Research · 4 min read · 2026-10-12
4 × 4, 4 × 8 or 4 × 16 minutes?
In a seven-week study, 4 × 8 minutes at a hard but controlled effort gave more than both shorter, harder and longer intervals.
Short and very hard, or longer and a little less hard? A Norwegian study compared three interval sessions where the athletes worked as hard as they could hold, and found that the hardest one was not the most effective.
What the researchers did
Thirty-five trained recreational cyclists were split into four groups for seven weeks. One group only trained at low intensity. The three others did two interval sessions a week, plus easy training: 4 × 4 minutes, 4 × 8 minutes or 4 × 16 minutes. They were told to ride each interval at the highest intensity they could sustain for the whole session.
That made the short intervals hardest. The 4 × 4 group worked at about 94 percent of their maximum heart rate, the 4 × 8 group at about 90 percent and the 4 × 16 group at about 88 percent.
What they found
The 4 × 8 group improved most. Across maximal oxygen uptake, power at maximal oxygen uptake and power at a set lactate level, they improved by 11.4 percent on average. The other groups improved by 4 to 6 percent.
In other words, 32 minutes of work at about 90 percent of maximum heart rate gave more than 16 minutes at about 95 percent, even though it felt less hard.
What it means for you
Harder is not automatically better. Intervals of several minutes at a hard but controlled effort, where you can hold the pace through every repetition, can give more than short intervals you only just survive.
If the last repetitions are clearly slower than the first ones, the session was probably too hard for its purpose.
What the study can't tell you
The participants were cyclists, not runners, and the study lasted seven weeks. The 11.4 percent is a combined result across three measures, not a promise of a faster race time. And 4 × 8 minutes is not the right session for every runner or every phase of training.
How we use it
Interval sessions in PX7 Training have a target and a range for each work period, so you can hold the pace through all of them. The purpose of the session and the total time decide the intensity, not a wish to make every session as hard as possible.
Source
- Seiler et al. (2013), Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. Scandinavian Journal of Medicine & Science in Sports.
Written by Jørgen Larsen for PX7 Training.