What happens to your body when doing interval training?

Classic interval training builds speed. When performed, a lot of lactate or lactic acid is produced. What kind of “beast” is this and how to handle it correctly?

Lactate is lactic acid, which is actively produced during intense exercises. You feel lactate when your muscles start to burn. The more lactate is produced, the more it burns. It is the acid that creates a burning feeling in the muscles.

With increasing load, the concentration of lactate in the blood and muscles increases. The body can successfully process lactate and turn it into energy on the go. But until a certain point. After it, the processing of lactate can’t keep up with the speed of its creation and it begins to accumulate. The burning in the muscles intensifies. This moment corresponds to the specific heart rate level called the threshold heart rate.

The threshold heart rate is different for everyone, but the level of lactate that corresponds to it is always the same. This is about 4 mmol per 1 liter of blood. On this lactate level, you can continue training for a long time without any problems. But once lactate level continue to grow above 4 mmol/l you will find it harder to continue with workout. At this point, the muscles most often start reaching the limit of saturation with lactic acid and refuse to continue running. This means the end of the workout. You just can’t run any further.

The threshold HR is the maximum intensity level (pace or power) that you can hold for 1 hour — 1 hour 30 minutes, depending on your fitness level. You will be able to move for longer, but much slower. For those who run a half marathon faster than 1 hour and 30 minutes, the average heart rate for the distance will be approximately equal to the threshold HR.

The threshold HR in cycling is different from running, but it has the same meaning. This is the heart rate and power you can sustain for 1 hour.

You can continue running for a significant amount of time at the threshold HR. The higher your heart rate is above the threshold, the higher your lactate concentration, and the less time is left until you are done. The heart rate zone right above the threshold is VO2Max. You can run for about 2–3 minutes on this HR level. On top of VO2Max zone, there is a maximum heart rate. Most people are able to stay in this zone for 10–20 seconds, reaching the maximum of their capabilities, touching the maximum HR and fully stop. At this point, the concentration of lactate is maximum.

You can accurately determine the level of lactate during a workout using a lactometer or a gas analyzer. Professional athletes, when performing interval training, measure this value by taking blood from the earlobe or finger with the help of medical personnel. Amateurs can occasionally measure the lactate during functional testing with a gas analyzer.

There are several ways to find out your heart rate at the threshold. The Conconi test consists of running intervals of a certain length and intensity. Alternatively, you can 30-minute run at a maximum pace. Here the average heart rate for the last 20 minutes will approximately correspond to the threshold level. The FTP Test for cyclists will also show your threshold power level. It is 95% of the average power of 20 minutes of maximal pedaling interval.

A less accurate, but the most accessible way for most people to estimate threshold HR, is to plug the numbers into the formula. Knowing your resting heart rate (morning heart rate after waking up) and the maximum heart rate (it can also be estimated using the formula), you can determine the approximate heart rate zones and, accordingly, the threshold heart rate.

We estimate your threshold heart rate based on the data that you upload to the app: maximum heart rate, training data, etc. If any data is not correct, update it in the Settings section of the app. The accuracy of your individual training tasks depends on this.