Evaluation of heart rate and rhythm during exercise

Heart rate and rhythm can readily be monitored during exercise. Although there is considerable variation depending on the athletic discipline, exercise can be considered to be one of the most physiologically demanding times for the cardiovascular system. Assessment of heart rate during exercise typically provides information regarding fitness and the intensity of exercise, and on some occasions may provide an early indication of disease. Cardiac causes of poor performance occur relatively infrequently in comparison with disorders of the musculoskeletal and respiratory systems. Nevertheless, exercise induces cardiac hypertrophy, which predisposes athletes to valvular regurgitation and arrhythmias. Consequently cardiac murmurs and arrhythmias are frequently present in equine athletes, whereupon the clinical significance can be difficult to determine. Undertaking an exercise test to identify exercise‐induced arrhythmias is important in the assessment of poor athletic performance and the risk of sudden cardiac death during exercise. This paper describes the assessment of heart rate and rhythm during exercise. Most research has been undertaken on racehorses but, where data are available for other disciplines, they have been included. Considerations regarding the choice, type and design of exercise test were detailed in the first paper in this series.     

Nutrition and fuel utilization in the athletic horse

Substrate depletion and end product accumulation are two important factors in exercise fatigue. Fatigue during long-term exercise results from a depletion of muscle and liver glycogen and coincides with an inability to maintain blood glucose levels. During high intensity exercise, the rapid catabolism of carbohydrate and the resultant production of lactate and hydrogen ions cause a reduction in muscle pH that inhibits maximum force generation. Dietary manipulations that can influence carbohydrate status or lactate accumulation may be beneficial to performance. In human athletes, carbohydrate loading and carbohydrate supplementation can enhance endurance time during long-term exercise. These practices have not been explored extensively in the equine athlete, although glycogen loading does not enhance the performance of horses during short-term intense work. Short-term work can be detrimentally affected if glycogen levels are inadequate. The most marked effect of exercise on nutrient requirements is in the energy requirement. Horses in heavy training may require more energy than they can consume on a conventional diet. Fat has been added to horse diets to increase energy density, usually at levels between 6% and 12% of the total diet. Although protein requirements may be slightly increased in the working horse, supplementing protein as a means of adding calories is not an efficient practice. In addition, although studies with horses are not available, human studies indicate that there are no benefits to vitamin supplementation above required levels. At this point, more is unknown than is known about feeding performance horses. Most information on fuel utilization is extrapolated from studies with rats and humans. Areas that have received little attention but are critical to optimizing feeding practices are the timing of pre-event feeding and the determination of ideal body composition in equine athletes of different types.     

Arrhythmias in Thoroughbreds During and After Treadmill and Racetrack Exercise

The objectives of the study were to develop information regarding the frequency of recurrence of exercising arrhythmias and the relationship of arrhythmia development to exercise intensity and type of exercise in Thoroughbred horses. Electrocardiograms (ECGs) were recorded on nine Thoroughbreds during maximal or submaximal exercise on a racetrack (Ra) and treadmill (Tm). The frequency of arrhythmias on a Ra and Tm was compared, and their relationship to exercise intensity (expressed as HR/HRmax [%]) was evaluated. Sixty-five workouts were analyzed: 46 workouts were on a Tm and 19 on a Ra; median number of workouts/horse was four, and the range was 2–14. Exercising arrhythmias were detected in 4/9 horses (12/65 workouts), and there were postexercise arrhythmias in 7/9 horses (19/65 workouts). Arrhythmias were detected at some point in 8/9 horses. For 7/9 horses, the same rhythm result was obtained during exercise in repeated recordings. For 7/9 horses, the postexercise rhythm was variable: postexercise arrhythmias were present in median: 21%; range: 0%–75% of workouts. The presence of arrhythmias was positively related to exercise intensity (P = .01; odds ratio = 1.2) and all occurred during workouts at ≥94% of HR/HRmax (%). Arrhythmias during exercise were more frequent on the Ra than on the Tm (P = .009). A single ECG did not always display all the arrhythmias detected over several exercise tests. The presence/absence of exercising arrhythmias was more consistent than postexercise arrhythmias. Arrhythmias were more likely to be detected at maximal or near-maximal intensities and during gallops on the Ra. A larger population needs to be studied before more definitive conclusions are drawn.     

