"The Burn" is Not a Flex if You Want to Get Stronger
May 16, 2024
You may have heard that the burning sensation you feel in your working muscles during exercise is from a build-up of lactic acid. You might also be under the impression that it means you're getting stronger because it feels so darn uncomfortable, therefore it MUST be working. While that burning sensation isn't necessarily a bad thing depending on your goal, it might not be what you want and it's important to understand what it is a sign of so that you can make the most of your time and energy in your workouts. Understanding this might also reduce any unnecessary discomfort during your workouts.
In order to discuss this topic you need to understand energy systems and their metabolic by products. There are 3 primary metabolic pathways/energy systems:
- Phosphagen (ATP-PC) System
- Glycolytic (Anaerobic Glycolysis) System
- Oxidative (Aerobic Glycoysis) System
Each of these have different fuel sources, process the fuel in different ways in order to convert it into energy, and leave behind different metabolic byproducts in the process. Let’s dive in…
Phosphagen (ATP-PC) System
Breakdown: At the onset of activity your body will utilize the Phosphagen system primarily, with phosphocreatine as its fuel source. Your body stores this in very small amounts in your skeletal muscle so it really can only last around 10 seconds. It is an extremely fatiguable pathway, but also can be replenished quickly.
This system is used at the onset of ALL activity and during high-intensity short-term work with longer rest times. Think: very heavy lifting, sprinting, plyometrics.
Mechanism: It works by breaking down ATP stored in skeletal muscle's myosin cross-bridges to release energy for muscle contraction. This process leaves adenosine diphosphate (ADP) and one phosphate (Pi) as byproducts. Phosphocreatine (PC) is broken down by the enzyme creatine kinase into Creatine and Pi. The energy released during this process allows the AD(di)T and Pi to combine and create new ATP (energy). This regeneration can only happen if there is a rest time of at least 2 minutes.
Byproduct: Healthy kidneys filter creatinine out of the blood and it is excreted in urine.
FUN FACT: a urine test may show high levels of creatanine not only if someone has diabetes or problems with their kidneys, but also if they have high muscle mass. Here is an example of how this process happens in the body:
Glycolytic System
Breakdown: Following the Phosphogen system, the Glycolytic System kicks in (around the 10 second mark) and lasts maybe a few minutes. “Glyco” has to do with “glucose” or “glycogen” as in what carbs and sugar are broken down into for energy. “Lytic” refers to loosening or dissolving. So in short, the fuel source is glucose (mobile in the body) and later on glycogen (stored glucose from muscle and liver) which ultimately comes from carbs.
Mechanism: In order to derive energy from the glucose it must first be broken down in to 2 molecules of pyruvate. The end result is 2 pyruvate, 2 APT, and 2 NADH molecules. Within this process there is a byproduct produced called...
Byproduct: Lactate (not lactic acid) which can create the “burning” sensation you feel when you are approaching the limit of this pathway.
FUN FACT: carbohydrates are the only macronutrient whose storage form (glycogen) can generate energy (ATP) anaerobically (without the presence of oxygen). Therefore, this pathway functions anaerobically.
Lactate vs. Lactic Acid
Photo Credit: useca.com
The only difference is a hydrogen atom. But it’s very important. To be classified as an acid, a molecule needs an extra hydrogen ion to donate. If lactic acid donated that extra proton it has from the hydrogen atom, it would become lactate.
This is important to note because while lacate is a byproduct (not immediately helpful) it can be reused via two different cycles. One of those cycles can convert it into glucose via gluconeogenesis in the liver and create more ATP!
So this “burn” you feel is you approaching your current lactate threshold. Your body can actually recycle this “waste” product to create more energy (ATP!)
Depending on what your training goal is, this could be a beneficial sign, but it is not if you are training for strength.
Oxidative System
Breakdown: After approx. the 2 minute mark, the oxidative system will begin to take the lead using carbs and fats as the primary fuel source. This is also the metabolic pathway that takes the lead at rest since as it takes the longest to produce ATP. It cannot keep up with high-intensity exercise, but has the highest capacity for energy production. This system differs from the glycolytic pathway in that it can only produce energy (ATP) in the presence of oxygen, so it can also be called the aerobic system.
Mechanism: The workhorses of the cell for oxidative metabolism are the mitochondria. This form of energy production is contingent on the ability of the CR system to deliver oxygen and nutrients and the cell’s ability to process that oxygen. Just like in glycolysis, the glucose is transported into the mitochondria where it is taken up into the Krebs cycle for ATP. Glucose can also be broken down and cycled into the electron transport chain for ATP production.
Byproduct: The only byproducts of this system are CO2 and water and your body disposes of them by breathing, sweating, and urinating. Think: Why do you sweat so much more with long-duration exercise?
You can see how they are similar yet energy production differs in the presence of oxygen.
Photo Credit: fsps.muni.cz/emuni/data/reader/book-6/05.html
Is the "burn" a bad thing?
Not at all. But it is important to understand what it is.
There seems to be a misconception that if one is using weights and they are fatigued that this means they are strength training, but this is not the case.
Depending on the duration of the specific exercise and movement and the rest time involved, the energy system/metabolic pathway being used will be different.
Feeling that burning sensation simply means you are approaching your lactate threshold with-in anaerobic glycolysis. Now that you know what “the burn” is, what are you going to do with that information?
Photo Credit: https://www.fitnessgenes.com/blog/energy-systems-used-during-exercise
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