Equine Sports Medicine and Surgery, 1e

1344 Pages·2004·107.6 MB·English

by  Kenneth Hinchcliff, Andris Kaneps, Raymond Geor

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CONTRIBUTORS
Dorothy M.Ainsworth DVM PhD Jean-Marie Denoix DVM PhD Agrégé Michael Hamlin BPhEd MHMS PhD
DACVIM Professor,Centre of Imaging and Research on Senior Lecturer in Sport Science,Environment,
Professor of Medicine,College of Veterinary Equine Locomotor Diseases (CIRALE),France Society and Design Division,Lincoln University,
Medicine,Cornell University,Ithaca,USA Canterbury,New Zealand
Norm G.Ducharme DVM MSc
Brian H.Anderson BVSc MVSc MS Diplomate ACVS Joanne Hardy DVM Diplomate ACVS
Diplomate ACVS Medical Director and Professor of Surgery,Equine Diplomate ACVECC
Partner,Ballarat Veterinary Practice,Ballarat, Hospital,Cornell University,Ithaca,New York, Large Animal Medicine and Surgery,Texas A&M
Victoria,Australia USA University,College Station,Texas,USA
Fabrice Audigié DVM PhD Mary M.Durando DVM PhD DipACVIM Kevin K.Haussler DVM DC PhD
Maître de Conferences (Associate Professor), Assistant Professor,Sports Medicine,New Bolton Lecturer,Department of Biomedical Sciences,
Centre of Imaging and Research on Equine Centre,University of Pennsylvania School of College of Veterinary Medicine,Cornell University,
Locomotor Diseases (CIRALE),France Veterinary Medicine,USA Ithaca,New York,USA
Eric Barrey DVM PhD Jack Easley DVM MS Diplomate American Kenneth W.Hinchcliff BVSc(Hons) MS
Research Associate,National Institute of Board of Veterinary Practitioners (Equine) PhD Diplomate ACVIM
Agricultural Research (INRA),Laboratory of Shelbyville,Kentucky,USA Professor,Department of Veterinary Clinical
Genes and Training Interactions,Evry University, Sciences,College of Veterinary Medicine,The
France Patricia M.Ellis BVSc MVSc MACVSc Ohio State University,Columbus,Ohio,USA
Veterinary Consultant,Australian Racing Board,
Lance H.Bassage II VMD Diplomate Victoria,Australia Susan J.Holcombe VMD MS PhD
ACVS DACVS DACVECC
Staff Surgeon,Rhinebeck Equine,Rhinebeck,New Howard H.Erickson DVM PhD Associate Professor,College of Veterinary
York,USA Professor of Physiology and Roy W.Upham Medicine,Michigan State University,East Lansing,
Professor of Veterinary Medicine,College of Michigan,USA
Laurie A.Beard DVM MS Diplomate Veterinary Medicine,Kansas State University,
ACVIM Manhattan,Kansas,USA David W.Horohov PhD
Associate Professor,Department of Veterinary William Robert Mills Chair,Department of
Clinical Sciences,The Ohio State University, David Evans BSc PhD Veterinary Science,Maxwell H.Gluck Equine
Columbus,Ohio,USA Associate Professor,Faculty of Veterinary Science, Research Center,University of Kentucky,
University of Sydney,Sydney,New South Wales, Lexington,Kentucky,USA
Brendon Bell BVSc MS MACVSc Australia
Specialist Equine Surgeon,Southern Veterinary John A.E.Hubbell DVM MS
Centre,Invercagill,New Zealand Jonathan H.Foreman DVM MS Associate Dean for Academic Affairs;Professor of
Diplomate ACVIM Veterinary Anesthesia,Department of Veterinary
Alicia L.Bertone DVM MS PhD Associate Professor,Department of Veterinary Clinical Sciences,The Ohio State University,
Diplomate ACVS Clinical Medicine,University of Illinois,Urbana, Columbus,Ohio,USA
Trueman Family Endowed Chair,Professor of Illinois,USA
Equine Orthopedic Surgery,The Ohio State Seppo Hyyppä DVM
University,Columbus,Ohio,USA Michael A.Foss DVM Senior Scientist,Equine Research,Ypäjä,Finland
Partner,Wolf,Davidson & Foss PC,Hood River,
Eric K.Birks DVM PhD Oregon,USA Brad R.Jackman DVM MS Diplomate
Assistant Professor of Equine Exercise Physiology, ACVS
Sports Medicine and Imaging,School of Veterinary Raymond J.Geor BVSc MVSc PhD Co-owner and Managing Partner,Pioneer Equine
Medicine,University of Pennsylvania,Kennett Diplomate ACVIM Hospital,Oakdale,California,USA
