatial power correlation structure between samples (0, 10, 15.3, 20, 24, 29, 39, and 54 min). The repeated factor was horse × day × rider. The responses were compared to the level at rest. Data for plasma lactate concentration for both gaits were log transformed before differences between samples were analyzed with model 2, as variance of the samples was different and the residuals were not normally distributed. PROC REG was used to calculate linear regressions between stride frequency and speed and also stride length and speed. Student’s t test was used to compare the stopwatch speed and the GPS speed. Results are expressed as least squares means with their SE from model 1 unless otherwise stated. A Tukey test was used for comparison, and the level of statistical significance was set to P < 0.05, with a tendency if P < 0.1.

A total of 31 tests (16 in trot, 15 in tölt) were performed in the study, and when possible (Table 2), speed at a plasma lactate concentration of 4 mmol/L (V4) and 2 mmol/L (V2) and a HR of 140 beats/min (V140) and 180 beats/min (V180) was calculated. The HR at a plasma lactate concentration of 2 mmol/L (HR2) was also calculated within gait and rider. Microsoft Excel 2010 (Microsoft, Redmond, WA) was used to calculate the exponential regressions of V4, V2, V140, and V180 and the linear regression of HR2.


View Full Table | Close Full ViewTable 2.

Numbers of tests (out of 32) used for calculation of fitness parameters, where the calculated parameter was not reached, and with missing data1

 
V4 V2 V180 V140 HR2
Used for calculations 18 28 12 19 18
Calculated parameter not reached 11 1 7 0 1
Missing data2 3 3 13 13 13
1Fitness parameters calculated were speed at plasma lactate concentration of 4 mmol/L (V4) and 2 mmol/L (V2) and at a heart rate of 140 beats/min (V140) and 180 beats/min (V180) and heart rate at a plasma lactate concentration of 2 mmol/L (HR2).
2Heart rate, incorrect gait, and lack of data from 1 horse in 1 test.

Oxygen consumption based on HR for individual horse at each gait and speed was calculated by using the equations presented by Coenen et al. (2011) (VO2 = oxygen consumption in mL·kg−1 body mass·min−1 = 0.002816 × HR1.9955; r2 = 0.911) and Eaton et al. (1995) (VO2 = 0.833 × HR − 54.7; r2 = 0.865). The Eaton et al. equation has been suggested to give better estimates of oxygen utilization at high HR, and the Coenen et al. equation has been suggested to give better estimates at lower HR (NRC, 2007). Energy expenditure was estimated by transforming oxygen into joules on the basis of 20.1 kJ/L VO2 (Coenen et al., 2011).


RESULTS

Speed, Stride Frequency, and Stride Length

In accordance with the experimental plan, the speed of the 3 phases increased from Speed3 to Speed5, (P < 0.05; Table 3), and within each phase the speed was not different between the 2 gaits (P > 0.05; Table 3).


View Full Table | Close Full ViewTable 3.

Speed, stride frequency, and stride length (least squares means ± SE) of Icelandic riding horses in three 642 m phases (Speed3, Speed4, and Speed5)2 of an incremental exercise test performed in tölt and trot

 
Item n1 Tölt n1 Trot P-value
Speed3
    Speed, m/s 15 3.19 ± 0.03 16 3.20 ± 0.03 0.81
    Strides per min 14 106.9 ± 1.2a 15 94.1 ± 1.2b <0.0001
    Stride length, m 14 1.79 ± 0.02a 15 2.04 ± 0.02b <0.0001
Speed4
    Speed, m/s 15 4.09 ± 0.05 16 4.08 ± 0.05 0.82
    Strides per min 14 115.9 ± 1.3a 15 105.7 ± 1.2b <0.0001
    Stride length, m 14 2.13 ± 0.02a 15 2.33 ± 0.02b <0.0001
Speed5
    Speed, m/s 15 5.54 ± 0.09 14 5.61 ± 0.09 0.58
    Strides per min 14 126.2 ± 1.5a 13 115.8 ± 1.6b <0.0001
    Stride length, m 14 2.64 ± 0.04a 13 2.93 ± 0.04b 0.0001
a,bWithin rows, least squares means with different superscripts differ significantly (P < 0.05).
1Number of tests.
2 Speed3 ∼ 3.0 m/s, Speed4 ∼ 4.0 m/s, and Speed5 ∼ 5.0 m/s.

