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Cardiovascular effects of intermittent or continuous treadmill conditioning in horses
Refereed
CARDIOVASCULAR EFFECTS OF INTERMITTENT OR CONTINUOUS TREADMILL CONDITIONING IN HORSES A.V. Rodiek, PhD1; L.M. Lawrence, PhD2 and M.A. Russell, PhD3
SUMMARY
Nine mature Quarter Horse mares were trained for ten weeks by continuous (n=4) or intermittent (n=5) treadmill work. Both groups of horses trotted at 3.3m/sec. The continuous work was performed at a 3% incline while the intermittent work was done at a 9% incline. Heart rate, cardiac output and blood lactate concentration were measured during a standard exercise test that included work on both treadmill grades before and after five and ten weeks of training. The two conditioning programs did not produce differences in the measured parameters of the two groups of horses. However, there was an overall conditioning effect observed in both groups of horses throughout the ten weeks of treadmill conditioning. Heart rate decreased (P<.05) during exercise on both treadmill grades after ten weeks of training. Lactate concentration decreased (P<.05) during the 9 percent grade trot after five weeks of training. Only slight further decreases occurred between week five and week ten of conditioning. Cardiac output increased (P<.05) between rest and exercise on both grades but there was no conditioning effect. INTRODUCTION
Interval training has been widely acclaimed for its ability to improve the performance of human athletes7. In interval training and other types of intermittent conditioning, the Authors' address: 1Departmentof Animal Science,CaliforniaState University-Fresno,Fresno,California93740; 2Departmentof Animal Science, Universityof Illinois,Urbana, Illinois61801; 3Depaltment of Animal Science,Purdue University,West Lafayette,Indiana 47907. 14
savings in fatigue accompanying intermittent work can be converted to an increase in the intensity of work performed, which is directly related to the amount of improvement in the energy capacities of the muscles35 Some horsemen are presently trying to adapt the concepts of interval training to conditioning programs traditionally used to train race horses. Unfortunately, only a few studies have been done to investigate the physiological changes induced by different exercise loads and conditioning regimes on horses.lOd3 Comparisons of training methods have been hampered by a lack of a clear consensus of what physiological parameters are most valuable in estimating fitness in horses. Many authors cite changes in cardiovascular parameters such as heart rateS,14,19 and cardiac output2 or metabolic parameters such as lactate' concentration16, I7 as being useful monitors of fitness. However, few studies have attempted to measure several parameters simultaneously to elucidate the interrelationship of physiological changes during exercise.6,16 Additionally, few attempts have been made to make measurements during exercise itself and to conduct reproducible, standardized exercise tests to reduce the effects of outside variables on the measured parameters.2, 3 Under the different testing conditions used in many studies results often disagree or are difficult to compare. Discrepancies in experimental findings highlight the need for controlled and reproducible test conditions such as those of a standard exercise test. The purpose of this study was to examine the effect of two treadmill conditioning programs: an interval-type intermittent work and rest program and a more conventional continuous work regime, on the response of selected cardiovascular parameters to a standard exercise test performed on EQUINE VETERINARY SCIENCE
TABLE 1 Conditioning Schedule* Intermittentwork (9% grade)
Continuous work (3% grade)
Weeks Phase I - increasingwork time (rest interval = 3 min) 1 2 3 4 5 6 7
SET 1 1.0 minx 3 reps = 3.0 1.5 minx 3 reps = 4.5 2.0 minx 3 reps = 6.0 2.5 minx 3 reps = 7.5 3.0 minx 3 reps = 9.0 SET 2
SET 1 9.0 13.5 18.0 22.5 27.0 SET 2
Phase II - decreasingrest time (workinterval = 3 min) 8 9 10 11 12 13
9 min / 2.0 min rest 9 min / 1.5 min rest 9 min / 1.0 min rest 9 rain / 0.5 min rest 9 min / 0.0 min rest SET 3
27.0 27.0 27.0 27.0 27.0 SET 3
*Trotting time in minutes;horses were worked three times per week (Mon., Wed., Fri.)
a treadmill. A further aim was to assess changes in these parameters to the general conditioning effects of treadmill work. MATERIALS AND METHODS Nine mature Quarter Horse mares were randomly assigned to either continuous (four horses) or intermittent (five horses) treadmill exercise. All the mares were either four or five years old and ranged in body weight from 470 to 570 kg. The horses conditioned by continuous work trotted on a treadmill a set at a 3 percent incline. The horses conditioned by intermittent work trotted at the same speed (3.3m/sec) but at a 9 percent grade. Physical work (W) performed on the treadmill was calculated by the formula: W = F x d, where F was body weight and d was the vertical distance the animal theoretically travelled on the treadmill. Since vertical displacement is proportional to treadmill grade and all horses trotted at the same speed, then by the equation, d = t x v, the vertical distance (d) travelled on each grade was determined by the time (t) spent trotting. In order to maintain total physical work approximately equal between the two groups, trotting time was inversely proportional to treadmill grade. The study was divided into two 5-week segments with a standard exercise test (SET) administered at the beginning of the study and at the end of each part. In the first phase, work time was gradually increased by 4.5 minute increments weekly for the horses conditioned by continuous work and by 0.5 minute increments in each of three work bouts for the intermittently conditioned horses. The rest aAnamill Co., Vermilion, Ohio 44089
Volume 7, Number I
periods between work bouts were each three minutes. In the second phase, total work was maintained constant but the rest intervals were progressively shortened for the horses conditioned by intermittent work. The continuously conditioned horses trotted 27 minutes while the intermittent horses trotted three 3-minute periods. The rest intervals were progressively decreased (by 0.5 minutes per period per week) until after five weeks the intermittently conditioned horses were trotting nine consecutive minutes (Table 1). A two-part SET was used to assess the horses at two work intensities, the 3 and 9 percent grades. After a twominute rest period and a three-minute warm-up walk, the horses trotted for 15 minutes at 3.3m/sec on a 3 percent grade. A six minute standing recovery period was followed by ten minutes of rest in a restraint stock while the treadmill was raised to a 9 percent grade. The horses rested and walked for one minute each and then trotted for five minutes at the same speed and recovered for six minutes on the steeper grade. The