Changes in muscle activation with graded surfaces during canter in Thoroughbred horses on a treadmill.

Understanding how muscle activity changes with different surface grades during canter is essential for developing training protocols in Thoroughbreds because canter is their primary gait in training and races. We measured the spatiotemporal parameters and the activation of 12 surface muscles in the leading limb side of 7 Thoroughbreds. Horses were equipped with hoof strain gauges and cantered at 10 m/s on a treadmill set to grades of -4%, 0%, 4%, and 8%, randomly, for 30 seconds each without a lead change. Integrated electromyography (iEMG) values during stance and swing phases were calculated and normalized to mean iEMG values during stride duration at 0% grade in each muscle. The iEMG values at each grade were compared using a generalized mixed model. Stride duration significantly decreased due to shorter swing duration on an 8% grade (P < 0.001) compared to all other grades, where no significant changes were observed. Compared to a 0% grade, the normalized iEMG values during the stance phase on an 8% grade in five muscles significantly increased (Musculus infraspinatus; +9%, M. longissimus dorsi (LD); +4%, M. gluteus medius (GM); +29%, M. biceps femoris; +47%, M. flexor digitorum lateralis; +16%). During the swing phase, the normalized iEMG values in six muscles significantly increased on an 8% grade compared to a 0% grade (M. splenius; +21%, M. triceps brachii; +54%, LD; +37%, GM; +24%, M. semitendinosus; +51%, M. extensor digitorum longus; +10%). No significant changes were observed in iEMG values on -4% and 4% grades compared to the 0% grade. Although +/- 4% grades had little effect on neuromuscular responses, 8% uphill canter reduced stride duration due to decreased swing duration and required increase of muscle activation during either stance and swing phase. Canter on an 8% grade might strengthen equine muscles to increase propulsive force and stride frequency.

Heat acclimation improves exercise performance in hot conditions and increases heat shock protein 70 and 90 of skeletal muscles in Thoroughbred horses.

This study aimed to determine whether heat acclimation could induce adaptations in exercise performance, thermoregulation, and the expression of proteins associated with heat stress in the skeletal muscles of Thoroughbreds. Thirteen trained Thoroughbreds performed 3 weeks of training protocols, consisting of cantering at 90% maximal oxygen consumption (VO2max) for 2 min 2 days/week and cantering at 7 m/s for 3 min 1 day/week, followed by a 20-min walk in either a control group (CON; Wet Bulb Globe Temperature [WBGT] 12-13°C; n = 6) or a heat acclimation group (HA; WBGT 29-30°C; n = 7). Before and after heat acclimation, standardized exercise tests (SET) were conducted, cantering at 7 m/s for 90 s and at 115% VO2max until fatigue in hot conditions. Increases in run time (p = 0.0301), peak cardiac output (p = 0.0248), and peak stroke volume (p = 0.0113) were greater in HA than in CON. Pulmonary artery temperature at 7 m/s was lower in HA than in CON (p = 0.0332). The expression of heat shock protein 70 (p = 0.0201) and 90 (p = 0.0167) increased in HA, but not in CON. These results suggest that heat acclimation elicits improvements in exercise performance and thermoregulation under hot conditions, with a protective adaptation to heat stress in equine skeletal muscles.

Metabolomic responses to high-intensity interval exercise in equine skeletal muscle: effects of rest interval duration.

High-intensity interval training has attracted considerable attention as a time-efficient strategy for inducing physiological adaptations, but the underlying mechanisms have yet to be elucidated. By using metabolomics techniques, we investigated changes in the metabolic network responses in Thoroughbred horses to high-intensity interval exercise performed with two distinct (15 min or 2 min) rest intervals. The peak plasma lactate level was higher during high-intensity exercise with a 2 min rest duration than that with a 15 min rest duration (24.5±6.8 versus 13.3±2.7 mmol l-1). The arterial oxygen saturation was lower at the end of all exercise sessions with a 2 min rest duration than that with a 15 min rest duration. Metabolomic analysis of skeletal muscle revealed marked changes in metabolite concentrations in the first and third bouts of the 15 min rest interval conditions. In contrast, there were no metabolite concentrations or pathways that significantly changed during the third bout of exercise performed with a 2 min rest interval. Our findings suggest that the activity of each energy production system is not necessarily reflected by apparent changes in metabolite concentrations, potentially due in part to a better match between metabolite flux into and out of the pathway and cycle, as well as between metabolite production and disposal. This study provides evidence that changes in metabolite concentrations vary greatly depending on the number of repetitions and the length of rest periods between exercises, even if the exercises themselves are identical.

Physiological and skeletal muscle responses to high-intensity interval exercise in Thoroughbred horses.

