Equine Exercise Physiology - understanding basic terminology and concepts

Anaerobic work is performed at heart rates above 150 BPM and involves explosive power such as short sprints, acceleration, and fast galloping. A Quarter Horse running 2 furlongs would be deriving energy 60% anaerobically and 40% aerobically. The primary anaerobic fuel source is glycogen without the presence of oxygen. Typically a horse can perform purely anaerobic work for a short duration.

MUSCLES AND STRUCTURE

Horses have 700 individual muscles, and in thoroughbreds, muscles make up as much as 55% of the horse’s total body mass. The skeletal muscle consists of bundles of long spindle shaped cells called muscle fibres that attach to bone by tendinous insertions. The blood vessels and nerves that nourish and control muscle function run in sheets of connective tissue that surround bundles of muscle fibres. Each nerve branch communicates with one muscle fibre at the motor end. The nerve and all muscle fibres that it supplies are together termed a motor unit. Each time that a nerve is stimulated all of the muscle fibres under its control will contract. One motor nerve will supply from 10-2000 muscle fibres. A muscle’s unique ability to contract is conferred by the highly organized parallel, overlapping arrangement of actin and myosin filaments. These repeating contractile units or sarcomers extend from one end of the cell to another in the form of a myofibril. Each muscle fibre is packed with myofibrils that are arranged in a register giving skeletal muscle a striated appearance under a microscope. Muscle contraction occurs when the overlapping actin and myocin filaments slide over each other, serving to shorten the length of the muscle cell from end to end and mechanically pulling the limb in the desired direction. The sliding of the filaments requires chemical energy in the form of ATP.

MUSCLE FIBRE TYPES

The horse has three basic muscle fibre types: Type 1, Type 2A, and Type 2B. These fibres have different contractile rates and metabolic energy characteristics. Type 1 fibres, also known as “slow twitch” or “red fibres” have high oxidative capacity and are resistant to fatigue in part related to their high density of mitochondria which can utilize fuels aerobically and have the highest oxidative capacity. Mitocondria are the small organelles in the muscle cells that convert fuels (fats and glycogen) into ATP. They have the highest lipid stores, highest densities of capillaries, and the lowest glycogen stores. They have the lowest glycolytic enzyme capacity of the three fibre types. Type 2A are the “intermediate fibres” in terms of both contractile speed and metabolic properties between Type 1 and Type 2B. These fibres are aerobic, but also use a combination of glycogen and fat for energy generation.

The thoroughbred has a high percentage of these “intermediate” fast twitch oxidative fibres that can produce speed and still utilize large amounts of oxygen and resist fatigue. Type 2B “fast twitch” fibres have the fastest contractile speed, the largest cross-sectional area, the highest glycogen stores and glycolic capacity. They are ideally suited to short fast bursts of power. They have a low aerobic capacity and tend to depend on anaerobic glycolysis for energy generation. Genetics determine muscle type and composition and is 95% inheritable in humans, and is thought to be highly inheritable in horses (Snow and Guy). In evaluating the fibre type distribution in a number of breeds of horses, heavy hunters had a very large proportion of Type 1 fibres, while Thoroughbreds and Quarter horses had few Type 1 fibres and a large number of the faster contracting 2A and 2B types.

The percentage of each fibre type that a particular breed has in its muscle depends on the type of performance for which the breed is selected. Thoroughbreds have the highest number of the highly aerobic 2A fibres, illustrating the importance of oxygen utilizing pathways in the thoroughbred racehorse. Researchers also found that thoroughbred stayers have a higher number of Type 1 fibres than either sprinters or middle distance horses. Unfortunately, within a breed, the spread in fibre type distribution is so small that fibre typing as a predictor of performance is probably of limited value.

Muscle strength, size and shape can be predictive of muscle fibre ratios. Although each muscle may have a fibre type mix, generally a higher percentage of the “fast twitch” (Type 2) fibres are found in the horse’s hindquarters providing power, whereas the “slow twitch” (Type 1) are found in the forelimbs providing stride, rhythm and a weight bearing role.

