Building a stronger racehorse with DYNAMIC MOBILISATION EXERCISES
Author - Kimberley Marrss
Yoga, Pilates, cross training, pre-habilitation—whatever you prefer to call it—are strength- training exercises also known as dynamic mobilisation exercises, which can greatly benefit your racehorses. You can perform these exercises on your horses to help with correcting posture, gaining strength, flexibility and core stability. All of these positive gains will help a racehorse be more balanced in his movements. Incorporating these exercises daily will of course help alleviate tensions in the body that could potentially turn into problems or pathologies down the road, while reducing the risk of repetitive use injuries.
The concept is the same as cross training in human athletic training. A horse also needs several types of exercises to help keep the body balanced and able to handle the intensity of competition, reduce performance issues and aid in the body's recovery after competition. As human athletes have known this, one size—one exercise—does not fit all. The body's systems all need to be challenged to produce an all-around balanced athlete. This logic needs to be addressed with training racehorses as well. Regular sessions of these types of exercises will result in the improvement of the horse's posture and ability to carry a rider in a balanced way. A horse with good posture will allow for optimal performance for a longer period with less exhaustion and muscle fatigue.
Muscles at work
A dynamic mobilisation stretch is mostly done using a bait and are referred to sometimes as carrot stretches. While performing a stretch, certain muscles are activated so that they move, and stabilise the intervertebral joints.
Then the activated abdominal muscles help to bend and round the back. This is especially important for a racehorse unlike horses who are trained to carry a long low frame, which will help them engage their body’s core. Racehorses will tend to train with their head high and with a hollowed back. Over time, this can predispose a horse to back pain, kissing spines, lumbosacral issues—all which can then lead to compensatory issues.
If you take a series of two-year-old thoroughbred racehorses, a high proportion of them will have radiological evidence of kissing spines, and quite a lot will have associated back pain.
So joint stabilization is particularly important to help with improving performance and the prevention of injuries. Research has also shown that with regular performance of these exercises, the equine back becomes enlarged. Subsequently, this will strengthen the back muscles and enable the horse to carry the rider more efficiently with a lessening of back stress. To demonstrate this, I have a person put a 11 to 22 kg sack of feed over their shoulder. Next, I have them walk between 7 to 9 metres as they would normally walk. Then I have the person stand up straight, hold in their core (abdominals) with correct posture and walk back 7 to 9 metres. The difference is usually substantial, and the person can feel less stress on the limbs with a straighter and more balanced way of moving. Now they get why the horse's core strength is so important!
Get started on a routine
I have been doing these for some time and have seen amazing results in my horses' overall posture, toplines, reduction of back pain and suppleness in their musculature. To get started with your dynamic mobilisation routine, keep in mind that performing these exercises are safe. Do take care though: If your horse has a musculoskeletal or neurological issue, clear it with the vet first before starting dynamic mobilisation exercises. Also keep in mind that many of these exercises are also utilised in rehabilitation from injuries.
When implementing these exercises, ensure the horse follows the bait in a nice smooth manner, and get the horse to hold the stretch. This can sometimes be easier said than done. Some horses will try to snatch and then snap their head back forward. So be patient; there is a learning curve with each individual horse. The more you perform these moves, the better you’ll get, and the smoother the stretches will become.
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The Equine Heart - Anatomy, Function and Performance
Exciting new advances in ultrasound image technology have provided a better understanding of both the anatomy and function of the heart at rest and during exercise. In the last 30 years many veterinary clinics and universities with equine departments that study equine physiology are able to study the heart of the equine athlete in their own sports performance laboratories, while exercising on a high-speed treadmill.
Robert Keck (European Trainer - issue 19 - Autumn 2007)
Exciting new advances in ultrasound image technology have provided a better understanding of both the anatomy and function of the heart at rest and during exercise. In the last 30 years many veterinary clinics and universities with equine departments that study equine physiology are able to study the heart of the equine athlete in their own sports performance laboratories, while exercising on a high-speed treadmill. Considering that heart rate is one of the most frequently measured physiological variables in exercise tests, Thoroughbred racehorse trainers have largely failed to take advantage of the heart rate monitor as standard equipment. However, heart rate monitors are commonplace in eventing and sport horses. Understanding the heart’s function, and its response and adaptation to training, can provide trainers with a competitive edge.