Stride frequency was greater in tölt than in trot at all speeds (P < 0.001; Table 3), whereas stride length was greater in trot than in tölt at all speeds (P < 0.001; Table 3). There was a positive linear relationship between stride frequency (strides/min) and speed in both tölt (r2 = 0.88; P < 0.001) and trot (r2 = 0.82; P < 0.001) and between stride length and speed in tölt (r2 = 0.97; P < 0.001) and trot (r2 = 0.95; P < 0.001).

Plasma Lactate Concentration

Plasma lactate concentration was greater after tölt than after trot for Speed3 (P < 0.05), but there were no differences at any other time point (P > 0.05; Fig. 1A). Plasma lactate concentration increased after Speed5 compared with after Speed4 and Speed3 and was back to resting level after 15 min for both gaits, but there was a tendency that it had not recovered until after 30 min for tölt (P = 0.0675; Fig. 1A). For trot, plasma lactate concentration after Speed5 was greater than at all other time points (P < 0.001), whereas for tölt plasma lactate concentration after both Speed5 and 5 min recovery was greater than at all other time points (P < 0.001).

Figure 1.
Figure 1.

(A) Plasma lactate concentration, (B) heart rate, and (C) hematocrit (least squares means ± SE) before, during, and after an incremental exercise test consisting of three 642 m phases; at 3.0 m/s (Speed3), 4.0 m/s (Speed4) and 5.0 m/s (Speed5) in 8 mature Icelandic riding horses. Blood samples were collected at rest; at end of warm-up; at end of Speed3, Speed4, and Speed5; and at the end of 5-, 15-, and 30-min recovery. Heart rate was recorded during the last minute of warm-up, Speed3, Speed4, and Speed5 and 5-, 15-, and 30-min recovery. The HR at rest was the minimum HR measured before the exercise test. An asterisk (*) indicates significant difference between gaits; bars within a gait with different letters are significantly different (P < 0.05).

 

Heart Rate, Hematocrit, Calculated Oxygen Consumption, and Energy Expenditure

Heart rate was not different between tölt and trot during Speed3, Speed4, and Speed5 or after 5-, 15-, and 30-min recovery (P > 0.05; Fig. 1B). Mean HR during the total 30-min recovery period did not differ (P > 0.05) between tölt and trot, and it was not back to resting level after 30 min (Fig. 1B). For individual horses, there was a strong linear relationship between speed and HR within gaits (tölt range: r2 = 0.90 to 0.99, trot range: r2 = 0.93 to 1.00). There was no difference in the slope of the regression line between tölt and trot (20.4 and 22.0, respectively; P > 0.05).

The calculated oxygen consumption and energy expenditure did not differ between tölt and trot at the 3 speeds for either of the equations used (Table 4).


View Full Table | Close Full ViewTable 4.

Calculated oxygen consumption and energy expenditure (least squares means ± SE) in Icelandic horses during an incremental exercise test (Speed3, Speed4, and Speed5)1 performed in tölt and trot2

