In the last issue we considered some of the adaptations that occur in horses during training. To recap, we know that improvements in cardiovascular and muscle function occur quite quickly after the onset of training. In fact, maximum aerobic capacity (VO2max) can increase by up to 10% after as little as two weeks of a regular exercise program. On the other hand, strengthening of the horse's supporting structures (i.e., bone and tendon) occur much more slowly, and injury problems can develop if these structures are subjected to excessive overload too early in the training program. Thus, while the cardiovascular and energy-generating systems adapt quickly, we must design training programs that allow sufficient time for adaptation of supporting structures while at the same time provide the stimulus necessary for attainment of peak fitness. Training programs that provide a solid foundation of moderate intensity distance work before beginning any speed work will allow this goal to be realized.
To use this disciplined approach, we need methods for judging how hard the horse is working during training bouts. As well, it is useful to be able to measure improvements in fitness during a training program. By doing so, the speed or intensity of the workout can be stepped up in a controlled manner, helping reduce the risk of over-training or injury.
In laboratory settings, a large number of fitness parameters can be measured, but most of us do not have access to such facilities. Fortunately, much information can be gained by monitoring your horse's heart rate. Heart rate is perhaps the best, and certainly the most practical, means for judging work effort during exercise. As we will discuss later, heart rate during exercise and recovery is a very useful guide to a horse's progress during training.
Resting Heart Rate
Resting heart rate in most horses ranges between 25 and 40 beats per minute. However, a large number of environmental factors can influence resting heart rate, and the demeanor and temperament of the horse must be considered when interpreting these values. Sudden excitement or fear will result in a rapid increase in heart rate, with values of 80 to 100 beats per minute or more. In human athletes, training and increases in physical fitness are associated with a lowering of resting heart rate. However, research in horses has suggested that there is minimal change in resting heart rate with improving fitness.
Nonetheless, knowledge of your horse's normal resting heart rate is important because it serves as a benchmark for assessment of recovery following a workout. Inexpensive stethoscopes are available at most pharmacies. Buy one, and become comfortable using it. Position the head of the stethoscope on the left side of the chest, just under the point of the elbow. When measuring heart rates, note that each beat produces two very distinct sounds--lub-dub. The "dub" sound is the louder of the two. Count beats for at least 20 seconds (e.g., 12 beats in a 20-second period is a heart rate of 36 per minute).
To establish a true resting heart rate, the horse must be resting quietly, usually standing in his stall with measurements taken by a person familiar to the horse. If you are new to this practice, develop a pattern of taking heart rate measurements on a daily basis, trying to standardize the circumstances under which these recordings are made. It is also a good idea to develop a health record that includes daily heart rate values together with other observations concerning the horse's general attitude, appetite, etc. Over time, a normal range of resting heart rates can be developed for an individual horse, and that becomes a useful tool for detection of early signs of illness or over-exertion during training.
Heart Rate Monitors
A heart rate monitor is required for measurement of heart rate during exercise. Several companies sell heart rate monitors that have been specifically designed for use on a horse. Pricing depends on the number of "bells and whistles," but most fall within the $125 to $200 range. The main components are two small, flat sensing electrodes attached to wires that are connected with a small transmitting device. That device sends signals to a digital "wristwatch" worn by the rider. Basic models detect and display real time heart rate, thus allowing the rider to precisely monitor work effort during a ride. Upgraded models have watches that double as a regular watch/stopwatch and record details of the workout (and recovery) for review after completion of the ride. Computer software also is available allowing this information to be uploaded and analyzed. This capability is invaluable for monitoring progress during training.
Heart rate monitors are incorporated easily into the horse's tack. The sensing electrodes are positioned at two sites on the left side of the chest. One electrode is positioned under the girth strap at the level of the heart (at the point of the elbow), the other just below the withers where it can be seated under the arch of the saddle pommel. Typically, heart rate monitor kits come with Velcro patches that help to anchor the sensor/lead wire in the appropriate positions. The lead wires, which are available in different lengths depending on the application, are plugged into the transmitter unit, which is contained within a pouch that can be attached to the saddle or a breast plate.