ANATOMY AND FUNCTION
The heart of a Thoroughbred weighs about 1% of the horse’s bodyweight but can be as high as 1.3-1.4% in elite animals. Therefore an average 1000 pound horse has a heart weighing between 8-10 pounds. The horse has a proportionately larger heart per unit of body mass as compared to other mammals. The horse’s heart rate is 20-30 beats per minute at rest and may have a maximal heart rate of 240 beats per minute during maximal exercise. The fact that the horse is able to increase heart rate by nearly 10 times the resting heart rate is a contributing factor to their athletic superiority. As in all mammals, the heart consists of four chambers with valves that open and close as the heart muscle relaxes and contracts to insure blood flows in the right direction. The two pumping chambers are the left and right ventricles, and the two receiving chambers are the left and right atria. The left ventricle is larger than the right ventricle.
Specialized cells within the heart conduct electrical activity that coordinates the muscles of the heart to contract in order to optimize blood pumping. Electrical impulses of both the atria and ventricles are isolated by a fibrous ring; preventing them from contracting simultaneously. The only point at which electrical activity can pass between the atria and the ventricles is via the Purkinje fibers found in the wall between the left and right ventricle. When the atria contract, blood is delivered to the larger volume ventricle that lies beneath. The right side of the heart receives unoxygenated blood from the body and pumps it to the lungs to allow the red blood cells to uptake oxygen. Oxygenated blood returns to the left side of the heart, and the left ventricle pumps it out the aorta to the rest of the body. The cardiac cycle consists of a contraction/ejection phase (systole), and a relaxation/filling phase (diastole). Stroke volume (SV) is the volume of blood pumped in each beat, and is influenced by the muscular contraction of the ventricles, their resistance to flow during systolic ejection, and their ability to fill during the diastolic relaxation. The structural integrity of various anatomic components of the heart such as the valves and septa between the chambers affect heart function. Stroke volume in a 500 kg Thoroughbred is approximately 1.3 litres and can increase by 20-50% during exercise. Cardiac output (CO) is stroke volume (SV) multiplied by heart rate (HR); therefore CO = SV x HR. At rest the cardiac output is approximately 6.6 (25 litres) gallons per minute and increases to an amazing 79 (300 litres) gallons per minute in elite athletes during exercise. A horse’s total blood volume is approximately 10 gallons, representing 10% of its body weight.
At rest 35% of the horse’s blood volume is red blood cells, however they can amazingly increase their red blood cell count on demand to 65% of their blood volume during a race, with up to 50% of the total red blood cells stored in the spleen. The horse has a proportionally larger spleen per unit of body mass as compared to other mammals. The red blood cells are void of a nucleus and have the large protein haemoglobin that transports oxygen. The horse’s heart is able to handle the increased viscosity of the blood. During exercise blood is diverted away from internal organs such as the intestines and kidney to working muscles used in motion.
THE HEART AND VO2 MAX
The heart is a major determinant in VO2 max, a measure of aerobic capacity. VO2 max is the maximal rate of oxygen consumption that can be consumed by the horse. VO2 max is determined by cardiac output (stroke volume x heart rate), lung capacity, and the ability of muscle cells to extract oxygen from the blood. During exercise the oxygen requirement by muscles can increase to 35 times their resting rate. Sydney University studies have shown that training can increase a Thoroughbred’s VO2 max by 20% or more, with this improvement highly attributable to the heart’s pumping capacity. VO2 max expressed as millilitres of O2 per kilogram of bodyweight per minute (or second). At rest the horse absorbs 3 millilitres of oxygen per kilogram of body weight per minute. Maximal rates of oxygen intake vary within breeds and training state, but fit Thoroughbreds have a VO2 max of 160-170 ml./min./kg and elite horses can achieve 200 ml./min./kg. By comparison elite human athletes have a VO2 max of about half or 85 ml./min./kg. Pronghorn antelopes have a VO2 max of 210-310 ml./min./kg. VO2 max is a high indicator of athletic potential, and has been found to be highly correlated with race times in Thoroughbred horses. A horse with a higher VO2 max had faster times (Harkening et al, 1993). The ability of the horse’s muscle mass to consume oxygen far exceeds the ability of the heart and lungs to provide oxygenated blood. Therefore cardiac output is a limiting factor in performance. Conditions that improve cardiac output positively impact VO2 max.
