Effect of restraint inside the transport vehicle on heart rate and heart rate variability in Thoroughbred horses.

This study aimed to investigate the effect of transportation and restraint in a van on heart rate (HR) and HR variability in Thoroughbreds. Eight healthy Thoroughbreds were exposed to four conditions, each for a duration of 30 min: stall rest (REST), restraint inside a van (VAN), restraint inside a van with the engine running (V + E), and road transportation (TRANS). Electrocardiograms were recorded to determine HR, low-frequency (LF) power, high-frequency (HF) power, and LF/HF ratio. During TRANS, HR was significantly greater than during REST and V + E. There was a significant increase during VAN compared with REST. These results demonstrated that restraint inside the transport vehicle was one of the major stressors that may cause physiological changes during transportation.

Relationship between outcome in acute stroke patients and multiple stroke related scores obtained after onset of stroke.

[Purpose] The purpose of this study is to examine the effectiveness of a stroke-related scale with regard to outcome, onset, and timing of stroke patients. [Participants and Methods] The participants included 583 out of 996 patients who were admitted to the stroke care unit.The outcomes and 3 stroke scale (National Institutes of Health Stroke Scale: NIHSS, Functional Independence Measure: FIM, modified Rankin Scale: mRS) scores immediately at hospitalization, on day 7 after onset, and on day 30 after onset were investigated. This study was analyzed using a generalization linear model with a binomial distribution. The comparisons between outcomes were made in terms of home discharge versus convalescence, and convalescence versus hospital transfer. [Results] Comparisons of home discharge versus convalescence hospital transfer showed a significant difference in the NIHSS and mRS scores at the time of hospitalization, and a significant difference in the NIHSS scale score on day 7 after onset. In comparisons between convalescence and hospital transfer, significant differences were observed in NIHSS and FIM scores at hospitalization, and the FIM scale score showed significant differences on day 7 and day 30. [Conclusion] The study suggested the efficacy of using multiple scales for prediction of stroke outcome with higher accuracy.

Studies on the exercise physiology of draft horses performed in Japan during the 1950s and 1960s.

Although the total number of horses raised in Japan dramatically decreased after World War II, because draft horses were still used for farm work in paddy fields and on farms during the period of the 1950s and 1960s, a performance test for selecting better draft horses was needed. In order to determine the most suitable size of draft horses for Japanese farm conditions, the working power of horses weighing from 185 to 622 kg was evaluated by performing an endurance test, several kinds of working power tests, and maximum pulling power tests. Oxygen consumption during draft exercise was measured by the Douglas bag method in order to evaluate effects of draft workload under the conditions of different types of work (14- and 18-cm plow depths, cultivator, and tillage), traction methods (shoulder traction, shoulder-trunk traction, and chest-trunk traction), walking speeds (40, 60, 80, 100, and 120 m/min), and depths of water (0, 18, 36, and 54 cm) on energy expenditure. The relationship between energy consumption and pulse rate during exercise was also evaluated. A study of a performance test for draft horses was conducted to establish a new approach for evaluating draft horse performance using heart rate as an index. For this study, a beat meter for measuring heart rate was developed, and experimental protocols were used to evaluate the relationship between heart rate and workload. Although the research results obtained from these studies do not have particular relevance in the current day, these studies are valuable for understanding the history of equine exercise physiology in Japan.

Studies on exercise physiology and performance testing of racehorses performed in Japan during the 1930s using recovery rate as an index.

