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Science

The science behind aerozone.

Why aerobic base training matters, how to measure your effort, and why coaching you can hear helps. Each section ends with a short note on how strong the evidence is, and every figure links to its source. Last reviewed October 2026.

Why building your aerobic base matters

Regular aerobic training can improve endurance and running performance, and lower-intensity sessions are an important part of many effective training programmes. In a 10-week study of 30 recreational runners, two different training approaches both improved 10 km performance. The group doing more low-intensity running improved by 5.0%, compared with 3.6% for the group doing more moderate-intensity running. The difference between the two approaches was not statistically significant.[1]

There’s more than one good way to structure it. A 2025 review of 13 studies (348 athletes, 150 of them recreational) found no clear winner between the main approaches built on low-intensity training.[2] Another review found strict “80/20” training had a small edge for aerobic fitness, mainly in highly trained athletes.[3] What they share is plenty of time in the aerobic zone.

Evidence: moderate From fairly fit recreational runners and trained athletes.

How aerobic training improves fitness

Regular aerobic training grows your engine. Across 28 trials and 723 adults, continuous endurance training raised VO2max, a key measure of aerobic fitness, by about 4.9 ml/kg/min compared with no training.[4] In 16 previously untrained men, six weeks of endurance training raised red blood cell volume by 8%, peak cardiac output by 7%, capillaries per muscle fibre by 18% and mitochondrial volume by 43%.[5] Across 191 studies, exercise lowered resting heart rate by roughly 3 to 6 bpm.[6]

Harder training builds fitness too: minute for minute, intervals produced similar or slightly larger gains in VO2max.[4] One advantage of lower-intensity training is that it can support a substantial volume of consistent exercise. In a small study of highly trained runners, recovery markers returned towards baseline sooner after lower-intensity sessions than after harder sessions.[8]

Evidence: strong for VO2max, moderate for the changes behind it. From untrained and general adults.

Understanding your aerobic training zone

Exercise scientists split effort into three domains, separated by two thresholds.[9]

Your aerobic zoneBelow the first threshold
Between the thresholdsHarder, still sustainable
Above the secondHeld only for a limited time
Lower effortHigher effort
  • Below the first threshold: your aerobic zone. Lactate stays close to resting levels, and your body settles into a steady state within a few minutes. You can keep going for a long time.
  • Between the thresholds. Lactate rises and settles at a higher level. It’s still sustainable, but harder.
  • Above the second threshold. Lactate keeps climbing and your body can’t reach a steady state, so you can only hold the effort for a limited time.

These thresholds aren’t an on-off switch between aerobic and anaerobic exercise. Your body uses both energy systems across different intensities. As effort increases, the contribution from anaerobic energy pathways generally becomes greater, while aerobic metabolism remains important.

You’ll feel it. Most people find exercise below their breathing threshold pleasant, and above it nearly everyone finds it unpleasant.[7] It shows in recovery too: in highly trained runners, heart rate variability, a marker of recovery, returned to normal within 5 to 10 minutes of a run below the first threshold, even a two-hour one. After threshold or interval sessions it took about 30 minutes.[8]

Where the first threshold sits varies from person to person. In 100 adults, it fell anywhere from 60% to 90% of max heart rate.[10]

Evidence: strong on the three domains (lab physiology). Recovery comes from one study of 9 highly trained runners.

Why staying in your zone can be difficult

Planned low-intensity sessions can sometimes feel harder than intended. A review of 29 studies found athletes felt planned low-intensity sessions as harder than their coaches intended, while moderate and hard sessions matched.[11]

In training data from 119,452 marathon runners, faster runners did more of their training at low intensity, and more time at moderate and hard efforts went with slower marathons.[12]

Other people play a part too. Across more than a million runners, seeing friends’ runs nudged people to run farther and faster.[13] And in races, runners misjudge pace: among 1.7 million marathon finishers, a third ran their first 5 km as their fastest.[14]

Evidence: moderate that it happens. The marathon data show a link, not proof of cause.

Measuring intensity: heart rate, pace and feel

Each way of judging effort has a weak spot.

  • Pace is useful, but the same pace can require very different levels of effort on hills, in hot conditions or when you’re fatigued. GPS watches also misjudged distance by up to about 9% among buildings.[15]
  • How it feels tracks heart rate fairly well across large groups[16], but it varies from person to person and is hard to judge mid-run.
  • Heart rate responds to exercise intensity as well as factors such as heat and fatigue. It can lag behind changes in effort and gradually drift upwards during longer sessions. It takes at least 3 to 4 minutes to settle when you start.[17] And it drifts upwards on long runs: in 82,303 marathon runners, heart rate relative to speed was about 16% higher in the last 5 km than early on. The drift began around 25 km, and runners who drifted less finished faster.[18]

That’s why aerozone gives you a warm-up before it coaches you, and shows pace drift after longer runs. It’s also why the talk test is a useful check: if you can’t hold a conversation, you’re working harder than moderate, whatever the number says.[19]

Evidence: strong on how heart rate behaves.