HEART RESPONSE TO TRAINING
The heart has two initial responses to exercise, a rise in blood volume pumped and dilation of the blood vessels. The heart rate increases, and beats stronger. The stroke volume may increase from 20-50% above resting rates. Through training the heart becomes more efficient at delivering oxygenated blood to exercising muscles. Heart mass has been shown to increase with training. This hypertrophy (enlargement) in the heart comes in two ways, a thickening of the heart walls, and an increase in the size of the chambers, especially the left ventricle. Although the effects of training on the heart are not clearly understood, heart mass has been shown to increase up to 33% in 2-year old horses after only 18 weeks of conventional race training (Young, 1999). The increase in heart size results in increased cardiac output. Stroke volume has been shown to increase by 10% in as little as 10 weeks of training (Thomas et al, 1983).
Although not yet proved, it is likely that in addition to the strengthening, improved filling capacity of the pumping chambers when the heart is relaxed may contribute to the increases shown in stroke volume. Interestingly, maximal heart rate does not increase with training, and resting heart rates (unlike humans) do not decrease with training. Training can improve VO2 max from 10-20% in the first 6-8 weeks of training, after which further improvement is limited. The relationship between VO2 max and velocity is highly correlated, but the differences found in speed and performance of two Thoroughbreds with equal VO2 max can be explained by differences in biomechanics and economy of locomotion. Although the heart plays an important role in determining several physiological factors related to performance, it is merely one variable in the whole physiological equation that describes the equine athlete. Not only does the heart change and adapt with the rigors of training, but a myriad number of adaptations take place in the muscle fibers at the cellular level. As a result of training, oxidative enzymes in the muscles increase, along with the size and density of mitochondria, the powerhouse of the cell. Enhanced oxidative capacity results in increased utilization of fat and less reliance on blood glucose and muscle glycogen, being an advantage at both submaximal and maximal exercise, because fat is a more efficient energy fuel. An improved network in the number and density of capillaries provides more efficient blood flow and transit time to working muscles, which also become more efficient in buffering lactate in anaerobic exercise. Muscle, bone, tendons and ligaments modify their structure with the stresses of training. Depending on the event, the horse develops “metabolic specificity” and neuromuscular coordination for his chosen discipline.
EVALUATING THE HEART - ULTRASOUNDS
When evaluating the equine heart, ultrasound has become an extremely valuable non-invasive tool, revolutionizing equine cardiology. The heart’s anatomical structure and physiology can be readily determined as well as measurements in heart size, wall thickness, and identifying defective cardiac valve function. Findings can determine pathology of the heart and the cause of poor performance. The ultrasound examination of the heart (echocardiogram) is now considered an integral part of cardiovascular evaluation of equine athletes. An ultrasound machine works by emitting a beam of high frequency sound waves (>20,000 Hz) from an ultrasound transducer into the body tissues. In general, the waves can penetrate to a maximum of 15 inches (40 cm) and they interact with various tissue types in different ways. The waves can be scattered, refracted or attenuated. The reflected waves are transmitted back to the ultrasound transducer. This information is interpreted by the ultrasound machine which produces a two-dimensional black and white image called a sonogram. The frequency of the ultrasound waves emitted by the transducer markedly influences the quality of the image, depending on the depth of the tissues. Higher frequency ultrasound waves have a shorter wavelength and yield better resolution of small structures close to the skin surface. However, more energy is absorbed and scattered with high frequency, therefore high frequency transducers have less penetrating ability.
