It is widely accepted that warm-up exercise should be performed before the main bout of activity during athletic competition. The benefit of such practice is that the warm-up exercise may improve performance, although there is limited evidence as to the mechanisms causing these effects.1,2 How warm-up exercise should be structured in terms of its intensity, duration, and mode and the recovery time between the warm-up and performance has not been systematically and objectively evaluated, particularly for horses. Several studies in humans3,4 and horses5,6 have revealed that warm-up accelerates kinetics of 
O2 during subsequent intense exercise. However, the mechanisms causing this acceleration have not been clearly delineated. It has been proposed that increases in muscle blood flow and temperature following a prior exercise bout enhance oxygen delivery and use in working muscle.3,7
Although the aerobic contribution to total metabolic power during supramaximal exercise in human athletes has been estimated to be < 50%,8 Eaton et al9 reported that the aerobic contribution to total energy expenditure in Thoroughbreds was > 80% for 120-second sprint exercise. This estimate suggests that horses quickly produce high aerobic power and have rapid kinetics of respiratory gas exchange during exercise, compared with other larger mammalian species that have been studied, despite the allometric tendency for physiologic events to take longer to occur in larger animals.10 Thus, increasing O2 kinetics with warm-up exercise would be a substantial advantage during supramaximal exercise in horses, and horses might be good experimental models in which to determine the mechanisms by which these effects take place.
Previous studies5,6,11,12 that have examined the effects of warm-up exercise in horses used warm-up durations of ≥ 5 minutes. However, warm-up routines usually performed in conjunction with Thoroughbred races are different, and the duration of warm-up prior to racing in Japan is usually much shorter (≤ 1 minute). The duration of warm-up exercise is an important component when considering its effects on metabolism and, potentially, on performance in subsequent highintensity sprint exercise.
We hypothesized that altering the intensity, duration, or presprint interval of warm-up exercise would alter the kinetics of aerobic power, the contribution of aerobic power to total metabolic power, and the peak aerobic power attained in subsequent supramaximal exercise. Thus, the aim of this study was to compare the effects of different warm-up exercise protocols on aerobic metabolism in Thoroughbreds. The duration of prior exercise used in this study was shorter than that of previous studies and was consistent with actual warmup protocols used in Thoroughbred races in Japan.
Materials and Methods
Protocols for the study were reviewed and approved by the Animal Welfare and Ethics Committee of the Japan Racing Association Equine Research Institute where the study was conducted.
Horses—Eleven Thoroughbreds (3 males, 2 geldings, and 6 females; mean ± SD age, 5.3 ± 2.6 years) were used in this study. The horses underwent surgery to move a carotid artery from the carotid sheath to a subcutaneous location to facilitate arterial catheterization. After recovery from surgery, the horses were trained to run on a motorized treadmilla while wearing an open-flow mask.13 At least 1 year passed between the surgery and treadmill experiments. Horses were exercised 5 d/wk on a treadmill inclined at a 10% grade and were pastured in 2-hectare pastures for approximately 6 h/d on the other 2 days for 14 weeks before the onset of the study. The training program consisted of a warmup (1.7 m/s for 1 minute and 3.5 m/s for 3 minutes), cantering for 5 minutes, and a cool-down (1.7 m/s for 3 minutes). The speed of cantering increased as the training progressed (weeks 1 to 7: 6 m/s for 5 minutes; week 8: 80% O2max of each horse for 5 minutes; weeks 9 to 10: 80% O2max for 3 minutes and 100% O2max for 2 minutes; weeks 11 to 14: 80% O2max for 3 minutes and 100% O2max for 2 minutes on 2 d/wk and 60% O2max for 1 minute, 80% O2max for 1 minute, and 100% O2max for 1 minute on 3 d/wk).
Treadmill measurements—Following the carotid loop surgery and familiarization with the treadmill, horses performed an incremental step protocol to identify each horse's O2max and speed required to attain it (speed at 100% O2max). During this procedure, the horses wore an open-flow mask for measurement of O2 and CO2 and a jugular catheter was used to draw venous blood samples for determination of [La] and 
lactate.