Square,PA,USA Associate Professor,Department of Biomedical
Sciences,Ontario Veterinary College,University of Leo B.Jeffcott MA BVetMed PhD DVSc
Laurent L.Couëtil DVM Diplomate Guelph,Guelph,Ontario,Canada VetMedDr(hc) FRCVS
ACVIM Professor of Veterinary Clinical Studies;Dean of
Associate Professor of Large Animal Medicine, Carol Gillis DVM PhD the Veterinary School,University of Cambridge,
Equine Sports Medicine Center,Purdue University Equine Referral Practice,Vacaville,California,USA Cambridge,UK
School of Veterinary Medicine,Indiana,USA
Allen E.Goodship BVSc PhD MRCVS Eduard Jose-Cunilleras DVM
Antonio M.Cruz DVM MVM MSc Professor of Orthopaedic Science,Royal Diplomate ACVIM
DrVetMed Diplomate ACVS Diplomate ECVS Veterinary College;Director,Institute of Musculo- Research Associate,College of Veterinary
Assistant Professor Large Animal Surgery, Skeletal Science,University College London, Medicine,The Ohio State University,Columbus,
Department of Clinical Studies,Ontario London,UK Ohio,USA
Veterinary College,University of Guelph,Guelph,
Ontario,Canada Mary E.Gordon MS Andris J.Kaneps DVM MS PhD
Equine Science Center,Cook College,Rutgers – Diplomate ACVS
Elizabeth J.Davidson DVM DACVS the State University of New Jersey,New Staff Veterinarian;Parrott Equine Associates,
Assistant Professor of Sports Medicine,New Brunswick,New Jersey,USA Hamilton,Massachusetts;Former Assistant
Bolton Center,School of Veterinary Medicine, Professor,Equine Orthopedic Surgery,College of
Kennett Square,PA,USA Veterinary Medicine,The Ohio State University,
Columbus,Ohio,USA
x
Contributors
Christopher E.Kawcak DVM PhD Kenneth Harrington McKeever Sidney W.Ricketts LVO BSc BVSc DESM
Diplomate ACVS PhD FACSM DipECEIM FRCVS
Assistant Professor,Orthopedic Research Center, Equine Exercise Physiologist;Associate Professor Senior Partner,Rossdale & Partners,Beaufort
Colorado State University,Fort Collins,Colorado, and Associate Director for Research,Equine Cottage Laboratories,Newmarket,UK;Visiting
USA Science Center,Rutgers – The State University of Professor,University of Bristol,School of
New Jersey,New Brunswick,New Jersey,USA Veterinary Science,Bristol,UK
Kevin G.Keegan DVM MS Diplomate
ACVS Alfred M.Merritt DVM MS Andrea Ritmeester BVSc MSc
Associate Professor,E.Paige Laurie Endowed Appleton Professor in Equine Studies, Diplomate ACVS
Program in Equine Lameness,College of Department of Large Animal Clinical Sciences, Specialist Equine Surgeon,Veterinary Associates
Veterinary Medicine,University of Missouri, College of Veterinary Medicine,University of Equine and Farm,Takanini,New Zealand
Columbia,Missouri,USA Florida,Gainesville,Florida,USA
José-Luis L.Rivero DVM PhD
Janene K.Kingston BVSc MVS DVSc Cliff Monahan DVM PhD Professor of Veterinary Anatomy;Head of the
PhD MACVSc Diplomate ACVIM Assistant Professor,College of Veterinary Laboratory of Muscular Biopathology,University
Senior Lecturer in Equine Medicine,Veterinary Medicine,The Ohio State University,Columbus, of Cordoba,Cordoba,Spain
Teaching Hospital,Massey University, Palmerston Ohio,USA
North,New Zealand Bonnie R.Rush DVM MS Diplomate
Rustin M.Moore DVM PhD Diplomate ACVIM
Cynthia Kollias-Baker DVM PhD ACVS Professor;Assistant Dean,Career Development,
DACVCP Professor of Equine Surgery;Service Chief,Equine Equine Internal Medicine,Kansas State University,
Associate Professor and Program Director,Racing Medicine and Surgery;Director,Equine Health Manhattan,Kansas,USA
Laboratory,University of Florida,Gainesville, Studies Program,School of Veterinary Medicine,
Florida,USA Louisiana State University,Baton Rouge,Louisiana, Katarina Schuback PhD
USA Assistant Professor and Clinician,Department of
Joanne Kramer DVM Diplomate ACVS Large Animal Clinical Sciences,Swedish University
Clinical Assistant Professor,Equine Surgery, Paul S.Morley DVM PhD DACVIM of Agricultural Sciences,Uppsala,Sweden
