Artyku³ przegl¹dowy Review
Survival of horses in natural conditions was con-nected with their ability to bolt when facing a preda-tor. This task, the key one for survival of the species, could be done only due to numerous adaptation me-chanisms that have been formed in the course of the evolution. Besides, it is interesting that this adaptation often outweighs the adaptability of other animals and humans. During the peak exercise of a horse, its heart beat count may increase even eightfold (from 25-30 to 220-240 bpm), while top athletes bpm increases only less than fourfold (6, 33). The high fitness level of these animals is testified by oxygen maximal consumption (in ml/kg/min) and lung capacity in ratio to body mass, which are also more than twice as high as those of humans (20).
Since the main forms of utilization of animals are connected with exercise, it is no surprise that evalu-ation of its impact on the animal was a subject of special interest for many physiologists. Originally, the research on equine exercise physiology was stimula-ted by utilization of the animal in farming. At present, the animals significance for this area is rather low, and its participation in sport orients the research into the area of genetics, biomechanics of movement, struc-ture and composition of muscle tissue, gas exchange, and appropriate nutrition. Moreover, much research has focused on evaluating the efficiency of the respiratory and circulation systems as well as on morphological
and biochemical analysis of blood of horses subjected to various loads.
The aim of this paper was to assess the effect of physical exertion on horses.
Effect of physical exercise on the horses organism. Adaptive changes in the respiratory and circulatory
system
It was found that consumed oxygen volume (VO2) remains in a linear and directly proportional relation to intensity of equine submaximal and maximal exer-cise (15, 16, 19). During fast run the VO2 value may increase even 35-fold in comparison to resting level, which is about 3-4 ml/kg/min (14, 19). Such a sub-stantial demand of tissues for oxygen is linked to tachycardia (even up to 400 bpm), and the result is nearly a ninefold increase of heart minute volume (from 40 to more than 350 l/min) (3, 49). At the same time, it was found that the stroke volume (SV) is not subject to major changes during submaximal loads (16). During maximal exercise even a decrease of the SV value was recorded, which may be connected with a significantly increased heart beat count (11). Although the maximal heart beat frequency does not depend on the training level, the endurance training through an increase of SV may result in a maximal heart minute volume twice as large as other animals which were not included in a training programme (14). The
re-Fundamentals of exercise physiology in horses
BOGDAN JANICKI, ANNA KOCHOWICZ, DOROTA CYGAN-SZCZEGIELNIAK, WIES£AW KRUMRYCH*
Department of Small Ruminant Biology and Environmental Biochemistry, University of Technology and Life Sciences, ul. Mazowiecka 28, 85-084 Bydgoszcz, Poland
*Department of Pathophysiology of Reproduction and Mammary Gland, National Veterinary Research Institute, ul. Powstañców Wlkp. 10, 85-090 Bydgoszcz, Poland
Janicki B., Kochowicz A., Cygan-Szczegielniak D., Krumrych W.
Fundamentals of exercise physiology in horses
Summary
The aim of this paper was to assess the effect of physical exertion on horses. In the course of evolution the horse was forced to develop a number of adaptations which enabled it to survive under unfavourable natural conditions. These adaptations surpassed the ones developed by other animals and of man. During a maximal effort the horses heart rate as well as maximal oxygen consumption increase significantly, which is possible, among others, due to a favourable lung volume to body mass ratio in this animal. As the horses use always involved and still involves its physical exertion, the assessment of respiratory and circulatory system efficiency in this animal remains a vital subject of numerous studies. They analyze biochemical and blood morphology parameters, as well as the issues of the biomechanics of movement, composition and construction of muscle tissue, gas exchange.
search carried out showed that horses are characte-rized by a very high maximal oxygen consumption value (VO2 max) of about 130 ml/kg/min, and in the case of race horses even more than 196 ml/kg/min (16). In comparison, average values of this index for humans, rats, and dogs are: 43, 83, and 87 ml/kg/min respecti-vely (15). Equine ability to consume large amounts of oxygen is confirmed by a proportionally larger supply thereof to the mitochondria of muscle cells and bigger oxidation potential of those organelles than in is the case of other animals of similar size (8, 30). It was empirically confirmed that one of the results of a 6--week, intense training of horses is an increase VO2 max by about 25% (16).