Approximately 1 week following the step protocol, warm-up exercise studies were performed at weekly intervals to quantify O2 transport variables at speeds calculated to elicit approximately 115% O2max for each horse. During these studies, blood samples were drawn, and O2 and CO2 were measured before the run began and at 30-second intervals during the run, allowing the O2 transport system to come to steady state by the end of the first minute during both the warm-up run and then the sustained sprint. After catheters and transducers were connected and tested, the horse began its exercise. The horse would exercise at its warm-up speed or sprint speed on the treadmill while O2 was measured and arterial and mixed-venous blood samples were drawn simultaneously for the measurement of blood gases and [O2], from which 
and SV were calculated by use of the Fick Principle. Heart rate was determined from ECG recordings via bipolar electrodes across the long axis of the heart, which were amplifiedb and recorded on a personal computer by use of commercial hardware and softwarec sampling at 200 Hz. Following a run, the R-R intervals were analyzed during 15 seconds to calculate the mean HR at that sampling time.
Experimental design—The effects of warm-up on oxygen uptake and transport during exercise were examined in a 3-way randomized crossover study. Eleven horses participated in a preliminary measurement and each of 3 experimental treatment trials with protocols as follows: 1) preliminary incremental measurement run in which the horse warmed-up by walking at 1.7 m/s for 2 minutes and trotting at 3.5 m/s for 5 minutes, then cantering or galloping up a 10% incline for 1 minute each at 6, 8, 9, 10, 11, and 12 m/s until the horse could not maintain its position at the front of the treadmill; 2) NoWU trial (control trial) in which the horse walked on the treadmill for 1 minute at 1.7 m/s 10 minutes prior to the sprint; then for the measurements, the horse was galloped up a 10% incline at a speed calculated to elicit 115% O2max (based on the preliminary incremental measurement) until the horse could not maintain its position at the front of the treadmill; 3) MoWU trial in which the horse walked on the treadmill for 1 minute at 1.7 m/s, then cantered for 1 minute at a speed equivalent to 70% O2max (based on the preliminary incremental measurement) 10 minutes prior to the sprint to warm up; then for the measurements, the horse was galloped up a 10% incline at a speed calculated to elicit 115% O2max (based on the preliminary incremental measurement) until the horse could not maintain its position at the front of the treadmill; and 4) HiWU trial in which the horse walked on the treadmill for 1 minute at 1.7 m/s, then galloped for 1 minute at a speed equivalent to 115% O2max (based on the preliminary incremental measurement) 10 minutes before the sprint to warm up; then for the measurements, the horse was galloped up a 10% incline at a speed calculated to elicit 115% O2max (based on the preliminary incremental measurement) until the horse could not maintain its position at the front of the treadmill. After performing the initial incremental performance measurement, each horse completed each of the experimental protocols once in random order with ≥ 6 days between trials for individual horses.
Oxygen consumption—Horses wore a 25-cm diameter open-flow mask on the treadmill through which a rheostat-controlled 3.8-kW blower drew air. Air flowed through 20-cm diameter tubing and across a 25-cm-diameter pneumotachographd connected to a differential pressure transducerd; this was used to ensure that the bias flows during measurements were identical to those used during calibrations. Bias flow was set to keep changes in [O2] and CO2 concentration < 1%. Oxygen consumption and CO2 were measured by use of an O2 and CO2 analyzer,e and gas calibration was carried out by use of the N2-dilution/CO2-addition mass-balance technique.14 Gas analyzer and mass flowmeter outputs were also recorded on personal computers.
