If you’ve ever exercised at high intensity, you’ll be familiar with that sensation of fatigued muscles. It can feel uncomfortable, but the cause – lactic acid – is actually your ally, helping you move faster and lift heavier.

When it comes to athletic performance, lactic acid has historically been viewed as the enemy – the culprit behind DOMS (Delayed Onset Muscle Soreness) and fatigue. We think of it as a waste product that holds us back and prevents us from achieving our best. But what if, rather than hindering your performance, the production of lactic acid in fact makes you a better athlete?

Here’s the lowdown: when we do strenuous exercise, we breathe faster in order to transfer more oxygen to the working muscles. In most cases our bodies naturally prefer to generate energy using the aerobic system (meaning, “with oxygen”). However, when our bodies are under stress – trying to lift heavy weights or perform fast sprints – we switch to the anaerobic system (“without oxygen”) to produce this energy. When this happens, the body produces a substance called lactate which allows the breakdown of glucose – and the production of energy – to continue.

But it’s complicated. Higher blood lactate levels actually slow down the muscle’s capacity for more work. If it seems counter-intuitive that the body would produce something that actually reduces its ability to perform, it’s not. It turns out that lactic acid is a natural defence mechanism that prevents us from over-doing it … and doing ourselves permanent damage.

We caught up with exercise scientist Associate Professor Andrew Kilding to find out more about this vital but often misunderstood aspect of exercise.

SARAH SHORTT: Why does lactic acid get a bad rap?

ANDREW KILDING: The accumulation of lactic acid in the muscles has long been incorrectly associated with fatigue during exercise, as well being linked with delayed-onset muscle soreness (DOMS). Even today you’ll hear sports commentators saying, “athlete X must be fatiguing/tiring because of lactic acid build up”. We know now that this is not the case, as lactic acid has no direct role in causing these exercise-related symptoms.

More recent schools of thought consider that lactate is no longer a so-called “harmful waste product”, but rather is a supplemental fuel – it has been referred to as the “forgotten fuel”. Lactate produced during exercise can be used as a fuel source both during the exercise itself, depending on intensity, and during rest. The human body is extremely efficient and can recycle produced lactate for oxidation in the heart and brain.

What are the benefits of lactic acid?

The production of lactate serves to reduce acidity in the blood and muscle in an attempt to maintain an optimal pH level in the muscle, and to allow the muscle to keep contracting at high rates. However, this “buffering” can’t last forever, so when pH in the muscle starts to drop and hydrogen ions accumulate, this is when the sensation of “burning” is felt as the disruption to the muscle’s ability to contract starts to occur.

Lactate also helps to preserve other fuel stores and is a direct source of energy for the muscles, heart and brain. The body is efficient at re-using lactate and can even “shuttle” lactate to different parts of the muscle and between tissues.

From a training perspective, lactate has been viewed as an important “signalling molecule” for promoting adaptation. What I mean by that is, the production of lactate during exercise triggers a series of metabolic changes that will enhance the ability of the muscle to oxidize it.

How does it contribute to exercise performance?

It depends on the sport or exercise. For endurance-based sports you want to minimize the production of lactate and be able to clear it quickly. Endurance cyclists and runners are the best at doing this because they typically have a high proportion of well-conditioned slow twitch “oxidative” fibers containing lots of mitochondria and oxidative enzymes. These help to produce aerobic energy without the accumulation of lactate.

Short duration/power athletes, however, often have more fast twitch “glycolytic” fibers, and these fibers will naturally produce high amounts of lactate so they can perform high-intensity movements such as sprinting.

Which types of exercise are best for producing lactate?

Exercises that involve the upper limbs typically induce higher levels of lactate, compared to the legs, as upper body muscles typically have a greater number of fast twitch fibers, which are predisposed to glycolytic energy production.

However, this is not a reason to only perform upper body exercise. Rather, you should aim to perform exercises that recruit the large muscle groups relevant to the sport or exercise goals you have – utilizing the correct work-to-rest ratios.

How can you increase the capacity of your lactic system?

You can enhance the capacity of the lactate producing system by performing repeated bouts of sustained 20 to 60 second supra-maximal exercise – exercise performed above 100 percent of your maximum fitness capacity. This maximally stimulates the glycolytic system and, in doing so, challenges the enzymes that limit glycolytic energy production.

You will need to provide lots of recovery between repetitions to allow full recovery of the system so that it can be taxed heavily again – work-to-rest ratios of 1:4 to 1:8. Too little recovery in these types of sessions will mean the aerobic system will be increasingly engaged, and it’s unlikely you will get the glycolytic stimulus you were aiming for.

Do better athletes produce more or less lactate?

In Olympic sports, two athletes who are quite physiologically different can achieve the same performance level but in a different way. For example, one may have a slightly lower ability to produce energy using the glycolytic system, and would likely have lower lactate levels, but can offset this by having a slightly higher aerobic ability that allows them to sustain a given power.

It’s hard to say if more is better from a performance perspective, as each individual has a unique set of physiological/metabolic machinery that they’ve “tuned” through habitual training.

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