Respiratory System Overview
10 Things You Might Not Know About the Horse’s Respiratory System
- Horses can only breathe through their nose. A horse does not breathe through its mouth and nose like we do.
- At a canter and gallop horses take one breath perfectly in time with one stride. This is referred to as respiratory-locomotory coupling. One breath = One stride. Anything that affects the horse’s breathing has the potential to shorten its stride.
- The amount of air moved in and out of the lungs increases in direct proportion to how fast the horse is running. If a horse runs twice as fast it must move twice as much air in and out.
- During exercise, when horses inhale, around 90% of the resistance to air movement is in the upper airways namely, the nostrils, the nasal passages, the larynx and the trachea. The nose is a major contributor to the resistance.
- If you tighten a horse’s girth too much, it will affect performance not because of constricting the chest and preventing the lungs from expanding, but because it decreases the effectiveness of the muscles around the front of the chest and shoulder that move the forelegs. More strides equates to more breaths.
- During canter and gallop, horses do not breathe by expanding and contracting their chest. They expand and contract the chest when breathing at rest, when breathing at walk and trot, and perhaps most noticeably when blowing hard after exercise. But during a fast canter and gallop, all air movement comes from movement of the legs and the diaphragm. The air moves in and out along the lines of a syringe; with the stiff wall of the syringe representing the chest and the plunger the diaphragm.
- During jumping, horses hold their breath over jumps and do not breathe again until they land, starting with breathing out (exhaling).
- You cannot train the respiratory system of the horse. Plenty of books will tell you that you can. Scientific studies show the reverse. The amount of air moved in and out by an unfit horse at a fixed speed will be the same when that horse is fully fit.
- The blood pressure in the blood vessels within the horse’s lung (referred to as pulmonary blood vessels) during galloping is about 4-5 times greater than when at rest.
- This increased pressure puts stress on the very thin walls of the blood vessels and leads to some of them rupturing, commonly referred to as exercise induced pulmonary hemorrhage (EIPH or “bleeding”). Horses that bleed rarely show blood at the nostrils. Bleeding occurs deep within the lung tissues.
How it Works
By the time a horse crosses the finish line in a 5 furlong race, has completed a Grand Prix show jumping round, or gone 1/6th of the way around a 3-star cross-country course, it will have moved somewhere around 1,800 liters (475 gallons), or 6 bathtubs, of air in and out of its lungs. This equates to moving two buckets of air into and out of the lungs every second!
The air inhaled during a race will consist of around 380 liters (100 gallons) of oxygen of which the horse will absorb around a quarter (i.e. 95 liters) into the blood. The rest of the air is made up of nitrogen.
The oxygen absorbed by the lungs is used to perform the process of aerobic metabolism, or getting energy from stored glucose (carbohydrates) into the muscle cells. Of the total amount of energy the racehorse needs to get from the starting gate to the finish in a 5 furlong race, around 70% of this will come from aerobic metabolism (also around 70% for showjumping and 90% for cross-country).
The remaining energy comes from anaerobic metabolism, which also breaks down glucose to generate energy, but can do so without oxygen. Anaerobic metabolism is very fast but inefficient, and can be used for only a short period of time due to buildup of lactic acid. Aerobic metabolism is not so fast, but very efficient at generating the energy to run.
Even in a race or jumping round lasting less than a minute, the majority of the energy generated must come from using oxygen to “burn” carbohydrates. This underlines the importance of a well-functioning respiratory system.
The harder a horse works, the more oxygen it needs and the more air it must move into and out of the lungs.
In fact, these are so tightly coupled that if a horse doubles its speed, it will need to double the amount of air moved into and out of the lungs.
As illustrated above, air enters the body by first passing through the upper respiratory system including the nostrils, the nasal passages and larynx and then into the trachea (or windpipe) and finally into the alveoli of the lungs.
