What is lactic acid and what does it have to do with running? We’re exploring everything you need to know about lactic acid, lactate threshold training, and why it matters for running (along with self tests to determine your lactate threshold!)
When you hear the words lactic acid you might be thinking about certain ingredients in food. While some foods do contain lactic acid, the kind we’re referring to isn’t an ingredient in your snack: it’s a metabolic byproduct in your body.
Lactic acid is a natural product of strenuous anaerobic exercise, and your lactate threshold can help determine when you start to fatigue when you exercise hard.
We’re going to explain what lactic acid is, what a lactate threshold is, and how it affects your running.
What is lactic acid?
Lactic acid is natural byproduct of metabolic processes in your body called cellular respiration. Cellular respiration is provides the energy you need for daily life and exercise.
During daily living and submaximal exercise (exercise that is not difficult to maintain a steady pace), your body converts glucose and oxygen into energy with aerobic respiration. Byproducts of aerobic metabolism are heat and carbon dioxide.
When exercise intensity increases and the amount of oxygen your muscles demand to keep moving can no longer be met, your body switches gears to anaerobic respiration that doesn’t require oxygen, but the byproduct of anaerobic respiration is lactic acid.
Lactic acid inhibits energy production when it build up in your muscles. When your muscles produce more lactic acid than can be removed, lactic acid begins to accumulate, causing fatigue.
Once your oxygen consumption returns to normal, lactic acid is then converted to other compounds by your liver to use in regular aerobic respiration.
Fatigue can be caused by a variety of other reasons, too, but fatigue from lactic acid buildup is more likely the cause if you’re exercising at an intensity above aerobic capacity.
Takeaway: when your body’s oxygen consumption exceeds demands, lactic acid is produced, resulting in fatigue.
Scientists have also discovered that different muscle fibers produce different amounts of lactic acid as well, so it’s not just cellular respiration/metabolism that comes into play.
Fast twitch muscle fibers (the ones used in activities like sprinting) produce more lactic acid in the muscle cells than slow twitch fibers do (slow twitch muscle fibers are the ones that are used mostly for longer and slower endurance exercise.
So activities that use more fast twitch muscle fibers than slow twitch muscle fibers will result in more lactate buildup. Another reason sprinting causes fatigue faster than an easy jog!
Lactic acid and muscle soreness
People used to believe that muscle soreness was caused by residual lactic acid that didn’t leave the muscles adequately.
But new science shows that muscle soreness is actually caused by micro-tears in your muscles and has little to do with lactate build up at all.
If you’re sore, it can be helpful to do gentle movement to help increase blood circulation and speed healing of muscle tissue, but it doesn’t necessarily move lactic acid out of sore muscles as once believed.
What is lactate threshold?
Lactate threshold is the point where lactic acid begins to accumulate in your muscles – your body switches from aerobic respiration where lactic acid is easily moved out of the muscles, to anaerobic respiration where lactate builds up.
During exercise, we’re never really in just one type of cellular respiration, there’s always a little bit of anaerobic metabolism going on along with aerobic metabolism, but it’s usually pretty small.
Once we reach our lactate threshold, the majority of our cellular respiration switches to the anaerobic kind that doesn’t use oxygen and we start to fatigue.
On average, regular people reach lactate threshold at 50-60% of their VO2 max, recreational athletes reach lactate threshold at about 65-80% of their VO2 max, and elite endurance athletes reach their lactate threshold at 85-95% of VO2 max (source).
Why does lactate threshold matter for runners?
Lactate threshold matters to runners because, similar to VO2 max, the higher your lactate threshold, the better your running economy, speed, and exercise recovery.
Less conditioned runners or exercisers have a lower lactate threshold. So a person who is new to exercise will fatigue faster than someone who is well conditioned.
The well conditioned runner will be able to run at a higher intensity with less fatigue than their less conditioned counterpart (barring things like medical conditions). So increasing your lactate threshold through strategic planning can help you become a faster and more efficient runner!
How to test your lactate threshold
Knowing what your lactate threshold is can help your training for a couple of reasons:
- It allows you to develop a strategic training plan that helps you train smarter
- You can track your progress over time
When you exercise above an easy and resting level, your body goes through a couple different ventilatory thresholds (times when your breathing noticeably changes): the first ventilatory threshold (V1) and the second ventilatory threshold (V2).
V1 occurs when your breathing rate increases and you can’t talk comfortably, but you can still put several words together while exercising.