Evaluating the Benefits of Equine Massage Therapy: A Review of the Evidence and Current Practices

Following the lead of human athletic training, equine massage therapy is becoming a more common part of the management of equine athletes and pleasure horses alike. The basic science rationale for massage is supported by research indicating that massage may affect a number of physiologic systems as well as cellular and fascial components of the muscular system. Equine therapeutic massage, or sports massage, employs a number of techniques first developed in humans and has been reported to increase range of motion and stride length, reduce activity of nociceptive pain receptors, and reduce physiologic stress responses. Additional preliminary research indicates that massage therapy also may improve some aspects of exercise recovery. Although important evidence has begun to document the potential benefits of massage therapy for equine athletes, the current review may say less about the true clinical effects of massage therapy than it does about the current state of research in this field. Additional prospective study of massage therapy using sufficient scientific rigor will be necessary to provide veterinarians, trainers, and owners with definitive data and scientifically based confidence in the use of equine massage. In the meantime, the preliminary research, anecdotal positive effects, and case studies indicating potential benefit are not to be ignored; equine massage therapy already plays a valuable practical role in the care and training of many equine athletes.     

Effects of acute exercise on angiotensin I-converting enzyme (ACE) activity in horses

Angiotensin I-converting enzyme (ACE) level measurement in blood samples is an important tool in human medicine for the detection, treatment and control of diseases such as sarcoidosis and hypertension. Recently ACE has been advocated as being correlated to athletic aptitude in human athletes and a genetic polymorphism has been shown to be responsible for the enzymatic levels in the circulation. The objective of this research was to evaluate the effects of acute exercise in horses in order to increase the understanding of a possible correlation between ACE levels in plasma and performance in equine athletes. A standardised exercise test (SET) to fatigue was conducted on 8 horses and repeated venous blood collections carried out for ACE activity measurements before, during and after the SET. Our results show an increase in ACE activity up to fatigue and a return to baseline values at 30 min post exercise.     

Hormonal responses to acute exercise, training and overtraining. A review with emphasis on the horse

Overtraining is an imbalance between training and recovery leading to symptoms associated with a neuroendocrine dysbalance called the overtraining syndrome, a disease characterized by behavioral, emotional and physical symptoms similar with depression. Although the prevalence of overtraining is high in human and equine athletes, at present no sensitive and specific test is available to prevent or diagnose overtraining. Nowadays, it is believed that combination of different (hormonal) parameters appear to be the best indicators of overtraining. Therefore, this review provides a summary of previous literature examining the response of the hypothalamic-pituitary-adrenal (HPA) axis and the growth hormone-insulin-like growth factor-I (GH-IGF-I) axis to acute and chronic exercise as well as overtraining in humans and horses. The exercise induced hormonal responses seem to be equal for the equine as well as the human athlete, which makes comparisons possible. Repeated bouts of exercise are suggested to provide a way to detect subtle changes in hormonal responses in the individual athlete, which may make them an important tool in detecting early overtraining. This should be combined with corticotropin releasing hormone (CRH) stimulation tests and basal ACTH and GH pulsatility determination. Further research is needed to establish the correct training intensity and rest period for the exercise test in equines.     

Pathologic and electrocardiographic findings in sudden cardiac death in racehorses.