 
Item Unit n3 Tölt n3 Trot P-value
Speed3
    Oxygen consumption
        Coenen et al. (2011) equation mL·kg−1·min−1 11 49.7 ± 2.2 10 50.0 ± 2.3 0.86
        Eaton et al. (1995) equation mL·kg−1·min−1 11 56.9 ± 2.5 10 57.2 ± 2.5 0.81
        Per meter based on Coenen et al. mL/m 11 125.9 ± 6.4 10 123.1 ± 6.7 0.50
        Per meter based on Eaton et al. mL/m 11 144.0 ± 7.1 10 141.0 ± 7.4 0.49
    Energy expenditure
        Coenen et al. (2011) equation kJ·kg−1·min−1 11 1.00 ± 0.04 10 1.00 ± 0.05 0.86
        Eaton et al. (1995) equation kJ·kg−1·min−1 11 1.14 ± 0.05 10 1.15 ± 0.05 0.81
        Per meter based on Coenen et al. kJ/m 11 2.53 ± 0.13 10 2.47 ± 0.13 0.50
        Per meter based on Eaton et al. kJ/m 11 2.89 ± 0.14 10 2.83 ± 0.15 0.49
Speed4
    Oxygen consumption
        Coenen et al. (2011) equation mL·kg−1·min−1 13 67.0 ± 2.5 11 68.2 ± 2.6 0.42
        Eaton et al. (1995) equation mL·kg−1·min−1 13 75.1 ± 2.5 11 76.2 ± 2.6 0.41
        Per meter based on Coenen et al. mL/m 13 131.2 ± 5.6 11 131.6 ± 5.7 0.87
        Per meter based on Eaton et al. mL/m 13 146.8 ± 5.8 11 147.0 ± 5.8 0.96
    Energy expenditure
        Coenen et al. (2011) equation kJ·kg−1·min−1 13 1.35 ± 0.05 11 1.37 ± 0.05 0.42
        Eaton et al. (1995) equation kJ·kg−1·min−1 13 1.51 ± 0.05 11 1.53 ± 0.05 0.41
        Per meter based on Coenen et al. kJ/m 13 2.64 ± 0.11 11 2.64 ± 0.12 0.87
        Per meter based on Eaton et al. kJ/m 13 2.95 ± 0.12 11 2.95 ± 0.12 0.97
Speed5
    Oxygen consumption
        Coenen et al. (2011) equation mL·kg−1·min−1 13 92.4 ± 3.9 9 96.4 ± 4.4 0.27
        Eaton et al. (1995) equation mL·kg−1·min−1 13 97.6 ± 3.3 9 101.1 ± 3.7 0.27
        Per meter based on Coenen et al. mL/m 13 133.3 ± 5.2 9 135.9 ± 5.3 0.24
        Per meter based on Eaton et al. mL/m 13 140.8 ± 4.4 9 142.6 ± 4.6 0.36
    Energy expenditure
        Coenen et al. (2011) equation kJ·kg−1·min−1 13 1.86 ± 0.08 9 1.94 ± 0.09 0.27
        Eaton et al. (1995) equation kJ·kg−1·min−1 13 1.96 ± 0.07 9 2.03 ± 0.07 0.27
        Per meter based on Coenen et al. kJ/m 13 2.68 ± 0.10 9 2.73 ± 0.11 0.23
        Per meter based on Eaton et al. kJ/m 13 2.83 ± 0.09 9 2.87 ± 0.09 0.36
1Speed3 ∼ 3.0 m/s, Speed4 ∼ 4.0 m/s and Speed5 ∼ 5.0 m/s
2Calculated oxygen consumption is based on heart rate recordings during the last minute of each phase (Speed3, Speed4, and Speed5) and the Coenen et al. (2011) and Eaton et al. (1995) equations. Energy expenditure is calculated from oxygen consumption (1 L O2 = 20.1 kJ; Coenen et al., 2011).
3Number of tests.

Hematocrit was greater after tölt than after trot at Speed3 (40% ± 1% vs. 39% ± 1%; P < 0.05) but did not differ between gaits at other time points (Fig. 1C). Hematocrit was back to resting level after 15-min recovery after trot and after 30-min recovery after tölt (Fig. 1C).

Rectal Temperature and Respiratory Rate

There was a tendency that rectal temperature was greater for trot than tölt both before and after the exercise test (before test: 37.6°C ± 0.1°C vs. 37.5°C ± 0.1°C, respectively; P = 0.0941; after test: 38.6°C ± 0.1°C vs. 38.5°C ± 0.1°C, respectively; P = 0.0528), but there was no difference after 30-min recovery (38.1°C ± 0.1°C and 38.1°C ± 0.1°C, respectively, P > 0.05). Rectal temperature increased from rest until after the tests and decreased during the 30-min recovery period (tölt: 37.5°C ± 0.1°C vs. 38.5°C ± 0.1°C vs. 38.1°C ± 0.1°C; P < 0.001; trot: 37.6°C ± 0.1°C vs. 38.6°C ± 0.1°C vs. 38.1°C ± 0.1°C; P < 0.001), but was still greater than before the tests (P < 0.001). The decrease in rectal temperature from the end of exercise and until 30-min recovery was smaller after tölt than trot (0.30°C vs. 0.44°C; P < 0.05).

Respiratory rate did not differ between tölt and trot (P > 0.05) after the test (70 ± 7 vs. 75 ± 7 breaths/min, respectively) and after 30-min recovery (29 ± 3 vs. 25 ± 3 breaths/min, respectively). Respiratory rate increased after the gait test compared with resting value (tölt: 70 ± 7 vs. 21 ± 2 breaths/min; P < 0.001; trot: 75 ± 7 vs. 22 ± 2 breaths/min; P < 0.001) and after 30-min recovery, it was still elevated for tölt (29 ± 3 vs. 21 ± 2 breaths/min; P < 0.05) but not for trot (25 ± 3 vs. 22 ± 2 breaths/min; P > 0.05).