Blood sampling—Before leading a horse onto the treadmill, an 18-gauge × 6.4-cm catheterf was placed in the horse's carotid artery and an 8.5-F × 9-cm introducerg in the jugular vein. A Swan-Ganz catheterh was passed via the jugular vein so that its tip was positioned in the pulmonary artery, confirmed by measuring pressure at its tip with a pressure transducer.i Mixed-venous blood samples were drawn from the tip of the SwanGanz catheter and arterial samples from the 18-gauge carotid catheter at timed intervals into heparinized syringes and stored on ice until measurements were made following the experiment. Blood samples were analyzed for blood gases by use a blood gas analyzerj and for SaO2, [O2], and [Hb] with a hemoximeter.j Accuracy of the blood gas analyzer was verified with blood samples tonometered with precision gas mixtures, and accuracy of the hemoximeter set to its equine blood algorithm was verified by comparing tonometered samples with direct measurements of [O2] made with a galvanic cell.k Following measurement of blood gases and oximetry, the blood was sampled for PCV determination by use of microcentrifugation and for [La] by use of a lactate analyzer.l The lactate was calculated as the change in [La] per minute of sprint exercise.
Temperature measurements—The Swan-Ganz catheter in the pulmonary artery was connected to a computerg so that its thermistor registered TPA, which was recorded at each blood sampling and used to correct the blood gas measurements. Blood temperatures detected by use of the thermistor were corrected for systematic bias by calibrating the catheter thermistor immediately following each run in a water bath via an Hg thermometer, with calibration traceable to the Japan Bureau of Standards.
Statistical analysis—Data are presented as mean ± SD. Data were analyzed for differences between warmup regimens and time within a regimen by use of 2-way repeated-measures ANOVA, and pairwise comparisons were made by use of the Tukey test. Values reported as fatigue were the last sample taken before a horse stopped running for a given protocol. Statistical analyses were made with commercial software,m and significance was defined as P ≤ 0.05.
Results
Preliminary exercise test—All horses completed the initial incremental step procedure without difficulty. Mean specific O2max and speed to achieve 100% O2max of the horses were 3.00 ± 0.10 mL/(s kg) and 10.7 ± 0.3 m/s (up a 10% incline), respectively.
Exercise test and time—Mean work intensity during the high-speed runs was 113.1 ± 1.7% O2max. Two horses failed to complete 90 seconds of exercise in NoWU; therefore, the values at 90 seconds for NoWU group are the mean ± SD for 9 horses. Mean run time to fatigue was significantly greater in MoWU than in NoWU; there was no difference in time to fatigue among the 3 protocols (statistical power, 0.95).
Blood temperature—Warm-up exercise resulted in a significant increase in TPA. Ten minutes after completion of the warm-up, TPA in MoWU and HiWU was 0.68 ± 0.31°C and 1.20 ± 0.31°C higher, respectively, compared with that of NoWU (Figure 1). These between-treatment differences were maintained during the sprint.
Mean (± SD) TPA of 11 horses before (pre) and after (post) warm-up protocols and during a sprint at a speed eliciting 115% O2max Horses received NoWU (triangles), MoWU (squares), or HiWU (circles). Horizontal error bars indicate SD of the run time to fatigue.*Significant (P ≤ 0.05) difference between NoWU and MoWU. *Significant (P ≤ 0.05) difference between MoWU and HiWU. §Significant (P ≤ 0.05) difference between NoWU and HiWU.
Citation: American Journal of Veterinary Research 69, 5; 10.2460/ajvr.69.5.690
Gas exchange measurements—The O2 was significantly higher in HiWU and MoWU than in NoWU throughout the sprint exercise (Figure 2). The CO2 was significantly higher in NoWU and MoWU than in HiWU during the initial 30 seconds of the sprint exercise; however, the peak values for CO2 were significantly higher in MoWU than in NoWU and HiWU. The RQ was significantly higher in NoWU and MoWU than in HiWU during the initial 30 seconds of the sprint (Figure 2).
