Cycling
FTP is a population average. It only matches reality at E ≈ 0.62.
Functional Threshold Power assumes all riders can sustain their lactate threshold for approximately 60 minutes. This is wrong — threshold duration depends on endurance. TrueZone replaces FTP with Lactate Threshold Power (LTP), an individualized metric that separates threshold intensity from threshold duration.
FTP critique
FTP is a 60-minute assumption. LTP is your actual threshold.
Threshold duration depends on endurance — ~35 min for novices, 140+ for elites. TrueZone replaces a population assumption with an individualized fit.
The Problem
FTP assumes everyone's threshold lasts 60 minutes. It doesn't.
Functional Threshold Power is defined as the highest power a rider can sustain for approximately one hour. But how long a rider can actually sustain their lactate threshold depends on their aerobic endurance — and that varies enormously between individuals. FTP conflates two independent physiological qualities: threshold intensity and threshold duration.
FTP overestimates low-endurance riders
38 cyclists with E < 0.65 cannot sustain their lactate threshold for 60 minutes. FTP assumes they can, inflating their threshold estimate and setting zones too high.
FTP underestimates high-endurance riders
186 cyclists with E > 0.75 can sustain threshold well beyond 60 minutes. FTP caps their capacity, setting zones too low and leaving performance on the table.
FTP is only correct at E ≈ 0.62
The 60-minute assumption matches reality for riders whose endurance puts their lactate threshold duration at 60 minutes. Under the full TZPD model this is E ≈ 0.62. For everyone else — the majority — it is systematically wrong.
The Solution
Lactate Threshold Power is individualized.
TrueZone separates threshold intensity from threshold duration. LTP is the power at your actual lactate threshold. How long you can sustain it depends on your endurance (E). The model derives both from ordinary ride data — no 20-minute test, no ramp protocol, no calibration ride.
FTP gets two things wrong, not one. First, the threshold power itself (because duration is assumed, not measured). Second, the zone boundaries around it. FTP-based zones use fixed percentages of FTP — the same ratios for every rider. But endurance shifts where thresholds actually fall relative to each other. A high-endurance rider has their aerobic zones compressed closer to threshold; a low-endurance rider has them spread further apart. TrueZone derives zone boundaries geometrically from each individual's threshold alignment, not from population averages.
LTP sustainable duration by endurance
FTP's fixed 60-minute assumption is only correct at E ≈ 0.62. For the majority of cyclists, it is systematically biased. LTP eliminates this bias by modeling threshold intensity and duration independently.
FTP vs LTP across the endurance spectrum
The Zone Problem
Even if FTP were correct, the zones would still be wrong.
Every platform uses the same fixed percentages of FTP to define power zones. Zone 2 is always 56–75%. Zone 4 is always 91–105%. But these boundaries assume everyone's physiology distributes power the same way. It doesn't.
A high-endurance rider's zones compress near the top — their aerobic system dominates, and the gaps between thresholds are narrow. A low-endurance rider's zones spread wide from bottom to top. Same FTP, completely different zone shapes. Fixed percentages can't capture this.
TrueZone's zone boundaries are set at actual metabolic thresholds: Fatmax, LT1, LT2, vVO₂max—not arbitrary percentages of a single number. Each boundary corresponds to a real physiological transition where the body shifts from one energy system to another.
Standard zones vs individualized zones — adjust endurance to compare
Standard power zones (fixed % of FTP — same for everyone)
TrueZone individualized power zones (E = 50% — LTP below FTP)
Low endurance (50%): Zones spread wide below threshold. This rider's real lactate threshold is well below FTP, so every standard zone overestimates their intensity. What Zwift calls “Zone 2” is actually above their real threshold — they're burning carbs when they think they're building base.
Validation
297 cyclists. 13,887 sessions. Median MAE 6.0 bpm.
Validated on the GoldenCheetah OpenData dataset — real-world cycling data from a diverse population of riders. The model achieved a 99.5% session success rate, with 71% of riders reaching full parameter convergence.
Mean endurance (E)
The GoldenCheetah population skews toward experienced cyclists. Mean E well above the 0.62 population average confirms that FTP systematically underestimates their threshold durability.
Mean peak aerobic power
Peak aerobic power (~VO₂max intensity) derived from the model. Sprint power is estimated separately using an endurance-dependent multiplier.
Median MAE
Median absolute error in heart rate prediction across all fitted sessions. Comparable to running validation and sufficient for accurate zone and threshold derivation.
Bayesian Learning
Parameters converge from ordinary rides.
No FTP test required. Each ride refines the model's estimate of the rider's endurance. The confidence interval narrows as evidence accumulates, stabilising within 3–10 rides.
Interactive
Power-duration explorer
Set your lactate threshold power and endurance to explore how the model predicts sustainable power across durations. See how LTP and FTP diverge as endurance moves away from the population average.
LTP vs FTP
LTP (Lactate Threshold)
Sustainable for 1:12:08
FTP (60-minute power)
Sustainable for exactly 60 min
VO₂max power
~5 min sustainable
Sprint power (est.)
3.0× aerobic peak
This rider's high endurance (E = 0.70) means they can sustain their lactate threshold for 72 minutes — well beyond 60. Their true 60-minute power is 10 W above LTP. FTP underestimates this rider's threshold durability.
Power-duration curve
Predicted sustainable power across durations, derived from your lactate threshold power and endurance. Power values account for aerodynamic and rolling resistance physics. LTP sits where the rider can actually sustain threshold; FTP sits at 60 min. They only coincide near E ≈ 0.70.
| Duration | Power | % of LTP |
|---|---|---|
| 5 min | 449 W | 180% |
| 10 min | 387 W | 155% |
| 20 min | 332 W | 133% |
| 30 min | 303 W | 121% |
| 60 min(FTP) | 260 W | 104% |
| 90 min | 238 W | 95% |
| 2 hr | 224 W | 90% |
| 3 hr | 206 W | 82% |
Power thresholds
Exercise thresholds reflect transitions between muscle fiber recruitment patterns. T2 is the lactate threshold — the boundary between sustainable and unsustainable intensity. “Sustainable for” is the full-TZPD prediction of how long this rider can hold each threshold; “iLevel zone” maps each threshold to Hunter Allen's individualized power levels (used in WKO5/TrainingPeaks and Zwift workouts).
| Threshold | Description | Power | Sustainable for | iLevel zone |
|---|---|---|---|---|
| T1 | Endurance threshold (LT1) | 114 W | 6+ hr | Endurance |
| T1.5 | Aerobic threshold | 173 W | 6+ hr | Tempo / Sweet Spot |
| T2 (LTP) | Lactate threshold | 250 W | ~72 min | Threshold (iFTP) |
| T2.5 | VO₂max threshold | 349 W | ~16 min | Sweet Spot Hard / iAnT |
| Sprint | Neuromuscular peak | 1349 W | ~10 sec | Neuromuscular (iPmax) |
Replace estimated FTP with measured LTP.
Give your cyclists individualized threshold power and endurance-adjusted zones. TrueZone is SDK and API ready today.