College of Veterinary Medicine,University of Associate Professor;Director of VTH Biosecurity,
Missouri,Columbia,Missouri,USA Colorado State University,Fort Collins,Colorado, Colin C.Schwarzwald DrMedVet
USA Clinical Instructor,College of Veterinary Medicine,
Federico G.Latimer DVM MS The Ohio State University,Columbus,Ohio,USA
Associate Clinical Professor,Department of J.Richard Newton BVSc MSc PhD
Veterinary Clinical Sciences,Veterinary Hospital, DLSHTM FRCVS Roger K.W.Smith MA VetMB PhD DEO
Columbus,Ohio,USA Veterinary Epidemiologist,The Animal Health Diplomate ECVS MRCVS
Trust,Newmarket,Suffolk,UK Professor of Equine Orthopaedics,Department
Guy D.Lester BVMS PhD Diplomate of Veterinary Clinical Sciences,The Royal
ACVIM Christopher B.O’Sullivan BVSc(Hons) Veterinary College,North Mimms,Hertfordshire,
Associate Professor of Equine Medicine,Murdoch Equine Surgery Resident,The Ohio State UK
University,Perth,Western Australia University,Columbus,Ohio,USA
Scott Stanley
Michael I.Lindinger BSc MSc PhD Nigel R.Perkins BVSc(Hons) MS Dip Assistant Professor,California Animal Health and
Associate Professor,College of Biological ACT FACVSc Food Safety Laboratory System,Davis,California,
Sciences,University of Guelph,Guelph,Ontario, Associate Professor,Veterinary Epidemiology, USA
Canada EpiCentre,Massey University,Palmerston North,
New Zealand
Mireia Lorenzo-Figueras DVM Hugh G.G.Townsend MVM MSc
Research Associate,Department of Large Animal Duncan F.Peters DVM MS Professor,Department of Large Animal Clinical
Clinical Sciences,College of Veterinary Medicine, Veterinarian;Owner Partner,Pioneer Equine Sciences,University of Saskatchewan,Saskatoon,
University of Florida,Gainesville,Florida,USA Hospital,Oakdale,California,USA Saskatchewan,Canada
Jonathan M.Lumsden BVSc DVCS MS Richard J.Piercy MA VetMB MS DACVIM Tracy A.Turner DVM MS DACVS
Diplomate ACVS MRCVS Professor of Equine Surgery,College of Veterinary
Equine Surgeon,Randwick Equine Centre,New Wellcome Research Fellow,Dubowitz Medicine,St Paul,Minnesota,USA
South Wales,Australia Neuromuscular Unit,Imperial College London,
London,UK Honor A.Walesby DVM MS DACVS
David J.Marlin BSc PhD Assistant Professor,Equine Surgery,School of
Head of Physiology,The Animal Health Trust, David C.Poole BSc MS PhD DSc FACSM Veterinary Medicine,Louisiana State University,
Newmarket,Suffolk;Visiting Professor in Professor of Kinesiology,Anatomy and Physiology, Baton Rouge,Louisiana,USA
Respiratory and Cardiovascular Physiology, Department of Kinesiology,Kansas State
University of Bristol,Bristol,UK University,Manhattan,Kansas,USA John P.Walmsley MA VetMB CertEO
DECVS MRCVS
Ben B.Martin,Jr VMD DACVS Rebecca E.Posner BS BVetMed MRCVS Senior Partner,Liphook Equine Hospital,Liphook,
Associate Professor Sports Medicine,College of Equine Veterinarian,Genesee Valley Equine Clinic, Hampshire,UK
Veterinary Medicine,University of Pennsylvania, New York,USA
Kennett Square,PA,USA Keith L.Watkins BVSc MRCVS
A.Reeta Pösö PhD Head of Veterinary Regulation & International
L.Jill McCutcheon DVM PhD Professor of Veterinary Physiology,University of Liaison,The Hong Kong Jockey Club,Equine
Professor,Department of Pathobiology,Ontario Helsinki,Helsinki,Finland Hospital,Sha Tin Racecourse,New Territories,
Veterinary College,University of Guelph,Guelph, Hong Kong SAR,China
Ontario,Canada
xi
Contributors
Steven J.Wickler PhD DVM James L.N.Wood BSc BVetMed MSc Lesley E.Young BVSc DVA DVC DECVA
Professor,Department of Animal and Veterinary PhD MRCVS DLSHTM DipECVPH PhD MRCVS
Sciences;University Veterinarian;Director of Head of Epidemiology,The Animal Health Trust, Senior Scientist and Cardiologist,The Animal
Equine Research,California State Polytechnic Newmarket,Suffolk,UK Health Trust,Newmarket,Suffolk,UK
University,Pomona,California,USA
FOREWORD