The quality of contact between the sensing electrodes and the horse's skin has a large bearing on the reliability of heart measurements obtained. The electrodes must be firmly attached to the skin so that they do not slip during exercise. Application of a special conducting gel (or even a solution of water and table salt) improves the quality of the signal. Actually, horse sweat is an excellent conductor, so the heart rate signal often will improve once the horse starts to sweat under the saddle/girth area. On the other hand, a long hair coat will decrease the quality of the signal. This is not a problem during the summer months, but some clipping of the hair coat might be required in winter to ensure proper function of the heart rate monitor. If the heart rate values are very erratic while the horse is working at a fairly constant speed, suspect poor electrode contact due to movement of the sensors and/or lack of conducting substance.
Using Heart Rate To Assess Work Effort
Heart rate changes in a fairly predictable manner with exercise. In some horses, particularly youngsters and those not accustomed to the exercise routine, heart rate will increase in anticipation of the start of exercise with values up to 90 to 100 beats per minute. This response might not occur in older campaigners. Heart rate will increase rapidly during walk-to-trot or trot-to-canter transitions. In fact, there tends to be an "overshoot" in heart rate during these transitions. If a steady speed then is maintained, heart rate will slow after two to three minutes. Therefore, heart rate measurements during this initial period are unreliable for assessing work effort. The size of the overshoot tends to be larger in unfit horses.
The usefulness of heart rate as a means to quantify work effort comes from knowledge that there is a linear relationship between heart rate and running speed (or work effort if we consider other factors that will influence the intensity of exercise, such as deepness of footing, terrain, and environmental conditions). This relationship is depicted in Figure 1 on page 86 (top). In general, the linear relationship holds true at heart rates between 120 and 210 beats per minute. During light exercise, heart rate might not be a true reflection of work rate or speed, particularly in horses more interested in their surroundings than the task at hand!
With increasing running speed, this speed/heart rate curve will flatten out as the horse approaches its maximum heart rate (HRmax). Beyond this point, there will be no change in heart rate despite further increases in running speed. HRmax can vary greatly between horses. However, for most light breeds, HRmax will average 220 to 230 beats per minute. For an individual horse, HRmax can be determined by measuring heart rate during an exercise test involving a stepwise increase in running speed. From a practical perspective, however, knowledge of the true HRmax is not critical because it does not change with training and therefore does not provide a guide to fitness state.
In humans, maximal heart rate decreases with age. Whether a similar change occurs in horses is less clear, although it seems likely that older horses (greater than 15 years) have lower HRmax values. As a general guide, mature horses (three to 10 years) will, on average, have maximal heart rates of approximately 220 beats per minute, whereas horses greater than 15 years of age will have values of around 210 beats per minute. These numbers can be used as guide for setting exercise effort as a percentage of HRmax.
We also know that percentage HRmax is related to percentage maximum aerobic capacity (% VO2max)--see Figure 2 on page 86 (bottom). This relationship can be applied to a horse's training program. During the early part of training, the major goal is to establish a basic level of conditioning without overly stressing the horse's supporting structures. This is best accomplished by keeping the work effort well within the "aerobic zone." An adequate training stimulus occurs when the horse exercises at workloads equivalent to 40%-50% of maximum aerobic capacity (VO2max), which equates to 60%-70% of HRmax. If we assume that HRmax is 220 beats per minute, the target zone for working heart rate will be 135 to 155 beats per minute. Most heart rate monitors have "high-low" alarm settings, providing the rider with immediate feedback when the work effort moves out of the pre-set target zone.