Mean ± SD values for respiratory gas exchange variables (A, specific O2 consumption [O2/kg]; B, specific CO2 production [CO2/kg]; and C, RQ) and [La] (D) measured during warm-up and sprint exercise in the same horses as in Figure 1. *Significant difference (P ≤ 0.05) between NoWU and MoWU. †Significant difference (P ≤ 0.05) between MoWU and HiWU. §Significant difference (P ≤ 0.05) between NoWU and HiWU. STPD = Standard temperature and pressure, dry. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 69, 5; 10.2460/ajvr.69.5.690
[La]—During the first 90 seconds of exercise, [La] was significantly higher in HiWU than in MoWU and NoWU (Figure 2). After 90 seconds of sprinting, [La] was significantly higher in NoWU than in MoWU. The lactate was significantly higher in NoWU than in MoWU and HiWU, and lactate in MoWU was also significantly higher than in HiWU.
Blood gas measurements and circulatory oxygen transport—Horses hyperventilated more, or hypoventilated less, at all times in HiWU than in NoWU or MoWU, but still were often more acidotic in HiWU (Figure 3). Arterial hypoxemia was more pronounced in HiWU than in NoWU after 60 seconds of sprinting and at fatigue; HiWU horses had lower SaO2 than in NoWU at all times and MoWU at all times except fatigue. The PCV and [Hb] were higher at the start of the sprint in MoWU and HiWU than in NoWU, but those differences disappeared by 60 seconds of sprinting. The [O2]a was significantly higher in MoWU and HiWU than in NoWU at the start of the sprint but was similar by the time the horses became fatigued. The PvO2 and SvO2 were typically lower at all times in MoWU and HiWU than in NoWU. Specific (/kg) was lower in MoWU and HiWU than in NoWU at the start of the sprint, but after 60 seconds of sprinting, it was not significantly different among the 3 trials (statistical power, 0.32). Because of technical problems with the ECG instrumentation, HR was recorded for only 8 horses. Nevertheless, HR was significantly higher at all intervals in MoWU and HiWU than in NoWU. Both MoWU and HiWU horses increased SV by approximately 25% within 60 seconds of the start of the sprint and maintained the larger SV during their runs.
Mean ± SD values for arterial (a) and mixed-venous (v) blood gas measurements (A, PaO2; B, PvO2; C, PaCO2; D, mixed-venous carbon dioxide partial pressure [PvCO2]; E, [O2]a; F, mixed-venous oxygen concentration; G, SaO2; H, SvO2; and I, [Hb]) during warm-up and sprint exercise in the same horses as in Figure 1. White triangle = NoWU (arterial). White square = MoWU (arterial). White circle = HiWU (arterial). Black triangle = NoWU (mixed venous). Black square = MoWU (mixed venous). Black circle = HiWU (mixed venous). See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 69, 5; 10.2460/ajvr.69.5.690
Discussion
This study revealed that following warm-up exercise, horses used a greater proportion of aerobic power during subsequent sprint exercise. The contribution of greater aerobic energy during the sprint in MoWU and HiWU was evident from the higher peak O2. In addition, the lower lactate (MoWU and HiWU) and lower RQ (HiWU) in the sprint indicated reduced net anaerobic power during the sprint. Higher aerobic contribution to total energy expenditure in a sprint following a warm-up has been reported in horses,5,6 although with different warm-up protocols. The kinetics of O2 at the onset of high-intensity exercise in horses are faster than in human subjects,15 despite their larger size and allometrically expected slower physiologic time scale, and those kinetics are further accelerated by warm-up exerxise.5,6 The more rapid gas-uptake kinetics in horses may partly be a function of splenic release of erythrocytes at the onset of the exercise.15,16 The significantly higher PCV at the beginning of the sprint in MoWU and HiWU was consistent with the higher peak O2 in these trials.
Increased O2 kinetics during sprint exercise after a warm-up are thought to result from metabolic changes induced by the prior exercise.3 Because the horses had higher TPA after MoWU and HiWU, it is likely that their skeletal muscle temperatures were even higher.17 The measured differences in TPA between the treatments would be predicted to increase metabolic rates from NoWU by 5.8% for MoWU and 10.6% for HiWU, assuming a 10 of 2.3.18 The observed increases in O2 at 60 seconds of 4.9% and 6.9% are close to these estimates.
