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The Role of Lactic Acid in Energy Production

If you’re familiar with ‘the burn’ in the gym, then that means you’ve met lactic acid. But have you ever stopped to consider just what exactly lactic acid is, or how it might be hindering or helping your performance?

While this is something many of us won’t have given much thought, it’s actually a crucial focus for professional athletes as well as a hot topic for debate in the scientific community. Read on to learn more about this substance and what it means for your muscle.

What Is Lactic Acid?

To understand lactic acid (lactate), you first need to understand the role of the three energy systems in the body.

These are:

  • The ATP-PC system (adenosine triphosphate, phosphocreatine)
  • The lactic acid system (glycolytic)
  • Oxidative (aerobic)

The first two systems are used for shorter-term explosive movement and rely on energy already easily available to the muscles. If you begin to exercise, then first your body is going to use the stored ATP in your muscles. Creatine can help to recycle some of that ATP but it will only last for a few seconds before it completely depletes. Specifically, the ATP-PC can provide you with around 36 calories per minute but it won’t last that long.

Once that’s exhausted, the body will switch you to the glycolytic system. This means that the body will be relying on glucose in the blood and glycogen stored in the muscles and liver. This is broken down to provide more ATP and doesn’t require oxygen. The method is less efficient than the ATP-PC system and can provide up to 16 calories per minute for up to around 2 minutes. The problem with this is that it creates a by-product which is pyruvic acid – a substance that causes fatigue and that all-too-familiar burning sensation.

But there are two types of glycolysis. These are ‘fast glycolysis’ and ‘slow glycolysis’. Slow glycolysis generates less power but is able to convert pyruvic acid to acetyl coenzyme A (acA), which can be fed through the oxidative Krebs cycle to create more ATP and avoid extreme fatigue. So at this point, the body is starting to require some additional oxygen and exertion is beginning to trail off.

Finally, the body will then switch to the aerobic system when it is reaching the point of fatigue from glycolysis. This is the point at which you begin breathing heavily and it’s the energy system we will rely on if we’re running long distance.

Once you switch to this form of energy, you’ll be capable of exerting roughly 10 calories per minute. Through this system, the body can create energy through the krebs cycle, electron transport chain or beta oxidation. But that is a subject for another time!

Back to Lactate

Simply put, the form of energy we use for activities that last a moderate duration is the ‘glycolytic’ system or ‘lactic-acid’ system. That means it is what we use for things like lifting weights, or boxing for 2 minute rounds.

The body also reverts back to this system of energy production when the caloric demands are too great for the less efficient oxidation system to handle. For instance, if you break into a sprint then you will need more calories than the oxidative system can deliver. This is therefore called ‘anaerobic training’ as it forces you to rely on glycolysis once more.

In many ways, the glycolytic system is actually the most useful then, seeing as it allows us to use greater output than oxidation but for longer durations than ATP-PC.

The problem is that it causes the build-up of by-products such as lactic acid and pyruvate (actually, lactate is made from pyruvate).

The question then becomes: how long until that build-up of lactate is enough to cause fatigue?

The answer to this depends on the individual and a factor known as the lactate inflection point. The lactate inflection point is the point at which the build up of lactic acid in the bloodstream reaches a critical mass and starts to seriously hinder performance. Too much lactic acid can even lead to nausea and vomiting.

The good news is that this lactate inflection point (also known as the lactate threshold) can be trained. Elite athletes have a LIP of 80-85%.

Muddying the Waters

The only slight hitch in this theory is that it isn’t actually… accurate. While it has long been believed that muscle fatigue and soreness was the result of lactic acid build-up, the reality may not be so straightforward.

That’s because more recent research has found that the body doesn’t actually produce the acidic, ionised lactic acid, only the negatively-charged ion lactate. Lactic acid is produced by it is split immediately into its constituents, lactate and hydrogen and these do not remain in the muscle tissue.

What’s more, is that lactate actually has an important use in the body. Specifically, lactate is what is used to ‘clean up’ pyruvate. Pyruvate is the true byproduct of glucose broken down by glycolysis but it is by first converting pyruvate into lactate that the body is able to make use of it as an additional energy source. Pyruvate can be recycled via the KREBS cycle by the aerobic system but it can also be converted into lactate to be used as a direct fuel. Lactate can fuel the brain or the heart as a source of energy, or it can be converted to glucose in the liver. Gluconeogenesis is the formation of glucose in the liver and it is lactate that is the most important ingredient for this function.

But the simple fact remains that there is a correlation between lactic acid in the body and fatigue. So what is going on here?

The answer seems to be that muscle fatigue comes from certain metabolites, including inorganic phosphates, as well as the loss of potassium. Meanwhile, lactate and/or compounds that happen to correlate with lactate appear to be used as an indicator that the body is working ‘too hard’. After a few minutes, this causes us to feel nauseous to force us back into a sustainable level of intensity.

Lactate Threshold

So lactate and lactic acid are not responsible for the burning sensation in your muscles or for fatigue. Instead, lactate is actually a useful secondary fuel source that your body can use to fuel your muscles and your brain.

The reason the lactate threshold will correlate with muscle fatigue and burn out, is simply that the lactate builds up once you are placing greater demands on the body. At a certain point, you will be producing lactate faster than your body is able to use it. This also just so happens to be when the fatigue sets in due to the build-up of other metabolites. This is the point known as the lactate threshold: it is the fastest pace you can run while still relying on glycolysis and the associated lactate processing. This is therefore generally equivalent to your anaerobic threshold.

As you train, you can improve your lactate threshold. Endurance training stimulates the body to use more lactate more efficiently and in trained athletes, lactate is actually the preferred energy source for the body over glucose. This means you spare more glucose stores and can actually train for longer.

Professional athletes can actually invest in NIRS sensors, which will help them to monitor their lactate levels in real-time. But there are also numerous ways that you can calculate your own lactate threshold, which in turn can act as a very good indicator of cardio fitness.

For example, the average lactate threshold is going to be at 85% of MHR (maximum heart rate). Your maximum heart rate is the heart rate you reach during the highest intensity training. If your MHR is 196 then, your lactate threshold will likely be around 166.

Another interesting metric is your RSLT. This is your ‘Running Speed at Lactate Threshold’. As you might have guessed, this is the speed at which you are able to run to sustain up to the lactate threshold. One of the best ways you can measure this is with an exercise called the 30 minute test. Simply, this involves testing how fast you can run for 30 minutes. Your objective is to go all out for those 30 minutes to the point that you couldn’t go any faster. You then measure your average speed by the distance. This is a (mostly) accurate predictor of your RSLT because it tells you how fast you can go over that time and it would be impossible to use the oxidative system for much of that.

Training at or around your LT and RSLT is the best way to improve it as you condition the body to more efficiently convert lactate for energy. In turn, this can make the body significantly more energy efficient, allowing an athlete to train for longer periods using a highly efficient energy system and while maintaining glycogen stores.

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