Since time in memoriam, horses have been renowned for their 50 percent during exercise, and that the magnitude of this
athletic prowess and work capacity, and documenting and increase was related to workload.4,5 Their observations
understanding the physiologic basis for these abilities has were published in 1926. They also noted similar events in
occupied many industrial-age equine scientists for more than a dogs and observed that splenectomy eliminated this exercise-
century. An integrative approach is essential to the elucidation related hemoconcentration. The contractile nature of the
of the mechanisms by which horses work, compete, or perform equine spleen was subsequently confirmed by Steger in
the various recreational activities that contemporary society 1938.6These phenomena are still the basis of various studies
asks of them. The performance or ‘output’ of an equine athlete designed to evaluate different aspects of metabolism and the
is determined by many complicated interdependent biological dehydration status of horses under different exercise
processes. Understanding how these processes function and conditions.
relate to each other is mandatory if the horse is to be effectively As well as the above cited work, interested readers are also
trained and managed during its working or competitive life. referred to the work of two other teams of people in particular.
Such understanding is also pivotal to the clinical application of First, Samuel Brody and colleagues conducted a seminal series
basic physiologic and pathologic principles, and is therefore of studies at the University of Missouri with the aid of a tread-
necessary to ensure the successful diagnosis and management mill that they built. They focused on equine energetics, the ef-
of exercise-related diseases in horses. ficiency of metabolism, nutrition, work and growth in the
Modern day equine exercise science is generally regarded to second quarter of the 20th century and published their find-
have been born in 1967 with the publication of Sune Persson’s ings in a collection of 66 Missouri Agricultural Experiment
doctoral thesis.1This was the first work that documented data Station Research Bulletins. Among other things, Brody et al.
generated from horses exercising on a high-speed treadmill. The showed that the caloric cost of movement per unit live weight,
subsequent widespread availability ofsuch treadmills has had a per unit horizontal distance covered, is not affected by the size of
great deal to do with defining the current state of knowledge the animal and is independent of speed. He also demonstrated
with respect to basic and applied equine exercise physiology. The that minute ventilation is exponentially related to the energetic
inception of the quadrennial International Conference on cost of work or exercise.
Equine Exercise Physiology (ICEEP) in 1982 has also provided a The Russian scientists Karlsen and Nadaljak published a
regular forum at which investigators can describe new fun- series of papers from 1960–1965 that were, unfortunately,
damental and clinical findings pertaining to exercise in horses of difficult to obtain. Karlsen, with Brejtsen, provided the first
all ages. Despite the great strides in equine exercise science that ‘modern’ documentation of the synchrony of breathing and
have been made in the last 35 years, it is important to recognize stride frequencies7 and, with Nadaljak, displayed great
the contributions of the pioneers who helped develop many of ingenuity in conducting the first field study ofhorses exercising
the techniques upon which current scientific methodologies are at high speed.8Together these investigators recorded an oxygen
based. These people also made fundamental observations that consumption of 62.8 L/min in a Standardbred galloping on a
are still pivotal to much of the work that is being conducted track at 11.1 m/s.