Five racehorses in apparently normal condition succumbed to sudden cardiac death (SCD) during or shortly after intensive training exercise. Cardiopathologic examination was performed. In 1 of the 5 horses, the use of an electrocardiogram (ECG) recording taken continuously for 440 sec enabled us to analyze some of the arrhythmias in the terminal event of SCD. The ECG tracing exhibited the R-on-T phenomenon following a pair of ventricular premature contractions (VPCs). The phenomenon rapidly degenerated into ventricular fibrillation, which led to cardiac arrest. In all 5 horses cardiopathologic examination revealed the following lesions: (i) foci of myocardial fibrosis in the right atrium located close to the sinoatrial (SA) node, (ii) fibrotic and/or fibroplastic changes in the upper portion of the interventricular septum, including the atrioventricular (AV) conduction system, and (iii) arterio- and arteriolosclerosis of the SA and AV node vessels. Pathogenetically, the process by which the focal lesions of myocardial ischemia secondary to vascular sclerosis progressed into fibrosis and/or fibroplasia could play a major role in the genesis of arrhythmias. Presumably the fibrotic and/or fibroplastic changes in the area of the AV bundle and its bundle branches are closely related to the onset of fatal ventricular arrhythmias such as VPCs, deteriorating into ventricular fibrillation. SCD in training and racing Thoroughbred horses appears to be due to arrhythmia.     

Reliability of cardiorespiratory measurements with a new ergospirometer during intense treadmill exercise in Thoroughbred horses

This study investigated the reliability of measurements with a new equine ergospirometer (Quadflow). Heart rate and blood lactate responses during exercise in horses wearing the Quadflow and an open flow mask were also compared. The mean percentage error of the oxygen uptake measurements was 8.2% (range 2.1-12.5%). Percent error for peak expiratory flow rates ranged from 6.1% to 9.4 %, and for minute ventilation from 2.5% to 7.4%. The coefficients of variation of the means of four measurements in two horses exercising continuously at 9.0 m/s were <5% for variables related to pulmonary ventilation, and was 7.7% for oxygen uptake. The Quadflow mask resulted in small increases in blood lactate concentration and relative heart rate during submaximal exercise. It was concluded that between- and within-test reliability statistics for important measurements in equine clinical exercise testing were acceptable for routine use in a veterinary practice or research laboratory.     

Effects of athletic taping of the fetlock on distal limb mechanics

REASONS FOR PERFORMING STUDY: Athletic taping is used frequently by human athletes to stabilise, maintain or strengthen soft tissue structures, but empirical evidence supporting any changes in equine kinematics is lacking. OBJECTIVES: To assess the effects of athletic taping of the fetlock applied by an experienced athletic trainer on forelimb mechanics in healthy horses. HYPOTHESES: That athletic taping of the distal forelimb reduces 1) hyperextension of the fetlock joint during stance, 2) flexion of the fetlock joint during swing and 3) ground reaction forces during stance. METHODS: Ground reaction force and kinematic data were obtained for 6 healthy horses trotting at 3 m/sec for 4 sequential conditions (baseline, untaped; pre-exercise, taped; post exercise, taped post 30 mins trotting exercise; transfer, 4 h after tape removal). Data were analysed using 2-way mixed ANOVAs (condition; joint). RESULTS: A statistically significant interaction was identified for the fetlock during the swing phase (mean +/- s.d. peak flexion at baseline 157 +/- 4 degrees, reduced with taping to 172 +/- 4 degrees; P<0.05) compared with no differences across conditions for the other joints. Peak vertical force reduced significantly (P<0.05) with taping. CONCLUSIONS: Athletic taping of the fetlock does not alter the kinematics of the forelimb during stance, but does limit flexion of the fetlock during the swing phase. The decreased peak vertical force may be due to an increased proprioceptive effect. POTENTIAL RELEVANCE: Reduced peak vertical forces may be of benefit in preventing or reducing injury. Further investigation remains necessary before it can be concluded that taping should be applied for tendinous or ligamentous rehabilitation in equine patients.     

