Heart rate zones turn a single number on your watch into something useful: a read-out of how hard your body is actually working. Rather than guessing whether a session was easy, moderate or genuinely demanding, the zones let you anchor effort to a percentage of your maximum heart rate — and, in turn, to the physiological adaptations you are trying to trigger. At CAROL, we’re guided by science, so it is worth understanding what the zones measure, how to set yours accurately, and which ones actually move the markers that matter for health and longevity.
What are heart rate zones?
Heart rate zones are bands of exercise intensity, each expressed as a percentage of your maximum heart rate (HRmax) — the highest number of beats per minute your heart can reach during all-out effort. The most widely used model divides effort into five zones:
- Zone 1 (about 50–60% of HRmax) — very light, the pace of a gentle warm-up or recovery walk.
- Zone 2 (about 60–70%) — comfortable, conversational aerobic work.
- Zone 3 (about 70–80%) — moderate; breathing deepens and talking gets harder.
- Zone 4 (about 80–90%) — hard, threshold effort you can only hold for a few minutes.
- Zone 5 (about 90–100%) — maximal, the domain of short, all-out intervals.
The percentages are a practical convention rather than a law of nature, but they give you a shared language for intensity. Lower zones tend to draw more on fat as a fuel and feel sustainable for a long time; higher zones lean on carbohydrate and can only be held briefly. The key is that your zones are only as accurate as the maximum heart rate you build them from.
How do you find your maximum heart rate?
The familiar shortcut — 220 minus your age — is easy to remember but tends to be inaccurate, particularly as you get older. A large meta-analysis pooling 351 studies and more than 18,000 people found that HRmax is more accurately predicted by 208 − (0.7 × age), an equation that held regardless of sex or habitual activity level (Tanaka et al., 2001, https://doi.org/10.1016/s0735-1097(00)01054-8). The authors noted that the older 220-minus-age rule underestimates HRmax in older adults, which means it can quietly set your zones too low.
In practice, a 40-year-old has an estimated HRmax of around 180 bpm (208 − 28), and a 60-year-old around 166 bpm (208 − 42). Any formula is an estimate with individual variation, so treat it as a starting point and adjust against how effort actually feels.
Which heart rate zone builds fitness?
Time in the lower zones has real value for recovery and aerobic base, but the sharpest gains in cardiorespiratory fitness — measured as VO₂max, or maximal oxygen uptake, the maximum amount of oxygen your body can use during intense exercise — come from spending time high up the scale. A meta-analysis of 53 randomised controlled trials found that high-intensity interval training reliably improved VO₂max, and that even short intervals of 30 seconds or less, low training volumes of five minutes or less, and programmes as brief as four weeks produced clear benefits (Wen et al., 2019, https://doi.org/10.1016/j.jsams.2019.01.013).
That pattern holds with age. In a review of 29 trials covering 1,227 older adults, high-intensity interval training and continuous moderate training produced similar improvements across most fitness and health markers — and in the higher-quality controlled trials, the cardiorespiratory gains were greater with the higher-intensity work (Oliveira et al., 2024, https://doi.org/10.1016/j.archger.2024.105451). Reaching the top zones, even briefly, appears to be a particularly efficient way to raise VO₂max.
Why the top zone matters for longevity
This matters well beyond athletic performance. VO₂max is one of the strongest independent predictors of how long you live: a review of the evidence describes cardiorespiratory fitness as a key determinant of all-cause and disease-specific mortality (Strasser & Burtscher, 2018, https://doi.org/10.2741/4657). The size of the effect is striking. In an 11-year follow-up of middle-aged men, each 1 mL/min/kg higher change in measured VO₂max was associated with a 9% lower risk of death from any cause (Laukkanen et al., 2016, https://doi.org/10.1016/j.mayocp.2016.05.014). Protecting and improving VO₂max is, in effect, protecting your healthspan — and the higher heart rate zones are where that improvement is driven.
How REHIT reaches the top zone in minutes
The catch with the top zones is that most people assume reaching them requires long, punishing sessions. It does not. REHIT (Reduced Exertion High-Intensity Interval Training) is built around just two 20-second all-out sprints inside a short, low-volume session. Research on the acute response shows that a single REHIT bout depletes muscle glycogen and activates the same molecular signalling pathways — including PGC-1α — once thought to belong only to prolonged aerobic exercise (Metcalfe et al., 2015, https://doi.org/10.1007/s00421-015-3217-6). Those brief sprints briefly push you into that highest zone, where the stimulus for raising VO₂max is strongest.
Intensity that high is not one-size-fits-all. When researchers compared perceptual responses to a single REHIT session, people with above- and below-average fitness reported broadly similar enjoyment and effort, but the authors still recommend that REHIT is individualised for each person, as those with lower fitness were more likely to find it aversive (Astorino et al., 2019, https://doi.org/10.1016/j.physbeh.2019.112687). CAROL handles this by tailoring the resistance of each sprint to you, so you reach the top zone at a level your body is ready for.
The bottom line
Heart rate zones are a practical map of exercise intensity, and they are only useful if the maximum heart rate underneath them is accurate — so favour 208 − (0.7 × age) over the older 220-minus-age rule. The lower zones support recovery and aerobic base, but the evidence points to the higher zones as the ones that raise VO₂max, the marker most closely tied to longevity. The encouraging part is that you do not need hours in the red to get there. A few well-judged, individualised seconds at the top of the scale — the principle behind REHIT — can deliver much of the benefit, in a fraction of the time.