SET was clearly submaximal in intensity. However, the horses were very unfit at the beginning of the study having been maintained in paddocks without exercise for one year prior to the start of the experiment and appeared to be unable to complete a more strenuous test. Heart rate was monitored by hardwire electrocardiogramb the last ten seconds of every minute of the SET (except the ten-minute rest period between parts of the SET) and continuously during the t-~t minute of both recovery periods. Cardiac output was determined by the dye-dilution technique4. A length of polyethylene tubing (pe 2009 was introduced into the pulmonary artery and served as a catheter for mixed venous blood samples and dye injection. Arterial blood for cardiac output determination was withdrawn through a short catheter~ in the carotid artery which had previously been relocated to a subcutaneous position.22 Venous blood samples were drawn (for determination of lactate concentration), and cardiac output determinations were made at rest at the beginning of the SET and during the last minutes of trot and recovery on both treadmill grades. The effects of activity (rest, work, recovery), SET (before conditioning, 1; after five weeks of conditioning, 2; and after ten weeks of conditioning, 3; and conditioning group (intermittent or continuous work) were evaluated by a threeway analysis of variance. Duncan's procedure 21 for mean comparisons was used when significant F tests were found. All differences reported are significant at the five percent probability level. RESULTS Table 2 shows the mean heart rates of the horses conditioned by continuous or intermittent work in each activity (mean of all minutes within activity) during the three bRodiek, AV, Cardiovascularfitness of horses conditionedby treadmill work. Ph.D. Thesis, Univ of Illinois, Urbana, 1984 eClay Adams Div., Becton DickinsonCo. Parsippany,New Jersey 07054 d20 gauge-2 inch, E-Z Cath Catheter, The Deseret Co., Sandy, Utah 84070 15
SETs. Heart rates of the horses gained intermittently were consistently higher than the heart rates of the continuously Heart Rates of Horses Conditioned conditioned horses in all three exercise tests. Mean heart in Each Activity During the Three SETs * rates declined between SETs 1 and 3 in all activities (rest, walk, trot, and recovery). Decreases were significant during Activity and SET Heart Rate (beats per minute.) the 3 and 9 percent grade trots and during recovery minutes C I Mean 2 through 6 on the 9 percent grade. 3% G R A D E Neither cardiac output nor stroke volume decreased sigREST (2 min) nificantly with progressive conditioning nor were there SET 1 51+1.9 59+1.9 55 2 48+1.8 57+_2.5 53 significant differences between conditioning groups (inter3 48+_2.5 57+_3.9 53 mittent or continuous). Increases in mean cardiac output between rest and exercise averaged 210 percent on the 3 iiiii~Kiii~i ~iiiiiiii~iiiiii~i~!~iii~i~iiii~iiiiii!iiiiiiiiiiiiiiiiiiii~iiiiiiiiiii~i~iii~ii~!!iiii iiiiiiiiiiiiiiiiiiiiiiiiiiii~ percent grade and 290 percent on the 9 percent grade. Heart iiiiiiiiiSE~i~iiiiiiiiiiiiii!iiiiiiiiiiiiiiiiiiii iiiiiiiiiiiii7~3~i~iiiiii~i~i~i~i~ii!!!8~i3iiiiiiiiiii~ii~i~i~i~ii~i~i~i~i~i~i~i~ii~ii~i~i iiiiiiiiiiiiiiiii~!iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiliiiii iiii~~i!~iiiiiiiiiiiiiiiiiiiiiiiiiiiis~i~iiiiiiiiiiiiiiiiiiiiiiiiiiiii iiiiii~i~iiiiiiiiiiiiiiiiiiiiiiiiii rates measured concurrently with cardiac output increased iiiiiiiiiiiiiiiii~iiii!iliiiiiiiiiiiiiiiiiiiii~i~i~!~iiiiiiiiiiiiiiiiiii~ii~iiiiiiiiiiii~i~iii~ii~ii~ii~ii~ii~i~iii~ii~iiiii~ 190 and 240 percent, between rest and the 3 and 9 percent grade trots. Although stroke volume increased by TROT (15 min) approximately 10 percent between rest and the 3 percent SET 1 128_+0.8 138+1.1 133 grade trot and 15 percent between rest and exercise on the 2 125+1.1 135+1.5 130 3 123+1.2 125+1.2 124b steeper grade (Table 3), these changes were not significant. Within the first SET there were significant differences in the lactate concentrations of the groups of horses conditioned by intermittent or continuous work (Table 4). During the 9 percent grade trot and recovery, lactate concentration averaged 45 and 42 mg/100ml, respectively, RECOVERY (min 2-6) for the horses conditioned by intermittent work and 29 and SET 1 71+_2.5 72Y.1.5 72 24 mg/100ml, respectively, for the horses conditioned by 2 61+1.7 70-I-_1.4 66 continuous work. This discrepancy was caused by the 3 5 +8~_2.2 67+1.2 63 extremely high lactate value (75mg/100ml) of one horse in 9% GRADE the intermittent work group and by one horse with an especially low value (14mg/100ml) in the continuous work ~:4:: ::i :: 55~ :i::::~:~:~:~:i~i~i ~ ~::::::::::::::::::::::::::::::::::::::::::::::::::::: group. After 5 and especially after 10 weeks of conditioning : ~ iiiiiiiiiiii:: :: ii~.~ iiiiiiiiiiiiiii~ii~iiiiiiiii::iili :: ~ the lactate concentrations of the two groups became more : i ::::iiilliliiiiiiiii~i~ :~::i!ii::i i~i:: i ~::: ::: ii::i similar, and even though the intermittently conditioned WALK (1 min) horses tended to have higher values than the horses SET 1 965:4.7 104+_3.8 100 conditioned by continuous work, the differences were 2 100-&_3.2 97+_3.1 98 insignificant. Therefore the data from the two groups were 3 91±4.2 96+4.8 94 combined for analysis of the effects of progressive conditioning. Lactate concentrations decreased between SETs 1 and 2 i:i !~iiiiiiiiii :6iiiiiiii ~5~iiiiiiiiiiiiiiiiiiiiiiiiiiiiii::ii~ during the trot and recovery on both treadmill grades. The lactate production during the 3 percent trot in the first SET was approximately the same as the lactate values found RECOVERY (rain 1) SET 1 141+_5.3 116+4.3 115 during the 9 percent grade trot in the second SET and by the 2 107+_5.6 113+4.4 110 third exercise test lactate values did not rise significantly 3 103+_5.6 110-2_4.7 107 above the resting value during the entire test. TABLE 2
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Values are mean +s+_SE *C = Continuous work; I = InterrniUent work (average of all minutes
within activity) aindicates significant differences between means on the same line b(hOrSes conditioned by continuous or intermittent work) (P<.05) indicates significant differences between means within activity (intermittent and continuous values combined) (P<.05)
16
DISCUSSION AND C O N C L U S I O N S
Reports on the effect of conditioning on resting heart rate have been conflicting. Some experimental work has shown a training-induced resting bradycardia in horses 14 while other research has not.1630 In the present study, a trend was observed towards lower pre-exercise heart rates, but the changes were not significant. Part of the decrease observed may have been due to lessening of nervous anticipation as the horses became more accustomed to the conditioning and testing procedures. EQUINE VETERINARY SCIENCE
TABLE 3 Cardiac Output and Stroke Volume Values* Activity and SET C
#!