While at rest, the flow of blood through muscles is rather low and is about 3-5 ml/100 g/min. However, during breaks between intense contractions, it may increase even thirtyfold as soon as at the initial stage of load (23). This effect is obtained as a result of local metabolic changes (accumulation of lactates, release of potassium ions, increase of internal temperature), conditioning even a hundredfold increase of the number of expanded capillaries. This leads not only to slowing down the blood flow but also to shortening the distance the gases, nutritional substances, and meta-bolites have to cover, when diffusing between blood and muscle tissue (29). A local increase of tempera-ture and a decrease of blood pH assist oxyhemoglobin dissociation, due to which more oxygen is supplied to working muscles. That results in a more than threefold bigger arterially-venous difference in oxygen satura-tion of blood (40). Moreover, it should be stressed that, as a result of an increase in vascularization of muscle through capillaries as well as an increase in concen-tration of myoglobin and glycogen, regular training increases the availability of O2 for skeletal muscles. This is accompanied by metabolism changes (e.g. higher activity of enzymes in the Krebs cycle) and a bigger share of muscle fibres of higher oxidation potential, which in turn helps to save glycogen and decreases a risk of metabolic acidosis (24).
Another, commonly observed reaction of the ani-mal to exercise is the increased ventilation of lungs. This is primarily caused by stimuli sent from the pro-prioceptors of muscles, ligaments, and joints, accumu-lation of metabolism products, a decrease of blood pH as well as an increase in body temperature. During maximal exercise, ventilation of the equine respira-tory tract increases even fiftyfold, reaching the value of 1400-1800 l/min (39). An increase of the gradient of concentrations of vesicular and capillary gases facilitates oxygen bonding by haemoglobin in lungs and release of carbon dioxide (54). In the case of horses, there is a unique mechanism based on the quick contr-action of blood vessels and contrcontr-action of muscles expanding the upper respiratory tract which certainly decreases air flow resistance (56). This phenomenon is supported by the ability of the animal to decrease
resistance (viscosity) of upper respiratory tract dis-charge, which facilitates consuming even more oxygen during hyperventilation (36). Horses also have the ability to sustain the condition of arterial hypoxia, and even excess of CO2 in the blood during exhaustive exercise. It would appear that this strategy leads to limit the losses of metabolic energy, particularly during extremely ventilation of lungs (5). Additionally, horses are characterized by very quick kinetics of oxygen consumption. It was found that as early as one minute after starting an intense exercise, the values of VO2 reach approx. 95% of values recorded later at the stage of stabilization (21, 48). This phenomenon, as it seems, is mainly used during races and training done on short distances, as well as during the 60-70 second sprint finishing a test (48).
A very high value of VO2 max is conditioned not only by intensity of consumption and utilization of oxygen by skeletal muscle but also by an exercise-induced increase in haemoglobin content in the blood (4). This reaction is primarily possible due to the ability of horses to store and release the reserve of erythrocytes from so called visceral stores (mainly the spleen) to peripheral blood in response to the growing demand for oxygen. Due to this mechanism, horses may even double the number of erythrocytes circulating in the blood, which translates into facilitating the transport of oxygen in the body in the same proportion (20, 35). Exercise-induced spleen contractions also result in an increase in the value of the haematocrit index. It was shown that this reaction results not only from the direct connection of that index value with the number of erythrocytes, but also with high index value (c 80%) in blood stored in the spleen (49). Additionally, during exercise an increase in the haematocrit index is heightened by a decrease in the volume of blood circulating as a result of displacement of water from plasma beyond the vascular system (mainly to the muscles), as well as the loss of water together with sweat (55). At this point, it should be noticed that exercise is not the only factor generating the above mentioned phenomena. An increase in values of ery-throcyte indices as a result contractions of the spleen, although less noticeable, also was found in stressful situations, such as horses entering a racetrack, await-ing the start, blood sample takawait-ing (43).
