Lactic Acid – Help or Hindrance?

I had a great question last week as a client described getting that familiar quad burn during an exercise class: “What is lactic acid and what purpose does it serve” was the basis of the question. What a great topic to research a bit further and explain, as I reckon we’ve all felt “the burn” and resultant muscle fatigue at different stages of exercise.
Traditionally, lactic acid has negativity written all over it – the enemy of the athlete due to the effect it has on our performance – the burning tending to correlate with a deterioration in our performance, a heaviness of the muscles, and the potential for the infamous “DOMS: Delayed Onset Muscle Soreness”. Whilst these beliefs stem from research conducted in the 60’s and 70’s (and even earlier!), lactic acid is now understood to be an important fuel source that actually helps to support exercise performance.
Firstly, it is important to have a basic understanding of some of the different energy systems that muscles use to generate power. For sustained exercise, the muscles use aerobic power meaning that carbohydrate (or fat at lower intensities) is burnt in the presence of oxygen to provide energy. As intensity increases, at some point oxygen cannot be burnt quick enough to fuel this reaction and the muscle contractions become anaerobic (without oxygen). This anaerobic process occurs with 30 – 60 second sub – maximal sprint efforts, or for intermittent explosive movements. This is the system where lactic acid is produced, known as anaerobic glycolysis. In the laboratory we can exercise people through progressive levels, and can determine the ‘turning point’ where aerobic intensity is ‘maxed out’, and the anaerobic or lactate system takes over. This known as the Lactate threshold and occurs when the amount of oxygen going into the muscles equals the amount of carbon dioxide being expelled. Well trained endurance athletes are very adept at competing just below this threshold, where the lactate can be cleared at the same rate at which it is produced.
The idea that lactic acid is responsible for fatigue during exercise or muscle soreness has actually been scientifically refuted in a paradigm shift that began in the mid 80’s! So why the confusion? Yes, lactate is a by-product of anaerobic metabolism that, if not cleared, means our exercise intensity cannot be maintained. But it is not the lactate per se that causes the burn, fatigue or soreness. When the anaerobic energy system predominates, hydrogen ions (acid) and other metabolites are also produced. It is this acidification of the muscles that reduces their ability to contract, bringing on fatigue and burn, and preventing the athlete from maintaining such a high intensity. Well trained athletes have a strong ability to clear hydrogen ions and other metabolites in the hardest working muscles.
The lactate on the other hand which is being produced anaerobically by one muscle fibre type (fast twitch fibres) can be oxidised by other muscle fibre types (slow twitch) aerobically within the same muscle. This process, known as “shuttling”, can occur in other ways – the lactate can be taken up by the blood and used by other muscles and organs (including the brain and heart) and the liver can take it up and convert it back into glucose.
The aerobic process, which needs oxygen to turn glycogen into energy, is an efficient way to produce energy. The two fold problem is i/ you need enough oxygen input over time, and ii/ you need enough glycogen stored in the body. When we train to levels of oxygen depletion and lactate starts to build (at or below our lactate threshold) this is a trigger for the body to adapt to the oxygen debt and improve the muscles ability to then utilise that lactate for energy – in other words, by deliberately training at the limits of aerobic metabolism, our lactate threshold occurs at higher intensity and our muscles become more aerobic.

Ideally, especially in endurance based sport, you want to minimise the production of lactate and be able to clear it quickly, and continue using oxygen as your source of energy. Endurance cyclists and runners are usually the best at doing this because they tend to have a high proportion of slow twitch “oxidative” fibers that are very oxygen efficient. The more oxygen efficient, the more energy produced with less accumulation of lactate.
Short duration/power athletes, however, often have more fast twitch “glycolytic” fibers, and this muscle type will produce high amounts of lactate so they can perform high-intensity movements such as sprinting without relying on oxygen as much.
To add to the complexity, fat is another source of energy utilised by athletes, and to use fat we need oxygen. The great thing about fat stores is they are pretty much an endless source for many athletes during endurance events, but the process is not quite as efficient as the breakdown of glycogen. Many endurance athletes will train in specific ways to try and become more efficient at burning fat, so they can utilise this when the glycogens stores get low, which is usually around 2 hours into intensive exercise.
So how to you manipulate this system to improve performance?
High intensity interval training is well known to challenge aerobic metabolism and raise the lactate threshold (among other benefits to the working muscles, including maximising muscle fibre recruitment). You can however have too much of a good thing. If you perform too many high intensity interval sessions and not enough slower or moderate intensity workouts, you are actually not effectively training your body’s ability to recycle the lactate or remove the hydrogen ions and other metabolites produced. This is the reason (at least in part) that elite athletes perform 80 – 90% of their workouts at low to moderate effort, with only a small fraction of workouts at high intensity.
The length and frequency of intervals to be performed will depend on what an athlete needs to focus on, whether that be improving oxygen uptake or lowering lactate response. Ideally, having a VO2 max test can inform this approach. Coaches can be very good at advising on the best approach, whether that be shorter intervals 30 – 60 seconds with longer recovery or longer intervals of 2 – 4 minutes with shorter recovery. As a guide for athletes with a good fitness base wanting to add some interval training to their program, it may be beneficial to add in one high intensity session per week of 4 x 4 minute intervals with 2 minutes rest. The pace of the interval should be what can be maximally sustained during the interval and repeated consistently. This training program is progressed by adding intervals (not greater speed).

This article was co-written by Chris Rauch and Rob O’Donnell.
Rob O’Donnell is an SSPC Physio/Director with multiple Australian distance running representation both personally, and professionally as physio; Chris Rauch is our resident Clinical and Sports Dietitian at SSPC. Outside of SSPC he conducts laboratory exercise tests at Monash University with athletes of all levels from recreational to elite level. Whilst he does not give direct training advice, the above guidelines are based on research and laboratory findings.