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Energy used during exercise

Energy used during exercise
Have you ever wondered about the energy used during exercise, where and how it is produced and which type of energy is used depending on the type of exercise you do? Well this article will help answer those questions.

Where our bodies energy comes from

When we eat, our food is broken down for use by our body. Carbohydrates are broken down into glucose, protein into amino acids and fat into fatty acids.

When we are resting or doing very low intensity activities, our body actually burns more fat cells than any other energy source (unless you have consumed a huge amount of carbohydrates).

Carbohydrates however start to become your main source of fuel when you start to up your activity level and is used by your muscles, brain, central nervous system and other organs. They also help metabolise fat and spare protein from being used for energy. Protein is used by the body to build, maintain and repair the body’s tissues and organs. It also helps with many chemical and biological processes and functions within the body. Fat is mainly used as a long term fuel source. It is easily stored. But it also provides support for cell growth and function, protection for your organs and various biological processes within the human body to keep it healthy. Excess carbs and protein that isn’t used by your body will also be stored as fat.

All our bodies energy sources are broken down into a substance called ATP (adenosine triphosphate). ATP is your body’s energy source needed by your muscles to carry out all movement. What decides which ATP energy source is going to be predominantly used is the type and length of the training you do.

ATP-CP (Phosphagen System)

When you do an explosive movement, lift a heavy weight or sprint for example, the Phosphagen System uses the ATP that is already stored in our muscles and also creatine phosphate (CP) to create more ATP, but only a small amount is available and it doesn’t last long and just for a short burst of energy. The first 3 seconds use the stored ATP, the next 8-10 seconds use creatine phosphate to create even more. This system doesn’t need oxygen to help create ATP and is a very fast way to produce it.

To note, while you rest between sets of an intense exercise your body starts to replenish the lost creatine phosphate, ready to be converted to more ATP and used in the next set if required. Replenishment usually takes between 3-5 minutes.

Anaerobic (Glycolytic System)

This is also a fast energy source and uses glucose (glycogen from carbohydrates). This system is a chemical process of the partial breakdown of sugar in the body and doesn’t require oxygen to be present to produce ATP. The glycogen which is stored in our liver and skeletal muscles is broken down and converted to ATP and is used to supply energy for moderate to high intensity training that generally lasts up to 2 minutes, such as weight lifting or a fast run/sprint. This system also produces lactate in the form of lactic acid, which builds up and is the cause known as the burn. Oxygen is required to oxidise the lactic acid to produce carbon dioxide and water and why you will end up with an oxygen dept, making you breath deeply. Fatigue will also occurs at this point due to a number of other processes and chemical reactions, stopping contraction of the muscle. Lactic acid can hinder muscle use after around 30 seconds of intense exercise, it then takes around 15 minutes to recover the PH levels. So this is why you gradually are unable to carry on lifting.

This system also replenishes itself. If you where to do a fast moving intense exercise for 1 minute, replenishment of ATP would take around 2 minutes. However there is a gradual shift to fatigue due to the lactic acid buildup.

Aerobic (Aerobic Respiration System)

Once you start working out for more than a couple of minutes, your body will start to use more of the aerobic system to produce ATP for its prolonged energy requirements. This system is much slower to produce ATP as it requires oxygen to burn carbs and fat for energy use. However, although this process is much slower, the amount of energy it can supply is only limited by the amount of stored glucose/glycogen, fat and even protein you have.

Carbon dioxide and water again are produced as waste products of this process and combined with the need for oxygen to create ATP from glucose and fat, will increase your heart rate and you will breath harder.

How much glucose or fat is used for the body’s fuel supply will depend on the intensity of your workout. Low intensity workouts will use mainly body fat (triglycerides) as an energy source, as this is generally a long term sustainable fuel source. Your bodies adipose fat tissue is broken down into fatty acids and slowly converted to ATP. Higher intensity workouts will start to use more glucose for its energy source, as this is quicker to convert to ATP. An average person will have enough glucose stored for 90 minutes of moderate intense activity, after that you will what’s called ‘hit the wall‘, a time when you have no more glucose to supply your more immediate energy requirements.

However, your body stores a very large amount of body fat, which holds 9 calories per gram, while carbohydrates only have 4 calories per gram. An average person will have 10’s of thousands of calories of body fat stored, It takes an awful long time for your body to burn up a significant amount of body fat doing low intensity workouts. Also remember your body is still burning glucose at the same time, so it’s not just burning body fat, making the process even slower. And unless you use up all your body’s stored glucose (of which there can be quite a bit), then low intensity workouts are not going to be the most efficient method for fat loss on its own, as it’s metabolically efficient and therefore not great for burning excess body fat.

High intensity workouts use more glucose than body fat, so on the face of it, it seems counter intuitive to think this is better for burning fat. However, high intensity workouts such as HIIT and weight training while do not burn a greater amount of body fat than low intensity steady state aerobics during your workout session, use up a lot more total calories and create a process called excess post-exercise oxygen consumption (EPOC). This is where your body also continues to burn calories after your workout, giving you a big boost with fat burning. Effectively, it ups your metabolism and makes your body a calorie burning furnace. At rest or doing daily activities you are burning mostly fat cells and high intensity workouts will boost this process for many hours after your workout. The increase isn’t a huge amount on its own, but as it lasts for many hours (sometimes as long as 48 hrs), the total accumulation of this fat burning process is significant. Not to mention that developing more muscles also burns more calories.

Your body can also use protein from the breakdown of muscle and other body tissue to produce energy. However this is normally a last resort to survival when all other fuel sources are either used up or close to being used up. However if you workout hard enough and burn a significant amount of your stored glucose, your body can start to break muscle down to replenish this lost energy.


So there you have it, the food we eat supplies our body with the energy it needs and generally while the three energy systems are separate processes, they are all used at the same time in different ratios depending on the activity we are doing and its intensity.

Please also see the effects of exercise on the body article if you haven’t already done so.