During intense work of muscles, increased oxygen saturation of the blood does not always meets the mus-cle demand. Following a quick depletion of reserve of oxygen bonded in muscles with myoglobin (oxymyo-globin), required energy is obtained from anaerobic processes of muscle glycolysis (45). It is claimed that the anaerobic metabolism threshold appears during muscle work performed at 60% of maximal value, which is accompanied by an increase of concentration of lactates in the blood plasma above 4 mmol/l (38). It was proved that the concentration of lactic acid in the blood increases exponentially in relation to an
increase of intensity of exercise and after a race it may reach the value of the order of 20-30 mmol/l (13, 38, 41). An increase of concentration of lactate ions in cells leads to despite good ability of horses to accumulate them disintegration of the cytoplasmic membrane structures, including decomposition of mitochondria, and even the destruction of entire muscle cells (9). In the plasma it has been demonstrated by an increase in the activity of enzymes (mainly creatinine phosphatase, lactate dehydrogenase, and asparagine aminotrans-ferase), which got out of damaged muscle cells (52). Also an increase in the number of phagocytes in the muscular tissue was demonstrated, proving the remo-val of damaged cells in the process of phagocytosis (7). Post-exercise measurement of the content of lactates in the blood is seen as one of the preferred methods of body function evaluation and also of training effectiveness (13, 38). It is of particular signifi-cance, since many goals of regular application of loads include intensification of blood flow, an increase in oxyhemoglobin and oxymyoglobin content, as well as improvement of gas exchange processes occurring in muscles and lungs (50). This translates into a decrease in the amount of lactic acid produced and a decrease in oxygen debt, among other things, which results in a higher functional capacity (26).
The meaning of respiratory and vascular mechanisms during exercise comes down not only to meeting the demand of active muscle cells for oxygen and energy constituents. These mechanisms are also responsible for the current elimination of many metabolites (lactic acid, pyruvate acid, phosphoric acid, CO2) as well as thermal energy. The conversion of energy accumula-ted in the form of phosphate bonds in adenosintriphos-phate (ATP) and phosphocreatinine into mechanical energy is relatively ineffective in view of the fact that as much as 80% of this energy is lost in the form of heat (2, 22). Whats more, the heat has to be dissipa-ted in order to avoid life-threatening protein denatura-tion. Horses control this growth mainly through heat loss during sweat evaporation, expansion of blood vessels increasing blood supply to the skin, as well as an increase in breathing frequency (37). It is estimated that these mechanisms allow the dissipation of meta-bolic heat as much as 65-75%, generated during the intense exercise of horses (23). It was empirically proved that sweat evaporation from the skin surface caused a lowering of its temperature in relation to deeper body layers by 2-4°C (23). However, in high temperature and air humidity conditions exercise may induce thermal stress. This occurs when the amount of generated heat exceeds the dissipation thereof. During long-distance test exercise, the main mecha-nism of removal of heat excess is evaporation along with sweat. Art and Lekeux (2) state that during tests continuing for many hours and held in favourable climatic conditions, about 5 l/h of water is lost that way, though in the case of high temperature and air
humidity as much as 10-15 l is lost. Loss of water (both from extra- and intracellular liquids) after a race may exceed even 40 l, which constitutes more than 15% of the entire water content in the body.
Perspiration is always accompanied by a loss of elec-trolytes. Unlike human sweat, equine sweat is hyper-tonic in relation to blood plasma and contains 249 and 139 mmol/l Na+, 78 and 3.7 mmol/l K+, and 301 and
100 mmol/l Cl- respectively (46). Deficiency of 25 l
of water in extracellular liquid induces deficiency of chloride ion content, which is 4000 moles, which in turn may result in metabolic alkalosis. It should be stressed that exercise-induced changes of water con-tent and electrolyte status compromise body function, and may even be life-threatening (37). However, ob-servations made by many authors indicate that regular training triggers numerous adaptation mechanisms targeted at the improvement of thermoregulation effi-ciency, such as an increase in blood plasma volume, better stability of cardiac and vascular functions, as well as improvement of giving up heat in the process of perspiration (17).