Conversely, a lower frequency transducer will have greater depth of penetration but poor resolution. The transducer selected for echocardiography should be the highest frequency available that will penetrate to the depths needed to image the heart in its entirety. Frequencies generally used for veterinary echocardiography range from 2.25-3.5 Mhz for adult horses. The three main types of ultrasounds available to veterinarians and researchers are the M-Mode, Two-Dimensional (2-D), and Doppler. Although M-Mode yields only a one-dimensional (“ice pick”) view of the cardiac structures, it can yield cleaner images of cardiac borders, allowing the researcher to obtain very accurate measurements of cardiac dimensions and critically evaluate cardiac motion over time. Two-dimensional echocardiography allows a plane of tissue, with depth and width, to be imaged in real time. This makes it easier to appreciate the anatomic relationships between various structures. 2-D echocardiography makes available an infinite number of imaging planes of the heart. Doppler echocardiography records blood flow within the cardiovascular system when blood moving toward or away from the transducer causes a Doppler shift. From this shift, it is possible to calculate the velocity of the moving blood.
ELECTRO-CARDIOGRAM (ECG)
An ECG (electrocardiogram) is another tool commonly used in evaluating the heart. It measures the heart’s electrical conductivity can identify a part that is not contracting properly. It is the tool of choice for diagnosing arrhythmias. The ECG provides information to the researcher about the quality and rhythm of the heartbeat. The appearance of the ECG changes dramatically from rest to exercise. Cardiac contractions are the result of a well-orchestrated electrical phenomenon called depolarization. In the myocardium are specialized fibers that are very conductive and allow rapid transmission of electrical impulses across the muscle, telling them to contract. There is uniformity in the sequence and force of both the filling and ejecting chambers, relying on a single impulse initiated by the sinoatrial (S/A or sinus) node. Another node is the A/V node (atrioventricular node) situated between the two chambers. The ECG measures electrical activity from the P-Wave, QRS, and T-Wave.
The P-Wave represents the electrical impulse measured across the atria, whereas the T-Wave measures the repolarization of the ventricles. The QRS represents the electrical impulse as it travels across the ventricles. Measurements between these impulses include the PR and ST segments and the PR and OT intervals, all of which can reveal abnormal heart function. Electrodes are placed in strategic positions on the skin surface to pick up the heart’s electrical activity. In clinical practice, 12 leads may be used in a diagnostic ECG, but usually there are three standard leads, I, II and III, placed at different areas around the ribcage and chest. Placement of the electrodes are critical, and can change the size and shape of the ECG. HEART MURMURS AN ARRHYTHMIAS Vascular diseases in horses, such as atherosclerosis, which contributes to strokes and heart attacks, are rare. Two of the most common heart abnormalities are heart murmurs and arrhythmias. A heart murmur is the sound of turbulent blood flow, usually caused by an abrupt increase in flow velocity. This turbulence is caused by increased velocity due to a leak or obstruction in one of the heart valves or because of abnormal communication between different parts of the heart. Heart murmurs, which are fairly common, occur in horses of all ages. They are called “innocent” when they are soft, short and variable without any other cardiac pathology. One study detected cardiac murmurs in 81% of 846 Thoroughbred racehorses (Kriz, Hodgson, and Rose 2000). Congenital heart defects are abnormalities that are present at birth, the most common being ventricular septal defect (VSD) where a hole is found between the two ventricles.
