FTP, threshold heart rate, W/kg and critical power: the numbers behind the zones
Four numbers shape how you train and how you compare yourself. Here is what they actually measure, how to find them, and how far you can trust them.
One number sets every session
Every ZoneDeux zone is a percentage of FTP. Zone 2 is 55–80% of FTP and threshold 95–105%, and every session in the session library and the training programs is written the same way. One number therefore sets the intensity of everything you do.
If your FTP is set 5% too high, every session is 5% too hard. The zone 2 ride drifts up into zone 3, and the threshold intervals get heavier than they should be. That is why it pays to know what the number measures, how to find it, and how uncertain it is.
This article covers FTP, threshold heart rate, watts per kilo and critical power. For each number we look at the definition, what research says about its precision, and how to use it in training.
FTP: the definition and the reality
FTP stands for functional threshold power. It is defined as the highest power you can hold in a quasi-steady state for about one hour without fatiguing (Allen & Coggan 2010). The number is a practical estimate of the threshold, where blood lactate stops being stable.
At group level the estimate is good. In trained cyclists FTP was on average 1.4% from the power at maximal lactate steady state (MLSS), the highest load with stable lactate, and the correlation was 0.91 (Borszcz et al. 2019). For an individual, though, the difference could be around 9% in either direction, and that spread is the one that applies to you.
"One hour" is the definition, not a promise. When trained cyclists rode to exhaustion at their FTP, they lasted 51 minutes on average, with large differences between individuals (Borszcz et al. 2018).
In the ZoneDeux system the threshold sits at 95–100% of FTP, and FTP itself in the middle of zone 4. You do not need a laboratory to train correctly, but you should treat FTP as a good estimate rather than a fact.
How to find your FTP
Every test gives an estimate, and every test can miss. Choose one method and stick to it, so your results are comparable over time.
4 × 12 minutes: our recommendation
We recommend a hard interval session instead of a formal test: 4 × 12 minutes with 4 minutes easy between efforts. Ride so hard that you could have done one more effort as well as the last. FTP is roughly the average power of the efforts multiplied by 0.95. The method is based on our own experience, not on a published study. The advantage is that the test is also a good training session, so you can repeat it regularly without sacrificing a day to testing.
The 20-minute test
The most widely used test is 20 minutes as hard as you can sustain evenly, with FTP set to 95% of the average power (Allen & Coggan 2010). The factor is an average. In trained cyclists the 95% rule overestimated MLSS by 12 watts, while an individual factor of 91% matched it (Lillo-Beviá et al. 2022).
The warm-up also changes the answer. With the original protocol, where a 5-minute maximal effort comes before the 20 minutes, the average was 256 watts. With a self-selected warm-up it was 270 watts (Tramontin et al. 2022). Use the same warm-up every time you test.
The ramp test
The ramp test is the quickest. The load rises every minute until you cannot continue, and FTP is set to 75% of your best minute of power (TrainerRoad). The factor is fixed, so the test does not fit everyone equally well. We do not provide a file for it. Use the ramp test in your training app.
In the training calculator you will find the 4 × 12 test and two versions of the 20-minute test as Zwift files. There you can also enter your result and use the FTP directly in your zones.
→ Find your FTP in the training calculatorWhatever the method, rather set FTP slightly too low than slightly too high. Zones that are a little low still train in the right range, while zones that are too high push you up into the next zone. Retest every 4–8 weeks, preferably at the end of an easier week.
Threshold heart rate (LTHR)
Without a power meter you can use threshold heart rate, lactate threshold heart rate, instead. Joe Friel's test is simple: ride 30 minutes as hard as you can, on your own, and take the average heart rate of the last 20 minutes (Friel 2011). Do not watch your heart rate during the test, and do not do it with others or in a race, because that changes the result.
Heart rate at FTP matches heart rate at MLSS reasonably well. In trained cyclists it was 1.4% lower on average, but the spread between individuals was around 8% in either direction (Borszcz et al. 2019).
Heart rate has weaknesses that power does not. It creeps up during a steady session, and dehydration and heat can change the relationship between heart rate and load considerably (Achten & Jeukendrup 2003). Use power as your main measure when you have it, and heart rate as a check.
The training calculator converts LTHR to ZoneDeux heart-rate ranges and has a Zwift file for the test. The ranges are guidance, because our zones are defined on power.
→ See your heart-rate ranges in the training calculatorWatts per kilo
W/kg is FTP divided by body weight. A cyclist with an FTP of 300 W who weighs 75 kg has 4.0 W/kg. The number says most about how fast you climb, because you lift your whole body weight against gravity.
On flat roads absolute watts matter more. Among professional cyclists, the flat specialists had the highest absolute power and the climbers the highest power per kilo. Climbing ability followed plain W/kg, while ability on flat roads depended far less on weight (Padilla et al. 1999).