Calculated Fitness Parameters

The parameters V2 and V4 did not differ between tölt and trot (V2: 4.4 ± 0.1 vs. 4.5 ± 0.1 m/s respectively; V4: 5.4 ± 0.1 vs. 5.4 ± 0.1 m/s, respectively). The HR2 values did not differ between tölt and trot and were 162 ± 4 and 162 ± 5 beats/min, respectively. Also, V140 and V180 did not differ between tölt and trot (V140: 3.4 ± 0.2 vs. 3.4 ± 0.2 m/s, respectively; V180: 5.2 ± 0.2 vs. 5.2 ± 0.2 m/s, respectively).


DISCUSSION

The results obtained here show no major differences in the physiological response to tölt and trot in trained Icelandic horses. The differences observed were so small for all parameters that if discussed singly with respect to methodology and physiological impact, they must be considered to be of little or no importance. For example, lactate levels at the lowest speed (Speed3) were actually not different from the resting values but still were significantly greater in tölt than in trot. However, the overall number of differences between gaits was too large to be ignored, and they all pointed in the same direction. Exercise hematocrit and lactate were greater in tölt at the slowest speed (Speed3), and recovery of hematocrit, rectal temperature, respiratory rate, and perhaps also plasma lactate was slower after tölt. This indicates that tölt might be slightly more demanding than trot, partly confirming our starting hypothesis.

One explanation for tölt being slightly more demanding (more anaerobic response) is the greater stride frequency observed, which means that more muscle contractions are performed at a greater frequency. This greater stride frequency in tölt compared with trot (at the same speed) confirms previous findings in another study on Icelandic horses (Robilliard et al., 2007). Those authors also observed a longer stance phase of the limbs in tölt compared with trot, which implies a considerably decreased swing phase, causing increased inertial forces to move the limb forward. In a study using a treadmill with an integrated force measuring system recording mean limb contact time, researchers at Zürich University (M. Weishaupt, personal communication) have estimated that slightly higher metabolic power (∼5.5%) is needed for tölt compared with trot at the same speed (∼3.4 m/s). Moreover, the head and neck carriage in balanced tölt is generally higher than in trot, even when the rider uses minimal aids, as in our study. Higher carriage of the head, neck, and withers of the horse increases the demand, alters muscle activation in the neck (Wijnberg et al., 2010), and shifts weight to the hind limbs (part of collection; Weishaupt et al., 2006, 2009). This also leads to greater demands because the musculoskeletal system of the hind limbs is not primarily constructed for weight bearing but for propulsion.

The total distance ridden in the exercise test was 3,126 m (including warm-up), which might be similar to a short daily exercise session for an adult Icelandic leisure horse. The slight differences in the physiological responses between the 2 gaits might have been more pronounced if the distance ridden had been longer. Interestingly, the differences between gaits observed during exercise were during the slowest speed (Speed3). This could indicate that tölt is more difficult and requires more energy expenditure than trot at slow speed but not at higher speeds. Further studies are needed to compare the physiological responses to tölt and trot in adult trained horses when ridden at different speeds and for a considerably longer distance than in this study.

The work performed by the horses in our study was mainly in the HR range of 120 to 170 beats/min, which indicates that the horses were mostly working aerobically (Marlin and Nankervis, 2002) except at the fastest speed (Speed5), at which the average HR was >180 beats/min and increased anaerobic metabolism might be expected. However, although we observed small but significant differences between gaits in other parameters, HR was not affected by gait. The presumed greater (anaerobic) workload in tölt and the subsequent increased need for oxygen might therefore have been supported by an increase only in hematocrit.

The plasma lactate concentration data confirmed that the work performed was aerobic at the 2 slower speeds, as the horses did not reach the plasma lactate concentration of 4 mmol/L (V4) until after the fastest speed (Speed5) had been performed. To the best of our knowledge, this is the first comparison of V4 in tölt and in trot to be published. However, it should be noted that in 11 tests out of 31, the plasma lactate concentration did not reach 4 mmol/L even at the fastest speed (Speed5), indicating individual differences in the ability to exercise below 4 mmol/L. There was also an effect of rider in that the horses had greater V4 and reached V4 fewer times (7 compared with 11) when ridden by the lighter rider. Horses also showed significantly greater HR, plasma lactate, respiratory rate, and rectal temperature responses with the heavier rider (data not shown). It is not known whether this was an effect of weight, riding style, or both. However, it has been shown previously (Stefánsdóttir et al., 2014) that the BW ratio between rider and horse affects plasma lactate concentration after exercise in Icelandic horses, with a 0.4 mmol/L increase for every 1% increase in BW ratio.

