Interval training, or training with intervals, is a widely adopted method among endurance sports enthusiasts. There are countless possible combinations between effort and recovery durations, as well as many intensity parameters.
Three main types of effort intervals are generally distinguished:
- Short intervals (10 to 30 seconds)
- Medium intervals (30 seconds to 1 minute)
- Long intervals (more than one minute, usually up to 5 minutes)
Each effort period is paired with a rest interval, which can be:
- Complete (3 to 5 times the duration of the effort)
- Partial (equal to or up to twice the duration of the effort)
- Incomplete (shorter than the duration of the effort)
The intensity of the efforts varies inversely with their duration: the shorter the interval, the higher the intensity. For example, a 20-second effort can be performed at more than 130% of aerobic capacity, while a long interval will be performed at lower intensities, around 85 to 95% of this capacity.
Here, the percentage of aerobic capacity or maximal aerobic speed (MAS) is used, rather than the percentage of maximal heart rate. Indeed, the brevity of the intervals does not allow heart rate to accurately reflect the real intensity of the effort. Sometimes, heart rate is even higher during recovery than during the effort itself. This is why more immediate indicators such as MAS (in km/h) or MAP (power at aerobic capacity, in watts) are preferred, whether in running or cycling.
But what happens if you reduce the intensity during short intervals? Rather than always seeking to maximize effort, I chose to explore the minimal intensity level required to optimize gains. The idea is to progress as much as possible with the minimum necessary effort—an interesting challenge in training.
With this in mind, I conducted a research project to determine whether, for intervals of the same duration, a difference in intensity could lead to similar adaptations in aerobic capacity.
I therefore compared two interval training protocols (30 seconds of effort for 60 seconds of passive-active recovery) at two different intensities (85% vs 100% of aerobic capacity/MAS), with the same number of intervals (weeks 1 to 3: 8 pairs of intervals; weeks 4 and 5: 10 pairs), among 10 participants, with one session per week.
Aerobic capacity was estimated using a maximal 6-minute indirect running test, performed before the first week and at the end of the sixth. This test does not allow for precise distinction of the nature of adaptations (metabolic or mechanical), but this point matters little from a practical perspective: what counts is the improvement in running performance. For a more detailed analysis, a direct measurement of oxygen consumption would be necessary.
The intensities of 85% and 100% were converted into km/h, and each participant had to maintain the target speed during the effort intervals. The 60-second recovery was done by walking. The overall perception of effort (RPE) was collected at the end of the last pair of intervals.
Table 1. Results
Table 1 shows that both groups improved their performance in the running test, indicating a significant increase in aerobic capacity over a short period (5 weeks). The difference in progression between the groups is not significant, neither statistically nor clinically: a difference of 0.2 mLO2·kg⁻¹·min⁻¹ remains negligible in terms of performance. However, the perception of effort differs: group A found the sessions easier (6.6/10) than group B (8.0/10).
In summary:
- Group A trained at an intensity 15% lower than group B.
- Both groups performed the same number of intervals, with one session per week.
- Progress was similar in both groups.
- Group A found the sessions easier.
Although the sample is small (10 volunteer runners), the duration short (5 weeks), and the measurement of aerobic capacity indirect, these results raise an interesting question:
Should we pay more attention to the minimal intensity required to progress, rather than always seeking to train at maximal intensity?
In strength training, this debate is common (training to failure or not), but it is much less so in endurance.
This project does not address training volume, but focuses on intensity. A 15% difference in intensity, above 85% of MAS, does not seem to influence progression. Participants can therefore save 15% effort while progressing just as much, which is not negligible: less intensity also means less fatigue, less recovery needed, and potentially lower risk of injury.
Perhaps it is time to rethink our endurance training paradigms and focus on the optimal level of exertion to generate the desired adaptations, whether you are a beginner or an experienced athlete.
Of course, this is not about advocating laziness or always training at low intensity: running at 85% of your MAS is still demanding. But within the high-intensity range (85-100%), aiming for the lower end may be enough to achieve similar adaptations, at a lower cost.
What do you think ?