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The Aerobic Energy System

While your body’s anaerobic system doesn’t require oxygen to provide you with short bursts of energy, the aerobic energy system uses oxygen to enable you to exercise at a level of intensity for sustained periods of time. The word “aerobic” literally means “with oxygen”.

Think of running, cycling and swimming. All of these are aerobic, or cardiovascular activities, and short energy bursts would not be sufficient for the distances that we would normally do. Cardiovascular exercise increases the heart’s capacity to send blood, and therefore oxygen, to our muscles. It’s the aerobic energy system that makes it possible for us to run marathons, whereas the anaerobic system is vital for sprinting.

The aerobic energy system utilises fats, carbohydrates, glucose and proteins from the food we eat, in conjunction with the circulatory system, to produce adenosine triphosphate, which is also known as ATP. ATP provides the energy that our bodies use for various cellular processes including muscle contraction. Although the aerobic system isn’t able to produce ATP very quickly, it has the capacity to continually produce this compound so that our supply is virtually never-ending.

The aerobic energy system can be broken up into three stages, with each one producing a certain amount of ATP.

Step 1: Aerobic Glycolysis

In this first stage that occurs in the cytosol or intracellular fluid, stored glycogen is converted into glucose. Enzymes then break down the glucose into a chemical compound called Pyruvate. As oxygen becomes present, the pyruvate is converted into acetyl coenzyme A. During this stage in glycolysis, two ATP molecules are produced.

Step 2: Krebs Cycle

In the mitochondria, a series of chemical reactions convert the fatty acids from fats, and the amino acids from proteins into further acetyl coenzyme A. Together with the acetyl coenzyme A from glycolysis in stage one, they enter the Krebs cycle and produce ATP as well as the by-products hydrogen and carbon dioxide. The carbon dioxide is expelled through breathing, but the hydrogen combines with two enzymes and moves to the next stage, the electric transport chain.

Too much hydrogen causes a build-up of acidity in the cells. The main purpose of the Krebs cycle is to release hydrogen for the electric transport chain so that the amount of acidity can be controlled and the production of ATP can continue.

Step 3: Electric Transport Chain

This final stage of the aerobic energy system is the most complex of the three but produces the most amount of ATP. In fact, for every molecule of glucose that is used, 34 molecules of ATP are generated.

Carrier molecules bring the hydrogen ions from the Krebs cycle into the electric transport chain. Once there, they are transferred to other carrier molecules and go through a series of chemical reactions to create a hydrogen ion gradient. The hydrogen ions move across the gradient and in doing so ATPase adds another phosphate group to, or phosphorylates, ADP to create ATP.

The by-products of the production of ATP are water and carbon dioxide. These are usually expelled successfully from our bodies: water is transferred from muscles into our blood as we lose water in the form of sweat, and carbon dioxide is transferred into our lungs and released through breathing.

Rate of Recovery

Recovery refers to the amount of time between the end of an exercise session and your cardiovascular system’s return to a state of rest. It includes certain physiological changes that take place, and that differ from the physiology in either the exercising or resting state. In other words, your body goes through certain changes during the recovery period, compared to how it normally functions.

Recovery is important to return the fuel (glycogen) stores in your body to the levels at which they were before exercise. It requires the ingestion and digestion of fuel, as well as the transportation of this fuel to the relevant muscles and cells within the body.

The rate of recovery can be a few hours, or a couple of days. The amount of time varies according to the length and intensity of your exercise session, and your fitness level. An athlete participating in interval training may require just a few minutes in between sessions, whereas a competitive endurance athlete may take a few weeks to fully recover.

There are various methods that can be used to enhance the rate of recovery, including ingesting carbohydrates, either by themselves or with proteins; and cold water immersion to lessen muscle fatigue and soreness, thereby reducing the amount of time needed for recovery.

Training Your Aerobic Energy System

Endurance athletes undergo regular training to that they are able to store more ATP molecules and therefore exercise for longer periods of time.

There are various types of training that can be implemented to improve your aerobic energy system.

Interval training consists of short, intense bursts of activity followed by an equal or slightly longer recovery time, or period of lower intensity exercise. For example, run two minutes at a high intensity (sprint) and then two to three minutes at a relaxed jog or walk. The high intensity interval should be such that holding a conversation is difficult, with the low intensity period giving you a chance to catch your breath and prepare for the next burst.  The benefits of interval training are endurance and motivation as well as fat-burning. Netball and rugby are examples of sports that utilise interval training.

Continuous training is exercise that lasts for a longer period of time and maintains a constant intensity. The intensity should be just outside your comfort zone, in between being able to have a conversation while exercising and not being able to talk at all. The exercise should last for 15 – 20 minutes. Continuous training helps to improve focus and mental strength. Sports that require continuous training are long distance running, rowing and swimming.