After completion of this foundation training (as a guide, 200 to 250 miles of aerobic conditioning exercise for a horse coming back after a lengthy lay-up), the training program can be geared toward the specific activity for which the horse is being prepared. For endurance horses, this will involve further increases in distance covered with some work at higher heart rate zones (160-180 beats per minute). Working heart rates in the range of 170-180 beats per minute (75%-80% of HRmax) will be close to a horse's lactate threshold. The lactate threshold is defined as the point at which lactate, the product of anaerobic metabolism, begins to accumulate in the blood. This threshold is reached when blood plasma lactate concentrations exceed 4 mmol/l (see Figure 3 on page 88). At work loads greater than 80% of HRmax, there will be marked increases in blood lactate, indicating substantial anaerobic metabolism during exercise. Training at or just below lactate threshold results in further improvements in aerobic capacity and the efficiency of fuel utilization during exercise.
Event horses and racehorses will require training at higher speeds--galloping and all-out sprint work. This type of exercise is necessary to stimulate adaptations in the anaerobic energy system and allow development of the coordination and mechanical efficiency required for running at high speed. These workloads will elicit HRmax, and heart rate measurements become a less useful indicator of work effort. Nonetheless, heart rate monitoring is still invaluable for evaluating recovery following hard work (more information and detailed descriptions of training programs for specific sports appears in books such as Conditioning Sport Horses by Hilary Clayton, BVMS, PhD).
Be aware that factors other than true work effort can alter the heart rate response to exercise. For example, lameness and some cardiac (i.e., disorders of heart rhythm) and respiratory (i.e., chronic obstructive pulmonary disease) ailments will result in exercise heart rates that are higher than the horse's norm for a given workload. Very hot conditions also tend to place higher demands on the horse's cardiovascular system, resulting in higher heart rates.
This point serves to emphasize the importance of good recordkeeping. Higher than normal heart rates during exercise might suggest over-exertion (the horse is being pushed too hard in training) or the need to have your veterinarian evaluate the horse more closely.
Using Heart Rate Responses To Monitor Changes In Fitness
Improvements in fitness do alter the heart rate response to exercise. Most training studies in horses have shown that an increase in fitness is associated with a reduction in a horse's heart rate at a given running speed or exercise intensity. In other words, the horse is able to achieve a higher running speed at a specific heart rate (see Figure 4 on page 90). As well, the improvement in fitness will be reflected in a more rapid decline in heart rate after cessation of exercise.
Standardized treadmill exercise tests are perhaps the best tool for gathering information regarding the fitness of an individual horse, particularly when such tests are repeated at regular intervals during training. Unfortunately, access to facilities with a high-speed treadmill is quite limited. On the other hand, with a little forethought and assistance, it is possible to conduct a field test that will provide useful information.
For example, by working a horse over a set distance (i.e., a half-mile) at different speeds, it is possible to gather data on the horse's heart rate response to exercise of different intensities. Each interval is followed by a 10-minute jog-walk rest interval that allows heart rate recovery and ensures that fatigue does not influence the heart rate response during subsequent gallops. Either the rider or an assistant uses a stopwatch to record running time, from which velocity is calculated.
It is important that the conditions for these tests are standardized. The horse should be ridden by the same person and, as much as is possible, environmental conditions such as air temperature and relative humidity and underfoot conditions should be similar. Wide variation in these conditions will make between-test comparisons less reliable.
The most useful values are the speeds or velocities (V) that result in heart rates of 140, 160, 180, and 200 beats per minute (termed V140, V160, V180 and V200, respectively). Of course, it is unrealistic to choose running speeds that precisely "hit" these target heart rates. However, as discussed earlier, we know that running speed is linearly related to heart rate over the range of 140 to 200 beats per minute. So, after the test is completed, the data for running speed (x axis) and heart rate (y axis) are plotted, as shown in Figure 4 on page 90. These graphs can be constructed by hand or, better still, by use of graphing features contained within most computer spreadsheet programs. A best-fit straight line (linear regression) is drawn through the data points. Using this line as the point of reference, it then is possible to calculate V140, V160, V180, and V200.
