The Endurance Athlete’s Guide to Training Zones

How to test your zones, manage intensity and make sure every training session is matched with your physiology and goal.

By Matt Warnock, endurance coach based in the Netherlands, qualified with British Triathlon, IRONMAN University and TrainingPeaks (Level 2).

The intensity trap

There is a common belief in endurance sport that if you don't finish a session tired, it didn't count. Performance gets tied to volume and suffering, and a lot of athletes end up training too hard, too often.

It's even more common when time is short. If you only have six or eight hours a week, it feels like every session has to be hard to be worth it. In my experience coaching more than 100 athletes, that's the single most common reason age-groupers stall.

Every zone triggers specific adaptations. Doing a Zone 2 session in Zone 3 doesn't make it better. You pay a bigger fatigue bill for little extra aerobic benefit, and your next hard session suffers for it.

And if that session's purpose was to improve your fat-burning, going harder works directly against it. As intensity rises, your body shifts its fuel use away from fat and towards carbohydrate. Physiologists call this the "crossover" point (Brooks & Mercier, 1994). Fat burning peaks at a moderate, conversational intensity (Achten, Gleeson & Jeukendrup, 2002), so push past it and you spend the session practising the wrong thing.

When easy sessions are too hard and hard sessions lack intent, the results are predictable:

• Chronic fatigue: you never quite feel fresh, and quality sessions suffer.
• Plateaus: performance stalls even as training load goes up.
• Injury risk: accumulated fatigue leaves less margin for error.

The fix is to understand what each intensity does, set your zones from real data, and test regularly so they stay accurate.

Where training zones come from

Every zone system, whether it has 3, 5, 6 or 7 zones, is built on two turning points in your physiology:

• The first threshold (LT1 / VT1): the point where blood lactate first rises above resting levels and your breathing first deepens. Below it you can keep going for hours; this is "easy".
• The second threshold (LT2 / MLSS / critical power): the highest intensity at which lactate production and clearance stay in balance. Above it, fatigue builds quickly and the clock starts ticking.

These two points split exercise into three domains: easy (below the first threshold), moderate to hard (between the two) and severe (above the second). The six zones below are a more detailed version of the same picture.

The six training zones

I use a six-zone model because it's detailed enough to be precise and simple enough to use day to day.

Ideally, I set an athlete's zones from a lactate test or metabolic analysis, because that measures their own thresholds and adaptation intensities directly. If an athlete isn't ready for that, or can't get to a test, we can still estimate their zones pretty well using percentages of threshold from a simple field test. That's what the table shows.

Bike ranges follow the widely used Coggan power levels (Allen & Coggan). Run and swim bands are the ones I use with my athletes, anchored to your threshold pace and critical swim speed (CSS) from the field tests below. Run bands are a percentage of threshold speed, so a lower number means slower: with a threshold pace of 5:00/km, Zone 2 is roughly 5:41–6:25/km. With a CSS of 1:50/100 m, Zone 2 is 1:58–2:05/100 m. All of these are averages: your own first threshold might sit at 65% of FTP or at 80%, which is exactly why testing matters. The free toolkit helps you calculate your personal numbers for all three sports.

Zone 1: Active recovery. Very easy movement that increases blood flow and helps you recover. Use it for recovery sessions, warm-ups and cool-downs.

Zone 2: Aerobic foundation and fat metabolism. Builds mitochondria and capillaries and improves your ability to burn fat. It also trains your fast-twitch muscle fibres to behave more like slow-twitch ones: more fatigue-resistant and better at using oxygen. The goal: more power or speed at the same low heart rate.

Zone 3: Tempo. Steady, "comfortably hard" work. It improves sustainable race pace and muscular endurance, but it's also where easy sessions drift to by accident. Typical recovery: 12–36 hours.

Zone 4: Threshold (around LT2 / MLSS). Directly targets and raises your threshold. FTP is a practical estimate of this point, not the threshold itself. Typical recovery: 24–48 hours.

Zone 5: VO2max and anaerobic capacity. Short, hard intervals (roughly 30 seconds to 8 minutes) that raise your aerobic ceiling and your tolerance for high-intensity efforts.