VO2 MAX

VO2 Max is 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. 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). Training increased VO2 Max. (Evans and Rose, 1987) VO2 Max is determined by measuring oxygen during exercise as increasing speed and/or incline of a high-speed treadmill incrementally increases the workload. VO2 Max expressed as millilitres of O2 per kilogram of body weight per minute (or second). At rest a horse absorbs 3 millilitres of oxygen per kilogram of body weight per minute. Maximal rates of oxygen intake vary within breeds and vary with breed and training state, but fit thoroughbreds have a VO2 Max of 160-170 ml./min./kg. By comparison elite human athletes have a VO2 Max of about half or 80 ml./min./kg. Pronghorn Antelopes have a VO2 Max of 210-310 ml./min./kg. When VO2 Max is determined, the speed at which VO2 Max is achieved is also measured. Comparing two (2) individuals with the same VO2 Max, one individual will have a higher speed at which the VO2 Max is achieved. VO2 Max calculations enable researchers to evaluate the fitness of a horse and its ability to utilise oxygen for energy.

ANAEROBIC THRESHOLD

Anaerobic threshold (also know as lactate threshold) is the level of effort usually expressed as a percentage of VO2 Max at which the body produces more lactate than can be removed. Anaerobic work is performed at a heart rate approximately above150 BPM and at intensities above 70% VO2 Max. At lactate threshold the cardiovascular system can no longer provide adequate oxygen for all exercising muscle cells and lactic acid starts to accumulate in those muscle cells (and subsequently in the blood as well). Lactate threshold research has recently focused on blood lactate threshold (LT) as a reflection of an individual’s level of training. There are always certain cells within muscles that are relatively deficient in oxygen and are therefore producing lactic acid, but at levels small enough to be quickly metabolized by other cells that are operating on an aerobic level. At some point the balance between the production of lactic acid and its removal by body systems shifts towards accumulation. Lactate threshold is usually slightly below VO2 Max, and will improve with training. Horses with increased LT not only experience less physical deterioration in muscle cell performance but also use less glycogen for ATP production at any level of performance.

TRAINING RESPONSES

Through training physiological changes take place in most of the horse’s systems. Major training responses take place in the blood, heart, muscles, and cardiovascular, neuromuscular and skeletal systems. The first 2-4 months of training increases the total amount of blood volume, red cell count, and hemoglobin concentrations and creates a more efficient circulatory system. Increased blood plasma in the first weeks of training contributes to improved thermoregulation and sweating capacity. After training for 3-6 months, an improved network in the number and density of capillaries provide more efficient blood flow and transit time to working muscles. After 4-6 months of training a multitude of adaptations take place at the cellular level. Oxidative enzymes in the muscles increase along with the number, size and density of mitochondria in the muscle cells. The 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. Training regimens that include speed work, and increased acceleration at intensities close to VO2 Max will also result in the increase of glycolic enzymes needed for anaerobic energy production.

Training at these higher anaerobic levels will improve the buffering capacity in the muscle cells. Buffers are chemicals that limit lowering of pH when lactic acid accumulates. The clearing and removal of lactic acid and wastes also becomes more effective. Heart mass has been shown to increase with training. Hypertrophy (enlargement) in the heart physically comes in two ways, a thickening of the heart walls, and an increase in the size of the chambers, especially the left ventricle. 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% after as little as 10 weeks of training (Thomas et al, 1983). A study has also shown that heart size is also correlated with VO2 Max using an ECG (Young et al, 2002). VO2 Max increases from 10-20% in the first 6-8 weeks of training after which further improvement is limited. Although the relationship between VO2 Max and velocity is highly correlated, the differences found in the speed and performance of two thoroughbreds with equal VO2 Max values can be explained by differences in biomechanics, and economy of locomotion. Horses with a high VO2 Max and efficient gait will use less energy to attain the same speed.

