FTP vs Lactate Threshold vs Critical Power

FTP, lactate threshold, and critical power appear on nearly every training platform that cyclists, runners, and triathletes use — from TrainingPeaks dashboards to lab-issued test reports on Intervals.icu. Coaches reference them interchangeably. Athletes treat them as synonyms. Most software quietly conflates them.

They are not the same thing. Each metric is measured differently, reflects a different physiological reality, and points to different decisions when it comes to setting training zones. The numbers often land close to each other, which is precisely why the confusion persists — and why it matters to untangle them.

What is FTP, and how is it measured?

Functional Threshold Power (FTP) is the highest average power, in watts, a rider can sustain for approximately one hour. The concept was established by Coggan and Allen in Training and Racing with a Power Meter and has since become the default reference point for structured cycling and triathlon training.

Several methods exist for estimating it. Platforms like Intervals.icu can auto-detect FTP from recent hard efforts — a reasonable starting point. The classic 20-minute test asks athletes to ride all-out, then multiply average power by 0.95. The ramp test increases power each minute until failure, with the app calculating FTP from the stopping point.

FTP is a field-based estimate, not a direct physiological measurement. Its real value lies in anchoring training zones and computing Training Stress Score (TSS). If your FTP is 250 W, your threshold zone sits at roughly 235–265 W, and sweet spot — defined by Coggan as 84–97% of FTP — falls around 210–240 W. Get the number wrong and every zone downstream is wrong too.

What is lactate threshold, and why does it split into LT1 and LT2?

Lactate threshold (LT) is the exercise intensity at which blood lactate begins to accumulate faster than the body can clear it. Measuring it directly requires blood samples — typically a fingertip or earlobe prick — taken during a ramp test as intensity increases progressively. The standard OBLA (Onset of Blood Lactate Accumulation) marker sits at 4.0 mmol/L.

The threshold is not a single point. Physiologists distinguish two inflection points: LT1, the aerobic threshold, occurs around 2 mmol/L and roughly corresponds to 65–85% of maximum heart rate. Below LT1, the body clears lactate as fast as it produces it. LT2 — the anaerobic threshold or maximum lactate steady state (MLSS) — occurs around 4 mmol/L, marking the ceiling of sustainable hard effort.

Between LT1 and LT2 lies the tempo and threshold zone. Above LT2, lactate accumulates rapidly and fatigue follows. In the polarized training framework described by Seiler (2006), approximately 80% of weekly training volume sits below LT1, with hard work targeted above LT2.

LT2 is widely considered the closest lab-based equivalent to FTP. It requires blood sampling or a metabolic cart, though — a power meter alone cannot measure it directly.

What is critical power, and how does it differ from FTP?

Critical power (CP) is the highest power output that can theoretically be sustained indefinitely without fatigue. Unlike FTP — derived from a single timed effort — CP is a mathematical parameter, calculated by fitting multiple maximal efforts of different durations (3, 5, and 12 minutes, for example) to a hyperbolic power-duration curve. The method is rooted in the Monod-Scherrer model.

The model also produces W' (W-prime): the finite energy reserve available above CP, measured in kilojoules. W' determines how long a rider can sustain efforts above CP before exhaustion forces them to drop back below it.

CP typically sits 5–10% above a well-measured FTP. The reason is model optimism — CP assumes perfect pacing and full recovery between efforts, which is physiologically generous. Nobody sustains CP indefinitely in practice. Platforms like Golden Cheetah implement CP and W' for race-day pacing and fatigue modelling, though accurate results require several maximal tests on fresh legs.

How do FTP, LT2, and critical power compare?

For well-trained athletes, all three metrics typically land within 5–10% of each other. The gap widens for untrained individuals or athletes with unusual fatigue profiles. Attribution: Coggan and Allen for FTP zones; Seiler (2006) for the LT1/LT2 polarized framework; Monod and Scherrer for the CP concept.

Which metric should you use to set training zones?

For most self-coached cyclists and triathletes, FTP is the practical choice — easy to test, widely supported by apps including TrainingPeaks, Intervals.icu, and PlanWatts, and sufficient for accurate zone prescription. Joe Friel's periodization guidance recommends retesting every 4–6 weeks during a build phase to keep zones current.

Athletes with lab access will find LT2 from a blood-lactate ramp test to be the gold standard, particularly when combining polarized training with heart-rate or power targets at LT1. Direct blood measurement removes the guesswork that field tests introduce.

For advanced modelling and race pacing, CP and W' provide the most complete picture of fatigue dynamics. The trade-off is real: multiple all-out tests and familiarity with power-duration curve software are both required.

A practical rule of thumb: if your FTP test, LT2 lab result, and CP model agree within roughly 5%, use whichever you measured most recently. If they diverge significantly, retest FTP first and check for fatigue or pacing errors before drawing conclusions.