today. Nathan Zuntz was a Berliner professor of animal science I mention these things because, in the words of the famous
who, with a number of colleagues, notably Drs. Curt Lehmann Australian neurologist, Sir Sydney Sutherland (1910–93), it is
and Oscar Hagemann, investigated the metabolism of horses important to ‘honor those who go first even if those who come
during rest and work. These studies are truly extraordinary by later go further’. In the 66 chapters in this book, the reader will
today’s standards. Zuntz and Lehmann built the first Laufband, find the latest information regarding the physiologic responses
or treadmill, and used a facemask and tracheotomies to meas- and adaptations of the various equine body systems to exercise
ure oxygen consumption and carbon dioxide production in two and training. This information is also linked to exercise-related
horses at speeds up to 3.5 m/s.2Zuntz and Hagemann further clinical problems of the same body systems. The chapters have
refined these results by measuring oxygen consumption, carbon been written by a number of contemporary experts in these
dioxide production, arteriovenous oxygen content difference, fields. There are also sections on breed-specific activities and
aortic blood pressure, tidal volume, heart rate and respiratory other ‘applied’ aspects of equine sports medicine. This book
frequency in horses that were walking, trotting and walking represents an ambitious and valuable contribution to the body
backwards freely, and while pulling loads of 66–78 kg uphill, of equine exercise-related physiologic and clinical literature.
downhill and on the horizontal!3These data were impressively With it the reader has the opportunity to follow a subject from
similar to those that are determined under similar exercise its basic principles to its current state of knowledge in both the
conditions today. physiologic and clinical or applied sense. Producing such a vol-
The ability ofthe horse to increase its blood volume during ume is a major undertaking and the principal editors and con-
exercise and the associated rise in hematocrit are two tributors are to be congratulated on their efforts. However, our
hallmarks of equine exercise or work, particularly in warm- knowledge base is incomplete; i.e., it is not perfect. When
blooded horses. Scheunert, Krzywanek and Müller were the considered in terms of progress made over the last 120 years,
first to observe that hematocrit ofhorses could increase by up to one might even suggest that there have been few major
xiv
Foreword
breakthroughs and that, rather, new information has come 2. Zuntz N, Lehmann C. Untersuchungen über den Stoffwechsel
to light in an almost begrudging but inevitable fashion. In des Pferdes bei Ruhe und Arbeit. Landwirtsschaftliche
reality this is the essence of the scientific process and it is the Jahrbücher 1889; 18: 1–156.
3. Zuntz N, Hagemann O. Untersuchungen über den Stoffwechsel
principal reason that this book should prove to be so useful. It
des Pferdes bei Ruhe und Arbeit. Landwirtsschaftliche
is also the main reason that such books need to be regularly
Jahrbücher 1898; 27: 1–438.
revised and it is hoped that this will not be the only edition 4. Scheunert A, Krzywanek FW. Fluctuations in the amount of
of this very complete and valuable text. blood corpuscles. Pflügers Arch 1926; 213: 198–205.
Warwick Bayly, College of Veterinary Medicine, Washington 5. Scheunert A, Müller C. Effect of activity on the blood of horses.
State University, Pullman, Washington Pflügers Arch 1926; 212: 468–476.
6. Steger vonG. Zur Biologie der Milz der Haussäugetiere. Deutsche
Tierärztl Wchnschr 1938; 46: 609–614.
7. Karlsen G, Brejtse N. Synchronizitat der Rhythmen von
References Atmung und Bewegung – Grundlage fur die Entwicklung eines
schnellen Trabes (in Russian). Konevodstvo i Konesport 1965;
35: 22–24.
8. Karlsen GG, Nadaljak EA. Interchange of gaseous energy and
1. Persson S. On blood volume and working capacity in horses. respiration of trotters at work. Konevodstvo i Konesport 1964;
Acta Vet Scand Suppl 1967; 19: 1–189. 34: 27–31.
PREFACE
The diagnosis and treatment of disorders of the equine Our belief in the importance of integrating both the basic
athlete is a specialty requiring not only the ability to and clinical sciences dictated the structure of this book.
recognize and treat clinical abnormalities, but also an Each of the major body systems is described beginning
understanding of the physiologic demands of exercise and with detailed coverage of the physiologic responses to acute
requirements of competition and training. The science of exercise and to conditioning. This is then followed by one
equine exercise physiology has progressed to the stage that it or more chapters describing the important clinical abnor-
now provides a sound, scientific basis for much of equine malities of equine athletes. Our belief is that knowledge
sports medicine. The current level of knowledge, while still of the fundamentals of exercise science is essential for an
incomplete and imperfect, of the physiologic processes understanding of the clinical abnormalities of the equine
underlying the acute responses to exercise and the athlete. However, for those readers with little interest in the
mechanisms and effects of exercise conditioning, provides a clinical abnormalities of athletic horses, the basic science
sound, fundamental understanding of the workings of the chapters can be read alone and will provide a sound under-
equine athlete. Contemporary equine exercise physiology is standing of the physiology of equine athletic performance.