It is important to consider that the group of horses used in this study consisted of experienced (age: 15.3 ± 1.6 yr) and highly trained school horses. Even though the Icelandic horse is a 4- or 5-gaited horse, it has to be trained to tölt in a regular and clear 4-beat rhythm while carrying a rider (Feldmann and Rostock, 1990). It is possible that more inexperienced horses would have shown greater differences in physiological responses between the gaits, in favor of trot, and also more instability (e.g., unclear beat and switching between gaits). Further studies are needed to compare physiological responses to tölt and trot in different groups of Icelandic horses on the basis of age, genetic and training background, and fitness level. In addition, the responses might be affected by individual factors, for example, conformation, stride length, natural gait ability, and balance.

It is known that ponies change gait according to speed and select speed within a gait to optimize their energy expenditure (Hoyt and Taylor, 1981). It has not been studied how speed in tölt affects the energy expenditure of the Icelandic horse. However, on the basis of the linear relationship between speed and HR observed in our study, it is likely that speed has an effect on energy expenditure in tölt similar to that in trot, although there were indications of small differences between the 2 gaits at the slowest speed. The linear relationship observed here between speed and HR confirms previous observations in horses (Lindholm and Saltin, 1974; Persson, 1983; Kobayashi et al., 1999), but to the best of our knowledge, a relationship between speed and HR in tölt has not been reported previously.

There were no differences in calculated energy expenditure between the gaits in our study, a not unexpected finding because there were no differences in HR. Following recommendations from NRC (2007) on the applicability of the Coenen et al. (2011) and Eaton et al. (1995) equations, the latter, which gave significantly greater values for oxygen consumption at all speeds in our study, might give more reasonable values at higher HR (e.g., Speed5 in our study). Using both equations, the energy expenditure at the fastest speed was approximately twice as high (significant, Student’s t test) as that at the slowest speed when estimated in kilojoules per kilogram per minute. However, when energy expenditure was estimated in kilojoules per meter, the difference was only significant when using the equation of Coenen et al. (2011). This indicates that the Eaton equation does not produce reliable results for the entire speed range tested here but might still be functional at the higher speeds (i.e., HR), as suggested by NRC (2007).

In our study, the HR of all horses was lowest at Speed3 in both gaits, and this speed was similar to the one Hoyt and Taylor (1981) found to be most energetically optimal in trot for smaller ponies (110 to 170 kg) without a rider. It has also been argued that the ideal range of speed of the marcha gaits in ridden Brazilian horses (BW ∼378 ± 8 kg) is 3.0 to 4.0 m/s because this speed allows a good rhythm and stability in the gaits (Wanderley et al., 2010). In the Icelandic horse, according to the breeding standards, it is desirable to have a wide range of speed at a clear 4-beat tölt (FEIF, 2002), ranging from a slow speed (∼2 to 4 m/s) to a fast speed (∼7 to 10 m/s), and speed is 1 of the major factors affecting the score for tölt in a breed evaluation field test (FEIF, 2002) and competitions (FEIF, 2014). However, there is no precise and objective definition of slow-, medium-, and fast-speed tölt for the Icelandic horse. Instead, speed is based on subjective evaluation of the riders and judges.

From an academic and theoretical point of view, a comparison of the physiological response to 2 gaits should preferably be done without a rider using an indoor treadmill for which all conditions could be standardized as much as possible. However, horses do not readily perform tölt without a rider, and the response to treadmill exercise cannot be directly transferred to practical conditions. Field tests have been suggested to be an advantage in sport horses, especially when technical skills are required (Sloet van Oldruitenborgh-Oosterbaan and Clayton, 1999; Munsters et al., 2014). We wanted this study to be of relevance in practice, and therefore, riders and a competition track were used. The use of 2 riders limited the likelihood of the results being influenced by the rider rather than the exercise and also ensured that the speed was the same for the 2 gaits. The test was performed on an even and firm oval track (accepted for international gait competitions). It is likely that a more uneven and/or heavier track would have altered physiological response to the gaits. This suggestion is also based on the experience of trainers, who indicated that most horses tölt more easily when the track is even and firm (preferably with elasticity) than when it is uneven and soft/heavy.

In conclusion, there were only minor differences in physiological responses to tölt and trot in this selected group of experienced adult Icelandic horses, and the biological and practical significance of the slightly elevated physiological responses to tölt and the slower recovery remains to be determined. Further studies are needed to evaluate the effect on gait responses of speed, age, training background, fitness level, different head and neck positions, conformation, rider technique, rider-horse weight ratio, and environmental conditions such as track conditions and distance covered.

 

References

Footnotes


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