Date: [__date__]

October Newsletter

Dear [__name__];

This month begins the first part in a series on the science behind training the thoroughbred racehorse. What makes the thoroughbred such an amazing athlete? How does their body adapt to training? How can we use our understanding of their physiology to measure fitness? What do we, as veterinarians look for in a poor performance examination? How can we use this information to win more races? Over the coming months we plan to answer these, and more questions on the training, fitness and management of athletic horses.

This month, we will review the physiologic reasons for increases in blood lactate and attempt to explain the science behind blood lactate testing. Lactate is more than just a number!

Practice news
Lara will be attending the Victorian Young Riders Dressage Squad training weekend later this month to present lectures on basic health care, fitness and management of performance horses. Lara is looking forward to meeting all the talented young riders again, after having such a good experience with them at the last training weekend.

Basic cardiovascular physiology
The equine heart is an enormous, muscular pump that takes water and oxygen to the working muscles and carries away carbon dioxide and metabolic waste products. The heart mass in thoroughbred horses is about 1 percent of body weight. In fact, there are definite breed dispositions for heart mass to body weight ratios. Racehorses have a relative heart mass of 0.86 compared to a draft horses who have a relative heart mass of 0.62!

Oxygen is carried to working muscles by the hemoglobin in red blood cells. The horse is specially adapted to provide working muscles with extra oxygen during exercise because horses store extra red blood cells in their spleen, which contracts in response to fear or exercise and adds additional red blood cells to the circulation. In effect, this means that horses have a high maximal oxygen consumption rates during exercise, compared to other species.

Heart rate measurements during exercise are used to measure fitness and describe the intensity of a given horses workload. So, what makes heart rates relevant to fitness? There is a direct relationship between the heart rate of a horse and it’s cardiac output. However, heart rate is not the only factor contributing to cardiac output. Stroke volume also contributes and the relationship is shown in this simple equation, HR x SV = CO. Stroke volume is the volume of blood ejected from the heart with each contraction. Cardiac output is the volume of blood ejected out of the heart each minute. In turn, oxygen uptake by muscles is directly related to cardiac output.

How does training affect heart rate and stroke volume?
Stroke volume increases in response to training. This is likely due, at least in part to increases in plasma (a component of blood) volume in response to training. Interestingly, studies have shown that plasma volume decreases by up to 15 percent for at least one hour after a large meal. Therefore, we advise against feeding horses a large meal within 1 to 2 hours of competition.

When we look at training in relation to heart rates, we think about a few different concepts. One is the concept of a maximal heart rate. Another is the concept of V200. This is the velocity (speed) of exercise at which the heart rate is 200 beats/min. Maximal heart rates are usually between 210 and 240 beats/min and each horse has its own intrinsic maximal heart rate. The maximal heart rate is not a good measure of fitness, because it is not affected by training. Instead, we use the V200 because this will increase as the horse gains fitness. In effect, this means that as the horse becomes more fit, it will be able to exercise at a faster speed before it’s heart rate reaches 200 beats/min.

The V200 is a very good guide for comparing the fitness of an individual horse with itself over time. However, it is not as useful for comparing fitness between individual horses. This is because maximal heart rates can vary between individual animals. So, when a horse with a maximal heart rate of 220 is exercising with a heart rate of 200 beats/min, it is exercising at 90% of its maximal heart rate. However, when a horse with a maximal heart rate of 240 beats/min is exercising with a heart rate of 200 beats/min, it is exercising at 83% of is maximal heart rate. The result is that the relative intensity of this work would be quite different for these two horses.

How does training affect oxygen uptake?
In untrained horses, expected maximal rates of oxygen uptake are between 80-140 ml/kg. Maximal oxygen uptake (VO2max) increases by between 10 and 20% after training. In the horse, maximal oxygen uptake is limited by the ability of the heart to pump blood to the working muscles. Therefore, the heart rate during exercise (as a percentage of maximal heart rate) is very highly correlated with relative oxygen consumption in exercising horses.

