Lactic Acid in Runners is normally misunderstood. Lactic Acid is perceived by many runners as something evil to be avoided, holding us back by making our legs hurt and slowing us down. This isn’t strictly true and what we as runners really need to do is get good at producing and reusing as much Lactic Acid as possible. In this article I’ll explain why and how Lactic Acid in runners occurs, why this process is immensely useful and how you can train to take best advantage of that Lactic Acid.
All of this will be explained in the context of how the body produces energy (essential for running!) and some of the article goes into the details, so for those that just want the high level messages, here they are:
- Your body produces Lactic Acid all the time as part of energy production.
- It happens a lot when the runs get tough because the process outputs new energy really fast.
- Your body reuses the Lactic Acid to create even more energy.
- The real name for Lactic Acid in the process is Lactate, but no one really cares.
- The burning sensation in your legs during a fast run is not caused by Lactic Acid! Read the details to find out the real cause.
- Runners can train their bodies to get really good at producing Lactic Acid by running near Lactate Threshold pace.
- Runners can train their bodies to get really good at reusing Lactic Acid for fuel by running at aerobic base pace. You need to do both.
What does Lactic Acid have to do with running?
For your muscles to do running (or anything), they need something called ATP (Adenosine Triphosphate) which makes them contract, so your limbs are forced to move at the joints. The speed at which this happens is mind boggling, as is the speed your brain can control it all, but that’s another topic. There are three main processes to produce ATP and they all happen, all the time, just in different amounts depending on the intensity of effort level. One of these processes also produces and uses Lactic Acid.
- Aerobic energy production.
- Anaerobic energy production, which is when the Lactic Acid stuff happens!
- Why Lactic Acid is unfairly blamed for sore and tired muscles.
- Alactic energy production.
- What happens to the Lactic Acid when you stop running?
- Does Lactic Acid make my legs sore the next day?
- How do I get better at making and reusing Lactic Acid?
Aerobic energy production
At low effort levels, the main source of ATP is when the body breaks down both carbohydrates and fats, using oxygen, so it’s called “Aerobic”.
Aerobic energy release is slow and happens mostly in the slow twitch muscles, but does create a lot of ATP per amount of fuel provided.
Anaerobic energy production
When you run harder, your body decides to make ATP much faster, so whilst continuing to use the Aerobic process, the body increases the amount of a second process called Glycolysis. Glycolysis works without Oxygen, so it’s referred to as Anaerobic Glycolysis.
Note that the body doesn’t decide to ramp up Anaerobic Glycolysis because there isn’t enough Oxygen – there’s plenty of that. Anaerobic Glycolysis only produces a fraction of the amount of ATP when compared to the Aerobic option, but it happens so fast that the actual output of ATP per second is greater. More ATP per second is required, so is the reason production of ATP using Anaerobic Glycolysis is scaled up when you run faster.
Anaerobic Glycolysis breaks down Glucose (carbohydrate fuel stored in the muscles with water as Glycogen), to release ATP, mainly in fast twitch muscle fibres, more of which are being “recruited” when you pick up the pace, so it’s targeting the right bits of the muscles too.
Apart from the ATP, one of the resultant chemicals of Anaerobic Glycolysis is technically Lactic Acid, but is only present in the body momentarily, because at normal body acidities, Lactic Acid immediately donates one of its Hydrogen Ions to become Lactate. However, it did collect another Hydrogen Ion along the way which is important.
When people talk about Lactic Acid and exercise, they really mean Lactate, but they are used so interchangeably that no one cares any more.
Note that Hydrogen ions are also being produced in other processes in the body too, for example in Aerobic energy release in slow twitch muscles, but because Oxygen is present in that process, they get removed as water.
Those Hydrogen Ions make muscle cells more acidic (depolarised), and if left to build up inside the cells, muscles would be less able to contract properly.
To keep the muscles moving properly, the body transfers the Lactate (transporting its collected Hydrogen Ion) to slow twitch muscle fibres nearby and then on via the blood, to the Liver, which uses Lactate as part of Glucose production (Gluconeogenesis). That Glucose is used again by both Aerobic energy production and Anaerobic Glycolysis, so the cycle restarts, and you can keep going for longer. Which is nice. Two issues sorted in one. It’s called the Cori Cycle.
Why Lactic Acid is unfairly blamed for sore and tired muscles
We learned above that Lactic Acid in runners (actually Lactate) is produced from Glucose during hard efforts as part of the Anaerobic energy system and is then sent to the Liver to be turned back into Glucose.