The calculator shows where your W/kg places you in Coggan's power profile. Use it as a rough guide, because the categories are broad and the table for women is an estimate.
Losing weight to raise W/kg is a risky shortcut. Among competitive male cyclists, 28% had low energy availability, too little energy for the training they did, and 44% had low bone density in the lower spine. Body fat percentage was not linked to performance (Keay et al. 2018).
Critical power and W′
Critical power (CP) rests on a different idea than FTP. The harder you ride, the shorter you last, and the relationship forms a curve. CP is the power the curve levels out towards (Vanhatalo et al. 2011).
Above CP you have a limited work reserve, called W′ and measured in kilojoules. The further above CP you ride, the faster the reserve empties, and when it is gone you have to ease off. CP therefore marks the boundary between load the body can keep in balance and load it cannot (Jones et al. 2019).
CP is calculated from several maximal efforts that typically last 2–15 minutes (Poole et al. 2020). Even though CP is a boundary for steady load, you usually cannot hold it for more than 20–30 minutes (Jones et al. 2019).
CP and FTP are not the same number. In one study CP was on average 7 watts above FTP, but for individuals the difference ranged from 19 watts below to 33 watts above, and the authors advise against using the two interchangeably (Karsten et al. 2020).
We use FTP as the anchor because it takes one test, and because all our zones, sessions and programs are written as a percentage of FTP. CP is useful when you want to plan efforts above threshold or see how much of the reserve an interval uses.
How the numbers fit together
| Number | What it measures | How to find it | Use it for |
|---|---|---|---|
| FTP | Estimate of the threshold in watts | 4 × 12 min, 20-minute test or ramp test | All power zones and sessions |
| LTHR | Heart rate at threshold | 30 min alone, average of the last 20 min | Heart-rate ranges when training without power |
| W/kg | FTP relative to body weight | FTP divided by weight | Climbing ability and comparison |
| CP and W′ | The boundary of steady load and the reserve above it | Several maximal efforts of 2–15 min | Planning and analysing efforts above threshold |
Test regularly with the same method, set FTP slightly low rather than high, and use power as your main measure and heart rate as a check. Then your zones are right, and your progress becomes visible over time.
Sources
- Allen H, Coggan A (2010). Training and Racing with a Power Meter, 2nd ed. VeloPress.
- Borszcz FK, Tramontin AF, Bossi AH, Carminatti LJ, Costa VP (2018). Functional Threshold Power in Cyclists: Validity of the Concept and Physiological Responses. Int J Sports Med 39(10):737–742.
- Borszcz FK, Tramontin AF, Costa VP (2019). Is the Functional Threshold Power Interchangeable With the Maximal Lactate Steady State in Trained Cyclists? Int J Sports Physiol Perform 14(8):1029–1035.
- Lillo-Beviá JR, Courel-Ibáñez J, Cerezuela-Espejo V, Morán-Navarro R, Martínez-Cava A, Pallarés JG (2022). Is the Functional Threshold Power a Valid Metric to Estimate the Maximal Lactate Steady State in Cyclists? J Strength Cond Res 36:167–173.
- Tramontin AF, Borszcz FK, Costa V (2022). Functional Threshold Power Estimated from a 20-minute Time-trial Test is Warm-up-dependent. Int J Sports Med 43:411–417.
- TrainerRoad. Ramp Test Makes FTP Testing More Efficient and Less Stressful. trainerroad.com/blog.
- Friel J (2011). Determining your LTHR. joefrieltraining.com.
- Achten J, Jeukendrup AE (2003). Heart rate monitoring: applications and limitations. Sports Med 33:517–538.
- Padilla S, Mujika I, Cuesta G, Goiriena JJ (1999). Level ground and uphill cycling ability in professional road cycling. Med Sci Sports Exerc 31:878–885.
- Keay N, Francis G, Hind K (2018). Low energy availability assessed by a sport-specific questionnaire and clinical interview indicative of bone health, endocrine profile and cycling performance in competitive male cyclists. BMJ Open Sport Exerc Med 4:e000424.
- Vanhatalo A, Jones AM, Burnley M (2011). Application of critical power in sport. Int J Sports Physiol Perform 6:128–136.
- Jones AM, Burnley M, Black MI, Poole DC, Vanhatalo A (2019). The maximal metabolic steady state: redefining the 'gold standard'. Physiol Rep 7:e14098.
- Poole DC, Rossiter HB, Brooks GA, Gladden LB (2020). The anaerobic threshold: 50+ years of controversy. J Physiol.
- Karsten B, Petrigna L, Klose A, Bianco A, Townsend N, Triska C (2020). Relationship Between the Critical Power Test and a 20-min Functional Threshold Power Test in Cycling. Front Physiol 11:613151.
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