Aerobic metabolism
Energy supply during exercise is dependent on molecules of adenosinetriphosphate (ATP). When an ATP molecule is split, it releases energy and this energy is used by working muscles during exercise. Aerobic metabolism is the process by which glucose and fats are used to produce ATP. Glucose is a very useful substrate for the production of ATP and when one molecule of glucose is oxidized (using oxygen of course) it results in the production of carbon dioxide, water and 36 molecules of ATP. Fats are not as easily oxidized as glucose and their contribution to aerobic metabolism only increases as the duration of exercise increases. The availability of oxygen to the working muscles is generally the limiting factor of the capacity of the horse to generate energy aerobically.

Anaerobic metabolism
Anaerobic metabolism results in the synthesis of ATP without oxygen. This pathway uses the metabolism of glycogen stored in muscles to produce molecules of ATP and lactate is a by-product of this process.

Anaerobic metabolism kicks in when a horse exceeds its aerobic threshold and aerobic metabolism can no longer meet the energy demands of the exercising muscles. Anaerobic metabolism is also active in the first 30-45 seconds of exercise, where it makes up the energy deficit whilst heart rate increases and splenic contraction mobilizes stored red blood cells.

It is important to remember that anaerobic metabolism is a normal response to fast exercise and that fast exercise is not purely anaerobic exercise. Anaerobic metabolism should be thought of as making up the energy deficit that results when aerobic metabolism cannot keep up with the demand from working muscles for ATP. However, it should be remembered that the aerobic threshold in fit horses increases, thereby resulting in a decrease in the energy deficit that will need to be met by anaerobic metabolism for a given work intensity.

Fatigue
Fatigue and a decrease in speed during fast exercise is associated with a decline in the concentration of ATP in muscle cells and an accumulation of other metabolic by products such as lactate and hydrogen ions. Hydrogen ions are produced as part of the anaerobic metabolic pathway. When the intrinsic threshold the body has for dealing with hydrogen ions is exceeded, they begin to accumulate. When lactate and hydrogen ions accumulate together, we term this ‘lactic acidosis’.

Increased lactic acid and decreased pH have a deleterious effect on muscle contraction. The horse will begin to fatigue and will slow down. However, this is not necessarily undesirable, because if there were no limitations to muscle cell activity during fast exercise, irreversible damage would occur to muscle cells and the horse would have no chance of recovering from exercise.

The challenge, is to train the horse to increase its maximal rate of oxygen consumption, increase the speed at which lactate begins to accumulate and thus increase the horses capacity for exercise before it begins to fatigue!

Blood lactate testing
Blood lactate concentrations in horses at rest are less than 1 mmol/L. Lactate measurements are used for more than just exercise testing in veterinary practice. For example, blood lactate measurements are very useful in cases of colic and neonatal sepsis to determine the need for referral to a hospital for intensive treatment.

When lactate is used for fitness testing in horses, we often think about the concept of VLa4. This is the velocity (speed) at which blood lactate concentration is 4 mmol/L. An appropriate intensity of strenuous training is that which results in a blood lactate concentration of between 4-8 mmol/L. The appropriate time to collect the blood for testing is 3-5 minutes after exercise.

Blood should be collected in a heparinised syringe to obtain the most accurate results with most hand held lactate meters. Heparin should be drawn up in the syringe and then evacuated before obtaining the sample. The sample should be analysed within 5 to 30 minutes of collection. This is because red blood cells continue to produce lactate, even after the blood has been collected. Prolonged intervals between collection and testing may result in artificially elevated blood lactate readings.

We hope you have enjoyed the October newsletter. Next month we plan to use our newsletter to discuss in detail, different fitness testing programs using both high speed treadmills and normal working tracks. In the meantime, if you would like to discuss fitness testing or exercise programs using high speed treadmills, please do not hesitate to contact our clinic to arrange a consultation.