The different phases of any eventing competition call for a variety of technical skills and physical abilities. Cross country involves an intermittent pattern of energy expenditure as the ground and terrain vary and the horse must be fit enough to cope with hills, turns, water and deep going. On the cross county phase heart rates can fluctuate between 140-200bpm and research has shown that this is irrespective of the level you are competing at.
The science bit!
To understand fatigue we need to understand how energy is produced. Energy generation is the process of converting stored chemical energy into mechanical energy for muscular movement. The basic unit of energy is Adenosine Triphosphate (ATP) and the main sources of energy are carbohydrates and fats which are broken down to yield ATP.
The breakdown of fat for energy can only take place with a constant supply of oxygen known as aerobic metabolism. Because oxygen is required the energy production process is slower but can be sustained for longer, meaning fat is an important energy source during low to moderate intensity exercise sustained over prolonged periods of time.
Carbohydrates are broken down into glucose units in the small intestine where they are absorbed into the blood (blood glucose) and then either oxidised directly to produce ATP or stored in the body in the form of glycogen. Muscle glycogen is an important fuel for energy generation during exercise.
For anything more than a slow canter the horse primarily relies on the glycogen stores in the muscles to produce energy. Glycogen can be broken down to produce energy either with or without oxygen. The process of energy production without oxygen is known as anaerobic metabolism. Anaerobic metabolism produces energy rapidly but also results in the production of lactic acid which, if too much accumulates, causes fatigue. This means it can only be used for short periods of time.
What the science means in practice
Anaerobic metabolism supplies the bursts of energy required to negotiate the obstacles, including jumping, turning and accelerating but the extent of anaerobic contribution depends on how the horse is ridden. A rider who changes pace quickly coming into a jump and then kicks on hard going away from the fence demands a greater anaerobic contribution from their horse than the rider who maintains a steady rhythm round the whole course.
The harder and longer the horse works the more glycogen is used up. With prolonged exercise both the liver and muscle glycogen stores decrease and blood glucose levels start to fall. This in turn leads to tiredness and fatigue. Horses replenish their glycogen stores relatively slowly and it may take 24-48 hours to be fully restored after a heavy training session or long cross country course. So we can see that fatigue occurs primarily due to the depletion of energy stores or the build-up of lactic acid (lactate).
During the type of high intensity exercise required on the Badminton cross country course lactate accumulates within the muscle fibres creating an acidic environment which interferes with metabolism and reduces the rate of energy production. Eventually, lactate levels reach a critical level of concentration and the muscle fibre is no longer able to contract. Lactate first starts accumulating in the type 2b muscles fibres (those muscle fibres required for high intensity work) and once the critical level is reached these fibres stop functioning and there is a noticeable drop in the power the horse can produce. Riding in a way that asks the horse for sudden acceleration, abrupt stops and starts and short bursts of speed uses up the glycogen stores and contributes to fatigue.
Muscular fatigue then is not just tiredness it is a decrease in the ability to perform work. You could see this at Badminton this weekend with horses slowing towards the end of the course, unable to maintain the gallop and unable to jump as high or as wide as they could at the earlier fences. From a welfare perspective, muscular fatigue increases the likelihood of hitting a fence, failing to clear a fence, tripping or falling. Interestingly a study looking at risk factors for horse falls in one day events found that competitors in a top 3 position before the cross country were more likely to have a horse fall than a competitor in any other position. Food for thought – how does a human’s competitiveness / ambition / drive to win impact the way they ride and as a result contribute to fatigue in their horse? This is why it’s so important for rider’s to take responsibility for pulling up tired horses but even more so for stewards to enforce the pulling up of a tired horse – you cannot nurse a tired horse home when they are physiologically incapable of performing the work being asked of them.
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