So we know we need to keep our heart rate up to improve fitness and stay in shape, but what’s actually happening in our body as our heartrate travels through the different training zones?
“I’ve just got no energy today”
We often refer to energy as something we tend to have a lack of, however we don’t really think about the metabolic processes our body goes through to provide the energy we have. How does energy actually work to help us exercise?
First off let’s get down with the lingo we’re going to be using.
ATP: This stands for Adenosine triphosphate and is what we use for energy. Every cell in our bodies uses ATP to stay alive.
How we use that ATP, is decided via 3 main energy systems, our aerobic, lactic and creatine phosphate energy systems. This is where things get a bit more techy.
The Aerobic Energy System
As soon as we start to exercise heart rate and breathing rate increases so more oxygen is getting to the muscles. Within a few minutes the muscles are supplied with enough oxygen for aerobic respiration to work. All we need for aerobic energy is oxygen, so we can last for a long time using this method. This energy system is used for low to medium intensity activity, with a long duration.
Your Aerobic System uses oxygen to break down food fuels, carbohydrates and fats. This gives off a high energy yield.
The transformation from food to energy is a complicated progress, involving 1 : Glycolysis Stage 2 : Kreb’s Cycle Stage 3 : Electron Transport Chain. This cycle produces a total of 38 molecules of ATP.
The aerobic system is the most efficient in energy production. The by-products (water and carbon dioxide) are easily expelled. The system relies on the availability of oxygen. Unless the body runs out of carbohydrate and fat stores, this system is unlimited. This is what is used during sessions where your heart rate is low - moderate and exercise is for a long duration.
The Lactate Energy System
The lactate system is anaerobic, this means without the presence of oxygen. Sometimes known as Anaerobic Glycolysis due to the initial process being the same as aerobic energy production (as mentioned above), only without oxygen. So, as before 10 chemical reactions occur within the Sarcoplasm which turn Carbohydrate into Pyruvic acid and 2 molecules of ATP. The difference now being the lack of oxygen meaning the carrier molecule can’t offload the Hydrogen (H+) by-product of glycolysis (stage 1 above) causing a build up in the cell.
To try to prevent an increase in acidity the pyruvic acid accepts the Hydrogen, forming Lactic acid. If oxygen was present the H+ would be transported to the Mitochondria for use in the Kreb's cycle. Due to lactic acid production, this energy system can only be predominant for up to 2 minutes, so is typically used in short sharp burst of intensity such as HIIT training. Heart rate during this exercise will typically be at a moderate to high level, depending on the individual.
Fun Fact: Lactic acid is your friend, it is what is produced to STOP the burning session. The burning session is actually caused by pyruvate.
The Creatine Phosphate Energy System
This energy system is also aerobic and lasts all of a few seconds. This energy system is the first one recruited for exercise and it is the dominant source of muscle energy for high intensity explosive exercise that lasts for 10 seconds or less. This energy system would be the main energy source for a 100m sprint, or a short set of a weightlifting exercise. It can provide energy immediately, it does not require any oxygen and it does not produce any lactic acid.
ATP energy is created via combination of ATP already stored in the muscles (about 1 or 2 seconds worth from prior cellular respiration during rest) and its subsequent rephosphorylation (about 8 or 9 seconds worth) after use by another molecule called phosphocreatine (PCr). PCr is a molecule that carries back-up phosphate groups ready to be donated to the already used ADP molecules to phosphorylate them back into utilisable ATP. Once the PCr stored in your muscles runs out the CP anaerobic energy system will not provide further ATP energy until your muscles have rested and been able to regenerate their PCr levels. Creatine supplementation is a method used to extend the duration of effectiveness of this anaerobic energy system for a few seconds by increasing the amount of PCr stored within your muscles, however your body can only store 120g creatine at a time. Bodybuilders use the supplement to create more power for their big lifts and therefore gain size and strength. More information on weight gain supplements.