What is fatigue?
In the context of horse sport, we use fatigue to describe the inability of a horse to continue to perform at the desired level. This may be manifested as a slowing down or not being able to jump as high or performing poorer movements as a test, course or race progresses. Fatigue in this context is different from exhaustion, which can result in an inability for a horse to even take another step or remain standing upright. Fatigue is a protective mechanism that is there to try and prevent a horse from exercising to a point where it causes damage to itself. In general terms, the harder a horse works the earlier it will fatigue. As an example, horses at the end of a 5 furlong flat race lasting under a minute are fatiguing as are endurance horses at the end of a 160km race lasting 8 hours. But the underlying causes of fatigue in each case will be very different.
Why is it so important to know when our horse is tiring?
As fatigue is protective, it’s a warning sign that the horse is struggling to maintain the intensity of exercise that the rider is asking for. Fatigue is not an all or nothing event. It tends to creep up. Once fatigue starts to set in, the rider has several options: decrease the pace and continue exercising, rest and resume exercise at the same pace, stop exercising or push the horse to continue at maximum pace possible. Because of the changes associated with the onset of fatigue, for example, changes in gait, there is an increased risk of injury as fatigue develops and becomes more pronounced. For example, a fatigued horse may be more likely to stumble and suffer a tendon injury. Horses that are pushed well beyond the point of fatigue in training or competition may also be at increased risk of developing other conditions after exercise, such as colic, tying-up, laminitis or heat exhaustion.
How do we recognise the signs?
Fatigue is not an all or nothing event. It tends to creep up on the horse and can therefore be difficult for riders to spot. The signs can also vary according to different disciplines but in general signs associated with the onset of fatigue can include:
What causes fatigue?
Fatigue can occur either in the brain (central nervous system) or peripheral nervous system or both, or most commonly, in the muscles themselves. In the latter case the brain sends the correct signals for what it wants to happen but the muscles cannot deliver. And we should consider that other muscles are important for exercise other than the locomotory (running/jumping) muscles. Fatigue of muscles in the upper airway for example may also result in reduced capacity for exercise (see below).
What precisely causes muscle fatigue is still not fully understood but can involve depletion of energy stores (e.g. muscle glycogen), accumulation of metabolic end-products and muscle acidification (e.g. lactic acid), failure of the contractile machinery (e.g. calcium imbalance) and disturbance to the internal environment of the muscle cell (e.g. electrolyte loss).
What factors influence the development of fatigue?
In simple terms, the harder a horse exercises, the earlier fatigue will start to occur. A large number of different factors can influence how hard a horse is working these can include:
One of the most important of these is fitness. As we train our horses and they become fitter, this makes them more resistant to fatigue. It will still occur, but the horse will be able to exercise for longer before the onset of fatigue. Training is also thought to be associated with a higher capacity to tolerate pain or discomfort. Horses are also likely to fatigue earlier, for example, if they are exercising on soft ground compared and or over hilly terrain. Rapid changes in pace will also hasten the onset of fatigue.
Fatigue in different disciplines
In dressage, fatigue may occur during a specific movement. For example, a horse who’s limbs initially achieve a good height in the piaffe, but gradually becomes lower and lower is probably fatiguing. Alternatively, a horse that achieves a 9 for extended trot at the start of a test but only 6 for the same movement later in the test, may do so because of the development of fatigue. In showjumping, many horses will begin to develop some degree of fatigue by the end of a round. This is because the muscles need a certain amount of time to recover between jumping efforts. The higher the jumps and the shorter the distance between them, the sooner fatigue will start to occur. And in showjumping fatigue can be expressed as something as subtle as just touching a pole.
In endurance, we have an apparently much simpler situation. However, in endurance fatigue may take many hours to develop and is likely to be due to depletion of energy stores, (glycogen) electrolyte loss, dehydration and a component of “brain” (central nervous system) fatigue. Fatigue in eventing during the cross-country phase may be the most complex to try and understand. A prolonged period of cantering, slowing down for jumps, accelerating after jumps, the jumping efforts themselves, hills, turns and water all make the effort highly variable. Good riders are generally those who are able to ride smoothly and delay the onset of fatigue.
The role of the respiratory system in fatigue
If muscles don’t get an adequate supply of oxygen for the level of work then they start to produce lactic acid. From studies of horses with very low grade respiratory disease exercising on treadmills, we know that these horses start to produce lactic acid at a lower speed/earlier on in exercise and as a result, they fatigue earlier. Whilst in untrained/unfit horses it is the heart and muscles which limit performance, in trained/fit horses it is the respiratory system which is the weakest link! So it naturally follows that even a small loss of function due to upper (e.g. “roaring”, “gurgling”) or lower respiratory tract disease (e.g. equine asthma, “bleeding” (EIPH)) can have a significant negative effect on performance. What is worse is that for the lungs, this frequently occurs without any indication that there is respiratory disease i.e. no cough, no abnormal noise and no nasal discharge.
One of the most common reasons why a horses’ respiratory system does not perform at its optimum and so could contribute to earlier fatigue is the presence of airway inflammation. This is usually due to inhalation of material in the stables, which in turn comes from forage, feed and bedding. This may include pollen, mould spores, bacterial toxins (known as endotoxin) plant material or physical dust (e.g. small fragments of sand, soil, etc). Feeds can be damped down to reduce inhalation of dust and rubber matting combined with low-dust bedding material, such as large chip wood shavings are two very good ways to improve the air quality in your horses stable, not forgetting good ventilation; it’s better to keep doors and windows open at all times and add extra rugs. Last but not least, one of the main sources of material that can lead to airway inflammation is FORAGE. Soaking is of course one option but leads to loss of minerals, vitamins and soluble sugars. In addition, soaking can be time consuming and it can be difficult or require significant effort to keep the soaking vessel clean. Many people have discovered steaming which kills spores and bacteria and significantly reduces dust whilst not impacting on the nutritional quality of the forage.
When it comes to fatigue, as a fight or flight animal the horse may often be willing to go further than a human athlete. One reason for this is that during hard exercise the levels of adrenaline in the horses’ blood are around 10 times higher than ours combined with high levels of endorphins (literally endogenous [within the body] morphine). Adrenaline and endorphins can help mask pain and allow a horse to push itself harder than may be beneficial for its own welfare. For this reason, it’s important that riders can recognise the signs of fatigue and know how to delay its onset through appropriate training, preparation and riding. As the respiratory system plays a central role in ensuring oxygen delivery to the muscles, it is essential that management of air quality is optimal.
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Dr. David Marlin discusses the impact working within a stable environment has on human respiratory health