Cardiac Output (l/rain) I
::::::::::::::::::::::::::::::::::::::::::: ~S ~::7i~ :: ::::::! :: :::::::: ~ ~ ~
Stroke Volume (l/stroke) C I
Mean
Mean
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TROT-- 3%
SET 1 2 3
713 _+ 14.0 196 _+ 14.0 155 + 25.1
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TROT 9% SET 1 2 3
164 _+12.1 202 _+22.2 165 -+ 7.2
210 _+ 12.0 218 _+ 4.6 228 _+ 5.0
168 200a 162
1.36 + .12 1.53 _+.06 1.28 -+ .71
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214 _+ 19.4 214 _+ 7.9 237 _+ 8.8
212 216 234a
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1.35 _+.10 1.40 + .09 1.55 + .17
1.22 -+. 88 1.48 _+.20 1.32 +. 04
1.28 1.50 1.31
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1.34 _+.09 1.33 _+.14 1.54 _+.08
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1.34 1.37 1.54
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Values are means_+SE;No significant differences between means of horses conditioned by continuous or intermittent work alndicates significant differences from SET 1 within activity (P<.05) *Values are during rest, exercise and recovery on two treadmill grades of the three SETs of horses conditioned by continuous (C) or intermittent (I) work
TABLE 4 The occurrence o f an exercise bradycardia with conditioning, as was f o u n d in this study, has b e e n previously reported.2a 4 D u r i n g submaximal exercise, decreased sympaActivity and SET Lactate concentration (mg/100ml) C I Mean thetic drive rather than increased parasympathetic tone, is REST thought to cause lower exercising heart rates after condiSET 1 9.54_+_2.07 9.58+1.66 9.56 tioning. 9,15 A decreased sympathetic drive could originate 2 5.99i-_1.11 6.81_+0.60 6.45 3 6.17_+0.67 6.49~0.67 6.35 from intracardiac or extracardiac factors.aS In the former situation an increase in stroke v o l u m e during s u b m a x i m a l work nOT ::3 ::::::::::::::::::::::::::::iiii::iiiii::i::i::ili::i which, in m a n results f r o m training, m e a n s that at the same g gTIiii: ::i~!8~_~5~::~ ::::~~~::~:~:i ~:88~{ ~:::::::::::::::::::::::~i~i~ :::::::::::::::::::::::::::::: ai99 a or slightly lower cardiac output, the drive for higher heart rate via sympathetic nervous stimulation to the heart is reduced. Extracardiac m e c h a n i s m s affect the heart indirectly RECOVERY-- 3% SET 1 14.84!3.84 14.29+__2.64 14.53a as a result o f alterations in the trained skeletal muscle. 4 In2 5.31-+0.34 8.07-+0.91 6.84 creased o x y g e n extraction andutilization, a n d neuronal feed3 5.60-+0.43 6.72-+0.65 6.22 back from the muscles and joints m a y alter sympathetic !! : i outflow. There was n o trend towards an increased stroke v o l u m e with c o n d i t i o n i n g in this e x p e r i m e n t or in other }{ longitudinal training studies36 However, between SETs 1 ::::::::::::::::::: 1:i5~ i::i7 :::::::::::::::::::::::::: ~::::::::::::::::::::::::::::::::::::::::::: ::4{4} ~!~i79 {::: ::{ i}i:::iiU:!::!:::#: ::::: ::::i::ii:::::::::::::::::::::::::::::: ::::::::::::~::5):: : : ;:i15:::ii::::::::::::::::::::::::::::::::::::::::::::::::: !~:{:::: and 2, lactate concentrations decreased significantly, indiRECOVERY--9% cating i m p r o v e d o x y g e n extraction and/or utilization. SET 1 23.54+_5.27 42.30-+12.09 b 33.96a Extracardiac rather than intracardiac factors appear to be re2 9.15+1.04 11.57_+1.21 10.49 sponsible for the exercise bradycardia f o u n d in this study. 3 8.50-+1.07 8.77_+0.76 8.65 Values arc means_+SE The decreased lactate concentration c o n c u r r e n t with un*Values are during rest, exercise and recovery on two treadmill grades changed cardiac output and stroke v o l u m e indicates that of the three SETs of horses conditioned by continuous (C) or intermittent (I) work either the distribution o f cardiac o u t p u t was altered to inaindicates significant differences between means within activity crease blood flow to the working muscles at the expense of (intermittent and continuous values combined)(P<.05) other organs or the m u s c l e cells b e c a m e m o r e efficient in bindicates significant differences between means on the same line (horses conditioned by continuous or intermittent work) (P<.05) oxygen utilization for the work o f contraction.
Lactate Concentration*
Volume 7, Number I
17
Neither cardiac output nor stroke volume changed consistently with progressive conditioning or between the groups of horses trained by intermittent or continuous work. Other longitudinal studies involving aerobic conditioning have shown either no change16 or a decrease2 in cardiac output. In humans, conditioning may cause no change or a slight decrease in cardiac output during submaximal work that requires the same oxygen consumption.15 Similar increases in stroke volume during submaJdmal treadmill work, as determined in this study, have been reported2,3 but no correlation has been found between stroke volume and cardiac output) In both dogs TM and manl the increase in cardiac output during exercise is attained primarily through tachycardia with little contribution by increased stroke volume. The physiological mechanisms responsible for the decreased accumulation of lactate during submaximal work following aerobic conditioning is not entirely known. Research on the adaptation of human skeletal muscle to endurance exercise has shown an increase in the capacity for aerobic metabolism. This is manifested as an increased number and size of muscle cell mitochrondia and an increase in the levels of a number of mitochondrial enzymes, including those involved in fatty acid oxidation, the citric acid cycle, and components of the respiratory chain. 12 This adaptation has also been reported in horses. 11 The relationship of heart rate to lactate concentration changes with conditioning. The lactate production at a given heart rate decreased markedly between SETs 1 and 2, but were approximately the same during SETs 2 and 3. There has been some speculation among horse researchers over the presence of an anaerobic threshold during exercise. Based on the relationship of lactate concentration to heart rate, the anaerobic threshold has been set at heart rates between 150 and 180 bpm. 8 From the results of this experiment, however, it seems that a heart rate for the anaerobic threshold would vary with the fitness of the subject. Work that was done at a given heart rate in the unfit horse produced a much higher lactate concentration than was found at the same heart rate after conditioning. No attempt was made in this study to assign a heart rate value to the anaerobic threshold. The wide range of heart rates set forth by other researchers for an anaerobic threshold may all be correct for the horses used in a given experiment depending on their level of aerobic fitness at the time. The intermittent or continuous work conditioning programs did not promote different physiological adaptations to submaximal exercise. The higher values of the cardiovascular and metabolic parameters measured in the horses conditioned by intermittent work appeared to be inherent in the horses themselves and not a function of the intermittent exercise as the difference between the intermittent and continuously conditioned horses were largest in the first SET (before conditioning began) and smallest in the third SET. It has been suggested that studies comparing intermittent and continuous conditioning on an equal work basis, as in the present study, do not reveal the advantages o f interval 18