Fartlek training is continuous exercise with variations in intensity (speed and effort), but with no rest periods. Fartlek means “speed play” in Swedish and allows you to fluctuate between intensity levels, or speeds if you’re running, thereby keeping the training interesting while improving speed, stamina and mental strength. It is very effective for both group and individual training. Fartlek training is excellent for sports like football and outdoor hockey.

Benefits of Aerobic Exercise

Aerobic exercise is not just for competitive athletes, or those who want to get fit. There are a number of excellent reasons to include aerobic exercise in your daily life.

1. Improves Vascular Health

Exercise is important to strengthen your heart so that it can pump blood throughout your body more effectively, thereby improving oxygen flow to the muscles. It also reduces the risk of heart disease, which, according to the World Health Organisation, is one of the biggest killers in the world. If cardiovascular or aerobic training is properly executed, it will improve the heart’s contraction strength, the elasticity of your blood vessels and the efficiency of your blood to carry oxygen around the body.

2. Lowers Blood Pressure

High blood pressure, or hypertension, can cause serious medical problems like strokes, heart failure and kidney failure among other conditions. Blood pressure refers to the force of the blood pressing on the vessel walls as it is circulated around the body. Hypertension means that the force is consistently too high. This can result in a blood vessel bursting and in worst cases can cause death. By doing some form of regular aerobic exercise, you will increase the strength of your heart muscle, meaning that it uses less force to pump the blood. This will exert less pressure on the blood vessels and therefore lower your blood pressure.

3. Assists in Weight Management or Weight Loss

We store energy in fat cells that increase in number and size if we continually consume high-calorie food and exercise very little. These fat cells store energy at a high rate and release it very slowly, without the intervention of exercise. Aerobic exercise increases our body’s ability to use fat as a source of energy which fuels new activity. Maintaining low intensity exercise allows us to burn more fat than if we do high intensity exercise. For example, you will burn more fat going for a steady 10km run than you will running the 100m sprint.

4. Improves Lung Capacity

Aerobic exercise can’t improve your lung function – once it’s gone it’s gone forever – but it can improve lung capacity. Lung capacity is the total volume of air that your lungs can hold. Through aerobic exercise we can increase our capacity by anything from 5 – 15%. This increased lung capacity makes it easier for us to breathe during exercise and we won’t find ourselves “out of breath” as quickly.

5. Decreases Resting Heart Rate

Your resting heart rate is the number of times your heart beats per minute when at rest. As your fitness level increases through aerobic exercise, your heart muscle will become stronger and able to pump more blood per contraction. It will therefore need fewer heartbeats to circulate the same amount of blood as before, thus lowering your resting heart rate.

Risks of Aerobic Exercise

Although there are many benefits of aerobic exercise, it is also important to consider whether or not there are risks involved. In general, aerobic exercise is pretty safe, unless you have a chronic illness or have not done any exercise for an extended period of time.

If you have a chronic condition, it is important to consult with your doctor before embarking on an exercise programme. If you have not done exercise for a long time, you should always start slowly and gradually build up your pace and distance so that you don’t put your aerobic energy system under too much pressure.

It is important to warm up before aerobic exercise so that your body is prepared for physical activity. Warm ups increase the heart rate, respiratory rate, cardiac output and blood flow to the muscles, but also increases your body’s temperature so that the joints are more mobile and the risk of injury is less. Exercising without having warmed up sufficiently can cause injuries like muscle strains or tears.

Aerobic vs Anaerobic Exercise

To achieve overall fitness, you can’t rely purely on your aerobic energy system. Although this system helps us to gain endurance, we also require the strength that we get from the anaerobic energy system. It supplies us with short bursts of intense energy interspersed with periods of rest. All top athletes utilise both the aerobic and anaerobic systems in their training programmes as each systems has benefits for fitness and general health. The aerobic system will contribute 5% to the energy requirements in a 200m runner, while the anaerobic system contributes 95%. Conversely, a marathon runner will take 98% of his energy from the aerobic system and only 2% from the anaerobic energy system.

Neither energy system is better than the other, but the frequency at which you use one compared to the other will depend on what you wish to achieve with your fitness. If your ultimate goal is to run a marathon, then you need to do more aerobic exercise than anaerobic in order to increase your fitness and endurance level. However, you still need to train your body to increase your lactate threshold. This is the intensity of exercise at which lactate starts to build up in your blood. Anaerobic exercise is utilised effectively in athletes to increase this lactate threshold so that they can exercise at higher intensities and at a faster pace.

Types of Aerobic Exercise

Aerobic exercises are those that require oxygen and utilise the cardiovascular system. 

Some of the most popular endurance exercises or sports are the following:

  • Running
  • Walking
  • Outdoor Cycling
  • Swimming
  • Cross-country skiing
  • Tennis
  • Football

It is clear that the aerobic energy system is a key element for overall health and fitness. By knowing how the system functions, we will be better able to decide the level at which we should be exercising, and can increase our fitness in a safe and meaningful way.

If you have any questions on the above or would like some advice on how we could help you with your fitness goal, don’t hesitate, visit our gym and try one of our personal fitness training sessions.

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