It is important to note that, unlike humans, horses can only breathe through their nose. During exercise, resistance to moving air into the lungs approximately doubles and 50% of the total resistance in the upper airway originates in the nasal passages. FLAIR® Strips support the tissues in the nasal passages to reduce resistance and make it easier to bring air into the lungs.
The horse’s windpipe is around 5-8cm in diameter nearest the larynx, but as it passes deeper in the lung it begins to divide to produce smaller and smaller airways, much like a tree on its side. When we get down to the level of the smallest airways, after perhaps 25 divisions, the airways are fractions of a millimeter in size.
When the air gets to this point in the chain from nostril to muscle cell, the oxygen in the air has to cross from the air space (“alveoli”) into the pulmonary capillaries (tiny blood vessels in the lungs). At this stage, the membranes separating the oxygen containing air in the alveoli from the red blood cells in the capillaries are only the thickness of 1/100th the width of a human hair.
The oxygen transfers from the alveoli across this thin membrane and into the blood by diffusion. The total area for oxygen to diffuse across the horse’s lung is equivalent to the area of 10 tennis courts!
A Note on Bleeding
Perhaps not surprisingly, the membranes that the oxygen must transfer across are so thin that they can rupture under the stress of exercise. This allows red bloods cells to spill from the capillaries into the alveoli, commonly referred to as EIPH or bleeding. Several independent clinical studies show that that by reducing nasal resistance at the nasal valve through the use of FLAIR Nasal Strips, these ruptures, and thus bleeding, is directly reduced.
Once in the bloodstream, the oxygen is bound to hemoglobin (the molecule inside red blood cells that makes blood red) and the oxygen rich blood is pumped around the body by the heart.
This oxygen rich blood must then reach the muscles to provide energy. At the muscle, the reverse process takes place with oxygen leaving the red blood cells and crossing into the muscle cells, again by diffusion. Finally in the muscle cells, the oxygen moves to a subunit of the cell (“organelle”) called the mitochondria.
By the time it gets inside the mitochondria, the level of oxygen may only be around 1/80th of that in the air outside the horse!
Efficient oxygen transfer from the airways to the red blood cells is therefore very important to maximize energy and a horse’s ability to exercise. In fact, some of the best racehorses (especially those racing over middle and longer distances) have large hearts and/or a high capacity to use oxygen. FLAIR Strips help to make sure that even horses with the biggest hearts and greatest capacity to use oxygen are bringing oxygen in as efficiently as possible.
Other Functions of the Respiratory System
A well-functioning respiratory system is not only important to maximize energy. One of the other important functions of the respiratory system is to get rid of carbon dioxide; a waste product produced within the mitochondria of muscle cells during exercise. This is effectively the same as the process for bringing oxygen in, but in reverse. Carbon dioxide moves out of the cells by diffusion. When the blood reaches the lungs, the carbon dioxide diffuses out across the membrane and into the airways. The carbon dioxide is then breathed out during exhalation. Accumulation of carbon dioxide is not a good thing and can contribute to the development of fatigue during exercise, so it’s important that as much carbon dioxide as possible is exhaled as fast as possible.
The lung is also a very important filter. All the blood in the circulation passes through the lungs when it comes back in veins from being pumped out around the body in arteries. As such, the lung is ideally placed to filter out any small blood clots (thrombi) or gas bubbles (emboli). It may not be great to have a pulmonary embolism (a gas bubble in the lung), but it’s still highly preferable for this to go through the lung and be filtered rather than lodging in a coronary (heart) vessel or the brain. The lung also has a better capacity to deal with bubbles and clots than most other organs in the body.
The lung is also able to activate or deactivate certain hormones in circulation, and in some cases the lung acts as an endocrine organ, actually releasing hormones which can have effects on the whole body.
The skin, the lung and the gastro-intestinal tract are the body’s interfaces with the outside world. The lung therefore has a highly developed immune system, different to that in other parts of the body, with specialized types of white blood cells to deal with things that could be inhaled, such as particles, bacteria, fungi and virus.