Zone 6: Neuromuscular power and sprint. Maximal efforts of a few seconds that build top-end power, coordination and technique.

Recovery times are starting points, not rules. They depend on session length, fitness, sleep and life stress. Triathlon adds a twist: much of the fatigue from a hard session is specific to the muscles you used, so a hard bike session doesn't rule out a quality swim the next day. Your whole-body fatigue still adds up, though, so count all three sports when you plan your hard days.

How much time should you spend in each zone?

What does the research actually show? When researchers analysed the power data of three top-five Giro d'Italia finishers in the months before the race, 84–91% of their training time was at low intensity, 7–11% in the middle and just 2–6% at high intensity (Gallo et al., 2022). Elite cross-country skiers show a similar picture, with about three-quarters of their sessions easy (Seiler & Kjerland, 2006), and a case study of an Olympic-level triathlete found the same emphasis on easy aerobic work (Mujika, 2014). In a 9-week training study, well-trained athletes who followed this kind of pattern improved more than groups doing mainly threshold, high-intensity or high-volume training (Stöggl & Sperlich, 2014).

One important caveat: treat these numbers as a description of how good athletes end up training, not as a recipe. The right mix for you follows from what you're trying to improve, whether that's your VO2max, your threshold or your fat-burning. The distribution is the result of good training, not the starting point.

In practice, most of your weekly hours should feel easy, sometimes almost embarrassingly so. If you finish a Zone 2 session feeling you could have done much more, you probably got it right.

How to find your own training zones

There are two broad ways to find your thresholds.

Lab testing (metabolic cart, lactate testing): the most precise, and it measures markers such as VO2max directly. The downsides are cost, how often you can realistically repeat it, and the fact that conditions in a lab differ from real-world training.

Field testing (time trials, critical swim speed): free or cheap, easy to repeat, and it reflects real conditions. The downside: it relies on estimates and assumptions.

A strong middle ground is field-based lactate testing or INSCYD profiling. INSCYD gives a fuller metabolic profile (VO2max, VLamax, fat and carbohydrate use) from data you can collect outside a lab.

Why formulas mislead

• "220 minus age" is a population average, not your number. A large analysis found a better equation (208 − 0.7 × age), but even that is an average. For any individual, the estimate can easily be off by 10 or more beats per minute (Tanaka, Monahan & Seals, 2001).
• Same percentage, different workout. Setting zones as a percentage of maximum heart rate or VO2max puts different athletes at different points relative to their own thresholds. Two people at "75% of max" can be doing very different sessions. Zones anchored to measured thresholds are more consistent (Mann, Lamberts & Lambert, 2013).
• FTP is an estimate. In trained cyclists, the classic 20-minute test × 0.95 overestimated maximal lactate steady state by about 12 W on average (Lillo-Bevia et al., 2022). That's fine for tracking progress, but it can push your "threshold" sessions above threshold.

If your zones came from a formula or an app's guess, there's a good chance some of your "easy" training isn't easy. A lactate test or INSCYD profile measures your own thresholds. I test athletes in person in the Netherlands and Belgium, and can also guide athletes outside those regions through a remote Power-Performance Decoder that gives high-level accuracy but can be performed at home before I analyse the data.

Field tests you can do yourself

Swim: critical swim speed (CSS). After a thorough warm-up, swim a 400 m time trial. Recover fully, then swim a 200 m time trial. Your CSS pace per 100 m = (400 m time − 200 m time) ÷ 2.

Bike: 20-minute test or ramp test. After a warm-up, ride 20 minutes as hard as you can sustain evenly. Your estimated FTP is about 95% of the average power. A ramp test (power rises every minute until you can't continue) is shorter; FTP is roughly 75% of the best one-minute power. Remember both are estimates.

Run: 30-minute time trial. Run 30 minutes solo, as hard as you can sustain evenly. Your average pace is a good estimate of threshold pace, and your average heart rate over the last 20 minutes estimates your lactate threshold heart rate (an approach popularised by coach Joe Friel).