The history of research on the exercise physiology of racehorses in Japan dates back to the 1930s. A research report entitled "Studies on exercise physiology and performance testing of the racehorse", published in 1933 by Shigeo Matsuba and Torao Shimamura of The University of Tokyo, was epoch-making and the most important study in the history of equine exercise physiology in Japan. Research results were reported from 92 Thoroughbred racehorses in a large-scale project during the period of 1928 to 1932 at the Shimofusa Imperial Farm and the Koiwai Farm, which were the two greatest racehorse farms at that time. A total of 20 physiological variables were measured to evaluate the fitness of Thoroughbred racehorses before exercise (Pre), just after exercise (Post), 1 hr after exercise (1 hr), 2 hr after exercise (2 hr), and 3 hr after exercise (3 hr) in order to calculate their recovery rates as an index of fitness and performance. The percentage of the Pre value at 1 hr, 2 hr, and 3 hr was calculated. When the percentage of a variable reached 95-105% of the Pre value, the variable was considered to be recovered. The percentage of the total number of variables that were recovered for each time period was calculated, and an overall average was calculated from them; Matsuba and Shimamura proposed calling this overall average the "recovery rate", which could then be applied to evaluate each horse. The effects of training on racehorses were subsequently evaluated by measuring the various physiological variables and the recovery rate.

Studies on exercise physiology of the racehorse performed in Japan during the period from the 1930s to the 1970s: respiration and heart rate during exercise and the effect of exercise on blood characteristics.

After publication of the epic report on equine exercise physiology by Matsuba and Shimamura in 1933, papers on exercise physiology of the racehorse in Japan began appearing in scientific journals and increased in number. In 1944, respiration during exercise at a walk, trot, and canter was measured by recording expiratory sounds with a microphone attached near the nostril. Respiratory frequency during cantering was synchronized with stride frequency, and expiratory sounds were found to occur during the stance phase of the trailing forelimb. Development of a radiotelemetry system in 1964 for electrocardiogram recording enabled the first recording of an equine electrocardiogram during field exercise that included fast galloping and calculation of heart rate (HR) during exercise. During low intensity exercise including walking, trotting, cantering and extended cantering, HR increased from 45 beat/min during pre-exercise to 150 beat/min at an extended canter. HR increased to 200 beat/min or more in most horses during 100 m of high-intensity sprint galloping. When blood lactate was measured after 3 days of draft work in 12 warhorses in 1934, no increase in blood lactate was found. The erythrocyte sedimentation rate (ESR) was decreased by intense exercise and also decreased as training increased. It was suggested that measuring changes in ESR and body weight in relation to training might become useful as a screening index of training, condition, and fatigue. This evaluation method was named the "ESR-body weight method."

History of research in Japan on electrocardiography in the racehorse.

Since the first recording of electrocardiograms (ECGs) of a horse in Japan was carried out in 1944, studies on ECGs have been performed intensively. During the early stages of research from the 1950s to 1960s, leads to use for ECG recording were evaluated using several different approaches including unipolar leads, bipolar limb leads, and bipolar chest leads. Based on these studies, the AB lead, which is oriented along the long axis of the heart, became the standard reference method in Japan. Electrodes of the AB lead are placed on the upper 1/4th point along a straight line between the withers and the left shoulder blade (base: B), and 10 cm posterior to the left olecranon (apex: A). The incidence of equine arrhythmias among racehorses has been surveyed, and details of the electrocardiographic characteristics of several arrhythmias have been investigated. In particular, atrial fibrillation (AF) has been extensively studied, and papers have reported findings such as that paroxysmal AF occurs during racing and described electrocardiographic changes that occur at the onset of AF during exercise. Development of a radiotelemetry system for ECG recording enabled the first recording of equine ECGs during galloping in 1964, the detection of arrhythmias, and calculation of heart rate during exercise. Studies on comparative and developmental changes of ECGs have described characteristics of the equine ECGs. Future research on changes in cardiac function, including autonomic function, that occur with aging may lead to new developments in equine electrocardiography and contribute to improving the health and welfare of the horse.

Effects of aluminum hinged shoes on the structure of contracted feet in Thoroughbred yearlings.

We applied aluminum hinged shoes (AHSs) to the club foot-associated contracted feet of 11 Thoroughbred yearlings to examine the effects of the shoes on the shape of the hoof and third phalanx (P III). After 3 months of AHS use, the size of the affected hooves increased significantly, reaching the approximate size of the healthy contralateral hooves with respect to the maximum lateral width of the foot, the mean ratio of the bearing border width to the coronary band width, and the mean ratio of the solar surface width to the articular surface width. These results suggest that the AHSs corrected the contracted feet in these yearling horses.

