How is afterburn estimated?

After a workout your body keeps using extra oxygen while it recovers, and that oxygen burns extra calories. ZonePoints estimates this afterburn from how long you spent in each zone, using the measured relationship between how hard you worked and how much oxygen you used afterwards.

What afterburn is

Physiologists call it EPOC, excess post-exercise oxygen consumption: the oxygen you use after exercise above what you would have used resting. It pays for refilling your muscles’ oxygen and energy stores, clearing lactate, cooling down and the extra work your heart and lungs do while all that happens. Every litre of that oxygen burns about 5 calories, as it does during exercise.

Two things decide how much afterburn a workout earns: how hard it was and how long it lasted. They work very differently.

Harder work earns a much bigger share

In “Effect of intensity of exercise on excess postexercise O2 consumption”, Bahr and Sejersted had six men cycle for 80 minutes on three separate days, at 29%, 50% and 75% of their maximum oxygen uptake (VO2 max), then measured them lying at rest for the next 14 hours[1]. The afterburn was 1.3 litres of oxygen after the easy ride, 5.7 litres after the moderate one and 30.1 litres after the hard one, and it lasted 20 minutes, 3.3 hours and 10.5 hours. Going from 50% to 75% of VO2 max increased the work by about half, but more than quintupled the afterburn. The authors describe the relationship as exponential.

Longer work earns more in proportion

Duration acts more simply. In their review “Effects of exercise intensity and duration on the excess post-exercise oxygen consumption”, LaForgia, Withers and Gore found that at intensities from around 50–60% of VO2 max upwards, afterburn rises in a straight line with the length of the workout[2]. Børsheim and Bahr’s review “Effect of exercise intensity, duration and mode on post-exercise oxygen consumption” describes the link with duration the same way: more linear than the link with intensity, especially at higher intensities[3]. Twice the time at the same effort earns about twice the afterburn.

Afterburn has a ceiling

The same review found that even the workouts that kept oxygen use raised for hours produced an afterburn of only 6–15% of the net oxygen cost of the exercise — the oxygen used during the workout above resting[2]. The authors conclude that afterburn’s role in weight loss has been overstated: nearly all the energy a workout costs is burned while you do it.

The formula ZonePoints uses

ZonePoints combines those three findings. For each zone you spent time in:

  • Your intensity. The middle of the zone, as a percentage of your heart rate reserve, gives your oxygen use as a fraction of your VO2 max, the same way the calorie estimate works it out.
  • The net cost. The calories that time cost above what you would have burned resting.
  • The afterburn share. 15% × e^(4.36 × (intensity − 0.75)), and never more than 15%. Fitted to Bahr and Sejersted’s three rides, it gives 2% of the net cost at 29% of VO2 max, 5% at 50%, 10% at about two-thirds and the full 15% from 75%.
  • The afterburn. The net cost multiplied by the share.

The zones’ afterburn is added together. Time below your training threshold earns none: at that effort Bahr and Sejersted measured 1.3 litres after 80 minutes, under 7 calories.

For a typical person with the standard zones, the share is about 7% in the Easy zone, 11% in Medium and the full 15% from Hard upwards. Harder zones still earn more afterburn per minute above that, because each minute costs more, but not a larger share of it.

Why it is usually about a tenth of the total

The calories ZonePoints shows for a session are what you burned during it plus the afterburn. Measured against that total, afterburn comes to about 6% for a session spent entirely in the Easy zone, 9% in Medium and 12% from Hard upwards. Most sessions mix zones, so most land near 10%. That is not a rounding shortcut: a narrow range is exactly what the research finds.

What the estimate leaves out

  • Very short, very hard intervals. Sprint intervals above your VO2 max can earn a larger share, but heart rate cannot tell them apart from steady hard work, so they are capped at 15% like everything else.
  • Short workouts. The straight-line rule holds on average. Individual studies find short workouts sometimes earn a little more than their length suggests.
  • How fit you are. The curve comes from a small group of healthy young men. A fitter or less fit person may recover faster or slower than the formula assumes.

Like every calorie figure from a heart rate, the afterburn is an estimate. It is a fair way to compare one workout with another, not a laboratory measurement.

References

  1. Effect of Intensity of Exercise on Excess Postexercise O2 Consumption
    Bahr, R., & Sejersted, O. M. (1991).
    Metabolism, 40(8), 836–841
  2. Effects of Exercise Intensity and Duration on the Excess Post-Exercise Oxygen Consumption
    LaForgia, J., Withers, R. T., & Gore, C. J. (2006).
    Journal of Sports Sciences, 24(12), 1247–1264
  3. Effect of Exercise Intensity, Duration and Mode on Post-Exercise Oxygen Consumption
    Børsheim, E., & Bahr, R. (2003).
    Sports Medicine, 33(14), 1037–1060