Feeling The Burn

Busting the lactic acid myth

Photo by Victor Freitas on Unsplash

Let’s face it. How many times have you heard someone say that they’re tired because of the lactic acid buildup in their muscles? I think I hear it at least five times each week. Many of these same people then expand on this initial observation by telling me that they should go for a walk or a massage to ‘flush it out of the system’. Maybe you, the person reading this article right now, are included in this category. You’d be forgiven for thinking that this is in fact correct, as, despite this research now being over one hundred years old, it is still the oversimplified truth that has been imprinted on the masses. Simply put, this isn’t quite how the body functions during exercise. To find out more, we first need to understand a little about the bioenergetics of anaerobic exercise.

Fuelling exercise

You may already be familiar with a molecule called Adenosine Triphosphate (ATP). This molecule is essentially the main source from which we derive our energy at the cellular level. Without ATP, not a lot gets done. The structure of ATP is pretty self-explanatory: One adenosine molecule (adenine + a ribose group) paired with three phosphate groups.

The energy from ATP comes via the breaking of the high-energy bonds between the phosphate groups, which is done by an enzyme called ATPase. It requires the presence of water, and is therefore known as hydrolysis. The net result is a molecule called adenosine diphosphate (ADP; because there are now 2 phosphate groups), a free inorganic phosphate which has been separated from the other two (P_i), a proton (H⁺) and energy.

Therefore, the breakdown of ATP goes as follows:

ADP can be further hydrolysed to form Adenosine monophosphate for more energy:

How to replenish ATP?

ATP can only be stored in small amounts. Therefore, the body needs a series of processes to replenish these stores and give a continual supply of energy. These processes can either occur in the presence of oxygen (aerobic) or without the presence of oxygen (anaerobic).

Anaerobic Systems

There are two main energy systems which take place anaerobically:

1.     The Phosphagen (ATP-PCr) system

2.     Glycolytic systems

The Phosphagen (ATP-PCr) System

This is by far the quickest system at our disposal. We use this system when performing short, intense bouts of exercise, such as sprinting or weightlifting. It is also the primary energy system at the start of bouts of pretty much any exercise, even endurance events.

It relies on two molecules. One of these is ATP, and we have already covered this. The other is called phosphocreatine (PCr). A molecule of PCr is incredibly valuable, because it possesses phosphate. This phosphate molecule can be taken from PCr and given to ADP to form ATP, thus replenishing ATP stores. It does this with the help of an enzyme called creatine kinase – see the full equation below. Although a very quick and effective system for short bursts of activity, it cannot be sustained for very long because the body doesn’t tend to store PCr in large quantities. As an aside, hopefully, you’ll be able to understand to some degree here, why supplementation of creatine before intense bouts of exercise can enhance performance by increasing the levels of circulating PCr and free creatine…

Glycolysis

If the Phosphagen system is unsustainable, then the body requires a longer-term solution to providing energy in the absence of oxygen. A quick reminder that since there is no oxygen present, these exercise bouts must be fairly short and intense. This is where glycolysis comes in. The name literally means ‘breaking down glucose’, which gives us an idea of the main substrate needed.

The glycolysis pathway is incredibly long and too much hassle to go into too much depth here, but feel free to give it a search on your browser. A short summary would look something like the following:

1.     Glucose or glycogen is eventually converted to another molecule called pyruvate.

2.     Pyruvate can then either be converted to lactate by the enzyme called lactate dehydrogenase, or;

3.     Pyruvate is shuttled into the cell mitochondria.

It is this conversion to lactate, not lactic acid, that has for so long been associated with burning muscles and exercise induced fatigue during intense exercise. Simply put, it cannot be lactic acid, or even lactate that causes this burning sensation - and this is why:

Yes – during bouts of high-intensity exercise, lactate concentrations are increased. However, lactate is the innocent party. At the same time, proton (H⁺)  concentrations are also elevated. Protons, by nature, are associated with acidity (acids are chemical compounds which are able to donate protons). This affects a series of processes, including slowing the rate of the reactions in the glycolytic pathway, inhibiting calcium biochemistry, and/or the cross-bridge recycling during muscle contraction. The net result is metabolic acidosis – which is now the preferred term for the reduction in skeletal muscle pH during intense exercise. Here’s the real clincher: research now shows that these protons don’t even originate from lactic acid. It is much more likely that the protons are from the other biochemical reactions during energy production, including ATP hydrolysis (the first equation). In fact, the conversion of pyruvate to lactate actually mops up protons, doing the exact opposite of what we once thought.

So, if not lactic acid, what is causing the burning sensation and fatigue during exercise?

The series of reactions that take place to produce energy and resynthesise ATP result in an accumulation of metabolites. These include: AMP, ADP, inorganic phosphate, lactate, and free radicals. It is know that these metabolites can impair neural drive, muscle excitation and contraction and also the energy supply itself. It is therefore likely that these metabolites are responsible for your muscular fatigue during exercise itself.

When it comes to muscle soreness after exercise, there is a different answer. Delayed onset muscle soreness (DOMS) is that familiar struggle to get down the stairs, put on a t-shirt, or get up and off the toilet a day or two after a gym session. Again, lactic acid is not the culprit, as it is quickly mopped up after exercise. Instead, the soreness is caused by actual structural damage to the muscle itself. Nothing serious, just micro-trauma. Incidentally, these small muscular tears are strongly associated with eccentric muscle contractions, such as downhill running on the quads, or the downwards phase of a push-up for the chest. It is this micro-trauma which is essential for the growth of new muscle fibres, as should not be seen as an injury or reason not to exercise.

So there it is. Next time you think of blaming lactic acid for all of the burning, aching and general difficulty during or after your exercise, think again. It is just the innocent bystander in this cascade of reactions, and is actually trying to help you more than hurt you!