The Effects of Inclination (Up and Down) of the Treadmill on the Electromyogram Activities of the Forelimb and Hind limb Muscles at a Walk and a Trot in Thoroughbred Horses.

It is important to know the effects of the inclination of a slope on the activity of each muscle, because training by running on a sloped track is commonly used for Thoroughbred racehorses. The effects of incline (from -6 to +6%) on the forelimbs and hind limbs during walking and trotting on a treadmill were evaluated by an integrated electromyogram (iEMG). The muscle activities in the forelimbs (5 horses) and hind limbs (4 horses) were measured separately. Two stainless steel wires were inserted into each of the brachiocephalicus (Bc), biceps brachii (BB), splenius (Sp), and pectoralis descendens (PD) in the forelimb experiment and into the longissimus dorsi (LD), vastus lateralis (VL), gluteus medius (GM), and biceps femoris (BF) in the hind limb experiment. The EMG recordings were taken at a sampling rate of 1,000 Hz. At a walk, the iEMG values for the forelimb were not significantly different under any of the inclinations. In the hind limb, the iEMG values for the GM and BF significantly decreased as the inclination decreased. At a trot, the iEMG values for the Bc in the forelimb significantly decreased as the inclination of the treadmill decreased. In the hind limb, the iEMG values for the LD, GM, and BF significantly decreased as the inclination decreased. Uphill exercise increased the iEMG values for the Bc, LD, GM, and BF, while downhill exercise resulted in little increase in the iEMG values. It was concluded that the effects of inclination on the muscle activities were larger for the uphill exercises, and for the hind limb muscles compared with the forelimb muscles.

In vivo measurements of flexor tendon and suspensory ligament forces during trotting using the thoroughbred forelimb model.

The purpose of this study was to create a lower forelimb model of the Thoroughbred horse for measuring the force in the superficial and deep digital flexor tendons (SDFT and DDFT), and the suspensory ligament (SL) during a trot. The mass, centers of gravity, and inertial moments in the metacarpus, pastern, and hoof segments were measured in 4 Thoroughbred horses. The moment arms of the SDFT, DDFT, and SL in the metacarpophalangeal (fetlock) and distal interphalangeal (coffin) joints were measured in 7 Thoroughbred horses. The relationship between the fetlock joint angle and the force in the SL was assessed in 3 limbs of 2 Thoroughbred horses. The forces in the SDFT, DDFT, and SL during a trot were also measured in 7 Thoroughbred horses. The mass of the 3 segments, and the moment arms of the SDFT and DDFT in the fetlock joint of the Thoroughbred horses were smaller than those of the Warmblood horses, whereas the other values were almost the same in the 2 types. The calculated force in the SDFT with this Thoroughbred model reached a peak (4,615 N) at 39.3% of the stance phase, whereas that in the DDFT reached a peak (5,076 N) at 51.2% of the stance phase. The force in the SL reached a peak (11,957 N) at 49.4% of the stance phase. This lower forelimb model of the Thoroughbred can be applied to studying the effects of different shoe types and change of hoof angle for the flexor tendon and SL forces.

Effect of Treadmill Exercise and Hydrogen-rich Water Intake on Serum Oxidative and Anti-oxidative Metabolites in Serum of Thoroughbred Horses.