We did not detect a significant influence of warmup on , other than increased rates in MoWU and HiWU at the start of the sprint. Nevertheless, at every time interval, horses with more intense warm-up (and higher TPA) had higher HR than those with lower warm-up intensity and TPA, and HR progressively increased with duration of the sprint. Maximum HR with NoWU was lower at fatigue (when the highest values were measured) than with MoWU and HiWU because HR with NoWU was the lowest of the 3 values for every horse. Lund et al12 also determined that the peak HR during supramaximal exercise at 105% O2max with a warm-up was higher than without a warm-up (208 ± 2 beats/min vs 194 ± 20 beats/min, respectively), suggesting the possibility of a higher aerobic contribution to total energy expenditure. The SV increased by approximately 25% with the onset of the sprint in horses that had warmed-up, but was not systematically different among protocols. This finding is consistent with measurements of SV made by use of sonomicrometers affixed to the left ventricle by Hiraga et al.19
In the present study, [O2]a-v was typically higher with warm-up than without warm-up. This occurred primarily because of the typical finding of decreased PvO2, PvO2, and SaO2. These results should be interpreted with caution because the 2 horses that became fatigued before running for 90 seconds in the NoWU group had markedly higher PvO2 and SaO2 values than when they ran with warm-up, increasing the means and SD of those NoWU data. Nevertheless, enhanced extraction of O2 following warm-up could result from increased perfusion of capillary beds in muscle, or could be related to the higher temperature of the muscles following warm-up. Increased [La], decreased muscle pH, and increased potassium ion concentration have all been reported to cause vasodilation and increase muscle blood flow.3,11,20–22 Increases in hydrogen ion concentration, PCO2, and 2,3-diphosphoglycerate in response to warm-up could also increase O2 delivery to the muscles by decreasing the O2 affinity of hemoglobin.21
Although HiWU exercise consistently results in acceleration of aerobic metabolism during subsequent heavy exercise in humans, LiWU has little effect on the kinetics of respiratory gas exchange.3,22 In contrast to these findings in human subjects, there has been no evidence that HiWU exercise is needed in horses to increase kinetics of O2. Tyler et al6 reported that even a low-intensity warm-up (50% of O2max) enhances the aerobic contribution to total energy expenditure in Standardbreds. Furthermore, McCutcheon et al11 reported that an HiWU did not provide any additional advantage during a subsequent sprint. In the present study, however, O2 at the end of the first 30 seconds of the sprint was significantly higher in HiWU than in MoWU, and RQ and lactate were significantly lower in HiWU than in MoWU. These results suggest that HiWU exercise can increase the onset of and peak value of aerobic power. A mechanism that might explain this observation is Bishop's23 suggestion that warm-up improves performance if it allows the athlete to commence the task with an increased O2 but otherwise recover from the warm-up. The intensity of the warm-up protocol in HiWU in the present study was high enough to increase baseline O2 and TPA, compared with MoWU and NoWU. However, we did not test the effects of varying the recovery period between the cessation of warm-up and the onset of sprint exercise, a variable that likely would affect the metabolic response during the sprint.