Respiratory diseases and their effects on respiratory function and exercise capacity

Given that aerobic metabolism is the predominant energy pathway for most sports, the respiratory system can be a rate‐limiting factor in the exercise capacity of fit and healthy horses. Consequently, respiratory diseases, even in mild forms, are potentially deleterious to any athletic performance. The functional impairment associated with a respiratory condition depends on the degree of severity of the disease and the equestrian discipline involved. Respiratory abnormalities generally result in an increase in respiratory impedance and work of breathing and a reduced level of ventilation that can be detected objectively by deterioration in breathing mechanics and arterial blood gas tensions and/or lactataemia. The overall prevalence of airway diseases is comparatively high in equine athletes and may affect the upper airways, lower airways or both. Diseases of the airways have been associated with a wide variety of anatomical and/or inflammatory conditions. In some instances, the diagnosis is challenging because conditions can be subclinical in horses at rest and become clinically relevant only during exercise. In such cases, an exercise test may be warranted in the evaluation of the patient. The design of the exercise test is critical to inducing the clinical signs of the problem and establishing an accurate diagnosis. Additional diagnostic techniques, such as airway sampling, can be valuable in the diagnosis of subclinical lower airway problems that have the capacity to impair performance. As all these techniques become more widely used in practice, they should inevitably enhance veterinarians' diagnostic capabilities and improve their assessment of treatment effectiveness and the long‐term management of equine athletes.     

Effect of Competition on Nitric Oxide, Carbon Monoxide, and Cyclic Guanosine Monophosphate Levels in Training Horses

The aim of this study was to compare the effect of exercise on nitric oxide (NO), carbon monoxide (CO), and cyclic guanosyl monophosphate (cGMP) levels in jumping and dressage horses involved in competition. Blood samples were collected from the jugular vein of 100 horses involved in jumping or dressage competition at three time points: baseline at rest, on reaching the schooling but before exercise, and over a jump or dressage course. Fourteen healthy horses not involved in competition were used as control group. Exercise increased plasma CO concentration in both jumping and dressage horses, and this effect was more apparent in dressage horses. Exercise also increased NO plasma concentration in jumping horses, whereas it did not significantly modify NO plasma concentration in dressage horses. After exercise, plasma cGMP concentration was higher in both groups. Our results show that different signaling pathways are initially activated by exercise and that this activation is specific to the different modes of exercise. This information can be used to optimize warm-up and cool-down procedures for sport horses or to optimize training programs for equine athletes.     

Dynamic respiratory endoscopy without treadmill in 68 performance Standardbred,Thoroughbred and saddle horses under natural training conditions

Reasons for performing study: High speed treadmill endoscopy provides a true assessment and diagnosis of the dynamic obstructions of the upper equine respiratory tract (DO‐URT). However, treadmills do not always allow reproduction of the exact conditions of dynamic collapse of the URT. The availability of on‐board endoscopes, which could be used without a treadmill, would make exercising endoscopy readily available to more equine practices. Objectives: To develop and validate an innovative endoscope which could be used for the examination of the URT at exercise in ridden and harnessed sport horses performing in natural conditions. Methods: Authors worked closely with engineers of a company manufacturing veterinary endoscopes. Over a 2‐year period several prototypes were tested on more than 20 cooperative horses performing either in trot, gallop, jumping or endurance. The final Dynamic Respiratory Scope (DRS) allows real‐time visualisation of the URT and video recordings for post test reviewing and archiving. The DRS was then tested in 2 equine livery yards. Performance horses presented for investigation of abnormal respiratory noises at exercise and/or poor performance were subjected to exercising endoscopy. Endoscopic images of the URT were recorded and video recordings of the URT were reviewed post test. Results: A total of 68 horses were examined: 39 harnessed Standardbred and 29 mounted horses (16 Thoroughbred and 13 saddle horses). Of these, 44 were diagnosed with a URT abnormality. Good quality videos were obtained even at maximum speed in all cases. The innovative insertion tube provided very stable images compared to those obtained with flexible video endoscopes on treadmills. Conclusions: This study validates the safety and the reliability of the DRS for imaging the equine URT during natural exercising conditions. Potential relevance: Most common causes of DO‐URT can be diagnosed easily during any type of performance without a treadmill and the DRS offers a great potential for further URT clinical research.     

