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Why Do Your Muscles Burn After An Intense Workout?

Discover the real reason why your muscles feel as though they are on fire when you have just finished that super-set.


We've all been there, you've just smashed a great set on the bench and you get up and quickly head over to the pec-dec to knock out a quick 12 reps to round off your superset and as you hit the 8th rep your chest is on fire. It feels as though your pecs are going to pop out of your chest and you're not sure if you can squeeze out the last 4 reps.


As you stand up you feel the blood fill your chest and your shirt instantly feels a size smaller as it squeezes around your chest. The pump. What a great feeling, but what is that burning sensations that comes with it and why does this happen?


Well, the first thing we need to learn is that the body has 3 main energy systems; the Creatine Phosphate system (CP, Phosphocreatine), the Lactate system and the Aerobic system. Each system is utilized by the body in different circumstances. The Creatine Phosphate system can supply energy for around 10 seconds during high-intensity bursts of exercise. The Lactate system provides energy for around 1-2 minutes during high-intensity exercise. The Aerobic system provides energy when performing exercises that are not as intense for a longer duration of time, such as running a marathon or going on a bike ride.


So from the info above, we can see that when weight lifting we will mainly be using the Creatine Phosphate system and the Lactate system. The average set of 8 reps can last between 30 seconds and 45 seconds and as such we will be using the Lactate system. Supersets will be about 90-120 seconds on average. The first 10 seconds will use the CP system before transferring over to the Lactate system.


Now we need to understand how the Lactate system works and why this leads to the familiar burning sensation we are all so used to.



When we start exercising and we have exerted the CP system our bodies then start to use carbohydrates to create the energy we are demanding. The energy we get from carbohydrates is glucose. A small amount of glucose is stored in the muscle itself and also transported to the muscle through the bloodstream as the main store of glucose in the body is the liver. This glucose is broken down to form ATP (Adenosine Triphosphate) which is our body's currency for energy, this process is called Glycolysis.


It takes 2 molecules of ATP for glycolysis to occur and the net yield of this is 2 ATP molecules, so for example; 2 ATP molecules in and 4 ATP molecules out.


A bi-product of Glycolysis is Pyruvic Acid. What happens to this Pyruvic Acid is dependant upon the presence of oxygen. If there is not enough oxygen present to further breakdown the Pyruvic Acid, as in the case of the Lactate system, then it gets turned into Lactic Acid.


The process of breaking down glucose molecules is the same as how the Aerobic system does it except there is a greater presence of oxygen which is then able to further breakdown the Pyruvate molecules, which yields a greater number of ATP molecules.


The Lactic Acid produced by the Lactate system either gets transported away by the blood or stays in the muscle until enough oxygen is present to be able to turn it back in Pyruvate molecules. This Lactic Acid causes a change in the pH of the muscle environment and turns it more acidic. This is known as Acidosis and causes the muscle burning sensation we have all come to accept and love when training.


So, why do we not always experience this sensation when training then?


The answer to this question depends on the intensity of your training. For example, if you completed a workout of 3x8, doing single exercises and resting for 2 minutes between sets then you probably wouldn't experience too much of a muscle-burning sensation. This is because the Lactic Acid is being produced at a slower rate and the body is able to get rid of it through the bloodstream and so it prevents the environment in your muscles becoming too acidic.


However, if you were to train 12 reps and do supersets or tri-sets with only a 60-second rest between your sets then you are far more likely to feel the burn. This is because there will be far more lactic acid that accumulates in the muscles as you will be using more and more energy which wi keep turning the glucose molecules being supplied by the body into more Lactic Acid.


Is this anything to worry about?


No, this is a completely normal and natural process of the body and something that the body is able to deal with, it just requires time to be able to supply the required oxygen to the muscles so it is able to turn the Lactic Acid back into Pyruvate molecules.


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