Retest every 6 to 8 weeks during a training block. Your thresholds move as your fitness changes, so zones from last season won't fit this one.

Two of the biggest mistakes endurance athletes make

1. Bleeding aerobic sessions into no-man's-land. Pushing the pace on long runs, base rides or recovery sessions creates an incorrect training effect and unbalances your programme.
2. The group effect. Letting your intensity be dictated by others: accidental match-sprinting and ego-driven pacing on group rides, club swims and track nights.

As coach Joe Friel teaches, every session should have a clear purpose. Before you start, ask yourself: what is this workout for?

It's not how much you train that matters most. It's how you train.

Frequently asked questions

What are training zones? Training zones are intensity ranges, set by heart rate, power or pace, that tell you how hard to train for a specific adaptation. Good zones are anchored to your own first and second thresholds rather than to a formula.

How often should I retest my zones? Every 6 to 8 weeks during a training block, and after any long break. Thresholds rise with fitness and fall with time off, so old zones drift out of date.

Are my heart rate zones the same for cycling and running? No. Most athletes reach a higher heart rate when running than cycling, so set separate zones for each sport from a sport-specific test.

Should I train with heart rate, power or pace? Power (cycling) and pace (running and swimming) respond instantly and are best for intervals. Heart rate lags behind effort but is useful for keeping easy sessions easy. Most athletes benefit from using both.

Ready to optimise your engine?

Whether you need a one-off metabolic deep-dive or ongoing coaching support, there's a route for your goals and budget:

• Performance testing (in person in the Netherlands and Belgium, or remotely with the Power-Performance Decoder): lactate testing and INSCYD metabolic profiling to find your real zones and limiters. See performance testing
• 1:1 coaching: training built around your life and adjusted every week from your data. Book a free intro call

Free Training Zones Toolkit: the printable guide, a zones calculator for swim, bike and run, and a 2-week intensity audit. Get it here.

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About the author: Matt Warnock is an endurance coach based in the Netherlands who has coached more than 100 triathletes, runners, cyclists and swimmers, from first-timers to Ironman and 70.3 World Championship qualifiers. He is training partner to ATAC and Taxandria Triathlon Oisterwijk, and certified by INSCYD, Runeasi and CORE. More about Matt.

References

1. Achten, J., Gleeson, M. & Jeukendrup, A.E. (2002). Determination of the exercise intensity that elicits maximal fat oxidation. Medicine & Science in Sports & Exercise, 34(1), 92–97.
2. Allen, H. & Coggan, A. Training and Racing with a Power Meter. VeloPress.
3. Brooks, G.A. & Mercier, J. (1994). Balance of carbohydrate and lipid utilization during exercise: the "crossover" concept. Journal of Applied Physiology, 76(6), 2253–2261.
4. Gallo, G. et al. (2022). How do world class top 5 Giro d'Italia finishers train? A qualitative multiple case study. Scandinavian Journal of Medicine & Science in Sports, 32(12), 1738–1746.
5. Lillo-Bevia, J.R. et al. (2022). Is the functional threshold power a valid metric to estimate the maximal lactate steady state in cyclists? Journal of Strength and Conditioning Research, 36(1), 167–173.
6. Mann, T., Lamberts, R.P. & Lambert, M.I. (2013). Methods of prescribing relative exercise intensity: physiological and practical considerations. Sports Medicine, 43(7), 613–625.
7. Mujika, I. (2014). Olympic preparation of a world-class female triathlete. International Journal of Sports Physiology and Performance, 9(4), 727–731.
8. Seiler, K.S. & Kjerland, G.Ø. (2006). Quantifying training intensity distribution in elite endurance athletes: is there evidence for an "optimal" distribution? Scandinavian Journal of Medicine & Science in Sports, 16(1), 49–56.
9. Stöggl, T. & Sperlich, B. (2014). Polarized training has greater impact on key endurance variables than threshold, high intensity, or high volume training. Frontiers in Physiology, 5, 33.
10. Tanaka, H., Monahan, K.D. & Seals, D.R. (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology, 37(1), 153–156.