Finally, perhaps one of the most important but often overlooked functions of the respiratory system is in regulation of body temperature. If a horse is taken from a cool climate to a warmer climate, one of the first things that can be noticed is an increase in the rate of breathing at rest. Respiratory heat loss is an important thermoregulatory mechanism for the horse. Although it is commonly believed that horses blow after hard exercise because they are trying to get more oxygen into the blood, the main purpose is to regulate how hot they are, not their blood oxygen level. FLAIR Strips help horses recover quicker by bringing cooler external air in more efficiently.
The Peculiarities of the Horse’s Respiratory System
To some extent the horse is still an enigma. There is no other animal that can carry the weight of a person (often representing an extra 10-15% of its own bodyweight) and itself at speeds of up to 35 mph or even more. It may therefore not be surprising that the horse’s respiratory system displays some curiosities, especially when compared to ourselves.
To learn more about the Equine Respiratory System, read The Growing Physical Demands of Modern Equestrian Sport, by Dr. David Marlin, Eventing, Issue Two, 2008.
Breathing easier helps maintain respiratory health and optimum performance. The benefits of breathing easier are important for horses competing at all levels of fitness and skill. Many riders report that horses wearing FLAIR® Strips are more relaxed and focused.
FLAIR Strips are scientifically proven to make breathing easier:
When using a FLAIR Strip horses do not breathe in more air. Rather, they take in the same amount of air but with less work. Think about the difference you feel breathing through a “stuffy” nose, as opposed to when your nose is clear. It definitely feels like more work to breathe through the narrowed nasal passages of a stuffy nose. Now, let’s look at why it feels that way and how it impacts health and performance of a horse.
Let’s start with the nose…
You may recall horses are “obligate nasal breathers”- meaning that during intensive exercise horses can only breathe through their nose, not their mouth. All the oxygen they need for exercise can only come through the nasal passages; a significant portion of which is unsupported by bone or cartilage. This unsupported portion of the nasal passage collapses inward for all horses when breathing in during exercise — reducing the size of the airway and greatly increasing resistance to airflow. This is significant because during exercise over 50% of resistance to air flow to the lungs comes from the nasal passages. Some studies suggest this percentage can be as high as 80%. In addition, pathological upper airway conditions (roaring, gurgling, nasal flutter, alar fold collapse) and functional obstructions (significant poll flexion) create additional increases in the work of breathing. This makes it more difficult to move air into the lungs.
FLAIR Strips work by providing a spring-like force that gently supports the nasal passages and reduces soft tissue collapse that causes narrowing of the airways during exercise. The Strips support the nasal passages including the nasal valve, which is the narrowest part of the nasal passages.
The photographs below are endoscopic views of the nasal passage of a horse. The photo on the left shows the nasal passage without a FLAIR Strip and with a FLAIR Strip on the right. Note the narrowed nasal passage in the photo on the left with unsupported tissues compared to the supported right nasal passage on the right.
FLAIR Strips and the Breathing and Stride Locomotory Coupling:
FLAIR Strips can impact stride efficiency due in part to the horse’s unique synchronization or “coupling” of stride and breathing at a gallop. At a walk and trot a horse’s respiratory rate is unrelated to its stride rate. However, at the canter and gallop, breathing and stride are linked. That is, horses take a single breath with each stride. As the illustration below shows, inhalation (red arrows pointing to air moving into the nose) occurs when the front legs are non-weight bearing and exhalation occurs when the front limbs are weight bearing (blue arrows pointing to air exiting the nose).
In a way, you can think of a galloping horse as a large bellows. As the front legs are in the non-weight bearing (or “flight”) phase (1-5) air is being pulled into the lungs like air moving into a bellows.