As fitness progresses, the horse will be able to attain a higher speed before reaching VO2 Max. An example would be a lightly trained thoroughbred hitting VO2 Max at 25mph, but after beginning a training program, the same horse would eventually be able to go 30 mph before reaching the limit. Although improvements in VO2 Max and aerobic capacity occurs early in the training stages, it’s not until 4-6 months that improvements are seen in bone and ligaments. This physiological mismatch is often the cause of many bone and soft tissue injuries. At maximal exercise levels, such as a gallop, increases are seen in bone density and mass. Bone density, shape and internal composition are related to strength. Medium tissues such as tendons and ligaments become thicker and more elastic. The modeling response of bone is stimulated by fast work, fortunately only short durations are necessary (Firth et al, 1999). Training at the trot or canter results in minimal changes in bone mass and density. Therefore, the trainer must gradually add speed work into the training plan with the goal of developing bone density. The peak time of bone development occurs between 2 and 3 years of age, with 50% of their primary structure replaced by their 3 year old year.

The ability of bone to adapt decreases with age, with some researchers believing that bone becomes more brittle with age, and young horses actually remodel bone more quickly and easily, and are at less risk than horses started later (McIlwraith). This idea is further supported by other researchers that found that tendons grow and adapt to the stresses of training more successfully prior to their 2 year old year (Smith, Birch, Patterson, Kane et al, 1999). Contrary to common belief, most current research indicates that early training may not only enhance bone and tendon development, but reduce the incidence of injury during training and racing, prolonging racing careers.

PERFORMANCE MEASURES

For over 30 years high speed treadmills have revolutionized the study of equine exercise physiology. Today many veterinary clinics and universities with equine departments are able to study the equine athlete in their own sports performance laboratories. The treadmill can easily evaluate the athletic potential of an equine athlete by standardizing variables used in an exercise test. A high speed treadmill can answer various questions relating to speed, ventilation, heart rate, VO2 Max, blood lactate, substrate (fuel) use, gait analysis, and endoscopic examination of the upper airway. The high speed treadmill will run at speeds in excess of 35 miles per hour, can be inclined at a 3-3.5% grade to simulate ground resistance and a rider’s weight. Treadmills equipped with a respiration calorimeter are used to measure gas exchange. Using indirect calorimetry, a loose fitted, padded face mask is attached to a motorized pump that monitors and analyses air breathed in each breath. The suction created by the pump ensures that expired air is collected and not re-breathed by the horse. The research team can design an exercise test tailored for desired performance measures.

The test can be designed as an incremental test, where horses are asked to perform at ever increasing high speed until reaching maximal exertion, or a longer endurance test. During a standard exercise test fitness can be monitored using heart rate, with a heart rate monitor. Heart rate is one of the most frequently measured physiological variables measured in exercise tests. Measurements of blood lactate, glucose concentrations, free fatty acids and pack cell volume can be taken throughout the test not just before and after. Knowing the horse’s weight is necessary in order to make calculations, and the horse is weighed prior to testing. During the test the airflow rate is measured in litres / minute. Both Oxygen (o2) intake and exhaled carbon dioxide (CO2) is measured. These measurements provide information to calculate VO2 (volume of oxygen), VO2 Max (maximal oxygen intake), and VCO2 (volume of carbon dioxide).