comprised of not only the physiologic responses to exercise Chapters in the last section of the book dealing with parasite
and training, but also nutrition, biomechanics, behavior and control, veterinary aspects of training the various breeds
pharmacology. This fundamental knowledge informs our of horse, aged athletes, and more, provide a pragmatic,
decisions regarding appropriate training, nutrition, care and utilitarian approach to the athletic horse.
treatment of the equine athlete. Finally, we thank the colleagues and students with whom
We recognized that equine exercise physiology and equine we have had the pleasure of working and who provided much
sports medicine had advanced to the stage where there of the knowledge contained within this book. Our profound
was a need for a comprehensive and integrated source of gratitude is extended to the authors of sections of this book
information for practitioners, students of veterinary with an appreciation of the effort that was required to
medicine, graduate students in equine exercise physiology, compile new comprehensive material on their designated
residents in training and well-informed lay horsemen and topic. We hope that they, and the readers, are pleased with
women. This book attempts to meet that need. As with the the final product.
first attempt at any major project, this book is imperfect and
will not be all things to all readers. However, we hope that we Kenneth W. Hinchcliff
have filled, at least partially, the requirement for a Andris J. Kaneps
comprehensive source that integrates the basic and clinical Raymond J. Geor
sciences of the equine athlete. 2004
1
CHAPTER
Integrative physiology
of exercise
Kenneth W. Hinchcliff and Raymond J. Geor
tances during multi-day races). In contrast, draft horses pull
The horse as an athlete 3
huge weights (1000kg or more) short distances in pulling
Integrative physiology of exercise 5
competitions, Warmbloods perform elegant, but demanding,
Physiology of training 6
dressage routines, and ponies pull lightly laden jinkers or
Factors limiting performance 7
buggies.
References 8
Regardless of their size, provenance or intended use, all
horses have in common an ability to perform physical activi-
ties, including running or jumping, at a level that surpasses
The horse as an athlete
that of most other animals of similar body size. The concept
of body size is important as many physiologic variables, and
especially the maximum values of these variables, do not
Comparative physiology scale directly with bodyweight but often more closely scale to
an exponent of bodyweight.3 Commonly, exponents range
The horse is an extraordinary athlete, a characteristic that is between 0.68 and 0.75. This exponent is derived empirically
the result of evolution of horses as grazing animals on the from the measurement of variables such as maximum
ancient prairies of North America. Survival in these open running speed or maximum rate of oxygen consumption
lands was enhanced by speed, to escape predators, and (V•O ). Typically, when expressed as a one-to-one function
2max
endurance, required to travel long distances in search of feed of bodyweight (i.e. per kg) values for many variables are
and water. These attributes are shared by pronghorn much higher for smaller mammals. The necessity to scale
antelopes, another species that evolved on the prairies. The variables allometrically has fascinating physiologic implica-
equid characteristics of speed and endurance were subse- tions and interpretations.3 However, direct comparison
quently modified or enhanced by selective breeding by among species is to some extent specious from the point of
humans. view of depicting differences in physical capacity, given that
Horses were domesticated on more than one occasion, the absolute values of these variables vary to such a large
based on analysis of mitochondrial DNA from a wide variety extent. Nonetheless, such comparisons are frequently made,
of current domestic breeds and Przewalski’s horses.1,2 if only to reinforce the magnitude of the maximal absolute
Domesticated horses were then selected and bred for certain values of these variables in the exercising horse (Table 1.1).
traits, depending on the intended use. Large, heavy breeds of The athletic capacity of horses is attributable to a number
horses were bred for draft work, such as pulling plows, sleds of physiologic adaptations. In some cases these adaptations
or carts, or military work, such as the chargers that carried are not affected by training, for example lung size, whereas
heavily armored knights into the battles of the Middle Ages. others change in response to training, for example blood
Lighter horses were bred for speed and endurance and were volume (see Chapters 28 and 38). The superior athletic ability
used for transportation, herding and sport. Horses have been of horses is attributable to their high maximal aerobic capac-
bred or adapted to a large variety of uses. Thoroughbred race ity, large intramuscular stores of energy substrates and in
horses run at high speed (18m/s, 64km/h) over distances of particular glycogen, high mitochondrial volume in muscle,
800 to 5000 meters, Standardbred horses trot or pace at high the ability to increase oxygen-carrying capacity of blood at
speed for distances up to 3600m, Quarter Horses sprint for the onset ofexercise through splenic contraction, efficiency of
400m or less at speed as high as 88km/h (see Chapter 32), gait, and efficient thermoregulation.