(photo credit: dynamicbalance.com)
Once the lead front leg contacts the ground, the front legs are in the weightbearing phase (6-10) and air is pushed out of the lungs. At speeds beyond a hand gallop, a horse increases its speed by increasing stride length, not by moving its legs faster. When a horse lengthens its stride to increase speed, it also takes deeper, longer breaths providing the lungs with more air. A horse struggling to move air in and out of the lungs may fatigue quicker or shorten its stride to compensate for the increased work of breathing in. By reducing resistance through the nasal passages, FLAIR Strips help make stride adjustability or lengthening easier.
FLAIR Strips and Lung Health Benefits:
FLAIR Strips provide proven health benefits to the lungs. One way FLAIR Strips help improve health is by reducing lung stress and reducing exercise induced pulmonary hemorrhage (EIPH or “bleeding”). For more information on EIPH and how FLAIR Strips help, click the EIPH button on the Learn page.
FLAIR Strips and Optimum Performance:
By reducing upper airway resistance allowing horses to breathe easier FLAIR Strips benefit respiratory health and help your horse performance his best.
Managing your horse’s recovery after training or competing can have a big impact on how your horse will perform, particularly if you are at a competition where he has to run several times on the same day (e.g. barrel racing or other rodeo event, showjumping, one day events) or where he has to compete over several days (e.g. 3-day events, rodeo event, showjumping, dressage, endurance). Effectively managing your horse’s recovery can also shorten the time to when he is ready to compete again. Proper recovery also can affect how a horse copes mentally with training and competing. A prolonged and uncomfortable period of recovery after training or competing could have a long term effect on his behavior and willingness to train or compete.
For a horse that has just completed competition or a long training session, recovery occurs in three phases. The first phase is relatively short and is the time from finishing exercise to being back in the stable. There is then a longer period of slower recovery over the next 24 hours and then finally a period of perhaps days to weeks until the horse is fully back to where he was before competing.
FLAIR® Strips provide benefits particularly in the initial phase of recovery.
How a horse behaves and how his body responds during this crucial first phase of recovery depends primarily on the intensity of the work and how hot he is. When cantering or galloping, your horse’s breathing and stride were locked such that breathing rate is the same as the stride rate. When the horse breaks from the canter, breathing will become dissociated from stride and is often slower and deeper (perhaps 60-80 breaths/minute) than during work. Unlike when galloping, the entire rib cage can move.
This deep breathing is commonly referred to as “blowing”. It is often suggested that “blowing” is caused by low arterial blood oxygen and/or high blood carbon dioxide. However, while oxygen and carbon dioxide do affect breathing, the main thing that controls this “blowing” after exercise is the horse’s body temperature. That is, although a horse’s oxygen may be low and carbon dioxide high during exercise, as soon as the horse begins to slow down the oxygen rapidly increases and the carbon dioxide rapidly falls. Body temperature, however, takes longer to go down and so blowing persists to help the horse to continue cooling down.
There will also be other signs that the horse is hot, such as sweating and feeling hot to the touch. And while evaporation of sweat helps dissipate heat, the loss of heat by evaporation of moisture in exhaled breath is also an important contributor. Moving air in and out of the lungs while “blowing” consumes significant energy. At this point you may be able to visibly notice that the soft tissue over the nasal passages gets sucked inwards, narrowing the nasal passages and increasing airflow resistance, so that more energy is needed for breathing.
The easier it is for your horse to move air in and out during the initial phase of recovery will help:
- the horse to cool quicker
- the horse’s heart rate to come down
- the horse to feel more comfortable
FLAIR Strips allow the horse to move air more easily and therefore recover faster while being less distressed and using less energy. Make sure to leave the Strip on until the horse is completely cooled down.
Exercised Induced Pulmonary Hemorrhage (“EIPH” or “Bleeding”)
WHAT IS A BLEEDER?