Oxygen-rich blood from the higher pressure left ventricle passes through to the lower pressure right ventricle and pulmonary artery during ventricular systole. Because some blood bypasses the lungs, it is not fully oxygenated and will have an adverse effect on cardiac function. Depending on the size of the hole, the horse may be fully capable of moderate activities without fatigue or shortness of breath. VSD is usually detected on the right side of the chest over the cranial part of the heart, and can be fully diagnosed with 2-D ultrasound and Doppler echocardiography. Atrial fibrillation is an electrical disorder of the heart rhythm, also know as an arrhythmia. Associated with diminished performance, the normally regular, organized atrial waves become irregular, disorganized and chaotic, and the atria fail to contract normally, leading to an unpredictable and irregular heartbeat. Accurate diagnosis using an electrocardiogram can determine type and severity, and often an oral or injectable drug such as quinidine can be administered to establish a normal rhythm. An arrhythmia can sometimes be caused by myocarditis, where part of the heart muscle tissue has died due to an infectious disease such as strangles, influenza or an internal abscess. Toxic damage to the heart muscle may occur from a severe deficiency of vitamin E or selenium. The most commonly recognized acquired structural heart disorders are degenerative valvular deformities. These defects, involving a thickening and deformity of the valve leaflets, cause inefficiency of one or more heart valves, resulting in dilation of the chambers trying to handle the regurgitated blood on either side of the damaged valve. If the leak is severe enough, the pressure in the veins leading to the affected side of the heart increases until fluid accumulation (edema) occurs.
HEART SIZE AND PERFORMANCE
For centuries, owners, breeders and trainers have been captivated by the idea that the horse’s heart may be the proverbial “Holy Grail” to understanding athletic performance, and predicting the future elite racehorse. The large hearts found in elite human athletes are well-documented. In the 1920’s the “Flying Finn” Paavo Nurmi, who won 12 Olympic medals in track including 9 Golds and set world records from 1500 meters to 20 kilometers, had a heart three times larger than normal (Costill). At postmortem, the legendary 7-time Boston Marathon winner Clarence De Mar was shown to have an enlarged heart and massive coronary arteries (Costill). In 1989, it was believed that Secretariat, American Triple Crown winner of 1973, had a heart weighing over 10 kg (22 lbs.), and may have had a VO2 max of 240 ml./kg./min. Autopsies showed that the great Australian racehorse Phar Lap had a heart weighing 6.4 kg. (14.1 lbs), 20% larger than normal, and Key to the Mint, American champion 3-year old of 1972 and excellent broodmare sire, had a heart weighing 7.2 kg (15.8 lbs). Secretariat’s rival and runner-up Sham had one of the heaviest hearts recorded, weighing in at 18 lbs. (8.2 kg). Some of the first studies that scientifically attempted to correlate heart size with race performance were conducted in the 1950’s and early 60’s.
The Heart Score concept was first discovered and developed by Dr. James D. Steel, a professor of veterinary medicine at the University of Sydney in Australia in 1953. Using ECG (electrocardiography) to studying herbivores, he began studying the occurrence of heart disease in racehorses. His examinations led him to the development of the “Heart Score” which was his term to describe the correlation between the QRS (intraventricular conduction time) complexes and the performances of several elite versus average racehorses at the time. He believed that the higher heart score number based on the QRS duration using the standard bipolar leads must be correlated with the larger heart size and weight found in superior racehorses. Steel developed a ranking system that placed male horses with a heart score of 120 or more (116 or more for fillies and mares) in the large heart category, between 103-120 in the medium to normal category, and 103 or less in the small heart category.
His conclusion was based on the assumption that the QRS represents the time required for the electric wave to spread and depolarize the ventricular mass. He believed that the QRS interval corresponds to the beginning and end of ventricular depolarization. As the ventricular muscle mass increases, a longer time will be necessary for the ventricular depolarization to take place. Therefore, he believed the higher the heart score the larger the heart mass (and size) Unfortunately, Steel was wrong! Steel’s conclusions seemed logical at a time when equine cardiology was in its infancy. But in the horse (and hoofed mammals) the depolarization process differs from that of small animals because of the very widespread distribution of the Purkinje network. These fibers extend throughout the myocardium and ventricular depolarization takes place from multiple sites. The electromotive forces therefore tend to cancel each other out; consequently, no wavefronts are formed, and the overall effect of the ventricular depolarization on the ECG is minimal. (Celia 1999) Today, we know that ECGs provide little or no information about the relative or absolute sizes of the ventricles.
An ECG cannot measure heart size and cannot be used to correlate its size and / or mass. In several studies, heart score showed a relationship neither with body weight nor with ventricular mass, as determined by echocardiograph. Heart score did not correlate with heart size and cannot be regarded as an index for predicting potential performance (Lightowler et al 2004). Although a study using Danish Standardbreds showed a correlation between heart score and Timeform ratings, using these scores to determine heart size has largely been disproved.