Why audio coaching helps

Real-time feedback helps people hold a target zone. In a study of 20 adults on exercise bikes, time in their heart rate zone rose from 43% without cues to 90% with sound cues.[20]

Studies comparing audio and visual feedback have reported similar results for staying on target.[21, 22] Audio’s practical advantage is that it allows runners to receive feedback without repeatedly checking a screen. Simple audio alarms kept people in their zone as well as music did.[23] And in a running test with 48 people, spoken pace cues kept runners on target more often than vibration alone (60% of the time against 53%).[24]

Evidence: low to moderate. Mostly short lab sessions with non-elite adults.

How aerozone estimates your training zone

aerozone starts from your max heart rate and your resting heart rate, and sets your zone at 60 to 70% of your heart rate reserve (the gap between the two). Using resting heart rate makes the zone more personal than a percentage of max alone.

If you don’t know your max, aerozone estimates it from your age. Age-based formulas give the average for people your age: against max heart rate measured in a lab, “220 minus age” was typically about 12 bpm out and the newer Tanaka formula about 11 bpm out, with some people 20 bpm or more out either way.[25] In 3,320 healthy adults, another formula’s typical error was 10.8 bpm.[26] So if you know your max heart rate from a proper test, enter it.

Because everyone’s first threshold sits in a different place[10], treat your zone as a starting point and fine-tune it. Guidelines for the general population count 60% of heart rate reserve and above as “vigorous”[27], so if your zone feels hard, lower it or walk. If you can’t talk in full sentences, ease off.

Evidence: strong. From healthy everyday adults.

Heart rate sensors

Chest straps read your heart’s electrical signal, the same signal an ECG uses. In a lab study of 10 people, a chest strap read 99.4% of heartbeats correctly during hard exercise.[28] Optical sensors on the arm or wrist use light instead. In a study of 16 people doing everything from sitting to jogging and intervals, an upper-arm band stayed within 5% of a chest strap 95% of the time, a forearm band 89% and a wrist watch about 72%.[29]

Evidence: moderate. Small studies, mostly of older devices.

Understanding the evidence

aerozone is designed to help runners follow their intended heart rate targets more consistently. The fitness benefits come from regular, well-structured training, and aerozone helps guide it.

Some popular claims go further than the research:

  • Injury. Low-intensity running hasn’t been shown to prevent injury. In a trial of 447 recreational runners, injury rates were similar whether runners built up harder running (16%) or more low-intensity running (19%).[30]
  • One perfect split. “80/20” isn’t proven better than other plans built on low-intensity training for everyday runners.[2, 3]
  • “Zone 2” and fat burning. Training in a specific so-called “fat-burning zone” has not been established as superior for long-term weight loss.[31]
  • Sound versus screens. In the studies cited, auditory and visual cues produced similar results for staying on target. Audio’s practical advantage is that it doesn’t require looking at a screen.[21, 22]

Who should take care

A heart rate zone assumes your heart rate responds normally to exercise. Some medicines (such as beta blockers) blunt that response, and people taking them are usually advised to go by effort or the talk test instead.[32] In pregnancy, heart rate is a less precise guide, and a single heart rate limit isn’t considered useful.[33] Heart conditions, irregular rhythms and implanted devices can also change how your heart rate behaves.