To most people a “bleeder” is a horse that has blood at the nostrils during or after training or racing. Many people believe that if a horse doesn’t show blood at the nostrils it’s not bleeding. However, bleeding is a silent injury that can go undetected by trainers and riders because it occurs deep in the lungs. It is best detected by lung washes or endoscopic examination. Endoscopy is also referred to as “`scoping”, a process of passing a long thin tube with a camera on the end for visualizing the upper airway and trachea. Blood in the lungs and lower airways has been shown to be an irritant that leads to further bleeding.
ARE ONLY A FEW HORSES BLEEDERS?
Numerous studies show that essentially all exercising horses experience some degree of EIPH during intensive exercise. Less than 5% of these horses show blood at the nostrils. That means in most cases you would not know if your horse bled without scoping or a lung wash. Symptoms of EIPH may include poor performance, coughing, extended cooling-out, and frequent swallowing. Many low and intermediate level bleeders may show no signs. However, each incidence of EIPH contributes to scar tissue formation within the lungs and potentially future bleeding episodes. The lung damage from repeated episodes of EIPH can shorten a horse’s competitive career.
FLAIR® Strips are proven to reduce EIPH. The Strips reduce resistance to breathing so less stress is put on the fragile blood vessels in the horse’s lung that rupture when a horse bleeds.
WHY DOES BLEEDING OCCUR?
When a horse begins to exercise two major things occur. First, the horse’s respiratory ventilation increases to bring more air into the lungs and to remove carbon dioxide. Secondly, more red blood cells (RBCs) are added to the circulatory system from the spleen to help carry the oxygen from the lungs to the heart, muscle and other organs requiring increased oxygen during exercise.
During inspiration, the horse’s stride and the diaphragm create a strong vacuum or negative pressure. This brings the air through the nasal passages into the alveoli (or “air sacs”) deep in the lungs where the oxygen is transferred from the lungs to the blood via the pulmonary capillaries.
Amazingly, during intensive exercise a horse breathes over 500 gallons of air into its lungs every minute. The strong negative pressure necessary to inhale this amount of air creates a strong suction force on the alveoli. The alveoli are separated from the small blood vessels in the lungs (pulmonary capillaries) by a thin membrane called the pulmonary capillary membrane or “PCM”. The PCM is very efficient at transporting oxygen and removing waste gases, carbon dioxide, between the alveoli and pulmonary capillaries because it is very thin- only about 1/100th the thickness of a human hair. This membrane, being very thin, is also very fragile.
In addition to the strong negative pressure on the alveoli side of the PCM, there is also a very strong positive pressure on the opposing side due to significantly increased blood pressure in the pulmonary capillaries. This increased blood pressure is due to an increased number of red blood cells (RBCs) and increased cardiac output. To increase the blood’s oxygen carrying capacity, the number of RBCs available is increased because the spleen contracts and releases its reservoir of RBCs into the bloodstream. (This is one reason warm up is so important- to ensure increased RBC availability during competition) Increased cardiac output is achieved in part through dramatic increases in heart rate. During intensive exercise a horse’s heart rate can increase to over 220 beats per minute (bpm) to circulate 75 plus gallons of blood through the lungs. Overall, these forces quadruple the blood pressure within the lungs from its resting pressure.
What this means is that there is a high positive pressure within the pulmonary capillaries pushing on the PCM, and on the alveoli side there is a high negative pressure pulling on the PCM.
When these opposing forces are applied across the fragile PCM during exercise the membrane can rupture spilling blood out of the blood vessels into the alveoli. This is exercise induced pulmonary hemorrhage.
Conditions like inflammatory airway disease, bronchitis and other inflammatory or infectious conditions can further increase the fragility of the PCM increasing the likelihood of pulmonary bleeding.
HOW CAN FLAIR STRIPS HELP WITH PRESSURE CHANGES OCCURRING DEEP IN THE LUNGS?