Introduction: The purpose of this study was to determine whether acute high-intensity interval exercise or sprint interval exercise induces greater physiological and skeletal muscle responses compared to moderate-intensity continuous exercise in horses. Methods: In a randomized crossover design, eight trained Thoroughbred horses performed three treadmill exercise protocols consisting of moderate-intensity continuous exercise (6 min at 70% VO2max; MICT), high-intensity interval exercise (6 × 30 s at 100% VO2max; HIIT), and sprint interval exercise (6 × 15 s at 120% VO2max; SIT). Arterial blood samples were collected to measure blood gas variables and plasma lactate concentration. Biopsy samples were obtained from the gluteus medius muscle before, immediately after, 4 h, and 24 h after exercise for biochemical analysis, western blotting and real-time RT-PCR. Effects of time and exercise protocol were analyzed using mixed models (p < 0.05). Results: Heart rate and plasma lactate concentration at the end of exercise were higher in HIIT and SIT than those in MICT (heart rate, HIIT vs. MICT, p = 0.0005; SIT vs. MICT, p = 0.0015; lactate, HIIT vs. MICT, p = 0.0014; SIT vs. MICT, p = 0.0003). Arterial O2 saturation and arterial pH in HIIT and SIT were lower compared with MICT (SaO2, HIIT vs. MICT, p = 0.0035; SIT vs. MICT, p = 0.0265; pH, HIIT vs. MICT, p = 0.0011; SIT vs. MICT, p = 0.0023). Muscle glycogen content decreased significantly in HIIT (p = 0.0004) and SIT (p = 0.0016) immediately after exercise, but not in MICT (p = 0.19). Phosphorylation of AMP-activated protein kinase (AMPK) in HIIT showed a significant increase immediately after exercise (p = 0.014), but the increase was not significant in MICT (p = 0.13) and SIT (p = 0.39). At 4 h after exercise, peroxisome proliferator-activated receptor γ co-activator-1α mRNA increased in HIIT (p = 0.0027) and SIT (p = 0.0019) and vascular endothelial growth factor mRNA increased in SIT (p = 0.0002). Discussion: Despite an equal run distance, HIIT and SIT cause more severe arterial hypoxemia and lactic acidosis compared with MICT. In addition, HIIT activates the AMPK signaling cascade, and HIIT and SIT elevate mitochondrial biogenesis and angiogenesis, whereas MICT did not induce any significant changes to these signaling pathways.

Acute exercise in a hot environment increases heat shock protein 70 and peroxisome proliferator-activated receptor γ coactivator 1α mRNA in Thoroughbred horse skeletal muscle.

Heat acclimatization or acclimation training in horses is practiced to reduce physiological strain and improve exercise performance in the heat, which can involve metabolic improvement in skeletal muscle. However, there is limited information concerning the acute signaling responses of equine skeletal muscle after exercise in a hot environment. The purpose of this study was to investigate the hypothesis that exercise in hot conditions induces greater changes in heat shock proteins and mitochondrial-related signaling in equine skeletal muscle compared with exercise in cool conditions. Fifteen trained Thoroughbred horses [4.6 ± 0.4 (mean ± SE) years old; 503 ± 14 kg] were assigned to perform a treadmill exercise test in cool conditions [COOL; Wet Bulb Globe Temperature (WBGT), 12.5°C; n = 8] or hot conditions (HOT; WBGT, 29.5°C; n = 7) consisting of walking at 1.7 m/s for 1 min, trotting at 4 m/s for 5 min, and cantering at 7 m/s for 2 min and at 90% of VO2max for 2 min, followed by walking at 1.7 m/s for 20 min. Heart rate during exercise and plasma lactate concentration immediately after exercise were measured. Biopsy samples were obtained from the middle gluteal muscle before and at 4 h after exercise, and relative quantitative analysis of mRNA expression using real-time RT-PCR was performed. Data were analyzed with using mixed models. There were no significant differences between the two groups in peak heart rate (COOL, 213 ± 3 bpm; HOT, 214 ± 4 bpm; p = 0.782) and plasma lactate concentration (COOL, 13.1 ± 1.4 mmoL/L; HOT, 17.5 ± 1.7 mmoL/L; p = 0.060), while HSP-70 (COOL, 1.9-fold, p = 0.207; HOT, 2.4-fold, p = 0.045), PGC-1α (COOL, 3.8-fold, p = 0.424; HOT, 8.4-fold, p = 0.010), HIF-1α (COOL, 1.6-fold, p = 0.315; HOT, 2.2-fold, p = 0.018) and PDK4 (COOL, 7.6-fold, p = 0.412; HOT, 14.1-fold, p = 0.047) mRNA increased significantly only in HOT at 4 h after exercise. These data indicate that acute exercise in a hot environment facilitates protective response to heat stress (HSP-70), mitochondrial biogenesis (PGC-1α and HIF-1α) and fatty acid oxidation (PDK4).