The present study aimed to clarify changes of oxidative stress and antioxidative functions in treadmill-exercised Thoroughbred horses (n=5, 3 to 7 years old), using recently developed techniques for measurement of serum d-ROMs for oxidative stress, and BAP for antioxidative markers. Also, the effect of nasogastric administration of hydrogen-rich water (HW) or placebo water preceding the treadmill exercise on these parameters was examined. Each horse was subjected to a maximum level of treadmill exercise in which the horses were exhausted at an average speed of 13.2 ± 0.84 m/sec. Blood samples were taken 4 times, immediately before the intake of HW or placebo water at 30 min preceding the treadmill exercise, immediately before the exercise (pre-exercise), immediately after the exercise (post-exercise) and at 30 min following the exercise. In all horses, both d-ROMs and BAP values significantly increased at post-exercise. The increase in d-ROMs tended to be lower in the HW trial, as compared to the placebo trial at pre-exercise. The increase in BAP was considerable at approximately 150% of the pre-exercise values in both the HW and placebo treatment trials. The BAP/d-ROMs ratio was significantly elevated at post-exercise in both treatment trials, while a significant elevation was also observed at pre-exercise in the HW trial. BAP, d-ROM, and the BAP/d-ROM ratio tended to decline at 30 min after the exercise, except BAP and BAP/d-ROMs in the placebo trial. These results demonstrate that the marked elevation of oxidative stress and anitioxidative functions occurred simultaneously in the intensively exercised horses, and suggest a possibility that HW has some antioxidative efficacy.

Effect of training and detraining on monocarboxylate transporter (MCT) 1 and MCT4 in Thoroughbred horses.

The aim of this study was to investigate the effects of training and detraining on the monocarboxylate transporter (MCT) 1 and MCT4 levels in the gluteus medius muscle of Thoroughbred horses. Twelve Thoroughbred horses were used for the analysis. For 18 weeks, all the horses underwent high-intensity training (HIT), with running at 90-110% maximal oxygen consumption (VO2 max ) for 3 min, 5 days week(-1). Thereafter, the horses either underwent detraining for 6 weeks by either 3 min of moderate-intensity training (MIT) at 70% VO2 max, 5 days week(-1) (HIT-MIT group) or stall rest (HIT-SR group). The horses underwent an incremental exercise test, VO2 max was measured and resting muscle samples were obtained from the middle gluteus muscle at 0, 18 and 24 weeks. The content of MCT1 and MCT4 proteins increased after 18 weeks of HIT. At the end of this period, an increase was noted in the citrate synthase activity, while phosphofructokinase activity remained unchanged. After 6 weeks of detraining, all these indexes returned to the pretraining levels in the HIT-SR group. However, in the HIT-MIT group, the increase in the MCT1 protein content and citrate synthase activity was maintained after 6 weeks of MIT, while the MCT4 protein content decreased to the pretraining value. These results suggest that the content of MCT1 and MCT4 proteins increases after HIT in Thoroughbred horses. In addition, the increase in the MCT1 protein content and oxidative capacity induced by HIT can be maintained by MIT of 70% VO2 max, but the increase in the MCT4 protein content cannot be maintained by MIT.

Free radical formation after intensive exercise in thoroughbred skeletal muscles.

Although high oxygen consumption in skeletal muscle may result in severe oxidative stress, there are no direct studies that have documented free radical production in horse muscles after intensive exercise. To find a new parameter indicating the muscle adaptation state for the training of Thoroughbred horses, we examined free radical formation in the muscle by using electron paramagnetic resonance (EPR). Ten male Thoroughbred horses received conventional training for 18 weeks. Before and after the training period, all horses performed an exhaustive incremental load exercise on a 6% incline treadmill. Muscle samples of the middle gluteal muscle were taken pre-exercise and 1 min, 1 hr, and 1 day after exercise. Muscle fiber type composition was also determined in the pre-exercise samples by immunohistochemical staining with monoclonal antibody to myosin heavy chain. We measured the free radical in the muscle homogenate using EPR at room temperature, and the amount was expressed as relative EPR signal intensity. There was a significant increase in Type IIA muscle fiber composition and a decrease in Type IIX fiber composition after the training period. Before the training period, the mean value of the relative EPR signal intensity showed a significant increase over the pre-exercise value at 1 min after the exercise and an incomplete recovery at 24 hr after the exercise. While no significant changes were found in the relative EPR signal intensity after the training period. There was a significant relationship between percentages of Type IIA fiber and change rates in EPR signal intensity at 1 min after exercise. The measurement of free radicals may be useful for determining the muscle adaptation state in the training of Thoroughbred horses.

