Competitive endurance rides are a test of the rider’s ability to safely manage their horse’s stamina and fitness while coping with the varying terrain, distance and weather conditions on the course. At elite level, rides can be up to 160km (100miles) long and split into 3-6 loops. At the start of the competition and at the end of each loop all horses must pass a veterinary examination before being allowed to continue. If a horse fails any of these ‘vet gates’ then their result is classified as FTQ or failure to qualify.
Research at international level events show that fast riding speeds in the early stages of a ride (loops 1 and 2) and when riding in large groups is a significant risk factor linked to negative outcomes for horses and tends to be followed by a sudden drop in speed in the following loop. In both cases it’s likely adrenalin takes over causing combinations to get ‘carried away’ and end up travelling at speeds they haven’t trained for or aren’t capable of maintaining for any length of time. More often than not this leads to an FTQ classification due either to lameness or fatigue. Fatigue is more than just tiredness, it’s the muscles inability to continue to work and occurs primarily due to the depletion of energy stores or the build-up of lactic acid. (You know that burning feeling after you’ve stacked a barn full of hay? That’s lactic acid build-up).
If you remember your high school science, you’ll know that energy generation is the process of converting stored chemical energy into mechanical energy for muscular movement. The main sources of energy are carbohydrates and fats. Fats can only be broken down to produce energy with a constant supply of oxygen, known as aerobic metabolism. Because oxygen is required the process is slower but can be sustained for longer, meaning fat is an important energy source during moderate intensity exercise (trot and slow canter) which is sustained over prolonged periods of time as in endurance.
In contrast, carbohydrates are broken down into glucose and either absorbed into the blood (blood glucose) or stored in the muscle cells in the form of glycogen. For anything faster than a slow canter the horse relies on these muscle glycogen stores to produce energy. Glycogen can be broken down 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. In essence, the harder the horse works the more glycogen is used resulting in depletion of the muscle’s glycogen stores, a fall in blood glucose levels and ultimately the inability to continue exercising. This means the production of energy using anaerobic metabolism can only be used for short periods of time, for example a burst of speed in a sprint finish. On the other hand aerobic metabolism, while not as fast, is very efficient at providing a constant, sustainable supply of energy for movement.
The successful endurance horse needs to generate the majority of its energy through the aerobic metabolism of fats and carbohydrates. The extent to which the horse is able to do this is partly genetic (which is why Arabs are so dominant in endurance) and partly due to training. Training enhances the muscles’ ability to use fats to produce energy leading to a glycogen sparing effect. It also results in a greater glycogen storage capacity. Being able to store more glycogen in the muscles and use fats to produce energy for longer means that glycogen depletion and the resulting fatigue is reduced or delayed. In other words, the horse gets better at producing energy aerobically allowing it to go faster over a longer distance without tiring.
If you ride in a way that ‘wastes’ the stores of glycogen (sudden accelerations, abrupt starts and stops or short bursts of speed) the muscles move to anaerobic metabolism and lactic acid begins to accumulate. Eventually, lactic acid reaches a critical level of concentration and the muscle fibre is no longer able to contract. When this happens not only is the horse unable to maintain its pace, hence the sudden drop in speed, he/she will also become stiff, sore and poorly coordinated. This may explain why some horses present with an irregular gait at the vet inspection or fail to qualify due to lameness but trot up sound the next day.
The moral of the story? Firstly, train at the speeds and over the distances required in competition. Secondly, resist the urge to ride too competitively in the early stages or when travelling in a group. Finally, allow a minimum of 2 days recovery between hard training sessions and in the run up to an event as it can take 24-48 hours for glycogen stores to be fully replenished.