VO2 Max provides information on aerobic capacity, and the speed at which VO2 Max is achieved. Being equipped with a heart rate monitor, the speed at which maximal heart rate achieved is also known. The relationship between running speed, heart rate and oxygen consumption is linear up to VO2 Max. Two commonly used variables that are used to describe the relationship between heart rate and velocity are V140 and V200. There is a high correlation between V200 (velocity at 200 beats per minute) and VO2 Max. These variables are simply used to describe speeds attained at different heart rates. Numerous graphs and charts can be generated to display a horse’s athletic progress over time. Similarly, the speed at which blood lactate reaches certain levels is also measured. Lactate levels at different speeds are used to measure anaerobic capacity. Onset of blood lactate accumulation (OBLA) is recorded as VLA4. This is the speed achieved when blood lactate concentrations reach 4 mmol./l. Elite thoroughbreds can tolerate lactate concentrations as high as 30 mmol/l. A sprint test on a thoroughbred may be run at supramaximal intensity of 115% VO2 max for a 2 minute period, near maximal heart rate, whereas an endurance horse such as an Arabian may be expected to run at 35-40% VO2 max for 90 minutes. Interestingly, Arabians have been found to use more fats as fuel than thoroughbreds (Kentucky Equine Research, Pagan). Using RQ (respiratory quotient) researchers can determine whether the horse is using fat or carbohydrate as a fuel source. Unlike oxygen, carbon dioxide varies tremendously with substrate (fuel) use. The RQ is calculated by dividing VCO2 by VO2. An RQ of 1.00 indicates that carbohydrates are being used as fuel, and an RQ of .7 indicates that fats are being used.

DESIGNING A TRAINING PLAN

By understanding the basics of equine exercise physiology, a racehorse trainer has the advantage of understanding how various physiological systems adapt and respond to training. In designing a comprehensive training plan for each horse the intensity, frequency, duration, and volume of the work is determined. The plan must also incorporate rest and recovery, and avoid overtraining. Each new level of training is maintained until the body has adapted to the added stress, after which further increase in training load can be applied. Alternating periods of increased workload with a period of adaptation is known as “progressive loading.” Training should be specific to the event in order to train the appropriate structures and systems, doing work that is similar to racing which elicits neuro-muscular coordination. Horses “learn” how to do the event. This principle of conditioning is known as “metabolic specificity.”

Most training programs are divided into three phases. Phase I is the long slow distance (LSD) phase, Phase II is focused around strength work, and Phase III involves sharpening and speed work. (Marlin and Nankervis, 2002) In Phase I, the primary focus is on long slow distance (LSD) and builds the foundation on which all other work is based. In their first year of training, Phase I may last from 3-12 months, with improvements in aerobic capacity seen in the first 6-8 weeks. Long slow distance is performed at slow canters at heart rates below 130-150 beats per minute. Even after this phase is completed LSD may comprise of 3-5 sessions per week lasting 20 minutes.

Phase I improves cardiovascular fitness and trains musculoskeletal structures decreasing the future risk of injuries. This phase also helps the horse’s mental attitude toward daily training. Phase I is primarily done at low intensities of aerobic levels. Phase II is the strength phase, where horses are trained with intensities from 150-180 beats per minute, and above 70% VO2 Max. Horses are usually working from a canter to a gallop over distances up to 1 ½ miles. This phase can be accomplished in 60-90 days. Aerobic and anaerobic systems are trained, with horses reaching anaerobic threshold levels during their workouts. These workouts over time will increase the time and speed at which lactate threshold is reached. Strength work may be performed 2 days a week with adequate rest between sessions.

Often in Europe hill work is added at this stage, increasing the intensity, without increasing the speed. Hill training strengthens the hindquarters, and working horses downhill strengthens the pectorals, shoulder, and working against gravity, the quadriceps in the hindquarters, become balanced. Phase III is the sharpening phase, where speed work is performed at heart rates and intensities at close to race speed, often reaching V200 and VO2 Max levels. Usually, depending on intensity, this type of work is performed only once every 1-2 weeks. Fast work can be performed as either continuous or interval training. Continous training performed at the racetrack involves distances from ¼, ½ mile, and 1 mile or more, usually with the last quarter at race speed. Interval training involves using multiple exercise bouts separated by relatively short recovery periods where the heart rate drops below 100 beats per minute.

CONCLUSION

Understanding basic equine exercise physiology and the metabolic systems of the horse not only benefits trainers, but owners, breeders and agents in training, breeding and buying a future thoroughbred athlete.