training. This is because an inherent quality of interval training is that the rest periods allow for more work to be done in each training session and therefore more conditioning can be accomplished at a faster rate. The results of this study indicate that an interval-type gaining program may be accomplished on a treadmill, but, at the workload used, it is no more successful than an equal work program of continuous trotting at improving cardiovascular and metabolic fitness. It is interesting that the horses conditioned at the lower intensity were as capable of coping with the higher intensity work as those conditioned at that intensity. From a practical standpoint, however, it took fewer minutes per horse per day on the interval program to get the same conditioning effects as the continuous program. This study also indicates that the largest proportion of the improvement in fitness occurred during the first five weeks of conditioning and was due either to changes at the cellular level (manifested as a decrease in lactate production) or changes in relative distribution of the cardiac output to the working muscles and other organs with only minor changes in total cardiac output. Further research is needed to elucidate the strengths a n d weaknesses of interval training and to establish appropriate work and rest intervals for specific training goals. It may be that work intensity and/or duration may have to be increased to illuminate the attributes of interval training as the work intensity in this study was far below the work levels at which interval training is usually employed, that is, for Thoroughbreds and Standardbreds in race training or for other high intensity events. REFERENCES
1. Astrand P, Rodahl OK: Textbook of Work Physiology. St. Louis: McGraw-HillBook Co. p 424, 1977. 2. Bayly WM, Gabel AA and Barr SA: Cardiovascular effects of subrnaximal aerobic training on a treadmill in Standardbred horses, using a standardized exercisetest. Am J Vet Res 44:544-553, 1983. 3. Bergsten G: Blood pressure, cardiac output and blood-gas tension in the horse at rest and during exercise. Acta Vet Scand (Suppl) 48:1-88, 1974. 4. Clausen JP: Effect of physical training on cardiovascular adjustments to exercise in man. Physiol Rev 57:779-815, 1977. 5. Engelhardt WV: Cardiovascular effects of exercise and training in horses. Adv Vet Sci 21:173-205, 1977. 6. Freeman JH, Bayly WM, Grant BD and Gollniek PD: Cardiopulmonary responses to a traditional method of training the Thoroughbred racehorse. Proc 8th Eq Nut and Physiol Symp, Lexington, p 193-198, 1983. 7. Fox E and Mathews D: Interval Training, Conditioning for Sports and General Fitness. Philadelphia: WB Saunders Co, p 1, 1974. 8. Fregin GF: Combined training panel: Cardiovascular response to exercise: A review. Proc 21st Ann Cony Am Assoc Eq Practitioners, Boston, p 311-313, 1975. 9. Frick MH, Elovainio RO and Somer T: The mechanism of bradycardia evoked by physical training. Cardiologia 51:46-54, 1967. 10. Gabel AA, Milne DW, Muir WW and Skarda RT: Interval compared to conventional training of Standardbred horses: Lactic acid and cardiovascular data (Abstract). Proc 1st Int Conf on Equine Exercise Physiol, Oxford, p 41, 1982. 11. Guy PS and Snow DH: The effect of training and detraining on muscle compositionin the horse. J Physiol 269:33-51, 1977. EQUINE VETERINARY SCIENCE
12. Holloszy JO: Adaptation of skeletal muscle to endurance exercise. Med Sci Sports 7:155-164, 1975. 13. Lindholm A and Saltin B: The physiological and biochemical response of Standardbred horses to exercise of varying speed and duration. Acta Vet Scand 15:310-324, 1974. 14. Marsland WP: Heart rate response to submaximal exercise in the Standardbred horse. J Appl Physiol 24:98-101, 1968. 15. Mathews DK and Fox EL: The Physiological Basis of Physical Education and Athletes (2rid Ed) Philadelphia: WB Saunders Co, p 250-286, 1976. 16. Milne DW, Gabel AA, Muir WW and Skarda RI: Effects of training on heart rate, cardiac output and lactic acid in Standardbred horses, using a standardized exercise test. J Am Vet Med Assoc 1:131135, 1977. 17. Milne DW, Skarda RT, Gabel AA, Smith LG and Ault K: Effects of training on biochemical values in Standardbred horses. Am
J Vet Res 37:285-2990, 1976. 18. Rushmer RF: Cardiovascular Dynamics (2rid Ed) Philadelphia: W.B. Saunders Co, p 195-196, 1967. 19. Sigler DI-I, Webb SP and Potter GD: Physiological response of horses in different conditioning regimes to a standard exercise tolerance test on a treadmill. Proc 6th Equine Nut and Physiol Syrup, College Station, p 48-54. 20. Skarda RT, Muir WW, Milne DW and Gabel AA: Effects of training on resting and post-exercise ECG in Standardbred horses, using a standard exereise test. Am J Vet Res 37:1485-1488, 1976. 21. Steel GD and Torfie JI-I: Principles and Procedures of Statistics. New York: McGraw-HiU Book Co, p 107-109, 1960. 22. Tavernor WD: Technique for the subcutaneous relocation of the common carotid artery in the horse. Am J Vet Res, 30:1881-1883, 1969.
ERRATA
frequency of inbreeding in wild horses. Evo 41:229-231, 1987. Summary: In the Granite Range feral horse population, incestuous, father-daughter matings are avoided by positive action of both sexes. Young females disperse from their rearing bands regardless of their Irue genealogic relationship to the band's resident stallion and, even when presented with the opportunity, stallions do not copulate with young females with whom they are familiar. Thus, as stated before (Berg op. cit.), the observed level of inbreeding in feral horse populations may be much less than that expected under a random breeding system. It is of interest to note that 21% of the females returned to their rearing band after copulating with non-harem males. The implication(s) of this latter finding is not yet known.