Arytenoid Cartilage Movement in Resting and Exercising Horses

Endoscopic examinations of the larynx were recorded on 49 horses at rest and while exercising on a 5% inclined high-speed treadmill for 8 minutes at a maximum speed of 8.5 m/sec. Subjective laryngeal function scores at rest and while exercising were based on the degree and synchrony of arytenoid abduction. Arytenoid abduction was expressed as a left:right ratio of rima glottidis measurements. Horses with arytenoid cartilage asynchrony at rest (grade 2) could not be distinguished from normal horses (grade 1) when exercising because full abduction was maintained throughout the exercise period. Five horses with incomplete left arytenoid abduction at rest (grade 3) maintained full abduction during exercise; one grade 3 horse had dynamic collapse of the left side of the larynx. All horses with laryngeal hemiplegia at rest (grade 4) had dynamic collapse of the left side of the larynx during exercise. Forty-two horses with a resting left:right arytenoid abduction ratio greater than or equal to .71 consistently had complete arytenoid abduction at exercise. Seven horses with a left:right ratio less than .71 consistently showed dynamic collapse at exercise. There was no significant difference in the exercising left:right ratio between normal horses (grade 1) and grade 2 or grade 3 horses. These results suggest that horses with arytenoid asynchrony at rest do not suffer progressive collapse of the rima glottidis during exercise, and that incomplete arytenoid abduction at rest is an unreliable predictor of such collapse. Surgical treatment of all grade 2 horses and some grade 3 horses may be inappropriate.     

Oxidant injury,nitric oxide and pulmonary vascular function: implications for the exercising horse

The athletic ability of the horse is facilitated by vital physiological adaptations to high-intensity exercise, including a thin (but strong) pulmonary blood-gas barrier, a large pulmonary functional reserve capacity and a consequent maximum oxygen uptake (VO2max) far higher than in other species. A high pulmonary artery pressure also serves to enhance pulmonary function, although stress failure of lung capillaries at high pulmonary transmural pressures, and the contribution of other factors which act in the exercising horse to increase pulmonary vascular tone, may lead to pathological or pathophysiological sequelae, such as exercise-induced pulmonary haemorrhage (EIPH). Reactive oxygen species (ROS) are an important component of the mammalian inflammatory response. They are released during tissue injury and form a necessary component of cellular defences against pathogens and disease processes. The effects of ROS are normally limited or neutralized by a multifactorial system of antioxidant defences, although excessive production and/or deficient antioxidant defences may expose healthy tissue to oxidant damage. In the lung, ROS can damage pulmonary structures both directly and by initiating the release of other inflammatory mediators, including proteases and eicosanoids. Vascular endothelial cells are particularly susceptible to ROS-induced oxidant injury in the lung, and both the destruction of the pulmonary blood-gas barrier and the action of vasoactive substances will increase pulmonary vascular resistance. Moreover, ROS can degrade endothelium-derived nitric oxide (NO), a major pulmonary vasodilator, thereby, with exercise, synergistically increasing the likelihood of stress failure of pulmonary capillaries, a contributing factor to EIPH. This review considers the implications for the exercising horse of oxidant injury, pulmonary vascular function and NO and the contribution of these factors to the pathogenesis of equine respiratory diseases.     

Evaluation of plasma alpha-2-macroglobulin and interactions with tumour necrosis factor-alpha in horses with endotoxemic signs.

