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Horse's muscle power and fatigue

Do you know what happens in the body of your horse while training? A lot more happens than you might think. In this blog we explain how the muscle works, where does it get's its energy from and why does a horse gets tired?



Summary

· The muscle of a horse needs glucose and oxygen to contract.

· A light training uses mostly the aerobic pathway, an intensive training mostly the anaerobic pathway.

· The brain signalizes fatigue through organs or muscles who are in danger.

How does a muscle works?

In order to move his body the horse needs to contract his muscles. To generate this power the muscles need energy. A muscle generates energy by using glucose and if available oxygen. We call this respectively anaerobic an aerobic metabolism. During the course of a training the pathway changes depending of the amount of oxygen and glucose that is available. We will discuss 4 stages:

  1. First explosion

  2. Light training

  3. Intensive training

  4. Brain fatique

1. A first explosion

If a horse would suddenly have to run off, for example because it spooks in the field. The muscle uses the glucose that is stored in the muscle. There is no oxygen yet, so the muscle uses the less efficient anaerobic pathway.


This is way it is important in regular training to start with a warming up. This allows the body to prepare the muscle and increase the blood flow with extra oxygen to arrive at the muscles.




2. Light training


In a light training, after warming up there will be enough oxygen transported to the muscle to allow for the highly efficient aerobe metabolism of glucose (see figure above). A horse can keep this up for a long time. In for example a 2-hour training in walk, the heartrate of your horse stays under 100 beats per minute.


The heart rate is a good indicator for the demand of oxygen in the muscle:

  • The more oxygen is required,

  • the more blood needs to be pump to the muscle to bring the oxygen,

  • the higher the heartrate will be.


3. Intensive training

As a training gets more intensive the muscles will first run out of oxygen. On average this happens at a heart rate of around 180, for example when you ride in canter for 6-8 minutes or you ride uphill or in a rough terrain. When a specific muscle runs out of oxygen varies greatly between horses and depends on the training level of the horse.


The fitter the horse to better it is able to bring oxygen to the muscles, the longer it can rely on the more efficient aerobe metabolism.

If the muscle runs out of oxygen it will switch to the less efficient anaerobic metabolism. It can now only generate 2 energy molecule in stead of 36 in that process extra acid (H+) is produced,. causing acidification and fatigue of the muscle. It is a misconception that lactate is the cause of acidification, lactate actually has many benefits for the muscles in this stage.


What does lactate do?

  • Lactate helps against acidification.

  • Lactate functions as a signal substance.

  • Lactate ensures more energy.

  • During intensive exercise lactate functions as an energy source for organs like the heart.

  • After training In an average of 3 hours the body will break down the lactate.

Figure 1. A research in 36 human cyclers showed no relationship between the onset of fatigue and lactate concentrations in the blood. Some athletes could keep on cycling way beyond the anaerobic threshold of 4 mmol/L blood lactate. Indicating that the blood lactate measuring is not a good tool to compare fitness levels between athletes or horses.

4. Fatigue in the brain

More and more research confirms that it are probably not the muscles causing the onset of fatigue and being the limiting factor in performance. Long before the muscles truly run out of glucose and lactate as their source of energy the body will have send warning signals to the brain. A lack of oxygen, overheating and a drop in glucose in the blood are all signals to the brain to slow the body down. This prevents the body from overtraining or injuries and is vital for the survival of the body. It is very hard for any athlete to ignore these signals and keep training passed the barriers of ‘central fatigue’ in the brain.


Keeping the brain motivated in training is there for a very important aspect of training. Creating a high standard of wellbeing and positive emotional state will translate directly in to better performance. And since horses do not care much for golden medals other motivational rewards need to be in place and applied in perfect timing with training. Horse are very sensitive to a pressure release or motivational stroke.


References:

Burgerhout, W. G. (2008) Afscheid van melkzuur, deel 1 27: 322. https://doi-org.has.idm.oclc.org/10.1007/BF03077607

Burgerhout, W. G. (2009). Afscheid van melkzuur, deel 2. Stimulus, 28(1), 37–49. https://doi.org/10.1007/s12491-009-0005-8

Burghout, W. (2017). Visies op vermoeidheid Deel 1: Waarom houdt dat verzuren maar niet op? Sportgericht, 71(5), 20–22.

Campbell, N. A., Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2014). Biology: A Global Approach. In 10.5 Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen (pp. 253–256). Essex: Pearson Education Limited.

Cairns, S. P. (2006). Lactic Acid and Exercise Performance. Sports Medicine, 36(4), 279–291. https://doi.org/10.2165/00007256-200636040-00001

Gladden, L. B. (2004). Lactate metabolism: a new paradigm for the third millennium. The Journal of Physiology, 558(1), 5–30. https://doi.org/10.1113/jphysiol.2003.058701

Hall, M. M., Rajasekaran, S., Thomsen, T. W., & Peterson, A. R. (2016). Lactate: Friend or Foe. PM&R, 8(3S), S8–S15.

https://doi.org/10.1016/j.pmrj.2015.10.018

Hinchcliff, K. W., Kaneps, A. J., & Geor, R. J. (2008). Equine Exercise Physiology: The Science of Exercise in the Athletic Horse. Edinburgh: Saunders/Elsevier.

Noakes, T. D. (2004). From catastrophe to complexity: a novel model of integrative central neural regulation of effort and fatigue during exercise in humans. British Journal of Sports Medicine, 38(4), 511–514. https://doi.org/10.1136/bjsm.2003.009860

Nybo, L., & Rasmussen, P. (2007). Inadequate Cerebral Oxygen Delivery and Central Fatigue during Strenuous Exercise. Exercise and Sport Sciences Reviews, 110–118. https://doi.org/10.1097/jes.0b013e3180a031ec

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