Berg W: Effective population size and inbreeding in feral horses: a preliminary assessment. J Eq Vet Sci 6:240-245, 1986. Page 240, column 2, line 12: "Nm = number of successfully breeding males;." Page 243, column 1, line 18: replace "impaction" with "impact." Page 245, column 1, line 36: delete "-" from the word "statistical." ADDENDUM
Berger J, Cunningham C: Influence of familiarity of
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Vth International Conference on Equine Infectious Diseases Lexington, Kentucky • October 7-10, 1987 Abstracts of scientific papers are being accepted before March I for consideration on the following topics: Herpesviruses • Strangles and other respiratory infections • Influenza • Vector-borne diseases • Gastrointestinal diseases • Infections of the Reproductive System For more information, contact: Donna A. Hall, Special Programs University of Kentucky, 204 Frazee Hall Lexington, Kentucky 40506-0031
(606) 257-3929
Sponsoredby Universityof KentuckyDepartmentof VeterinaryScience. Volume 7, Number I
lg
CARDIOVASCULAR EFFECTS OF INTERMITTENT OR CONTINUOUS TREADMILL CONDITIONING IN HORSES A.V. Rodiek, PhD1; L.M. Lawrence, PhD2 and M.A. Russell, PhD3
SUMMARY
Nine mature Quarter Horse mares were trained for ten weeks by continuous (n=4) or intermittent (n=5) treadmill work. Both groups of horses trotted at 3.3m/sec. The continuous work was performed at a 3% incline while the intermittent work was done at a 9% incline. Heart rate, cardiac output and blood lactate concentration were measured during a standard exercise test that included work on both treadmill grades before and after five and ten weeks of training. The two conditioning programs did not produce differences in the measured parameters of the two groups of horses. However, there was an overall conditioning effect observed in both groups of horses throughout the ten weeks of treadmill conditioning. Heart rate decreased (P<.05) during exercise on both treadmill grades after ten weeks of training. Lactate concentration decreased (P<.05) during the 9 percent grade trot after five weeks of training. Only slight further decreases occurred between week five and week ten of conditioning. Cardiac output increased (P<.05) between rest and exercise on both grades but there was no conditioning effect. INTRODUCTION
Interval training has been widely acclaimed for its ability to improve the performance of human athletes7. In interval training and other types of intermittent conditioning, the Authors' address: 1Departmentof Animal Science,CaliforniaState University-Fresno,Fresno,California93740; 2Departmentof Animal Science, Universityof Illinois,Urbana, Illinois61801; 3Depaltment of Animal Science,Purdue University,West Lafayette,Indiana 47907. 14
savings in fatigue accompanying intermittent work can be converted to an increase in the intensity of work performed, which is directly related to the amount of improvement in the energy capacities of the muscles35 Some horsemen are presently trying to adapt the concepts of interval training to conditioning programs traditionally used to train race horses. Unfortunately, only a few studies have been done to investigate the physiological changes induced by different exercise loads and conditioning regimes on horses.lOd3 Comparisons of training methods have been hampered by a lack of a clear consensus of what physiological parameters are most valuable in estimating fitness in horses. Many authors cite changes in cardiovascular parameters such as heart rateS,14,19 and cardiac output2 or metabolic parameters such as lactate' concentration16, I7 as being useful monitors of fitness. However, few studies have attempted to measure several parameters simultaneously to elucidate the interrelationship of physiological changes during exercise.6,16 Additionally, few attempts have been made to make measurements during exercise itself and to conduct reproducible, standardized exercise tests to reduce the effects of outside variables on the measured parameters.2, 3 Under the different testing conditions used in many studies results often disagree or are difficult to compare. Discrepancies in experimental findings highlight the need for controlled and reproducible test conditions such as those of a standard exercise test. The purpose of this study was to examine the effect of two treadmill conditioning programs: an interval-type intermittent work and rest program and a more conventional continuous work regime, on the response of selected cardiovascular parameters to a standard exercise test performed on EQUINE VETERINARY SCIENCE
TABLE 1 Conditioning Schedule* Intermittentwork (9% grade)
Continuous work (3% grade)
Weeks Phase I - increasingwork time (rest interval = 3 min) 1 2 3 4 5 6 7
SET 1 1.0 minx 3 reps = 3.0 1.5 minx 3 reps = 4.5 2.0 minx 3 reps = 6.0 2.5 minx 3 reps = 7.5 3.0 minx 3 reps = 9.0 SET 2
SET 1 9.0 13.5 18.0 22.5 27.0 SET 2
Phase II - decreasingrest time (workinterval = 3 min) 8 9 10 11 12 13
9 min / 2.0 min rest 9 min / 1.5 min rest 9 min / 1.0 min rest 9 rain / 0.5 min rest 9 min / 0.0 min rest SET 3
27.0 27.0 27.0 27.0 27.0 SET 3
*Trotting time in minutes;horses were worked three times per week (Mon., Wed., Fri.)
a treadmill. A further aim was to assess changes in these parameters to the general conditioning effects of treadmill work. MATERIALS AND METHODS Nine mature Quarter Horse mares were randomly assigned to either continuous (four horses) or intermittent (five horses) treadmill exercise. All the mares were either four or five years old and ranged in body weight from 470 to 570 kg. The horses conditioned by continuous work trotted on a treadmill a set at a 3 percent incline. The horses conditioned by intermittent work trotted at the same speed (3.3m/sec) but at a 9 percent grade. Physical work (W) performed on the treadmill was calculated by the formula: W = F x d, where F was body weight and d was the vertical distance the animal theoretically travelled on the treadmill. Since vertical displacement is proportional to treadmill grade and all horses trotted at the same speed, then by the equation, d = t x v, the vertical distance (d) travelled on each grade was determined by the time (t) spent trotting. In order to maintain total physical work approximately equal between the two groups, trotting time was inversely proportional to treadmill grade. The study was divided into two 5-week segments with a standard exercise test (SET) administered at the beginning of the study and at the end of each part. In the first phase, work time was gradually increased by 4.5 minute increments weekly for the horses conditioned by continuous work and by 0.5 minute increments in each of three work bouts for the intermittently conditioned horses. The rest aAnamill Co., Vermilion, Ohio 44089
Volume 7, Number I