HEART SIZE AND PERFORMANCE
Current research in the field of equine exercise physiology continues to investigate the heart and cardiac output. The size of the heart is a key determinant of maximal stroke volume, cardiac output and therefore aerobic capacity, and several new studies have proved this relationship. A recent breakthrough study demonstrated a significant linear relationship between British Horseracing Board Official rating or Timeform rating and heart size measured by echocardiography in 200 horses engaged in National Hunt racing (over jumps) (Young and Wood, 2001). It is the first study that positively correlates heart size to performance. Additionally, a significant strong relationship has been found between left ventricular mass (and other measurements of cardiac size) and VO2 max in Thoroughbred racehorses exercising on a high-speed treadmill. (Young et al 2002). Interestingly, no such relationships have been reliably been found when horses employed in flat racing were examined, suggesting that, as might be expected, VO2 max and heart size are more important predictors of performance for equine athletes running longer distances. It must be emphasized that these research studies were conducted on older racehorses that were already racing and training, very different from an untrained yearling.
CONCLUSION
Understanding the equine heart and its role in equine physiology will remain of great interest to breeders, owners and trainers. Future use of heart rate monitors and heart evaluations using ultrasound technology to identify heart pathology and abnormality will undoubtedly contribute to future breakthroughs in training and racing. The equine heart still remains just one variable in the elusive equation that makes for a great racehorse.
Horsewalkers - should they be round or oval?
Horsewalkers are used extensively in the management and training of horses. They permit controlled exercise of horses at walk and trot and are less labour intensive than most other forms of controlled exercise, such as walking in-hand, lunging, riding, swimming or running horses on treadmills.
Dr David Marlin & Paul Farrington (European Trainer - issue 18 - Summer 2007)
Horsewalkers (electro-mechanical devices that allow multiple horses to be exercised simultaneously in a controlled fashion) are used extensively in the management and training of horses. They permit controlled exercise of horses at walk and trot. They are less labour intensive than most other forms of controlled exercise, such as walking in-hand, lunging, riding, swimming or running horses on treadmills. The exception might be ride and lead, but this is not a widely used technique, except perhaps in polo.
Horsewalkers may be used for a variety of reasons including warming-up or cooling down prior to or following ridden exercise, as a way to relieve boredom in stabled horses, for controlled exercise as part of a rehabilitation programme and to supplement ridden exercise. Horsewalkers are often also used where ridden exercise is not desirable or possible, such as in preparation of young animals for sale or in animals that may have injury to the back and therefore cannot be ridden. The majority of horses can be trained to accept being exercised on a horsewalker within a short period of time.
Any form of exercise carries a risk of injury and whilst there does not appear to be any objective information on the safety of this form of exercise, it would generally be considered that the horsewalker is a very safe form of exercise. Until recently, horsewalkers have been exclusively of a round design in which the horse is constantly turning on a circular track. The radius (tightness) of the turn is determined by the diameter of the walker - the larger the walker, the more gradual the turn. At present commercial round horsewalkers vary from around 10 to 30 metres in diameter (i.e. 5-15 metres in radius). The conventional design is of a centre post from which radiate arms that support the moving dividers that separate the horses but also encourage them to walk as the centre post rotates, in turn moving the dividers.
Other designs do not incorporate dividers but horses are hitched to arms radiating from the centre post. Whilst the majority of walkers can operate in either a clockwise or anti-clockwise direction, on the walker the horse is still turning constantly. Exercising at walk or trot on a circle for prolonged periods of time must be considered to a large extent unnatural for a horse. Horses at pasture, whether grazing or exercising, move in all directions and never in one continuous direction. The same is true of ridden exercise. No rider would work his or her horse continuously for 30 minutes on a circle, even when working in a confined area. For example, a Dressage test incorporates many changes in rein and exercise in straight lines as well as on turns. Lunging is another mode of controlled, unridden exercise that is commonly used by horse owners or trainers.