The decreased rates of lactate during the sprint after MoWU and HiWU might reflect decreased rates of lactate production because of the higher peak O2; however, decreased [La] could also result from changes in the rate of lactate transport2 or increased rates of lactate uptake and oxidation. Increased muscle blood flow after warm-up exercise could increase the rate of lactate release from working muscle and increase perfusion of fibers with greater LDH-isozyme capacity for lactate oxidation. During intense exercise of 2to 3-minute duration, lactate efflux from working muscles has been estimated to be 25% to 35% of the total lactate production in humans.24
The run time to fatigue with MoWU was significantly longer than with NoWU. This result was consistent with higher peak O2, lower lactate, and higher PCV during the sprint exercise. Despite HiWU having similar advantages in terms of aerobic metabolic effects as with MoWU, the run time to fatigue following HiWU was not significantly longer than that with NoWU. This observation suggests that it may be critical to structure warm-up exercise to accelerate O2 kinetics and thus improve performance without amplifying factors that lead to fatigue. Performance may be impaired if the warm-up protocol is too intense or does not allow sufficient recovery time before commencing the sprint exercise so that the athlete begins the supramaximal effort closer to the fatigue threshold. Some studies suggest that a warm-up intensity of approximately 70% O2max is likely to be optimal for intermediate (10-second to 5-minute) performance in human athletes.25,26 If warmup intensity is too high, the subsequent metabolic acidemia impairs supramaximal performance and reduces the accumulated oxygen deficit.25 These events might be related to accumulation of hydrogen ions,27 decreased high-energy phosphates, or high body temperature.23,28 Results of the present study indicated higher peak TPA after HiWU than peak TPA following MoWU or NoWU and a lower arterial blood pH at fatigue following HiWU than the arterial blood pH in NoWU. These results are the opposite of what would be expected from previous studies on fatigue factors and prior exercise intensity. The fact that time to fatigue with NoWU was significantly shorter than that following 
oWU, but not following HiWU, suggests that the residual effects of HiWU (115% VO2max) with this protocol may be too intense to maximize subsequent sprint performance.
Warm-up exercise of 1-minute duration resulted in higher peak O2 and decreased lactate during subsequent sprint exercise in Thoroughbreds. Furthermore, an HiWU reduced RQ and increased [O2]a-v, contributing to the increased O2 uptake by the working muscle and the acceleration of O2 kinetics. These results differ from previous studies5,11 that involved use of longer warm-up protocols and that revealed no additional benefit derived from HiWU, compared with a warm-up at 50% O2max.
ABBREVIATIONS
| O2 | Oxygen consumption rate |
| O2max | Maximal oxygen consumption rate |
| CO2 | Carbon dioxide production rate |
| [LA] | Blood lactate concentration |
| lacture | Blood lactate accumulation rate |
| [O2] | Oxygen concentration |
| Cardiac output |
| SV | Stroke volume |
| HR | Heart rate |
| NoWU | No warm-up |
| MoWU | Moderate-intensity warm-up (1 minute at 70% maximal oxygen consumption rate) . |
| HiWU | High-intensity warm-up (1 minute at 115% maximal oxygen consumption rate) |
| Sao2 | Arterial oxygen saturation |
| [Hb] | Hemoglobin concentration |
| TPA | Pulmonary arterial temperature |
| RQ | Respiratory quotient |
| [O2]a | Arterial oxygen concentration |
| Pvo2 | Mixed-venous oxygen partial pressure |
| Svo2 | Mixed-venous oxygen saturation |
| [O2]a-v | Arterial mixed-venous oxygen concentration difference |
| pHa | Arterial pH |
Mustang, Kagra AG, Fahrwangen, Switzerland.
SM-29, Fukuda Denshi, Tokyo, Japan.
DATAQ, Akron, Ohio.
G. N. Sensor, LF-150B and TF-105, VISE Medical, Chiba, Japan.
METS-900, VISE Medical, Chiba, Japan.
Surflow, Telmo, Tokyo, Japan.
MO95H-8.5, COM-2 cardiac output computer, Baxter, Tokyo, Japan.
Viggo-Spectramed, Tokyo, Japan.
Statham P23d, Ohmeda, Madison, Wis.
ABL-505 and OSM3, Radiometer, Copenhagen, Denmark.
Lex-O2-Con K, Lexington Instruments, Waltham, Mass.
YSI 2300 STAT Plus, Yellow Springs Instruments, Yellow Springs, Ohio.
JMP, version 5.0.1a, SAS Institute Inc, Cary, NC.
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