Why HIIT Can Provide Quick Training Effects | Hardworkout Magazine

Key takeaways

- HIIT consists of short work periods at high intensity, broken up by rest or periods of lower pace.

- At very high intensity the body relies more on blood glucose and muscle glycogen than on fat, because carbohydrates can be converted into energy faster.

- Anaerobic glycolysis raises the production of lactate and hydrogen ions, which affects pH in the muscles and limits how long very high intensity can be sustained.

HIIT stands for high-intensity interval training, or high-intensity interval training. The training form consists of short work periods of high intensity interrupted by breaks or periods of lower pace. Although both HIIT and steady-state cardio can improve fitness, they affect the body in different ways.

At high intensity, the demands for both energy production, oxygen transport, and circulation increase much faster than with steady-paced training. Therefore, relatively short interval sessions can provide clear adaptations in both the heart, blood vessels, and muscle cells.

What Distinguishes HIIT from Zone 2

What is the difference between HIIT and zone 2 training?

During low-intensity endurance training, most of the energy is produced aerobically, that is, with the help of oxygen.

The intensity is low enough for the body to maintain energy release over a long period without significant changes in the acid-base balance in the muscles.

In high-intensity intervals, the energy demand increases much more rapidly. The muscles need energy at a much higher rate. Therefore, anaerobic energy production contributes more in addition to aerobic production. The body breaks down more glycogen to meet the demand.

At the same time, the production of lactate and hydrogen ions increases. Therefore, very high intensity can only be sustained for short periods at a time.

There is also an important difference between HIIT and zone 2 training. During lower-intensity endurance training, the body can keep energy release relatively stable over a long time, while high-intensity intervals cause large and rapid changes in both energy demand and oxygen consumption.

[Harvard](https://nutritionsource.hsph.harvard.edu/high-intensity-interval-training/) describes HIIT as interval training where the intensity often lies around 80-95 percent of maximum heart rate during work periods, followed by shorter periods of lower intensity or rest.

High Demands for Oxygen Transport

How does high intensity affect oxygen transport and VO2 max?

As intensity increases, the muscles require more oxygen to maintain the pace.

The heart therefore increases both the pulse and the amount of blood pumped out per minute. At the same time, the blood vessels must expand so that oxygen is transported efficiently to the muscles.

VO2max describes the body's maximum ability to take up, transport, and use oxygen during physical activity. The higher this capacity, the greater the work capacity of the body at high intensity.

High-intensity intervals are often used to improve this capacity because large parts of the training are conducted close to the maximum oxygen uptake.

An [overview article](https://www.mdpi.com/1660-4601/18/13/7201) published in the Journal of Cardiovascular Development and Disease in 2025 found that HIIT can improve VO2max, endothelial function, and several markers related to cardiovascular health.

The researchers point out that high intensity can improve the blood vessels' ability to expand when the need for oxygen increases, as well as strengthen the heart's pumping capacity.

This is an important reason why high-intensity intervals often improve fitness faster than longer endurance sessions at lower intensity.


What happens inside the muscle cells during a HIIT session?

High-intensity intervals affect how muscles produce energy during exercise.

ATP (adenosine triphosphate) is the direct energy source that drives muscle contraction, but the stores in the muscles are very small. Therefore, the body must produce new ATP very quickly when the intensity becomes high.

At low and moderate intensity, most of the energy production occurs aerobically with the help of oxygen. Fat contributes more as fuel because the body has enough time to produce energy at a slower pace.

When the intensity increases significantly, the need for ATP rises faster than fat burning can cover.

The body therefore becomes more dependent on glucose from the blood and glycogen stored in the muscles. Carbohydrates can be used more quickly for energy production than fat. Therefore, they become the primary energy source at very high intensity.

Then anaerobic glycolysis contributes more to ATP production. Glucose and glycogen are rapidly broken down to produce energy. At very high intensity, aerobic energy production alone cannot meet the energy demand quickly enough.

This anaerobic process simultaneously leads to increased production of lactate and hydrogen ions.

Eventually, this affects the pH balance in the muscles and contributes to the feeling many describe as lactic acid.

An [academic article](https://link.springer.com/article/10.1007/s42978-025-00352-w) published in Sports Medicine and Health Science in 2025 describes among other things how HIIT can affect mitochondrial function and improve muscle cells' ability to use glucose effectively during exercise.

Mitochondria account for large parts of the aerobic energy production in the cells. When their capacity improves, the muscles become better at maintaining energy production as intensity increases.

Why can short sessions give results?

An [overview article](https://www.mdpi.com/1660-4601/18/13/7201) published in the International Journal of Environmental Research and Public Health points out that lack of time is a common reason why many struggle to maintain regular exercise.

Researchers also show that short interval sessions can improve both fitness, blood sugar regulation, and insulin sensitivity even with relatively low total training time.

At high intensity, the body must produce energy and transport oxygen much faster than during lower intensity endurance training.

Therefore, relatively short sessions can provide clear physiological adaptations even if the total training time is lower than with many longer endurance sessions at moderate intensity.

Lower intensity endurance training also makes it easier to complete longer sessions and achieve a high total training volume without as great a need for recovery.

How much high-intensity training is appropriate will vary with training background, total training volume, and how high the intensity actually is.

How HIIT Can Be Varied

HIIT is not about a specific type of training, but about how the intensity is managed.

How long should the intervals be?

The principle can be applied in everything from running and cycling to circuit training, rowing machines, and sessions that combine strength and conditioning. How the intervals are structured also affects what adaptations the training provides the most.

- Short intervals of 20-30 seconds allow for very high intensity and are often used to develop explosive capacity, anaerobic energy production, and speed.

- Longer intervals of 3-4 minutes keep the body close to high oxygen uptake for a longer time and are often used to improve endurance and VO2 max.

- Circuit training with short breaks can combine high heart rate with strength exercises and places greater demands on both muscular endurance and conditioning at the same time.

The choice of activity also affects how the training feels for the body.

Running places high stress on the muscles, tendons, and joints in the legs, especially at high speeds and with many changes of direction.

Cycling, elliptical machines, rowing machines, and uphill intervals can make it easier to achieve a high heart rate with less strain on the knees, ankles, and hips.

Do short or long breaks between intervals work best?

The breaks between intervals also affect the intensity of the session:

- Short breaks keep the heart rate higher throughout the training.

- Longer breaks make it easier to maintain high speed or resistance in each interval.

- Active breaks with low intensity between intervals often make it easier to keep the body moving than a complete stop.

For individuals with little training experience, it may be wise to start with shorter intervals and moderate intensity before gradually increasing the pace.

High-intensity training requires more recovery than lower-intensity conditioning training, and too much high intensity over time can make it harder to maintain both quality and training volume.

How often such sessions should be used will vary with training level, total training volume, and the goal of the training.