Differences in Muscle Fiber Recruitment Patterns between Continuous and Interval Exercises.

We evaluated differences in muscle fiber recruitment patterns between continuous and interval training to develop an optimal training program for Thoroughbred horses. Five well trained female thoroughbred horses (3-4 years old) were used. The horses performed two different exercises on a 10% inclined treadmill: 90%VO2 max for 4 min (continuous) and 90% VO2 max for 2 min × 2 times with 10-min interval (interval). Muscle samples were obtained from the middle gluteal muscle before and immediately after the exercises. Four muscle fiber types (type I, IIA, IIA/X, and IIX) were immunohistochemically identified, and the optical density of periodic acid Schiff staining (OD-PAS) in each fiber type and glycogen content of the muscle sample were determined by quantitative histochemical and biochemical procedures, respectively. No significant differences were found in the OD-PASs and glycogen contents between the continuous and interval exercises, but the decreases in OD-PAS of fast-twitch muscle fibers were obvious after interval as compared to continuous exercise. Interval exercise may be a more effective training stimulus for the glycolytic capacity of fast-twitch muscle fiber. The data about muscle fiber recruitment can provide significant insights into the optimal training program not only for thoroughbred horses, but also for human athletes.

Sarcoplasmic Reticulum Ca(2+)-ATPase Activity and Glycogen Content in Various Fiber Types after Intensive Exercise in Thoroughbred Horses.

To find a new parameter indicating muscle fitness in Thoroughbred horses, we examined time-dependent recovery of glycogen content and sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity of skeletal muscle after intensive treadmill running. Two repeated 50-sec running sessions (13 m/sec) were performed on a flat treadmill (approximately 90%VO2max). Muscle samples of the middle gluteal muscle were taken before exercise (pre) and 1 min, 20 min, 60 min, and 24 hr after exercise. Muscle fiber type composition was determined in the pre muscle samples by immunohistochemical staining with monoclonal antibody to myosin heavy chain. SR Ca(2+)-ATPase activity of the muscle and glycogen content of each muscle fiber type were determined with biochemical analysis and quantitative histochemical staining, respectively. As compared to the pre value, the glycogen content of each muscle fiber type was reduced by 15-27% at 1 min, 20 min, and 60 min after the exercise and recovered to the pre value at 24 hr after exercise test. These results indicate that 24 hr is enough time to recover glycogen content after short-term intensive exercise. The mean value of the SR Ca(2+)-ATPase activity showed a slight decrease (not significant) immediately after exercise, and complete recovery at 60 min after exercise. There were no significant relationship between the changes in glycogen content of each muscle fiber type and SR Ca(2+)-ATPase. Although further studies are needed, SR Ca(2+)-ATPase is not a useful parameter to detect muscle fitness, at least in Thoroughbred horses.

Effects of warm-up intensity on oxygen transport during supramaximal exercise in horses.

OBJECTIVE: To determine whether warm-up exercise at different intensities alters kinetics and total contribution of aerobic power to total metabolic power in subsequent supramaximal exercise in horses. ANIMALS: 11 horses. PROCEDURES: Horses ran at a sprint until fatigued at 115% of maximal oxygen consumption rate (VO(2max)), beginning at 10 minutes following each of 3 warm-up protocols: no warmup (NoWU), 1 minute at 70% VO(2max) (moderate-intensity warm-up [MoWU]), or 1 minute at 115% VO(2max) (high-intensity warm-up [HiWU]). Cardiopulmonary and blood gas variables were measured during exercise. RESULTS: The VO(2) was significantly higher in HiWU and MoWU than in NoWU throughout the sprint exercise period. Blood lactate accumulation rate in the first 60 seconds was significantly lower in MoWU and HiWU than in NoWU. Specific cardiac output after 60 seconds of sprint exercise was not significantly different among the 3 protocols; however, the arterial mixed-venous oxygen concentration difference was significantly higher in HiWU than in NoWU primarily because of decreased mixed-venous saturation and tension. Run time to fatigue following MoWU was significantly greater than that with NoWU, and there was no difference in time to fatigue between MoWU and HiWU. CONCLUSIONS AND CLINICAL RELEVANCE: HiWU and MoWU increased peak values for VO(2) and decreased blood lactate accumulation rate during the first minute of intense exercise, suggesting a greater use of aerobic than net anaerobic power during this period.