The above mentioned examples of numerous me-chanisms of adaptation of horses to exercise result in the fact that the animal is able to cover a 400 m distan-ce at the speed of 70 km/h, while top athletes only a short sprint distance at the speed of about 36 km/h (10). At the same time, it is fascinating that these animals are not an oversized version of human sports-men, thus constituting a unique model for research in exercise physiology.
Progress in human sports medicine, utilizing the determination of many blood indices for the evaluation of health state, body function, and body adaptation to various loads and advanced training has resulted in many methods being transplanted to horses. It was demonstrated in numerous studies covering the evalu-ation of values pertaining not only to haematological indices (27, 54), but also the determination of many metabolites (13, 32, 41), macro- and microelements (1), hormones (27) and enzymes in the blood, among other things (1, 31, 52). Results of the above mentio-ned studies show that post-exercise changes of values of many indices are generally very significant: how-ever, they are usually short-termed. It seems that the nature of the changes is a result of the high efficiency of compensatory mechanisms in horses, which are usually additionally enhanced during reasonably per-formed training programs. Overly intense exercise results in the changes of blood indices being sustained for longer period of time and may be a sign of horse overtraining (53).
Impact of physical exertion on immune system in horses
In recent years, many researchers have focused on immunological studies in horses. The researchers point out a close connection between the size of physical
loads and the immunological potential of these ani-mals, since it was repeatedly confirmed that modera-tely intense exercise has an immunostimulating effect. This is manifested, e.g. by a small, post-exercise increase in the number of leukocytes caused by weak neutrophilia and lymphocytosis, as well as an increase in phagocytary and bactericidal activity of ortho-chromophil granulocytes (12, 34, 42). On the other hand, it was found that heavy exercise loads generally resulted in definite leukocytosis, neutrophilia, penia, together with a lower share of CD4+ lympho-cytes and the number and cytotoxic activity of NK cells (28, 34, 44). The immunosuppressive impact of exhaustive exercise is also confirmed by temporary a decrease in the phagocytary and bactericidal activity of neutrophilia and monocytes in the blood as well as lung macrophages, lower intensity of respiratory explo-sion of those cells, a decrease in the neutrophilia chemotactic index value as well as impairment of lymphocyte proliferation (28, 34, 42, 44). According to Horohov et al. (25), impairment of equine immuno-logical mechanism functioning is caused by exercise resulting in an increase in heart beat count > 200/min and an increase in concentration of lactic acid in the plasma > 4 mmol/l, although ontogenetic conditions in this subject are determined by multiple factors. An immunomodulation effect was also found during many-month training programmes, whereas its nature depended on, as it was the case with single exercise, the level of applied loads (12). There are numerous grounds to believe that impairment of immunological mechanism functioning occurring as a result of ex-haustive exercise may result in (as it is the case with humans) the effect of an open window, that is open for infection factors, thus increasing equine incidence. It seems to be confirmed by epidemiological data indicating, e.g. the significantly higher frequency of occurrence of upper respiratory tract infection among competitive animals when compared to those utilized for recreational purposes (18).
It should be anticipated that future research in equine exercise physiology will contribute to a better understanding of the mechanisms and consequences of various physical loads. Knowledge of these issues will also allow greater control and optimal utilization of the capacity potential as well as possible modifica-tion of training programmes applied. It should be expected that this will also be reflected not only by better sports achievements but also in the improve-ment of equine health state and subsequent wellbeing.
References
1.Art T., Amory H., Desmecht D., Lekeux P.: Effect of show jumping on heart rate, blood lactate and other plasma biochemical values. Equine Vet. J. 1990, 9, 78-82.
2.Art T., Lekeux P.: Exercise-induced physiological adjustments to stressful conditions in sports horses. Livestock Prod. Sci. 2005, 92, 101-111. 3.Art T., Lekeux P.: Pulmonary mechanics during treadmill exercise in race
ponies. Res. Vet. Commun. 1988, 12, 245-258.