The electrophoretic position and behavior of the native and activated forms of equine plasma alpha-2-macroglobulin (alpha 2M) were characterized and compared to human alpha 2M by nondenaturing polyacrylamide-gel electrophoresis (PAGE). Plasma alpha 2M was also compared between 6 normal horses and 6 horses with clinical signs of colic and endotoxemia due to volvulus or enteritis. Native and activated forms of alpha 2M were quantified by PAGE and densitometry. Binding of radio-labeled recombinant human tumour necrosis factor-alpha (125I-rhTNF-alpha) to native and activated forms of equine alpha 2M was also evaluated by autoradiography and densitometry of PAGE. Equine plasma alpha 2M migrated as a single band at a position equivalent to native human alpha 2M. Methylamine-reacted equine plasma samples resulted in faster migration of alpha 2M in a similar position to activated human alpha 2M. However, in methylamine-reacted equine plasma, an intermediate alpha 2M band was consistently present between the bands corresponding to native and activated alpha 2M. Amounts of plasma alpha 2M were similar in normal and endotoxemic horses, and remained in the electrophoretically slow or unreacted native form. The vast majority of 125I-rHuTNF-alpha did not bind to alpha 2M or other equine plasma proteins. 125I-rHuTNF-alpha bound weakly to both native and fast methylamine-reacted equine forms of alpha 2M, although binding was better to the activated form. This study indicates that: (1) equine plasma alpha 2M behaves similarly to human alpha 2M on PAGE, (2) plasma alpha 2M of horses can be activated to electrophoretically fast forms, but it is neither activated nor depleted during endotoxemia, and (3) the binding interactions between equine alpha 2M and TNF-alpha are too low to implicate equine alpha 2M as a regulator of TNF-alpha during endotoxemia in horses.     

Does a single bout of exercise cause adaptation of amino acid metabolism in pigs?

Amino acid responses to exercise stress in well-trained racehorses and human athletes are well characterised, but the knowledge of amino acid metabolism during and after exercise in inactive animal species is limited. To study this, plasma amino acid concentrations were measured in previously unexercised pigs which performed two exercise tests on a treadmill with an interval of one week. In general, the changes in amino acids were more pronounced after the second than after the first exercise bout. Alanine, glutamine, phenylalanine and tyrosine were elevated for one hour only after the latter exercise. Twenty-four hours after the second exercise isoleucine, leucine, phenylalanine, tyrosine and valine were increased, but only isoleucine was increased after the first test. These differences between the two tests might be explained by adaptation of the amino acid metabolism after a single exercise bout and suggest that domestic pigs are well suited to study the early effects of exercise.     

Oxidative Stress Biomarkers and Erythrocytes Hemolysis in Well-Trained Equine Athletes Before and After Exercise

The chronic exposure to regular exercise training seems to increase resistance to oxidative stress and improves the antioxidant defense system. The purpose of the present study was to investigate the effect of an exercise test of moderate intensity on oxidative stress biomarkers, antioxidant enzymes activity, and osmotic resistance of erythrocytes in well-trained equine athletes. Eighteen middle-aged horses of Ukrainian warmblood (8.3 ± 1.6 years) and Holsteiner (7.4 ± 1.9 years) breeds were used in this study. All horses have been in regular training for several years. The exercise test induced a significant increase of erythrocyte values, hemoglobin concentration, and hematocrit in horses of both breeds. Regular training induces activation the antioxidant enzymes and thereby can reduce oxidative stress in athletic horses. Our results suggest that the exercise test in horses of both breeds attenuates oxidative stress and accompanied with a significant decrease of lipid peroxidation and oxidatively modified proteins in erythrocytes after exercise. The findings of the present study demonstrated the elevated level of erythrocytes' catalase and glutathione reductase in Ukrainian warmblood horses, as well as decreased level of superoxide dismutase and glutathione reductase in Holsteiner horses reporting changes in levels of exercise-induced oxidative stress biomarkers in horses of both breeds. Statistically significant differences in the percentage of hemolyzed erythrocytes between pre-exercise and postexercise tests were observed and thereby signifying an oxidative stress-dependent impairment of erythrocyte stability. Our data suggest that oxidative stress and enzymatic antioxidant defense biomarkers can be used for the monitoring of fitness level, health benefits, and performance of equine athletes.