Lunging may be used in place of ridden exercise or to train riders or as a warm-up for the horse prior to it being mounted and ridden. Lunging may also be used in situations where a horse requires to be exercised but where fitting a rider and saddle is not desirable, for example, in the case of a sore back. However, prolonged lunging is not advisable and in addition, as with circular walkers, changing the rein frequently is common practice. Continual turning may be deleterious to the musculoskeletal system (muscles, bones, tendons, ligaments and joints). For example, it is widely recognised that signs of lameness are exacerbated in horses exercised on a circle. This is commonly used by veterinary surgeons in lameness investigations. It is also suspected that sharp turns may contribute to injury of distal limb structures (i.e. those structures furthest from the body such as the foot).
This implies that turning exercise changes the weight distribution through the limbs. The surface on which a horse is lunged may also determine whether lameness is apparent or not; a horse may not exhibit lameness when lunged on a soft surface but may do so when lunged on the same size circle on a firmer or uneven surface. Most research into how horses move has been concentrated in horses walking and trotting in straight lines, or on treadmills, and there are only a limited number of studies relating to horses turning on a circle. Only one kinematic (movement) study has evaluated the effects of turning a corner on the distal joint motions. Horses turning in a sharp (1.5m diameter) left circle showed a shorter stride length, but stance duration (the amount of time the foot is on the ground) was longer. This work also showed that the lower leg and foot rotate as the weight of the horse moves over the limb. Research from Australia showed that the outside edge of the cannon bone is not loaded significantly during exercise in a straight line on a flat surface. The same group of researchers also showed in a separate study that surface strains on the cannon bone vary between inside and outside forelimbs during turning.
On the inner surface of the cannon bone, compression of the bone is greatest in the outside limb, and stretching of the bone is greatest on the inside limb. On the outer surface of the cannon bone, both compressive and tensile peaks are largest on the inside limb, which also showed the largest recorded strains in compression. On the dorsal (front) surface of the bone (where bucked shins occur in young horses), compressive strains were largest on the outside limb, and were greater on larger circles. They concluded that turning exercise is required to maintain normal bone, in that low-speed exercise in a straight line only loads the outer edge of the cannon bone. In 2006 workers from the USA studied the effect of trotting in a circle on the centre of mass of the horse. The centre of mass is a point within or on the body at which the mass of the body is considered to act.
The centre of mass may vary according to gait, speed and direction of travel. The location of the centre of mass affects the distribution and size of the loads on the limbs. These researchers showed that in horses trotting on the lunge on a 6m diameter circle at a speed of ~2 metres/second, all horses leaned inwards at an angle of ~15°. The speeds attained by these horses at trot on a circle are lower than those typically seen for horses on a straight line. As the speed was slower, the implication is that stance proportion was increased (i.e. the weight bearing phase of the stride was longer on a circle than would be expected in a straight line). Furthermore, the researchers pointed out that “horses may behave differently when turning clockwise versus counter-clockwise due to asymmetries in strength, suppleness and neural programming…”. Thus, whilst it is often assumed that an equal amount of exercise on each rein on a circular horsewalker should be applied, this may not be the case for many horses and may actually be counter-productive. The potential negative impact of circular exercise has also been highlighted with respect to the muscular system: “Especially in the initial stages of a return to work avoid lunging, horse walkers, or work in tight circles, as well as hill work”; a quote from veterinary surgeon and muscle specialist Dr Pat Harris from the Equine Studies Group at the WALTHAM Centre for Pet Nutrition, UK. Exercising on a circle also requires more effort than exercising in a straight line (Harris, Marlin, Davidson, Rodgerson, Gregory and Harrison (2007) Equine and Comparative Exercise Physiology, in press).
For example, being lunged on a 10 metre diameter circle was around 25% more work than being ridden on a large oval track in an indoor school. In addition, being lunged on a 5m circle was around 12% more work than being lunged on a 14 metre diameter circle. Even accounting for the weight of the rider, lunging is harder work than ridden exercise, which is most likely due to the continual effort required by the horse to balance itself on a continual turn. Oval walkers are a new concept. The premise of using oval walkers is that continual exercise on a small circle is unnatural for horses and could even lead to injury and that a walker incorporating both straight line and turning exercise would represent a more appropriate form of controlled exercise.
