Exercise physiology Part 2: How do muscles utilise energy

As an equestrian sports coach (focusing on endurance) it is always interesting to visit rides without taking part as a competitor. My previous blog (Exercise physiology 1: the importance of warming up for our energy stores) was about my experiences at a ride I was helping out at. I talked about the importance of a proper warming up by comparing our fuel sources to wood, petrol and gas for their intensity in burning.

In this blog, I would like to tell you a bit more about muscles. Funny thing muscles… everybody has them (sometimes well hidden but still 😉 ). We know they are there, but most of us don't really know how they do what they do. Of course it is clear that muscles allow us to move, and if we move a lot or very fast we get tired, and might even be sore for a few days. If we do the same amount of moving a couple of times we get less tired and hopefully not so sore anymore. This process is training, and we apply it to our endurance horses as well. When we train our endurance horse, we want to get the muscles stronger and working more efficiently. How quick this will develop can be a little different per individual horse. To understand this, I will have to first explain about fuel and muscle types. Then we will go into the combination with heart and lungs, which will give a different athletic ability per individual horse. 

In the first blog, I have already told you a little about fuel. Let's focus on the primary sources of fuel for energy: carbs and fat. Carbs are blood glucose and glycogen. Fat is really available to utilise as fatty acids. Glucose in the blood is a non-stored source of energy, it is readily available in the bloodstream, but it is limited in amount. The body can also access power from stored sources. These are glycogen and fatty acids. Let's see what glycogen and fatty acids are and how they are utilised in the body:

• Fatty acids is our main supplier of energy for endurance because it will never run out. The body has a large store of these in the form of fat, even lean horses will have plenty. It will take a little while to get going well because it is slower to get going properly than carbs. So we need about 20-30 minutes of steady exercise to get this process up and running, perhaps longer depending on muscle type, heart size and lung capacity. To use this form of energy, the body needs some glucose to keep the process of burning fatty acids going. Also the body requires a lot more oxygen for the mitochondria to produce the ATP.
  
  
• Glycogen: is a stored form of carbohydrates in the cells of muscles. There is also a small amount stored in the liver to supplement blood glucose if needed. It is important to realise the store of glycogen in the muscles can run out.  Glycogen can be made available with oxygen (aerobic utilisation) and without oxygen (anaerobic utilisation).  Under normal circumstances, glycogen will be processed in the cell mitochondria into ATP (the actual usable metabolic energy for the muscles) using some oxygen, producing no harmful by-products. When energy is needed rapidly (either caused by adrenaline and/or sudden hard work/high speeds), glycogen can be turned into ATP within seconds without oxygen, but this will produce lactic acid, and the store of glycogen will run out faster.   

That brings us to the different types of muscles used for movement, and which fuel they use to provide energy:

There are 2 classes of muscles: Slow twitch and fast twitch (which is subdivided into types again). They respond differently to activity levels. All horses have all types, but selective breeding has lead to a predominance in the required muscle type for that breeds job. 

• Type I: slow twitch. Long distance, slow speed. Fuel: glycogen and but mostly fat. Aerobic metabolism.
• Type IIA: fast twitch. More capacity for speed. Fuel: glycogen and fat. More anaerobic metabolism
• Type IIB: fast twitch. For explosive speeds for a short period: Fuel: glycogen. Mainly anaerobic metabolism. (In equines this type is also called type IIX as it is slightly different than other mammals.)
• A third type, IIC is mainly present in foals and is thought to develop into the other types when growing up.

So you can see there is a scale in which muscles, and the fuel they use are deployed.

If we generalise and not take into account that different breeds had adapted differently in their required work by having a different percentage of a certain type of muscle (combined with heart and lung size) you can generalise in the use of energy in the different gaits. In rest and slow walk horses use type I muscles mostly, slow burning fatty acids that are available in the bloodstream. You can see this as a basic output that is always present. When activity increases so will the use of other types of muscles, and type II muscles are activated.

The fuel used is depending on the individual horse and its combination of muscle type. But generally you can say that fatty acids are used with calm work. Glycogen is used in the more active work and in extra amounts when the type IIB muscles are used as they burn anaerobically.

If we train our endurance horses well and ride nice and steady they will mainly use type I (burning fatty acid) and only a little type II (using depletable glycogen). The faster we go, and the less consistent we ride, the more we tap into our precious glycogen stores, risking fatigue and muscle aches.

In a future blog I will tell you more about the heart and lungs, as they are the organs that bring the much needed oxygen to the muscles. Because without oxygen there will be no sustained performance.