As the horse's fitness improves, the line representing the relationship between running speed and heart rate will be shifted to the right on the graph. Another very useful fitness measure is the V200 (velocity at a heart rate of 200 beats per minute). Research studies have shown that the V200 is directly related to a horse's maximum aerobic capacity (V.O2max). Thus, the higher the value for V200, the greater the horse's aerobic power. In a recent study of 2-year-old Thoroughbred horses (Kobayashi et al. 1999), the average V200 increased from 623 meters/minute to 691 meters/minute after five months of conventional race training.
Recovery heart rate is also very useful in the assessment of fitness and is accepted universally as a means to assess the ability of endurance horses to continue during competitions (the Cardiac Recovery Index). As fitness improves, heart rate will decline more quickly following completion of a standardized exercise test. Take note of the heart rate at two, five, and 10 minutes after completion of the exercise. In well-conditioned horses, the heart rate will be around 60 beats per minute at the end of this recovery period, even following reasonably strenuous workouts. However, a recovery heart rate of 72-80 beats/minute or more might indicate that the work effort was beyond the horse's current level of conditioning.
Again, a word of caution regarding climate. Heart rate recovery might be delayed when exercise is undertaken in hot conditions because of the effects of dehydration (sweat fluid losses) and a greater rise in body temperature compared to exercise in cool to moderate conditions.
Other Useful Measures Of Fitness
Many racehorse trainers rely heavily on repeated measurements of certain blood parameters for assessment of fitness and readiness for racing. In particular, red blood cell count, hemoglobin concentration, and packed cell volume (all indicators of the horse's capacity to transport oxygen in the blood) are measured on a routine basis. While it is true that exercise training is associated with small increases in these indices, resting values for red cell count and hemoglobin concentration cannot be used to assess fitness and are unreliable for prediction of the horse's capacity to transport oxygen during exercise.
Up to one-third of the horse's red blood cells are stored in its spleen; this red cell reservoir is pumped into circulation at the onset of exercise, providing a tremendous boost in oxygen-carrying capacity. Thus, only blood samples taken during exercise can serve as a reliable indicator of total red cell numbers.
Post-exercise respiratory rate also is not a reliable indicator of fitness. The respiratory tract is an important avenue for heat loss in the horse and the rate of decline in respiratory rate following exercise is related to the horse's body temperature. In hot conditions, particularly in hot, humid weather, the respiratory rate might remain high (greater than 80-100 breaths per minute) for up to 30 minutes after completion of long, strenuous exercise such as endurance rides or the speed and endurance test of a three-day event. These high-recovery respiratory rates indicate the need for active cooling of the horse (whole body washing with cool water).
Finally, but most importantly, ongoing assessment of general physical health is critical to the success of any training program. Working with your veterinarian and farrier, ensure that preventative medicine programs (i.e., vaccination and parasite control) and foot health are well maintained.
Conditioning Sport Horses, by Hilary M. Clayton. Sport Horse Publications, Saskatoon, Saskatchewan, Canada, 1991.
Eaton, M.D.; Evans, D.L.; Hodgson, D.R.; Rose, R.J. Effect of treadmill incline and speed on metabolic rate during exercise in Thoroughbred horses. Journal of Applied Physiology 79: 951-957, 1995.
Kobayashi, M.; Kuribara, K.; Amada, A. Application of V200 values for evaluation of training effects in the young Thoroughbred under field conditions. Equine Veterinary Journal Supplement 30: 159-162, 1999.
Seeherman, H.J.; Morris, E.A. Methodology and repeatability of a standardised treadmill exercise test for clinical evaluation of fitness in the performance horse. Equine Veterinary Journal Supplement 9: 20-25, 1990.
The Athletic Horse: Principles and Practice of Equine Sports Medicine, by David R. Hodgson and Reuben J. Rose. W.B. Saunders Company, Philadelphia, Pa., 1994.
About the Author
Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University
POLL: University Equine Hospitals