Effects of maintaining different exercise intensities during detraining on aerobic capacity in Thoroughbreds.

OBJECTIVE To determine whether racehorses undergoing regular exercise at 2 intensities or stall rest during a period of reduced training (detraining) would differentially maintain their cardiopulmonary and oxygen-transport capacities. ANIMALS 27 Thoroughbreds. PROCEDURES Horses trained on a treadmill for 18 weeks underwent a period of detraining for 12 weeks according to 1 of 3 protocols: cantering at 70% of maximal rate of oxygen consumption ([Formula: see text]o2max) for 3 min/d for 5 d/wk (canter group); walking for 1 h/d for 5 d/wk (walk group); or stall rest (stall group). Standardized treadmill exercise protocols (during which cardiopulmonary and oxygen-transport variables were measured) were performed before and after detraining. RESULTS Mass-specific [Formula: see text]o2max, maximal cardiac output, and maximal cardiac stroke volume of all groups decreased after 12 weeks of detraining with no differences among groups. After detraining, arterial-mixed-venous oxygen concentration difference did not decrease in any group, and maximal heart rate decreased in the walk and stall groups. Run time to exhaustion and speeds eliciting [Formula: see text]o2max and maximal heart rate and at which plasma lactate concentration reached 4mM did not change in the canter group but decreased in the walk and stall groups. CONCLUSIONS AND CLINICAL RELEVANCE Horses following the cantering detraining protocol maintained higher values of several performance variables compared with horses following the walking or stall rest protocols. These results suggested that it may be possible to identify a minimal threshold exercise intensity or protocol during detraining that would promote maintenance of important performance-related variables and minimize reductions in oxygen-transport capacity in horses.

Changes in heart rate and heart rate variability in Thoroughbreds during prolonged road transportation.

OBJECTIVE: To determine whether evaluation of heart rate (HR) and HR variability (HRV) during prolonged road transportation in horses provides a sensitive index of autonomic stimulation. ANIMALS: Five 2-year-old Thoroughbreds. PROCEDURE: ECGs were recorded as horses were transported for 21 hours in a 9-horse van. Heart rate, high-frequency (HF) power, low-frequency (LF) power, and LF-to-HF ratio from Fourier spectral analyses of ECGs were calculated and compared with values recorded during a 24-hour period of stall rest preceding transportation. RESULTS: HR, HF power, and LF power had diurnal rhythms during stall rest but not during road transportation. Heart rate was higher and HF power and LF power lower during road transportation than stall rest, and HR, HF power, LF power, and LF-to-HF ratio all decreased with time during road transportation. Heart rate during stall rest was weakly and inversely associated with LF power, but during road transportation was strongly associated with LF power, HF power, and LF-to-HF ratio. Neither LF power nor HF power was correlated with LF-to-HF ratio during stall rest, but LF power was strongly and HF power weakly correlated with LF-to-HF ratio during road transportation. High-frequency power and LF power were significantly correlated with each other during stall rest and road transportation. Heart rate was significantly influenced by LF power and LF-to-HF ratio during stall rest (R(2) = 0.40) and by HF power and LF-to-HF ratio during road transportation (R(2) = 0.86). CONCLUSIONS AND CLINICAL RELEVANCE: HR is influenced by different sympathovagal mechanisms during stall rest, compared with during road transportation; HRV may be a sensitive indicator of stress in transported horses.