4.Bayly W. M., Grant B. D., Breeze R. G., Kramer J. W.: Cardiovascular effects of submaximal aerobic training on treadmill in Standardbred horses, using a standardized exercise test. Am. J. Vet. Res. 1988, 44, 544-553.
5.Bayly W. M., Hodgson D. R., Schultz D. A., Dempsey J. A., Gollnick P. D.: Exercise-induced hypercapnia in the horse. J. Appl. Physiol. 1989, 67, 1958--1966.
6.Butler P. J., Woakes A. J., Smale K., Roberts C. A., Hillidge C. J., Snow D. H., Marlin D. J.: Respiratory and cardiovascular adjustments during exer-cise of increasing intensity and during recovery in Thoroughbred racehorses. J. Exp. Biol. 1993, 179, 159-180.
7.Chiaradia E., Avellini L., Rueca F., Spaterna A., Porciello F., Antonioni M. T., Gaiti A.: Physical exercise, oxidative stress and muscle damage in racehorses. Comp. Biochem. Physiol. B Biochem. Mol. Biol. 1998, 119, 833-836.
8.Constantinopol M,. Jones J. H., Weibel E. R., Taylor C. R., Lindholm A., Karas R. H.: Oxygen transport during exercise in large mammals II. Oxygen uptake by the pulmonary gas exchanger. J. Appl. Physiol. 1989, 67, 871-878. 9.Dean A. S., Harris C. R.: Adenine nucleotide degradation in the thorough-bred horse with increasing exercise duration. Eur. J. Appl. Physiol. 1992, 65, 271-277.
10.Derman K. D., Nokes T. D.: Comparative aspects of exercise physiology, [in:] Hodgson D. R. and Rose R. J. (eds.): The Athletic Horse. Saunders W. B. Company, Philadelphia, PA 1994, pp. 14-18.
11.Engelhardt W. von: Cardiovascular effects of exercise and training in horses. Adv. Vet. Sci. comp. Med. 1977, 21, 173-205.
12.Escribano B. M., Agûera E. I., Vivo R., Santisteban R., Castejón F. M., Rubio M. D.: Benefits of moderate training to the nonspecific immune response of colts. Equine vet. J. 2002, 34 (Suppl.), 182-185.
13.Eto D., Hada T., Kusano K., Kai M., Kusunose R.: Effect of three kinds of severe repeated exercises on blood lactate concentrations in Thoroughbred horses on a treadmill. J. Equine Sci. 2004, 15, 61-65.
14.Evans D. L.: Cardiovascular adaptations to exercise and training. Vet. Clin. North Am. Equine Pract. 2002, 1, 513-531.
15.Evans D. L., Rose R. J.: Cardiovascular and respiratory responses in thorough-bred horses during treadmill exercise. J. Exp. Biol. 1988, 134, 397-408. 16.Evans D. L., Rose R. J.: Cardiovascular and respiratory responses to
sub-maximal exercise training in the thoroughbred horse. Pflügers Arch. 1988, 411, 316-321.
17.Geor R. J., McCutcheon L. J.: Thermoregulatory adaptations associated with training and heat acclimation. Vet. Clin. North Am., Equine Pract. 1998, 14, 97-120.
18.Hamlin M. J., Hopkins W. G.: Retrospective trainer-reported incidence and predictors of health and training-related problems in Standardbred race-horses. J. Equine Vet. Sci. 2003, 23, 443-452.
19.Hanak J., Jahn P., Kobes R., Sedlinska M., Zert Z., Mezerova J., Chvatal O.: A field study of oxygen consumption and estimated energy expenditure in the exercise horse. Acta Vet. Brno 2001, 70, 133-139.