periods between work bouts were each three minutes. In the second phase, total work was maintained constant but the rest intervals were progressively shortened for the horses conditioned by intermittent work. The continuously conditioned horses trotted 27 minutes while the intermittent horses trotted three 3-minute periods. The rest intervals were progressively decreased (by 0.5 minutes per period per week) until after five weeks the intermittently conditioned horses were trotting nine consecutive minutes (Table 1). A two-part SET was used to assess the horses at two work intensities, the 3 and 9 percent grades. After a twominute rest period and a three-minute warm-up walk, the horses trotted for 15 minutes at 3.3m/sec on a 3 percent grade. A six minute standing recovery period was followed by ten minutes of rest in a restraint stock while the treadmill was raised to a 9 percent grade. The horses rested and walked for one minute each and then trotted for five minutes at the same speed and recovered for six minutes on the steeper grade. The SET was clearly submaximal in intensity. However, the horses were very unfit at the beginning of the study having been maintained in paddocks without exercise for one year prior to the start of the experiment and appeared to be unable to complete a more strenuous test. Heart rate was monitored by hardwire electrocardiogramb the last ten seconds of every minute of the SET (except the ten-minute rest period between parts of the SET) and continuously during the t-~t minute of both recovery periods. Cardiac output was determined by the dye-dilution technique4. A length of polyethylene tubing (pe 2009 was introduced into the pulmonary artery and served as a catheter for mixed venous blood samples and dye injection. Arterial blood for cardiac output determination was withdrawn through a short catheter~ in the carotid artery which had previously been relocated to a subcutaneous position.22 Venous blood samples were drawn (for determination of lactate concentration), and cardiac output determinations were made at rest at the beginning of the SET and during the last minutes of trot and recovery on both treadmill grades. The effects of activity (rest, work, recovery), SET (before conditioning, 1; after five weeks of conditioning, 2; and after ten weeks of conditioning, 3; and conditioning group (intermittent or continuous work) were evaluated by a threeway analysis of variance. Duncan's procedure 21 for mean comparisons was used when significant F tests were found. All differences reported are significant at the five percent probability level. RESULTS Table 2 shows the mean heart rates of the horses conditioned by continuous or intermittent work in each activity (mean of all minutes within activity) during the three bRodiek, AV, Cardiovascularfitness of horses conditionedby treadmill work. Ph.D. Thesis, Univ of Illinois, Urbana, 1984 eClay Adams Div., Becton DickinsonCo. Parsippany,New Jersey 07054 d20 gauge-2 inch, E-Z Cath Catheter, The Deseret Co., Sandy, Utah 84070 15
SETs. Heart rates of the horses gained intermittently were consistently higher than the heart rates of the continuously Heart Rates of Horses Conditioned conditioned horses in all three exercise tests. Mean heart in Each Activity During the Three SETs * rates declined between SETs 1 and 3 in all activities (rest, walk, trot, and recovery). Decreases were significant during Activity and SET Heart Rate (beats per minute.) the 3 and 9 percent grade trots and during recovery minutes C I Mean 2 through 6 on the 9 percent grade. 3% G R A D E Neither cardiac output nor stroke volume decreased sigREST (2 min) nificantly with progressive conditioning nor were there SET 1 51+1.9 59+1.9 55 2 48+1.8 57+_2.5 53 significant differences between conditioning groups (inter3 48+_2.5 57+_3.9 53 mittent or continuous). Increases in mean cardiac output between rest and exercise averaged 210 percent on the 3 iiiii~Kiii~i ~iiiiiiii~iiiiii~i~!~iii~i~iiii~iiiiii!iiiiiiiiiiiiiiiiiiii~iiiiiiiiiii~i~iii~ii~!!iiii iiiiiiiiiiiiiiiiiiiiiiiiiiii~ percent grade and 290 percent on the 9 percent grade. Heart iiiiiiiiiSE~i~iiiiiiiiiiiiii!iiiiiiiiiiiiiiiiiiii iiiiiiiiiiiii7~3~i~iiiiii~i~i~i~i~ii!!!8~i3iiiiiiiiiii~ii~i~i~i~ii~i~i~i~i~i~i~i~ii~ii~i~i iiiiiiiiiiiiiiiii~!iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiliiiii iiii~~i!~iiiiiiiiiiiiiiiiiiiiiiiiiiiis~i~iiiiiiiiiiiiiiiiiiiiiiiiiiiii iiiiii~i~iiiiiiiiiiiiiiiiiiiiiiiiii rates measured concurrently with cardiac output increased iiiiiiiiiiiiiiiii~iiii!iliiiiiiiiiiiiiiiiiiiii~i~i~!~iiiiiiiiiiiiiiiiiii~ii~iiiiiiiiiiii~i~iii~ii~ii~ii~ii~ii~i~iii~ii~iiiii~ 190 and 240 percent, between rest and the 3 and 9 percent grade trots. Although stroke volume increased by TROT (15 min) approximately 10 percent between rest and the 3 percent SET 1 128_+0.8 138+1.1 133 grade trot and 15 percent between rest and exercise on the 2 125+1.1 135+1.5 130 3 123+1.2 125+1.2 124b steeper grade (Table 3), these changes were not significant. Within the first SET there were significant differences in the lactate concentrations of the groups of horses conditioned by intermittent or continuous work (Table 4). During the 9 percent grade trot and recovery, lactate concentration averaged 45 and 42 mg/100ml, respectively, RECOVERY (min 2-6) for the horses conditioned by intermittent work and 29 and SET 1 71+_2.5 72Y.1.5 72 24 mg/100ml, respectively, for the horses conditioned by 2 61+1.7 70-I-_1.4 66 continuous work. This discrepancy was caused by the 3 5 +8~_2.2 67+1.2 63 extremely high lactate value (75mg/100ml) of one horse in 9% GRADE the intermittent work group and by one horse with an especially low value (14mg/100ml) in the continuous work ~:4:: ::i :: 55~ :i::::~:~:~:~:i~i~i ~ ~::::::::::::::::::::::::::::::::::::::::::::::::::::: group. After 5 and especially after 10 weeks of conditioning : ~ iiiiiiiiiiii:: :: ii~.~ iiiiiiiiiiiiiii~ii~iiiiiiiii::iili :: ~ the lactate concentrations of the two groups became more : i ::::iiilliliiiiiiiii~i~ :~::i!ii::i i~i:: i ~::: ::: ii::i similar, and even though the intermittently conditioned WALK (1 min) horses tended to have higher values than the horses SET 1 965:4.7 104+_3.8 100 conditioned by continuous work, the differences were 2 100-&_3.2 97+_3.1 98 insignificant. Therefore the data from the two groups were 3 91±4.2 96+4.8 94 combined for analysis of the effects of progressive conditioning. Lactate concentrations decreased between SETs 1 and 2 i:i !~iiiiiiiiii :6iiiiiiii ~5~iiiiiiiiiiiiiiiiiiiiiiiiiiiiii::ii~ during the trot and recovery on both treadmill grades. The lactate production during the 3 percent trot in the first SET was approximately the same as the lactate values found RECOVERY (rain 1) SET 1 141+_5.3 116+4.3 115 during the 9 percent grade trot in the second SET and by the 2 107+_5.6 113+4.4 110 third exercise test lactate values did not rise significantly 3 103+_5.6 110-2_4.7 107 above the resting value during the entire test. TABLE 2
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Values are mean +s+_SE *C = Continuous work; I = InterrniUent work (average of all minutes
within activity) aindicates significant differences between means on the same line b(hOrSes conditioned by continuous or intermittent work) (P<.05) indicates significant differences between means within activity (intermittent and continuous values combined) (P<.05)
16
DISCUSSION AND C O N C L U S I O N S
Reports on the effect of conditioning on resting heart rate have been conflicting. Some experimental work has shown a training-induced resting bradycardia in horses 14 while other research has not.1630 In the present study, a trend was observed towards lower pre-exercise heart rates, but the changes were not significant. Part of the decrease observed may have been due to lessening of nervous anticipation as the horses became more accustomed to the conditioning and testing procedures. EQUINE VETERINARY SCIENCE
TABLE 3 Cardiac Output and Stroke Volume Values* Activity and SET C
#!