However, if you run fast enough, your muscles do even more Anaerobic Glycolysis (because it’s quicker at releasing ATP than relying just on Aerobic energy production), so Hydrogen Ions get released even more quickly and Lactate is produced more quickly to help transport the Hydrogen Ions away.
Once the effort level reaches the point where the Liver is unable to create more Glucose quickly enough, the Lactate and Hydrogen Ions have nowhere to go, so the excess Hydrogen Ions stop the muscles contracting properly, eventually to the point of not contracting at all if you carry on. This is your muscles failing to respond when you tell them to go faster towards the end of your 5K race.
If you push this effort level, the excess Hydrogen Ions also start interfering with nerve endings, so you might experience a sensation in the legs, commonly referred to as “burning”. So you’ll note that it’s not the Lactate itself causing the burn, but actually the Hydrogen Ions, which the Lactate is valiantly scooping up as quickly as it can.
Alactic energy production
The third energy system relies on ATP and Creatine Phosphate that is stored in the muscles “for emergencies”. If you run in a flat out sprint, ATP sources are used up in two seconds and Creatine Phosphate recreates ATP inside the cells without external help for another few seconds until all the Creatine Phosphate is used up. This energy system thus lasts up to around eight seconds and because no Lactate is produced is called Alactic (“Without Lactic”)
What happens to the Lactic Acid in runners when you stop running?
When you slow down sufficiently, any spare Lactate and Hydrogen Ions start getting sent to the Liver and are used, along with other fuel sources, during more Gluconeogenesis to create new Glucose as the body gets back to normal energy production levels from all three processes – mostly Aerobic, a small amount of Anaerobic Glycolysis and a tiny amount of Alactic.
When you rest, that Gluconeogenesis continues making new Glucose from the spare Lactate and Hydrogen Ions (and other fuel sources) until the Lactate and Hydrogen Ion are cleared.
Glycogenesis also starts up, taking all that new Glucose and converting it all to Glycogen chains to be stored in the muscles, topping them up over a few days depending on how long you ran for, which is why you can’t run fast every day for more than a few days before being at huge risk of injury.
The terminology all gets a bit confusing, so just to recap:
- Glycolysis: getting ATP from Glucose without Oxygen, releasing Lactate and Hydrogen Ions. Happens all the time, but happens a lot while running hard.
- Gluconeogenesis: making new Glucose from Lactate and Hydrogen Ions, as well as other fuel sources, in the Liver. Happens all the time.
- Glycogenesis: getting any Glucose and turning it into Glycogen chains stored in the muscles. Happens while resting.
Does Lactic Acid make my legs sore the next day?
The way your legs feel after a few hours is not related to the acidity from the Hydrogen Ions or Lactate. Instead, any ache or soreness the next day is due to the micro-tears in your muscles. These heal over time and make the legs stronger, if you give them enough recovery. Lactic Acid in runners does not make your legs sore the next day.
How do I get better at making and reusing Lactic Acid?
For most runners, any race longer than around three to four hours is going to be at aerobic base pace to be in with a chance of finishing well. This predominantly uses the Aerobic energy system. The effort level associated with that pace is commonly called Zone2 (Z2) – an easy, conversational effort. The harder the effort, the higher the Zones go, so Z3 is harder/faster than Z2. You can find more about these and calculate your heart rate zones here. If you are going to run harder than Aerobic Base pace in a race, for example in a 5K or a 10K race, you are going to be using the Anaerobic energy system as well. Therefore to get faster, you need to stimulate that energy system in your training.
The three main aspects of the Anaerobic energy system, using Anaerobic Glycolysis, and how to improve them are:
- Producing ATP energy without Oxygen present in the process. This is improved most by training at Lactate Threshold somewhere in Zone 4 / Z4.
- Passing the Lactate and Hydrogen Ions from the fast twitch fibres to the slow twitch fibres. This is improved during threshold running, but can also benefit from Fartlek training where you push past the lactate threshold for a while, then drop below it again for a while and then repeat.
- Clearing the Lactate and Hydrogen Ions from the slow twitch fibres to the blood so that the Liver can recycle the Lactate as part of Glucose production, and to prevent the Ions causing the burning sensation. Oddly, this is improved most by training at Aerobic base in Zone 2 / Z2.
That’s a whole lot of words to basically say you need to incorporate two different types of running into your training to get faster, but now you know why. Hopefully, the facts are all presented correctly, but if any scientists are reading this and think I need to correct / add / remove anything, please let me know!
Another way to get faster is to improve your running form. Check out our running technique course that shows you how.