sometimes around figure of eight courses delineated by The maximal aerobic capacity (V•O ) of horses is
2max
barrels (barrel racing), and Arabians trot for up to 160km in approximately 2.6 times that of similarly sized cattle.8 The
a single day during endurance events (and over longer dis- larger aerobic capacity in horses is permitted by a larger
4
Integrative physiology and exercise testing
Table 1.1 Selected physiologic variables of athletic and non-athletic species4–7
·
Species Bodyweight Speeda Duration VO HRa Energy
2max
(kg) (km/h) of exercise (mL O /kg/min)(beats/min)expenditure
2
per day (kcal)
Thoroughbred 450 64 (max) 2min 180–200 240 (max) 30000
race horse
Endurance 400 15 12 h 180 38000b
race horse
Steer 470 80
Goat 32 80
Greyhound 34 64 (max) 60s Not reported 300a 2160
Sled dog 25 20 10 days 170 300 11000b
Human 70 36 (max) 9.4s 85 220 7000c
(Olympic class)
Pronghorn 32 65 10min 300
antelope
aDuring customary athletic activity.
bDay of racing.
cTour de France cyclists.
HR,heart rate;max,maximum value.
maximum cardiac output and stroke volume and higher weight, the greater is the oxidative capacity of muscle.
hemoglobin concentration.8Maximum heart rate is not dif- Muscle of horses contains approximately twice the concen-
ferent between horses and cattle. In addition to the cardio- tration of mitochondria as does muscle of cattle, a similarly
vascular differences between cattle and horses, horses also sized animal but with a much lower aerobic capacity.16This
have lungs that are twice as large as those of cattle with gas greater aerobic capacity in muscle, when supported by ade-
exchange surfaces 1.6 times those of cattle.9 Thus, horses quate substrate availability and oxygen delivery, permits a
have structural adaptations that enhance oxygenation of higher whole animal maximal aerobic capacity.
blood in the lungs, oxygen transport capacity of blood Oxygen transport from the lungs to exercising muscle is
and the ability to deliver oxygen to tissues. The oxygen trans- achieved by the circulation. In addition to cardiac output,
port chain, from air to muscle, of horses is suited to trans- oxygen delivery is limited by the oxygen-carrying capacity of
portation of the large volumes of oxygen required to sup- blood. Horses achieve rapid increases in the oxygen-carrying
port the high metabolic rate of strenuously exercising capacity of blood by increasing hemoglobin concentration
horses. through splenic contraction. Splenic contraction in anticipa-
Substrate is required to support these high metabolic rates tion of exercise and during exercise increases the circulating
during exercise. Substrate to support exercise is either carbo- red cell mass without concomitant increases in plasma
hydrate or fatty acids. Oxidation of fatty acids is limited and volume.17The resulting increase in hemoglobin concentra-
reaches maximal values in other species at a work intensity of tion increases the oxygen-carrying capacity of arterial blood
approximately 40–60% of V•O .10,11 It is likely that a by up to 50% during intense exercise. The beneficial effect of
2max
similar phenomenon occurs in horses. Additional work above this autoinfusion of red cells at the start of exercise is appar-
this exercise intensity is fueled solely by oxidation of carbohy- ent in horses from which the spleen has been removed.18–20
drates, predominantly glycogen.12 Horses have high intra- Splenectomized horses have lower hematocrits during exer-
muscular concentrations of glycogen, as do other athletic cise, altered systemic hemodynamics including lower right
species such as dogs.13Muscle concentrations of glycogen in atrial and pulmonary artery pressures, and reduced capacity
horses are approximately 140 mmol/kg of muscle (wet to perform strenuous exercise.