Upper airway structures including the nasal passages, larynx and trachea cause resistance to air moving into the lungs. The greatest resistance during exercise occurs in the nose. Specifically, over 50% or more of the resistance to breathing air into the horse’s lungs occurs in the nasal passages. FLAIR® Nasal Strips gently support the nasal passage to reduce resistance. Because resistance is reduced, the strong negative pressure on the alveoli side is reduced which helps reduce the pressure difference so the PCM is less vulnerable to rupture. Simply stated, FLAIR Strips are proven to reduce bleeding by making it easier to pull air through the nose during exercise. Several independent clinical studies have proven that by reducing nasal passage resistance, FLAIR Strips reduce pulmonary capillary ruptures and bleeding.
DO HORSES HAVE TO GALLOP TO BLEED?
No. Research in Japan showed that horses only cantering at speeds of up to 20 mph (a very slow canter for a racehorse) all had damage to their lungs as a result of broken blood vessels. Some studies report that horses bleed even when doing mild exercise such as trotting on a treadmill.
DOES DAMAGE TO THE LUNG INCREASE WITH SPEED?
Yes, The faster a horse gallops, the more damage that is potentially done to the lung. The more times a horse gallops, the more potential damage is done. Other factors can also increase how much damage might be done to the lung, such as the extremes of footing the horse is working on (very hard and very soft) and weight carried – the higher the weight, the worse the bleeding.
DOES DAMAGE TO THE LUNG INCREASE THROUGHOUT A HORSE’S CAREER?
Each time a horse does more than a slow canter, some blood vessels in the lung are broken. At first this damage only affects a small area at the top back part of the lung. However, with repeated cantering, galloping and racing, the damage does not stay in one place but accumulates and moves further towards the head, affecting more and more of the lung. Thus, the severity and frequency of bleeding observed by scoping after exercise or racing almost always increases with age.
WHAT DOES BLEEDING DO WITHIN THE LUNG?
The blood vessels that break in the lung are almost always the blood vessels of the pulmonary circulation. When the vessels are ruptured, they may become blocked or not function normally. If they repaired they may become stiffer, as scar tissue is not as flexible as normal healthy lung tissue. Damaged lung tissue, even if it is repaired, does not function as well, leaving the horse’s lung capacity and function reduced. As the lung is a limiting factor for performance in horses, even small losses of lung function can have a significant adverse effect on performance.
WHAT ARE THE BENEFITS TO REDUCING BLEEDING?
Reducing bleeding not only helps a horse perform better in the short term, but may also help long term by reducing the possibility of inflammatory airway disease and chronic lung damage due to repeated bleeding episodes.
WHAT ARE THE OPTIONS FOR TREATING BLEEDERS?
There is no known method to completely stop bleeding. The best we can do is to today is to reduce bleeding. There are only two proven ways to reducing bleeding: FLAIR Strips and the drug Furosemide (Salix, formerly Lasix).
As discussed above, FLAIR Strips reduce bleeding by normalizing pressure across the PCM. Studies have shown that horses affected by EIPH that wore a FLAIR Strip had fewer blood cells in their airways after exercise when compared to the same horses not wearing a FLAIR Strip. In addition, because the FLAIR Strip is a mechanical device, it will be equally as effective every time it is used and can be used with every hard workout.
Furosemide is a drug that has been shown to have protective effects the first time it is used. However, rather than normalizing airflow, Furosemide is a potent diuretic that works by reducing blood volume and subsequently reducing pulmonary vascular pressures. Unfortunately, the drug reduces blood volume by increasing urine production, which consequently reduces fluid in the tissues and organs of the body. Side effects of furosemide use include dehydration, weight loss, and electrolyte loss- particularly potassium. There is also no evidence that Furosemide is as effective if it is used repeatedly.
IN RACE HORSES IS THERE A BENEFIT TO USING FLAIR NASAL STRIPS DURING TRAINING EVEN IF THEY WILL NOT BE USED DURING A RACE?
YES. Damage to the lung occurs with every fast piece of work. This damage accumulates over time the more a horse canters and gallops. The more damage done to the lung, the greater its function is reduced, adversely affecting performance. Even if your horse doesn’t race with FLAIR Strips, using them in training will limit the damage occurring to the lungs so that they are in the best condition possible when it matters… in the race.