References

  1. Muñoz I, et al. Does polarized training improve performance in recreational runners? Int J Sports Physiol Perform 2014;9(2):265-272. Link
  2. Rosenblat MA, et al. Which training intensity distribution intervention will produce the greatest improvements? Sports Med 2025. Link
  3. Oliveira PS, Boppre G, Fonseca H. Comparison of polarized versus other types of endurance training intensity distribution. Sports Med 2024;54(8):2071-2095. Link
  4. Milanović Z, Sporiš G, Weston M. Effectiveness of high-intensity interval training and continuous endurance training for VO2max improvements. Sports Med 2015;45(10):1469-1481. Link
  5. Montero D, et al. Haematological rather than skeletal muscle adaptations contribute to the increase in peak oxygen uptake induced by moderate endurance training. J Physiol 2015;593:4677-4688. Link
  6. Reimers AK, Knapp G, Reimers CD. Effects of exercise on the resting heart rate. J Clin Med 2018;7(12):503. Link
  7. Ekkekakis P, Parfitt G, Petruzzello SJ. The pleasure and displeasure people feel when they exercise at different intensities. Sports Med 2011;41(8):641-671. Link
  8. Seiler S, Haugen O, Kuffel E. Autonomic recovery after exercise in trained athletes: intensity and duration effects. Med Sci Sports Exerc 2007;39(8):1366-1373. Link
  9. Hunter B, Meyler S, Maunder E, Cox TH, Muniz-Pumares D. The relationship between the moderate-heavy boundary and critical speed in running. Int J Sports Physiol Perform 2024;19:963-972. Link
  10. Iannetta D, et al. A critical evaluation of current methods for exercise prescription in women and men. Med Sci Sports Exerc 2020;52(2):466-473. Link
  11. Inoue A, et al. Internal training load perceived by athletes and planned by coaches: a systematic review and meta-analysis. Sports Med Open 2022;8:35. Link
  12. Muniz-Pumares D, et al. The training intensity distribution of marathon runners across performance levels. Sports Med 2025;55(4):1023-1035. Link
  13. Aral S, Nicolaides C. Exercise contagion in a global social network. Nat Commun 2017;8:14753. Link
  14. Smyth B. Fast starters and slow finishers: a large-scale data analysis of pacing at the beginning and end of the marathon for recreational runners. J Sports Analytics 2018;4(3):229-242. Link
  15. Gilgen-Ammann R, Schweizer T, Wyss T. Accuracy of distance recordings in eight positioning-enabled sport watches. JMIR Mhealth Uhealth 2020;8(6):e17118. Link
  16. Scherr J, et al. Associations between Borg’s rating of perceived exertion and physiological measures of exercise intensity. Eur J Appl Physiol 2013;113(1):147-155. Link
  17. Buchheit M. Monitoring training status with HR measures: do all roads lead to Rome? Front Physiol 2014;5:73. Link
  18. Smyth B, et al. Decoupling of internal and external workload during a marathon: an analysis of durability in 82,303 recreational runners. Sports Med 2022;52:2283-2295. Link
  19. Zuhl M. Tips for monitoring aerobic exercise intensity. American College of Sports Medicine, 2020. Link
  20. Shaykevich A, et al. Auditory feedback improves heart rate moderation during moderate-intensity exercise. Med Sci Sports Exerc 2015;47(5):1046-1051. Link
  21. Celikgögüs E, et al. Into the Zone: assisting runners with maintaining heart rate zones. Proc MUM 2025. Link
  22. Schubhan M, et al. Investigating gamification modalities in running. Proc ACM 2025. Link
  23. van der Vlist B, Bartneck C, Mäueler S. moBeat: using interactive music to guide and motivate users during aerobic exercising. Appl Psychophysiol Biofeedback 2011;36(2):135-145. Link
  24. Dobiasch M, Stafilidis S, Baca A. Effects of different feedback variants on pacing adherence in a field based running test. Int J Perf Anal Sport 2021;21(6):1015-1028. Link
  25. Shookster D, et al. Accuracy of commonly used age-predicted maximal heart rate equations. Int J Exerc Sci 2020;13(7):1242-1250. Link
  26. Nes BM, et al. Age-predicted maximal heart rate in healthy subjects: the HUNT fitness study. Scand J Med Sci Sports 2013;23(6):697-704. Link
  27. Ferri Marini C, et al. HRR and VO2R fractions are not equivalent: is it time to rethink aerobic exercise prescription methods? Med Sci Sports Exerc 2021;53(1):174-182. Link
  28. Gilgen-Ammann R, Schweizer T, Wyss T. RR interval signal quality of a heart rate monitor and an ECG Holter at rest and during exercise. Eur J Appl Physiol 2019;119(7):1525-1532. Link
  29. Schweizer T, Gilgen-Ammann R. Wrist-worn and arm-worn wearables for monitoring heart rate during sedentary and light-to-vigorous physical activities. JMIR Cardio 2025;9:e67110. Link
  30. Ramskov D, et al. Progression in running intensity or running volume and the development of specific injuries in recreational runners: Run Clever. BMJ Open Sport Exerc Med 2018;4(1):e000333. Link
  31. Purdom T, et al. Understanding the factors that affect maximal fat oxidation. J Int Soc Sports Nutr 2018;15:3. Link
  32. Porcari J. Ask ACE: how to help clients who have heart-related concerns exercise safely. American Council on Exercise, July 2024. Link
  33. Evenson KR, Hesketh KR. Monitoring physical activity intensity during pregnancy. Am J Lifestyle Med 2021;17(1):18-31. Link