V2 occurs when lactate begins to accumulate faster in your body than can be removed, and V2 threshold is considered your lactate threshold. Breathing becomes rapid and heavy, and you can no longer speak during exercise.
Traditional testing of lactate concentration in your blood was pretty invasive: doing blood draws at intervals during an exercise test. No thanks! But there are ways to estimate your lactate threshold without the blood draws.
Doing your own lactate threshold testing
There are a few different ways that you can determine your lactate threshold (LT) without going to a lab for testing:
- Using an online pace calculator to estimate your LT based on your fitness level and recent race times (this is the easiest method).
- Estimate your LT using your heart rate and the Karvonen formula
- Complete a 30 minute time trial to estimate your lactate threshold
There was once a device called the BSX Insight that promised to read lactate concentrations through your skin, but this device is discontinued and no longer on the market.
Estimating your LT using heart rate
If you know your resting heart rate, you can use the widely recognized Karvonen formula. This formula is helpful if you haven’t done a maximal exercise test to already determine your max heart rate, this helps to estimate it for you using your resting heart rate and age.
THR = [ (MHR – RHR) x % intensity] + RHR
MHR = 208 – (0.7 x age)
- THR = Target heart rate
- MHR = Max heart rate
- RHR = resting heart rate
Heart rate training zones in percentages:
- Recovery zone or aerobic training: 50-70% MHR
- Tempo and threshold runs: 71-85% MHR
- Intervals: >85% MHR
So if you’re a 25 year old person with a resting heart rate of 70 bpm and you want to do lactate threshold training at 80% MHR, here’s how that formula would look:
MHR = 208 – (0.7 x 25) = 190.5
THR = [ (190.5 – 70) x 80%] + 70
THR for 80% MHR training in estimate LT training zone = 166
30-minute time trial
You can also estimate your lactate threshold by doing a 30 minute time trial. Ideally you’d do this time trial when you’re not fatigued by heavy exercise the day before. Use a level track, level road, or a treadmill set at 1% incline and use a heart rate monitor.
Start by doing some light cardio activity for 10 minutes, like walking or an easy jog, to warm up your body and prepare it for exercise. Gradually Increase the intensity of your warm up.
Once you’re warmed up, start running at the fastest speed you can maintain for 30 minutes. This needs to be a steady pace over the course of the 30 minutes: make sure you don’t start out too fast and taper off as you get tired.
Measure your heart rate at 10 minutes into your run.
Continue your run and stop at 30 minutes and measure your heart rate.
Add your heart rate at 10 minutes to your heart rate at 30 minutes and divide by 2 to find the average. This is an estimate of your lactate threshold heart rate.
For example: if your heart rate after 10 minutes is 130, and your heart rate after 30 minutes is 170, then your LTHR (lactate threshold heart rate) would be 150 bpm.
Using your LTHR, you can determine training zones (remember, LTHR is not the same as max heart rate in these training zones!):
- Recovery Zone (Zone 1) = 65-85% LTHR
- Long endurance (Zone 2) = 85-90% LTHR
- Intense Endurance (Zone 3) = 90-95% LTHR
- Lactate threshold (Zone 4) = 95-102% LTHR (or same heart rate as time trial)
- Maximal/power training (Zone 5) = 102-110%
How to increase your lactate threshold
One great thing about your lactate threshold is that you can improve it over time with training, much like you can improve your VO2 max with strategic training.
There are 2 ways to improve your lactic acid threshold and how efficiently your body removes lactic acid from your muscles:
- Continue your aerobic steady state training
- Add in lactate threshold training 1-2 times a week
Continuing with your regular steady state aerobic training runs is important for lactic acid clearance because your body adapts to this type of exercise by increasing the number and density of mitochondria in your muscle cells.
Mitochondria are your cells power generators so to speak and where your cellular respiration takes place. The more mitochondria you have, the better your body becomes at using oxygen during exercise (meaning less lactic acid buildup at the same heart rates as before).
Another way to increase your lactate threshold is to add LT training 1-2 times a week into your training program. LT training sessions are higher intensity, so don’t do them too frequently and keep them fairly short, 20-30 minutes long.
The goal with lactate threshold runs is to keep your pace pretty steady throughout the run meeting that heart rate goal or pace goal consistently.
LT training is generally a bit below race pace and should be comfortably hard. They should be challenging but not maximal intensity like VO2 max training. This is where the pace calculators are handy since they help you determine at what pace you’d likely be LT training based on recent race times.
Now that you know how to determine your lactate threshold and how to improve it, you should be able to make improvements in your race times with these strategies!
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