Fatigue occurs in horses during both high intensity exercise for short periods of time as well as lower intensity exercise over prolonged periods of time. Practically speaking, the term “fatigue” during exercise is used to describe the sensation of tiredness and the accompanying decrease in athletic performance. It typically means the inability to continue exercise at a given level of intensity. Generally, the higher the intensity of exercise the earlier the onset of fatigue.
Much of our knowledge of fatigue in animals comes from horses because they can be readily trained to exercise on high-speed treadmills; allowing for controlled investigation of respiratory, cardiovascular, and metabolic responses. When fatigue occurs, changes in the horse’s gait, joint movements, muscular support, and willingness to perform can be seen. These changes are believed to be important factors contributing to structural fatigue injuries of the musculoskeletal system of performance horses. Structural fatigue injuries include pulled suspensory and other ligaments, bowed tendons and fractured bones.
Generally, exercise at an individual horse’s highest attainable speed cannot be maintained for more than about 30 to 40 seconds. After that, fatigue sets in and the horse slows down. However, the cause of fatigue during exercise is not the same for all horses or all events. For example, endurance horses competing over many hours or days experience fatigue, but the underlying mechanisms for fatigue is likely quite different than that of horses racing at maximal speeds lasting 3 minutes or less.
According to Dr. David Marlin, the use of the word “fatigue” in relation to exercise has very specific meanings and covers a wide range of manifestations including:
- the horse that will not move another step
- the endurance horse at the end of a 100 mile race that is reluctant to trot
- the racehorse that has slowed in the last eighth of a mile of a race, but when passing the post is still travelling in excess of 35 miles per hour
- the event horse on cross-country that slows down from a fast gallop pace, but when given a few strides at a slightly slower pace, is able to return to the original faster pace
Causes of Fatigue
Fatigue can result from the failure of one enzyme system, one cell, one organ or one body system, but is more often due to multiple factors contributing simultaneously. This in turn can place an excessive burden on other body systems as they try to compensate. In many ways fatigue is still poorly understood. However, it is generally accepted that no single mechanism explains all the different aspects of fatigue that are recognized. For example, a dramatic reduction of muscle glycogen can play a significant role in fatigue at the end of an endurance ride. But, fatigue also occurs when muscle glycogen concentrations are still high, for example in a show-jumper after completing its round.
There are also a number of factors unrelated to the intensity, duration or pattern of exercise that influences onset of fatigue including: metabolic myopathies (i.e. tying up and polysaccharide storage myopathy), overtraining, fitness, age, body condition and environmental conditions (eg. temperature, humidity, pollution or altitude).
Numerous different areas have been studied to try and understand the mechanisms of fatigue during exercise including:
1) Depletion of the energy generating systems inside and outside muscle cells
2) Accumulation of metabolic by-products (i.e. lactic acid, ammonia, H+ ions, and ATP metabolites) and failure of the muscular contractile mechanism
3) Disturbances to acid-base, electrolyte, hydration, and thermoregulatory homeostasis
4) Central and peripheral nervous system fatigue
In this section we will consider only the depletion of energy stores as a cause of fatigue. However, the reader should review the article by Dr. David Marlin for a more detailed discussion of the causes of fatigue in the performance horse.
Depletion of Energy Stores as a Cause of Fatigue
Ultimately, depletion of energy is one of the major causes of fatigue. In the body, the fundamental source of energy at the cellular level is a molecule called ATP (adenosine triphosphate). ATP supports almost all cellular-based active processes, including muscle contraction, by breaking down ATP to ADP (adenosine diphosphate). However the amount of ATP stored in the muscles is low and will only support exercise for several seconds of muscular activity. When ATP is broken down to ADP it must be regenerated back to ATP.