Use of a rigid-type polyethylene T-cannula, silicone tubing, and adhesion netting for surgical cannulation of ileum of the horse.

A rigid-type of polyethylene T-cannula was fitted into the anterior ileum of six horses in order to improve the cannulation techniques. A piece of polyethylene net was fastened onto the intestinal wall around the cannula to prevent dislodgment of the cannula by promoting a secure adhesion between the ileum and the abdominal wall. The cannula barrel sheathed with silicone tubing was exteriorized through a stab incision at the lateral ventral wall on the transverse line of the second lumber vertebra, and a flange was screwed onto the barrel. The feeding regime gradually increased concentrate without roughage prevented any colic signs. The use of these techniques succeeded in the ileal cannulation with no leakage of digesta.

Effects of initial handling and training on autonomic nervous function in young Thoroughbreds.

OBJECTIVE: To determine the effects of initial handling and training on autonomic nervous functions in young Thoroughbreds. ANIMALS: 63 healthy Thoroughbreds. PROCEDURE: All horses were trained to be handled and initially ridden in September of the yearling year and then trained until the following April by conventional training regimens. To obtain the heart rate (HR), electrocardiograms were recorded in the stable before initial handling and training and following 7 months of training; variations in HR were then evaluated from the power spectrum in terms of the low frequency (LF; 0.01 to 0.07 Hz) power and high frequency (HF; 0.07 to 0.6 Hz) power as indices of autonomic nervous activity. To evaluate the fitness, the V200 (velocity at HR of 200 beat/min), which is reflective of the aerobic capacity of the horse, was measured. RESULTS: Mean (+/- SE) resting HR decreased significantly from 41.5 +/- 0.8 to 38.7 +/- 0.4 beat/min following 7 months of training. The LF power of horses increased significantly from 1,037 +/- 128 milliseconds2 in September of the yearling year to 2,944 +/- 223 milliseconds2 in the following April. Similarly, the HF power increased significantly from 326 +/- 30 milliseconds2 to 576 +/- 39 milliseconds2 at the corresponding time points. The V200 increased significantly following training. CONCLUSIONS AND CLINICAL RELEVANCE: Increases in LF and HF powers indicate that parasympathetic nervous activity increases in horses by 7 months of training. The decrease in resting HR may be dependent on the training-induced increase of parasympathetic nervous activity in Thoroughbreds.

Effect of growth and training on muscle adaptation in Thoroughbred horses.

OBJECTIVE: To determine the effect of growth and training on metabolic properties in muscle fibers of the gluteus medius muscle in adolescent Thoroughbred horses. ANIMALS: Twenty 2-year-old Thoroughbreds. PROCEDURE: Horses were randomly assigned to 2 groups. Horses in the training group were trained for 16 weeks, and control horses were kept on pasture without training. Samples were obtained by use of a needle-biopsy technique from the middle gluteus muscle of each horse before and after the training period. Composition and oxidative enzyme (succinic dehydrogenase [SDHI) activity of each fiber type were determined by use of quantitative histochemical staining procedures. Whole-muscle activity of SDH and glycolytic enzyme (phosphofructokinase) as well as myosin heavy-chain isoforms were analyzed biochemically and electrophoretically, respectively. RESULTS: The SDH activity of type-I and -IIA fibers increased during growth, whereas whole-muscle activity was unchanged. Percentage of type-IIX/B muscle fibers decreased during training, whereas that of myosin heavy-chain IIa increased. The SDH activity of each fiber type as well as whole-muscle SDH activity increased during training. An especially noticeable increase in SDH activity was found in type-IIX/B fibers. CONCLUSIONS AND CLINICAL RELEVANCE: Changes in muscle fibers of adolescent Thoroughbreds are caused by training and not by growth. The most noticeable change was for the SDH activity of type-IIX/B fibers. These changes in the gluteus medius muscle of adolescent Thoroughbreds were considered to be appropriate adaptations to running middle distances at high speeds.