20.Hinchcliff K. W., Geor R. J., Kaneps A. J.: Equine Exercise. The Science of Exercise in the Athletic Horse. Saunders W. B. Elsevier, Edinburgh, London, New York, Oxford, Philadelphia, St. Louis, Sydney, Toronto 2008. 21.Hodgson D. G., Rose R. J., Kelso T. B., McCutcheon L. J., Bayly W. M.,
Gollnick P. D.: Respiratory and metabolic responses in the horse during moderate and heavy exercise. Pflügers Arch. 1990, 417, 73-78.
22.Hodgson D. R., Davis R. E., McConaghy F. F.: Thermoregulation in the horse in response to exercise. Br. Vet. J. 1994, 150, 219-235.
23.Hodgson D. R., McCutcheon L. J., Byrd S. K.: Dissipation of metabolic heat in the horse during exercise. J. Appl. Physiol. 1993, 74, 1161-1170. 24.Hodgson D. R., Rose R. J., DiMauro J., Allen J. R.: Effects of a submaximal
treadmill training programme on histochemical properties, enzyme activities and glycogen utilization of skeletal muscle in the horse. Equine Vet. J. 1985, 17, 300-305.
25.Horohov D. W., Dimock A., Guirnalda P., Folsom R. W., McKeever K. H., Malinowski K.: Effect of exercise on the immune response of young and old horses. Am. J. Vet. Res. 1999, 60, 643-647.
26.Hyyppä S., Räsanen L., Pösö A.: Resynthesis of glycogen in skeletal muscle from Standardbred trotters after repeated bouts of exercise. Am. J. Vet. Res. 1997, 58, 162-166.
27.Iversen P. O., Stokland A., Rolstad B., Benestad H. B.: Adrenaline-induced leukocytosis: recruitment of blood cells from rat spleen, bone marrow and lymphatics. Eur. J. Appl. Physiol. 1994, 68, 219-227.
28.Jensen-Waern M., Lindberg A., Johannisson A., Grondahl G., Lindgren J. A., Essen-Gustavsson B.: The effects of an endurance ride on metabolism and neutrophil function. Equine Vet. J. 1999, 30 (Suppl.), 605-609.
29.Karlstrom K., Essen-Gustavsson B., Lindholm A.: Fibre type distribution, capillarization and enzymatic profile of locomotor and nonlocomotor muscles of horses and steers. Acta Anat. 1999, 151, 97-106.
30.Kayar S. R., Hoppeler H., Lindstedt S. L., Claassen H., Jones J. H., Essen--Gustavsson B., Taylor C. R.: Total muscle mitochondrial volume in relation to aerobic capacity of horses and steers. Pflügers Arch. 1989, 413, 343-347. 31.Kêdzierski W.: Correlation of plasma creatine kinase activity and glucose level in exercised Purebred Arabian horses. Med. Weter. 2011, 67, 541-544. 32.Kêdzierski W., Bergero D., Assenza A.: Trends of hematological and bio-chemical values in the blood of young race horses during standardized field exercise tests. Acta Veterinaria (Beograd) 2009, 59, 457-466.
33.Kobluk C. N., Gross G. M.: Exercise intolerance and poor performance in western performance and sprint horses. Vet. Clin. North Am. Equine Pract. 1996, 12, 581-606.
34.Korhonen P. A., Lilius E. M., Hyyppä S., Räsänen L. A., Pösö A. R.: Produc-tion of reactive oxygen species in neutrophils after repeated bouts of exercise in Standardbred trotters. J. Vet. Med. A 2009, 47, 565-573.
35.Kunugiyama I., Ito N., Narizuka M., et al.: Measurement of erythrocyte volumes in splenectomized horses and sham-operated horses at rest and during maximal exercise. J. Vet. Med. Sci. 1997, 59, 733-737.
36.Lafortuna C. L., Saibene F., Albertini M., Clement M. G.: The regulation of respiratory resistance in exercising horses. Eur. A. Appl. Physiol. 2003, 90, 396-404.
37.McConaghy F.: Termoregulation, [in:] Hodgson D. R., Rose R. J. (eds.): The Athletic Horse: Principles and Practice of Equine Sports Medicine. Saunders W. B., Philadelphia 1994.
