High-Intensity Interval Training Explained Simply

1. What It Is About

In high-intensity interval training, short, very strenuous sections alternate with recovery phases. During recovery, one continues slowly or takes a break. The exertion sections can last only a few seconds, as in sprint interval training, or several minutes, as in classic HIIT. The evaluated studies begin in the year 2012.

How strenuous a section is is almost always determined in relation to one’s own performance capacity. For example, the proportion of the highest oxygen uptake or the highest heart rate achieved is measured. What constitutes a moderate exertion for a well-trained person may already be the upper limit for an untrained individual. The format is studied in supervised programmes, in rehabilitation after a cardiac event, in schools, and as walking exercises after a stroke.

Important for understanding: Comparisons are almost always made with another training form, usually gentler endurance training, and not with inactivity. Moreover, the programmes are hardly comparable because the length of the sections, number of repetitions, and total volume are chosen differently.

2. What Studies Show

Most frequently cited are studies on how the training is experienced. In a study with twelve recreationally active men and women, all completed two sessions on a cycle ergometer: one with intervals of eight one-minute exertions, each followed by one minute of recovery, and one with steady cycling for twenty minutes 3. After the intervals, enjoyment was greater, and eleven of the twelve participants preferred the intervals 3. Another trial with 44 participants compared three laboratory sessions: intervals, steady training, and very vigorous steady training 2. There, too, enjoyment of the intervals was greater, and more than half preferred them 2. The authors note that about half of people discontinue a training programme within the first six months, citing lack of time as the main reason 3.

For metabolism, review articles summarise many individual studies. A review of 18 randomised controlled trials in adults with obesity found that high-intensity training improved endurance performance more strongly and reduced body fat percentage more than extensive steady training; body mass index, the ratio of weight to height, did not differ between the two approaches 9. A second review with 13 studies and 345 patients with type 2 diabetes showed a more pronounced decrease in body weight, body mass index, and long-term blood sugar compared with gentler training 5. In older adults, endurance performance also improved more strongly with interval training than with steady training 8.

A study with 31 inactive adults with obesity shows how similarly both approaches can work. Training took place over twelve weeks with four sessions per week, and weight had to remain constant 7. Endurance performance increased by about one tenth in both groups, weight and fat mass remained unchanged. Insulin sensitivity — how well the body responds to insulin — was higher the day after the last session in both groups and returned to baseline after four days of rest 7.

3. What Remains Unclear

Mostly, outcomes such as endurance performance, body composition, or blood sugar control are measured, not the course of a disease or mortality. Whether the association between activity and mortality can be directly examined for interval training remains to be seen; randomised studies with sufficient duration are lacking 6.

The adaptation varies widely between individuals. In a study with 21 adults, endurance, lactate threshold, and heart rate improved under both programmes, but individual responses differed considerably; each participant responded to at least one of the two programmes 4. Mean values therefore obscure people without measurable adaptation.

For inflammatory processes, a controlled study with 26 people with insulin resistance is available: both training forms reduced inflammatory markers 1. Whether duration and volume or intensity are decisive remains to be clarified.

4. What This Means in Everyday Life

Interval training is a training form, not a medication. When reading the studies, it helps to keep in mind that comparisons are almost always made with another training form and not with inactivity: both approaches improved endurance, with intervals usually doing so slightly more strongly, often with a smaller total volume.

In the studies, enjoyment of short intervals was greater, and the majority preferred them 2 3. Whether this preference persists over weeks and months and whether it actually leads to more training remains open: enjoyment and preference were recorded, not training behaviour over weeks 2. It also remains to be clarified whether this preference exists in clinical groups, such as after a cardiac event, a stroke, or in diabetes, and whether short intervals change the perceived time commitment.

All details with study numbers and sources: detailed page on High-Intensity Interval Training (HIIT). For children: High-Intensity Interval Training (HIIT) for children.