Effect of speed and leading or trailing limbs on surface muscle activities during canter in Thoroughbred horses.

Given that Thoroughbred horses' canter is an asymmetric gait, not only speed but also leading or trailing limbs could affect muscle activities. However, the muscle activity during a canter remains poorly understood. Hence, we aimed to investigate speed and lead-side (leading or trailing) effects on surface electromyography (sEMG) during a canter. The sEMG data were recorded from left Musculus brachiocephalicus (Br), M. infraspinatus (Inf), long head of M. triceps brachii (TB), M. gluteus medius (GM), M. semitendinosus (ST), and M. flexor digitorum longus of seven Thoroughbreds with hoof-strain gauges at the left hooves. Horses cantered on a flat treadmill at 7, 10, and 13 m/s for 25 s each without lead change. Subsequently, the horses trotted for 3 min and cantered at the same speed and duration in the opposite lead side ("leading" at the left lead and "trailing" at the right lead). The order of the lead side and speed was randomized. The mean of 10 consecutive stride durations, duty factors, integrated-EMG values (iEMG) for a stride, and muscle onset and offset timing were compared using a generalized mixed model (P < 0.05). Stride durations and duty factors significantly decreased with speed regardless of the lead side. In all muscles, iEMG at 13 m/s significantly increased compared with 7 m/s (ranging from +15% to +134%). The lead-side effect was noted in the iEMG of Br (leading > trailing, +47%), Inf (leading > trailing, +19%), GM (leading < trailing, +20%), and ST (leading < trailing, +19%). In TB, GM, and ST, muscle onset in trailing was earlier than the leading, while offset in the leading was earlier in Br. In conclusion, different muscles have different responses to speed and lead side; thus, both the lead side and running speed should be considered during training and/or rehabilitation including canter or gallop.

Risk Factors for Epistaxis in Thoroughbred Flat Races in Japan (2001-2020).

We investigated the risk factors for epistaxis in Japanese flat races over a 20-year period. The veterinary records of horses identified as having epistaxis by endoscopy on the race day, and the official racing records of all flat races from April to September between 2001 and 2020, were reviewed. The racecourses (n = 10), surface type, surface condition, race class, race distance, race year, sex, age, two training centers, ambient temperature, and body weight on race days were assessed using multivariable logistic regression (p < 0.05). Of 475,709 race starts, 616 (1.30 cases per 1000 starts; 95% confidence interval [CI], 1.20-1.40) included an epistaxis event. Nine variables were significantly associated with epistaxis. Seven of the variables have been reported in previous studies: lower ambient temperature, soft surface conditions, shorter racing distances (≤1400 m), increasing age, females and geldings compared to males, training center, and race year. However, two novel variables were identified as significantly associated with epistaxis, increasing body weight per 20 kg (p < 0.001, odds ratio [OR], 1.33; 95% CI, 1.25-1.41) and the racecourses that the horses were running at (p < 0.001, especially Sapporo [OR; 4.74, 95% CI, 3.07-7.31], Hakodate [OR, 4.66; 95% CI, 3.05-7.11], and Kokura [OR, 4.14; 95% CI, 2.65-6.48] compared to the reference racecourse [Kyoto]). These results can facilitate developing interventions to reduce epistaxis in flat racing.

Decreased Virus-Neutralizing Antibodies Against Equine Herpesvirus type 1 In Nasal Secretions of Horses After 12-hour Transportation.

This study evaluated the effects of 12-hour transportation on immune responses to equine herpesvirus type 1 (EHV-1) and type 4 (EHV-4). Possible replication of EHV-1 and EHV-4 was monitored by real-time PCR of nasal swabs and peripheral blood mononuclear cells (PBMCs), and changes in systemic and mucosal antibodies were investigated. Six healthy Thoroughbreds with transport experience were transported in commercial trucks, repeating the same three-hour route four times. Blood samples for cortisol measurement were taken before departure and every three hours. Nasal swabs, PBMCs, nasal wash and serum samples were collected before departure, at unloading, two and six days after arrival. Cortisol concentration increased significantly after three and six hours of transport (P < 0.05), confirming acute transport stress. However, no evidence of viral replication or lytic infection was observed, and serum virus neutralization (VN) titers for EHV-1 and EHV-4 were unchanged, except for one horse that showed a four-fold decrease in titer against EHV-1 after transportation. Urea and total IgA concentration in nasal washes increased significantly after transportation (P < 0.05), while total IgA/protein ratio was unchanged. A transient, ≥4-fold decrease in VN titers for EHV-1 in nasal wash concentrates was observed in four out of six horses after transportation (geometric mean titer declined from 202 to 57, P < 0.05), suggesting suppression of VN capacity in the nasal mucosa may contribute to susceptibility to EHV-1 after transportation. VN antibodies against EHV-4 in nasal secretion were not detected at any timepoint.