Cardiac Output (l/rain) I
::::::::::::::::::::::::::::::::::::::::::: ~S ~::7i~ :: ::::::! :: :::::::: ~ ~ ~
Stroke Volume (l/stroke) C I
Mean
Mean
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TROT-- 3%
SET 1 2 3
713 _+ 14.0 196 _+ 14.0 155 + 25.1
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164 _+12.1 202 _+22.2 165 -+ 7.2
210 _+ 12.0 218 _+ 4.6 228 _+ 5.0
168 200a 162
1.36 + .12 1.53 _+.06 1.28 -+ .71
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214 _+ 19.4 214 _+ 7.9 237 _+ 8.8
212 216 234a
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1.35 _+.10 1.40 + .09 1.55 + .17
1.22 -+. 88 1.48 _+.20 1.32 +. 04
1.28 1.50 1.31
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1.34 _+.09 1.33 _+.14 1.54 _+.08
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Values are means_+SE;No significant differences between means of horses conditioned by continuous or intermittent work alndicates significant differences from SET 1 within activity (P<.05) *Values are during rest, exercise and recovery on two treadmill grades of the three SETs of horses conditioned by continuous (C) or intermittent (I) work
TABLE 4 The occurrence o f an exercise bradycardia with conditioning, as was f o u n d in this study, has b e e n previously reported.2a 4 D u r i n g submaximal exercise, decreased sympaActivity and SET Lactate concentration (mg/100ml) C I Mean thetic drive rather than increased parasympathetic tone, is REST thought to cause lower exercising heart rates after condiSET 1 9.54_+_2.07 9.58+1.66 9.56 tioning. 9,15 A decreased sympathetic drive could originate 2 5.99i-_1.11 6.81_+0.60 6.45 3 6.17_+0.67 6.49~0.67 6.35 from intracardiac or extracardiac factors.aS In the former situation an increase in stroke v o l u m e during s u b m a x i m a l work nOT ::3 ::::::::::::::::::::::::::::iiii::iiiii::i::i::ili::i which, in m a n results f r o m training, m e a n s that at the same g gTIiii: ::i~!8~_~5~::~ ::::~~~::~:~:i ~:88~{ ~:::::::::::::::::::::::~i~i~ :::::::::::::::::::::::::::::: ai99 a or slightly lower cardiac output, the drive for higher heart rate via sympathetic nervous stimulation to the heart is reduced. Extracardiac m e c h a n i s m s affect the heart indirectly RECOVERY-- 3% SET 1 14.84!3.84 14.29+__2.64 14.53a as a result o f alterations in the trained skeletal muscle. 4 In2 5.31-+0.34 8.07-+0.91 6.84 creased o x y g e n extraction andutilization, a n d neuronal feed3 5.60-+0.43 6.72-+0.65 6.22 back from the muscles and joints m a y alter sympathetic !! : i outflow. There was n o trend towards an increased stroke v o l u m e with c o n d i t i o n i n g in this e x p e r i m e n t or in other }{ longitudinal training studies36 However, between SETs 1 ::::::::::::::::::: 1:i5~ i::i7 :::::::::::::::::::::::::: ~::::::::::::::::::::::::::::::::::::::::::: ::4{4} ~!~i79 {::: ::{ i}i:::iiU:!::!:::#: ::::: ::::i::ii:::::::::::::::::::::::::::::: ::::::::::::~::5):: : : ;:i15:::ii::::::::::::::::::::::::::::::::::::::::::::::::: !~:{:::: and 2, lactate concentrations decreased significantly, indiRECOVERY--9% cating i m p r o v e d o x y g e n extraction and/or utilization. SET 1 23.54+_5.27 42.30-+12.09 b 33.96a Extracardiac rather than intracardiac factors appear to be re2 9.15+1.04 11.57_+1.21 10.49 sponsible for the exercise bradycardia f o u n d in this study. 3 8.50-+1.07 8.77_+0.76 8.65 Values arc means_+SE The decreased lactate concentration c o n c u r r e n t with un*Values are during rest, exercise and recovery on two treadmill grades changed cardiac output and stroke v o l u m e indicates that of the three SETs of horses conditioned by continuous (C) or intermittent (I) work either the distribution o f cardiac o u t p u t was altered to inaindicates significant differences between means within activity crease blood flow to the working muscles at the expense of (intermittent and continuous values combined)(P<.05) other organs or the m u s c l e cells b e c a m e m o r e efficient in bindicates significant differences between means on the same line (horses conditioned by continuous or intermittent work) (P<.05) oxygen utilization for the work o f contraction.
Lactate Concentration*
Volume 7, Number I
17
Neither cardiac output nor stroke volume changed consistently with progressive conditioning or between the groups of horses trained by intermittent or continuous work. Other longitudinal studies involving aerobic conditioning have shown either no change16 or a decrease2 in cardiac output. In humans, conditioning may cause no change or a slight decrease in cardiac output during submaximal work that requires the same oxygen consumption.15 Similar increases in stroke volume during submaJdmal treadmill work, as determined in this study, have been reported2,3 but no correlation has been found between stroke volume and cardiac output) In both dogs TM and manl the increase in cardiac output during exercise is attained primarily through tachycardia with little contribution by increased stroke volume. The physiological mechanisms responsible for the decreased accumulation of lactate during submaximal work following aerobic conditioning is not entirely known. Research on the adaptation of human skeletal muscle to endurance exercise has shown an increase in the capacity for aerobic metabolism. This is manifested as an increased number and size of muscle cell mitochrondia and an increase in the levels of a number of mitochondrial enzymes, including those involved in fatty acid oxidation, the citric acid cycle, and components of the respiratory chain. 12 This adaptation has also been reported in horses. 11 The relationship of heart rate to lactate concentration changes with conditioning. The lactate production at a given heart rate decreased markedly between SETs 1 and 2, but were approximately the same during SETs 2 and 3. There has been some speculation among horse researchers over the presence of an anaerobic threshold during exercise. Based on the relationship of lactate concentration to heart rate, the anaerobic threshold has been set at heart rates between 150 and 180 bpm. 8 From the results of this experiment, however, it seems that a heart rate for the anaerobic threshold would vary with the fitness of the subject. Work that was done at a given heart rate in the unfit horse produced a much higher lactate concentration than was found at the same heart rate after conditioning. No attempt was made in this study to assign a heart rate value to the anaerobic threshold. The wide range of heart rates set forth by other researchers for an anaerobic threshold may all be correct for the horses used in a given experiment depending on their level of aerobic fitness at the time. The intermittent or continuous work conditioning programs did not promote different physiological adaptations to submaximal exercise. The higher values of the cardiovascular and metabolic parameters measured in the horses conditioned by intermittent work appeared to be inherent in the horses themselves and not a function of the intermittent exercise as the difference between the intermittent and continuously conditioned horses were largest in the first SET (before conditioning began) and smallest in the third SET. It has been suggested that studies comparing intermittent and continuous conditioning on an equal work basis, as in the present study, do not reveal the advantages o f interval 18
