Energy Systems: How our body uses energy.
Physical exercise requires many different bodily functions to all work simultaneously for energy metabolism and to deliver adequate oxygen to muscle tissue and to remove waste products and heat as well as controlling fluid levels.
In order for your muscles to move you forward there is a multistep chain of events that occurs. The fundamental energy fuel for muscle contracting is called ATP (Adenosine TriPhosphate) and is the fuel ‘currency’ of our bodies. All other fuels are broken down to create ATP, which is how muscles contract. ATP is stored in the muscle in only small amounts but can be regenerated by Creatine Phosphate. This system is called the Phosphagen or Immediate energy system and is the first energy system used. The phosphagen system does not require oxygen to function so it is called an-aerobic. It can only provide energy to the muscle for a short period of time of approximately 10-15 seconds and would be used for a burst of energy, e.g. a golf swing, resistance training or a sprint.
The next energy system is called the Lactic Acid System and is also anaerobic. This system breaks down glycogen or uses blood glucose to form ATP, resulting in lactate. This energy supply can last for about 2-4 minutes, so again, perfect for short bursts of energy like sprinting, field games, high repetition resistance exercise etc.
During prolonged exercise, energy must come from fuel or substrates in the form of carbohydrate and fat or lipid. These fuel sources come from the food we eat and stored supplies as well. This energy system is called the Oxidative System as it uses oxygen (aerobic) to fuel muscles. It kicks in about 5 minutes into exercise and can last for hours so is the primary energy system for marathon running. You can see why, because you will be using this energy system, how important carbohydrates and fats are in your diet.
Our body has stores of fuel, mainly carbohydrate in the form of glycogen and fat in the form of fatty acids and triglycerides. As you can see from the table below, we have a far more abundant supply of fat store than glycogen. Muscle and Liver glycogen stores need to be replenished every day and can be fully depleted after 90 minutes of exercise.
So, you can see that even for the leanest person, we have far more usable fuel from fat than glycogen demonstrating that it is an efficient fuel to use for marathon running as the race takes longer than 1.5 hours. As your training develops and you become fitter, your body will adapt to use fat as fuel for longer, which can spare glycogen stores and ultimately offset fatigue from setting in. However, in endurance races, where the exercise intensity is higher for longer, we will still use our glycogen stores and blood glucose as fuel so it’s imperative to replenish glycogen after 90 minutes of exercise.