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MAS and the 45/15 session

Estimate your maximal aerobic speed from your 10K or a half-Cooper test, choose which MAS to use, and build the session of 45 seconds of effort and 15 seconds of recovery: formulas and example.

3 min read Updated October 9, 2026

Contents
  1. The principle
  2. The Azetta method
  3. Why this choice
  4. Limits
  5. What you set in the app
  6. References

The principle

Maximal aerobic speed (MAS) is the running speed at which your oxygen consumption reaches its maximum; you can hold it for about 4 to 8 minutes. It is a widely used reference in France for dosing short intervals. Azetta measures nothing itself: it converts a performance you know into MAS, and uses it to build the 45/15 session (known as the "Gacon"): 45 seconds fast, 15 seconds slow.

The Azetta method

Estimating your MAS

From a 10 km time, assuming the 10 km is run at 90% of MAS:

MAS (km/h) = (10 ÷ (time in min ÷ 60)) ÷ 0.90

From a half-Cooper test (running as far as possible in 6 minutes):

MAS (km/h) = distance (m) ÷ 100

MAS is bounded between 6 and 25 km/h.

Which MAS is used

  1. The more recent of a value you entered and a test.
  2. Otherwise, a value measured during a session, if it is less than 56 days old.
  3. Otherwise, the estimate from your 10 km time.

The app always tells you where the value comes from. A 45/15 session records a MAS data point if you did at least 60% of the planned work, with a GPS distance and an average effort speed between 5 and 30 km/h.

The 45/15 session

  • Effort: 45 s at 100% of your MAS; recovery: 15 s at MAS ÷ 3 (walking or very slow jogging).
  • Standard: 2 sets of 10 repetitions, 3 minutes between sets, 15 minutes of warm-up and 10 of cool-down.
Distance of one repetition = MAS ÷ 3.6 × 45 sPace = 3600 ÷ MAS (s/km)

Why this choice

  • Short intervals around MAS are one of the most effective ways to develop aerobic capacity; efforts of 30 s to a few minutes at this speed are well described in the literature on interval training.
  • The 45/15 lets you accumulate a lot of time close to MAS with a short recovery, which makes it accessible to runners of all levels.
  • The half-Cooper test is a simple field test, derived from Cooper's 12-minute test.

Limits

  • The 90% ratio between 10 km and MAS applies to runners who finish the 10 km in 40 to 65 minutes; it is a starting point, not a measurement.
  • A badly paced half-Cooper test (starting too fast) underestimates MAS.
  • No maximal aerobic power (MAP): that would require a power meter.
  • MAS is only used for the 45/15 session; the other paces come from your 10 km time.

References

  1. Billat LV. Interval training for performance: a scientific and empirical practice. Special recommendations for middle- and long-distance running. Part I: aerobic interval training. Sports Medicine, 2001;31(1):13-31.
  2. Cooper KH. A means of assessing maximal oxygen intake. Correlation between field and treadmill testing. JAMA, 1968;203(3):201-204.

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