As so little information exists on turning in horses, a study was designed by us [Dr David Marlin (Physiologist) and Paul Farrington (Veterinary surgeon)] to investigate turning stress in horses in more detail. The work was undertaken in collaboration with Dr Bob Colborne (a specialist in Biomechanics) at Bristol University, UK. A
SUMMARY OF THE RECENT RESEARCH ON TURNING
The purpose of this study was to record the forces acting on the lower limb as horses walked in a straight line, on a 14 metre diameter circle, and on a 10 metre diameter circle to provide insight into the horizontal forces transmitted up the limb during locomotion in a straight line and whilst turning. Three fit, sound Thoroughbred horses, ages 3, 5 and 12 years of age were used in the study. Horses were walked across a force-plate (a metal plate placed on the ground that measures the force with which the horses’ foot is placed on the ground) both in a straight line and on a 10 and 14 metre diameter turn. For the turns the horse was always walking on a left-turn. The results showed that the coffin joint had the greatest degree of abduction (movement of the limb away from the body), adduction (movement of the limb towards the body) and axial rotation (twisting movement) and that these movements were greatest at the time of impact and break-over. The first point of contact with the ground has a significant influence on the line of stress through the foot and up the limb, as does the position of the body at the same moment.
On a turn the horse abducts the inside forelimb away from the body towards the line of the circle with rotation of the foot in the direction of the turn. The stride length is dictated by the tightness of the turn, as is the stance time (when the foot is on the ground). As the horse then moves forward the horse’s body moves towards the inside limb increasing the loading on the limb. The results showed that on average the forelimbs tended to behave asymmetrically (i.e. the two front legs did not behave the same) on a circle so that the forces and movements differ to produce different torque effects (twisting forces). The hind limbs tended to behave more symmetrically except when the size of the circle was reduced from 14 to 10 metres in diameter.
IMPORTANCE OF HORSEWALKER SURFACES
The walking surface will likely have an effect on the stresses experienced by a limb. If the surface allows reasonably free twisting of the hoof when weight bearing, the stresses between the hoof and ground will be small. However, any ground surface that holds the hoof and impedes this horizontal rotation will probably impart higher loads to the joints of the lower limb. Large turning forces should be avoided when the limb is vertically loaded (i.e. when the weight of the horse’s body is over the limb and the limb is on the ground). It is also important that the walking surface is level to avoid tilting of the hoof during weight-bearing. A walking track that is worn in the middle and that causes rotation of the joints in the foot is likely to cause larger and uneven forces to the lower limb joints and associated tendons and ligaments.
IMPLICATIONS FOR OVAL VERSUS ROUND HORSEWALKERS
Our recent research and a review of other scientific studies show that turning is not equivalent to exercise in a straight line. Turning exercise is harder than exercise in a straight line and loads the bones in a different way. Furthermore, on small turns the inner and outer limbs may not behave in the same way as on larger circles. This may have implications for horses with pre-existing musculoskeletal injuries. The potential advantages of an oval walker is that it combines straight line and turning exercise that more closely mimics the exercise that a horse will do when being ridden or when free at pasture. The results of our small study have shown that the hind limb patterns were quite different on the tighter radius turns, indicating a different strategy for turning, and supporting the notion that both straight line and turning exercise should be recommended for overall loading patterns that are healthy for maintaining bone that can withstand loading forces in a variety of directions. The results also make clear that small diameter round walkers (~10 metre diameter or less) are less desirable than round walkers of 14 metre diameter or greater. Small diameter round walkers increase the loading and asymmetry and increase the work compared with larger diameter walkers. In conclusion, there appear to be significant advantages to using a walker of an oval design as opposed to a round design, as exercise on an oval loads the limbs with a combination of straight and turning movements, as would be experienced during riding or in free movement.