There are four different ways to regenerate ATP from ADP. The first two do not require oxygen; the last two do require oxygen. They are:
1. ATP Replenishment from other high energy phosphate molecules in the cells
This process does not require oxygen and hence is often referred to as anaerobic (meaning without using oxygen) energy production.
2. ATP Replenishment from Glycogen Breakdown to Lactic Acid
The second form of energy replenishment is also anaerobic. It is the rapid breakdown of intramuscular energy stores of glycogen to lactic acid. Accumulation of lactic acid, hydrogen ions, and ATP metabolites within muscles and the resulting reduction in muscle pH as well as significantly increased potassium in the blood, are believed to play a role in the development of fatigue (see later).
3. ATP Replenishment from Aerobic Metabolism of Muscle Glycogen or Blood Glucose
The complete aerobic breakdown of glycogen or blood glucose to carbon dioxide and water requires oxygen and occurs mainly in the mitochondria of the cells. It is within the mitochondria that the oxygen is used and the majority of ATP is regenerated. This process is much more efficient at regenerating ATP from ADP, but is slower than the anaerobic pathways described above. Around 90% of the total body stores of glycogen are stored within the muscles, with most of the remainder being stored in the liver.
4. ATP Replenishment from Aerobic Metabolism of Fat
The aerobic metabolism of triglycerides (fat) to regenerate ATP requires oxygen and takes place primarily within mitochondria. It also generates carbon dioxide and water as the end products. In contrast to glycogen, triglycerides stored in the muscle account for only around 10% of the total body stores, with the remainder being found in adipose tissue and subcutaneous fat depots. Triglycerides are broken down using oxygen to liberate free fatty acids (FFAs). FFAs are then transported in the blood to be taken up by working muscles.
While it is possible to deplete muscle glycogen stores, it is almost impossible to deplete stores of fat in a single bout of exercise. Repletion of muscle and liver glycogen in horses may take 24-72 hours. Depletion of brain glycogen has also been shown to occur in association with prolonged exercise. This has been suggested to be involved in central fatigue.
FLAIR® Strips reduce fatigue in exercising horses.
Early research on horses exercising on treadmills showed that horses using FLAIR Strips used 6-7% less oxygen and produced less carbon dioxide than when doing the same amount of work without a FLAIR Strip. Using less oxygen means the horse consumed less energy. It is believed that by reducing resistance to airflow (i.e. allowing horses to breathe easier) the work required of the respiratory muscles (e.g. the diaphragm) during exercise is reduced, thereby reducing the energy required for breathing. It was theorized that by reducing energy consumption, fatigue would be delayed since this energy would now be available to the exercising muscles. Subsequent studies have now shown that FLAIR Strips delay the onset of time to fatigue in exercising horses. This means that the energy conserved wearing a FLAIR Strip is available to sustain performance longer and potentially reducing the chance of fatigue related injuries.
As discussed above, there are many causes of fatigue. One of the major reasons is depletion of energy stores needed for the horse to perform. During less intensive, longer duration exercise FLAIR Strips help by allowing horses to get needed oxygen more efficiently to sustain aerobic work.
FLAIR Strips are also beneficial during shorter duration, sprint exercises (1/4 mile races or barrel runs) even though 60% of energy is generated by anaerobic metabolism, as 40% of the energy is still generated via aerobic pathways that require oxygen. However, at races over 5 furlongs (1000 meters) over 70% of energy is generated via aerobic pathways and at a mile (1600 meters) 80% of energy is generated via aerobic pathways. During longer distances even more energy is generated using aerobic pathways making efficient oxygen intake ever more important.
Oxygen is also very important in the recovery phase following intensive exercise to rebuild energy stores consumed during exercise. This means that the benefits that FLAIR Strips provide for efficient oxygen intake during intensive exercise continues through the recovery phase to continue providing efficient oxygen intake to rebuild energy stores. This is one more reason why it is important not to remove the Strip until the horse is fully cooled out and breathing has returned to normal.