38.Muñoz A., Castejõn F. M., Rubio M. D., Vivo R., Agûera E. I., Escribano B. M., Santisteban R.: How erythrocyte and plasma lactate concentrations are related in Andalusian horses during an exercise test and recuperation. J. Equine Sci. 1996, 7, 35-42.
39.Padilla D. J., McDonough P., Kinding C. A., Erickson H. H., Poole D. C.: Ventilatory dynamics and control of blood gases after maximal exercise in the Thoroughbred horse. J. Appl. Physiol. 2004, 96, 2187-2193.
40.Persson S.: Heart rate and blood lactate responses to submaximal treadmill exercise in the normally performing Standardbred Trotter age and sex variations and predictability from the total red blood cell volume. J. Vet. Med. A 1997, 44, 125-132.
41.Podolak M., Kêdzierski W., Janczarek I.: Intense training of Arabian horses and its effect on the level of selected biochemical indices in their blood and heart rate. Med. Weter. 2004, 60, 403-406.
42.Raidal S. L., Love D. N., Bailey G. D., Rose R. J.: Effect of single bouts of moderate and high intensity exercise and training on equine peripheral blood neutrophil function. Res. Vet. Sci. 2000, 68, 141-146.
43.Revington M.: Haematology of the racing Thoroughbred in Australia 1: Reference values and the effect of excitement. Equine Vet. J. 1983, 15, 141--144.
44.Robson P. J., Alston T. D., Myburgh K. H.: Prolonged suppression of the innate immune system in the horse following an 80 km endurance race. Equine Vet. J. 2003, 35, 133-137.
45.Roneus N., Essen-Gustavsson B.: Skeletal muscle characteristics and meta-bolic response to exercise in young Standardbreds. Am. J. Vet. Res. 1997, 58, 167-170.
46.Rose R. J., Arnold K. S., Church S., Paris R.: Plasma and sweat electrolyte concentrations in the horse during long distance exercise. Equine Vet. J. 1980, 12, 19-22.
47.Rose R. J., Evans D. L.: Training horses art or science? Equine Vet. J. 1990, 9 (Suppl.), 2-4.
48.Rose R. J., Hodgson D. R., Elso T. B., McCutcheon L. J., Reid T. A., Bayly W. M., Gollnick P. D.: Maximum O2 uptake, O2 debt and deficit, and muscle
metabolites in Thoroughbred horses. J. Appl. Physiol. 1988, 64, 781-788. 49.Sasimowski E., Budzyñski M.: Feeding horses. PWRiL, Warszawa 1987. 50.Schmidt W., Maassen N., Trost F., Boning D.: Training induced effects on
blood volume, erythrocyte turnover and hemoglobin oxygen binding proper-ties. Eur. J. Appl. Physiol. 1988, 57, 490-498.
51.Seeherman H. J., Morris E.: Application of a standarised treadmill exercise test for clinical evaluation of fitness in 10 Thoroughbred racehorses. Equine Vet. J. 1990, 9 (Suppl.), 26-34.
52.Stopyra A.: Gasometry indices and the activity of selected enzymes in horse serum during extreme exertion. Med. Weter. 2002, 58, 543-548.
53.Szarska E.: The changes of selected blood parameters after different distan-ces covered by endurance horses. Med. Weter. 2001, 57, 522-526. 54.Tyler C. M., Golland L. C., Evans D. L., Hodgson D. R., Rose R. J.: Skeletal
muscle adaptations to prolonged training, overtraining and detraining in horses. Pflügers Arch. 1998, 436, 391-397.
55.White C.: Thermoregulation and fluid balance in the horse. Dist. Int. 2001, 6, 20-23.
56.Wysocka B.: Treadmill endoskopy of the upper airways in the horse. ¯ycie Wet. 2006, 81, 198-201.
Corresponding author: prof. dr hab. in¿. Bogdan Janicki, ul. Mazowiecka 28, 85-084 Bydgoszcz, Poland; e-mail: janicki@utp.edu.pl