training. This is because an inherent quality of interval training is that the rest periods allow for more work to be done in each training session and therefore more conditioning can be accomplished at a faster rate. The results of this study indicate that an interval-type gaining program may be accomplished on a treadmill, but, at the workload used, it is no more successful than an equal work program of continuous trotting at improving cardiovascular and metabolic fitness. It is interesting that the horses conditioned at the lower intensity were as capable of coping with the higher intensity work as those conditioned at that intensity. From a practical standpoint, however, it took fewer minutes per horse per day on the interval program to get the same conditioning effects as the continuous program. This study also indicates that the largest proportion of the improvement in fitness occurred during the first five weeks of conditioning and was due either to changes at the cellular level (manifested as a decrease in lactate production) or changes in relative distribution of the cardiac output to the working muscles and other organs with only minor changes in total cardiac output. Further research is needed to elucidate the strengths a n d weaknesses of interval training and to establish appropriate work and rest intervals for specific training goals. It may be that work intensity and/or duration may have to be increased to illuminate the attributes of interval training as the work intensity in this study was far below the work levels at which interval training is usually employed, that is, for Thoroughbreds and Standardbreds in race training or for other high intensity events. REFERENCES
1. Astrand P, Rodahl OK: Textbook of Work Physiology. St. Louis: McGraw-HillBook Co. p 424, 1977. 2. Bayly WM, Gabel AA and Barr SA: Cardiovascular effects of subrnaximal aerobic training on a treadmill in Standardbred horses, using a standardized exercisetest. Am J Vet Res 44:544-553, 1983. 3. Bergsten G: Blood pressure, cardiac output and blood-gas tension in the horse at rest and during exercise. Acta Vet Scand (Suppl) 48:1-88, 1974. 4. Clausen JP: Effect of physical training on cardiovascular adjustments to exercise in man. Physiol Rev 57:779-815, 1977. 5. Engelhardt WV: Cardiovascular effects of exercise and training in horses. Adv Vet Sci 21:173-205, 1977. 6. Freeman JH, Bayly WM, Grant BD and Gollniek PD: Cardiopulmonary responses to a traditional method of training the Thoroughbred racehorse. Proc 8th Eq Nut and Physiol Symp, Lexington, p 193-198, 1983. 7. Fox E and Mathews D: Interval Training, Conditioning for Sports and General Fitness. Philadelphia: WB Saunders Co, p 1, 1974. 8. Fregin GF: Combined training panel: Cardiovascular response to exercise: A review. Proc 21st Ann Cony Am Assoc Eq Practitioners, Boston, p 311-313, 1975. 9. Frick MH, Elovainio RO and Somer T: The mechanism of bradycardia evoked by physical training. Cardiologia 51:46-54, 1967. 10. Gabel AA, Milne DW, Muir WW and Skarda RT: Interval compared to conventional training of Standardbred horses: Lactic acid and cardiovascular data (Abstract). Proc 1st Int Conf on Equine Exercise Physiol, Oxford, p 41, 1982. 11. Guy PS and Snow DH: The effect of training and detraining on muscle compositionin the horse. J Physiol 269:33-51, 1977. EQUINE VETERINARY SCIENCE
12. Holloszy JO: Adaptation of skeletal muscle to endurance exercise. Med Sci Sports 7:155-164, 1975. 13. Lindholm A and Saltin B: The physiological and biochemical response of Standardbred horses to exercise of varying speed and duration. Acta Vet Scand 15:310-324, 1974. 14. Marsland WP: Heart rate response to submaximal exercise in the Standardbred horse. J Appl Physiol 24:98-101, 1968. 15. Mathews DK and Fox EL: The Physiological Basis of Physical Education and Athletes (2rid Ed) Philadelphia: WB Saunders Co, p 250-286, 1976. 16. Milne DW, Gabel AA, Muir WW and Skarda RI: Effects of training on heart rate, cardiac output and lactic acid in Standardbred horses, using a standardized exercise test. J Am Vet Med Assoc 1:131135, 1977. 17. Milne DW, Skarda RT, Gabel AA, Smith LG and Ault K: Effects of training on biochemical values in Standardbred horses. Am
J Vet Res 37:285-2990, 1976. 18. Rushmer RF: Cardiovascular Dynamics (2rid Ed) Philadelphia: W.B. Saunders Co, p 195-196, 1967. 19. Sigler DI-I, Webb SP and Potter GD: Physiological response of horses in different conditioning regimes to a standard exercise tolerance test on a treadmill. Proc 6th Equine Nut and Physiol Syrup, College Station, p 48-54. 20. Skarda RT, Muir WW, Milne DW and Gabel AA: Effects of training on resting and post-exercise ECG in Standardbred horses, using a standard exereise test. Am J Vet Res 37:1485-1488, 1976. 21. Steel GD and Torfie JI-I: Principles and Procedures of Statistics. New York: McGraw-HiU Book Co, p 107-109, 1960. 22. Tavernor WD: Technique for the subcutaneous relocation of the common carotid artery in the horse. Am J Vet Res, 30:1881-1883, 1969.
ERRATA
frequency of inbreeding in wild horses. Evo 41:229-231, 1987. Summary: In the Granite Range feral horse population, incestuous, father-daughter matings are avoided by positive action of both sexes. Young females disperse from their rearing bands regardless of their Irue genealogic relationship to the band's resident stallion and, even when presented with the opportunity, stallions do not copulate with young females with whom they are familiar. Thus, as stated before (Berg op. cit.), the observed level of inbreeding in feral horse populations may be much less than that expected under a random breeding system. It is of interest to note that 21% of the females returned to their rearing band after copulating with non-harem males. The implication(s) of this latter finding is not yet known.
Berg W: Effective population size and inbreeding in feral horses: a preliminary assessment. J Eq Vet Sci 6:240-245, 1986. Page 240, column 2, line 12: "Nm = number of successfully breeding males;." Page 243, column 1, line 18: replace "impaction" with "impact." Page 245, column 1, line 36: delete "-" from the word "statistical." ADDENDUM
Berger J, Cunningham C: Influence of familiarity of
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Vth International Conference on Equine Infectious Diseases Lexington, Kentucky • October 7-10, 1987 Abstracts of scientific papers are being accepted before March I for consideration on the following topics: Herpesviruses • Strangles and other respiratory infections • Influenza • Vector-borne diseases • Gastrointestinal diseases • Infections of the Reproductive System For more information, contact: Donna A. Hall, Special Programs University of Kentucky, 204 Frazee Hall Lexington, Kentucky 40506-0031
(606) 257-3929
Sponsoredby Universityof KentuckyDepartmentof VeterinaryScience. Volume 7, Number I
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