weight) compared with 80–100mmol/kg in humans.14High Energetically efficient gait is challenging for large animals
intramuscular concentrations of substrate are important for because of the slow rate of contraction and low power output
fueling muscle contractions during exercise. The flux of of their muscles.21However, the gait of horses is energetically
glucose from blood into muscle and subsequently to the mito- efficient,22with the muscular work of galloping being halved
chondria provides only a small amount (< 10%) of the by elastic storage of energy in muscle and tendon units.23For
energy used during intense exercise,15 probably because of the forelimb, this use of stored energy and the subsequent
limits to the rate of transportation of these compounds catapult action mean that the biceps and brachiocephalicus
during exercise.13The presence of large amounts of readily muscles are less than one-hundredth the size that they would
available substrate in close proximity to mitochondria is need to be were there no use of stored energy.21
therefore essential for horses to undertake strenuous exercise. In summary, a large number of physiologic and anatomic
Mitochondria provide the energy for muscle contraction. features act in concert to endow the horse with extraordinary
The greater the quantity of mitochondria per unit of muscle athletic capacity. Optimal athletic performance is dependent
5
1 Integrative physiology of exercise
upon optimal integrated functioning of these physiologic and oxygen. Energy production can be achieved for brief periods
anatomic features. of time by anaerobic metabolism, but ultimately all energy
production is linked to substrate oxidation and an adequate
supply of oxygen.
Integrative physiology Production of ATP during exercise is proximately depend-
ent on supplies of substrate for oxidation and of oxygen. A
of exercise
schematic of factors influencing the supply of these fuels to
muscle is depicted in Fig. 1.1. The important concept is that
there are a number of steps in the process or transport chain
The detailed responses of each body system to acute exercise by which each of these products is delivered to the muscle
and to repeated exercise (conditioning or training) are cell. Because these processes are sequential and often non-
described in chapters throughout this book. These responses, duplicative, a limitation in one process or function will limit
although described in isolation for each body system, do not the rate of the whole system. In some cases these rate-limit-
occur in isolation, but rather occur as a component of a ing steps may be modified by training, in which instance the
complex and integrated response to exercise, the ultimate rate of oxygen or substrate delivery will be increased, or may
goal of which is to provide substrate for muscle contraction not be altered by training. The consequences of these differ-
while maintaining homeostasis. ences are discussed below under ‘Factors limiting perform-
Exercise results in coordinated changes in almost all body ance’.
systems. Fundamentally, exercise is associated with an At the onset of exercise there is a coordinated response by
increase in power output achieved by contraction of muscles. a large number of body systems to increase fuel availability,
Contraction of muscles consumes adenosine triphosphate maintain acid–base balance within acceptable limits, and
(ATP) and triggers an increase in metabolic rate to replace limit body temperature. These responses include a large
expended ATP. Increases in metabolic rate are dependent increase in flux of substrate, the nature of which depends on
upon an adequate supply of substrate and, ultimately, the intensity and duration of exercise. Increasing exercise
Muscle Liver Adipose Muscle Oxygen
glycogen glycogen tissue TG
.
IN VA
GL Blood glucose
HSL
CAT
LPL
G-6-P NEFA Lungs
CO, [Hb]
SaO2,SvO2
Acetyl CoA Tissue
PcO
2
PmO
2
ATP ADP Muscle contraction
CO Heat Skin
2
Lactate
HCO,CO
3
Lungs Storage Respiratory tract
Glucose CO
VA Glycogen 2
Air
Fig.1.1
Schematic of substrate and oxygen flux demonstrating the integrated and sequential nature of many processes.Substrate supply to
produce ATP that powers muscle contraction is through both lipid and carbohydrate.Carbohydrate is provided from muscle
glycogen by phosphorylase or from glucose in blood.Important controlling hormones are glucagon,insulin and the catecholamines,
in addition to control by local physicochemical factors.Lipid substrate is provided from both intramuscular and adipose tissues,with
the former being more important during exercise.Ultimately carbon molecules are delivered to the mitochondria wherein they are
oxidized to produce carbon dioxide,heat and work.Oxygen delivery to the mitochondria is dependent upon a chain of events
leading from the atmospheric air to the mitochondria.Muscle contraction is associated with production of work,carbon dioxide,
heat and lactate (under conditions of anaerobic metabolism).ADP,adenosine diphosphate;ATP,adenosine triphosphate;CAT,
catecholamines;CO,cardiac output;CoA,coenzyme A;CO ,carbon dioxide;G-6-P,glucose-6-phosphate;GL,glucagon;[Hb],
2
hemoglobin concentration;HCO ,bicarbonate;HSL,hormone-sensitive lipase;IN,insulin;LPL,lipoprotein lipase;NEFA,non-esterified
fatty acid;PcO ,capillary oxygen t3ension;PmO ,mitochondrial tension;SaO ,arterial oxygen saturation;SvO ,venous oxygen
saturation;TG,2triglyceride;V ,alveolar ventilat2ion. 2 2
A