What’s the difference between FTP and critical power?
FTP and critical power describe the top end of steady aerobic power, but they aren’t identical. Knowing how they’re defined, tested, and applied will help you set better training zones, choose smarter workouts, and pace climbs, TTs, and surges with confidence.
Definitions in plain language
Functional threshold power (FTP) is a practical benchmark for your maximal steady power. It’s intended to approximate the highest power you can sustain without a continual rise in lactate and ventilation (often linked to maximal lactate steady state, MLSS). Historically it was called your 60‑minute power, but in real riders the time to exhaustion (TTE) at FTP can range from about 35 to 75 minutes.
Critical power (CP) comes from a mathematical model fitted to multiple all‑out efforts of different durations. CP is the horizontal asymptote of your power–duration curve—the boundary between sustainable and unsustainable effort. The CP model also gives you W′ (W prime), your finite “battery” of work you can do above CP before you must slow down.
| Aspect | FTP | Critical power |
|---|---|---|
| Definition | Practical estimate of your maximal steady state (near MLSS) | Model‑derived aerobic steady state (asymptote of power–duration) |
| How it’s set | Single test (e.g., 20‑min x 0.95, ramp test, or open‑ended threshold test) | Multiple maximal efforts across durations (e.g., 3–5, 8–12, 20–30 min) |
| Extra parameters | None | W′ (anaerobic work capacity) |
| Typical TTE at value | ~35–75 min (individual) | ~30–70 min (individual, model‑dependent) |
| Sensitivity to anaerobic capacity | Varies by protocol (ramp and 20‑min tests can be skewed) | Lower if using adequate long durations; W′ captures anaerobic contribution |
| Common uses | Training zones, pacing steady efforts | Zones, W′ pacing for surges, interval design |
Key differences and why they matter
- Computation vs. protocol: FTP depends on the test you choose. A ramp test (from maximal aerobic power) can overestimate FTP in riders with big anaerobic punch and underestimate it in diesel time trialists. CP uses multiple efforts and is less biased by a single duration.
- Actionable add‑on: CP gives you W′. That lets you quantify how much work you can do above CP and how fast you deplete or recover that “battery.” It’s powerful for crits, punchy road races, and rolling TTs.
- Numbers don’t always match: In practice, CP and FTP often land within a few watts of each other, but either can be higher depending on the method and the athlete. The important part is using one anchor consistently and cross‑checking it against real‑world TTE.
Coach tip: whichever anchor you choose, validate it. If you can’t hold it for ~40–60 minutes on a good day, it’s set too high. If you can hold it for 75+ minutes, it’s likely too low.
How to test and set your numbers
Option A: set FTP
- Open‑ended threshold test (field or trainer): Warm up thoroughly. Settle at a hard but steady effort you believe you can maintain, then ride to exhaustion without surges. Average power for the steady portion ≈ FTP. Record your TTE at that power.
- 20‑minute test: Go all‑out for 20 minutes. Multiply average power by ~0.93–0.97 (95% is a starting point, but adjust based on your physiology and TTE).
- Ramp test: Use with caution. Convert peak one‑minute power with your platform’s formula, then validate with a steady 35–50 min effort within a week.
Option B: set CP and W′
Across 7–10 days, perform at least three fresh maximal efforts of different lengths:
- Short: 2–3 minutes (near VO2max)
- Medium: 8–12 minutes
- Long: 20–30 minutes (steady state)
Use those efforts to fit the CP model (many platforms do this automatically). Include at least one long effort; without it, CP may be biased high.
Applying FTP and CP to training
Training zones
- FTP‑anchored zones: Endurance (55–75% FTP), tempo (76–90%), sweet spot (88–94%), threshold (95–105%), VO2max (106–120%), anaerobic capacity/sprints above that.
- CP‑anchored zones: Similar boundaries, but with one key upgrade: you can target work above CP using W′ instead of only percentage zones.
Designing intervals
- Threshold development: 2–3 x 15–25 min at 95–100% FTP or ~CP, 5–8 min easy between. Progress by lengthening time in zone before raising power.
- VO2max: 4–6 x 3–5 min at 110–120% FTP or power that elicits near‑max respiration. With CP, aim to deplete ~30–50% of W′ across the set.
- W′ targeting (punchy racing): 6–10 repeats of 30–60 s at 140–170% of CP with 2–4 min easy. Track estimated W′bal between efforts; start the next rep when you’ve recovered to ~60–70% of W′.
- Sweet spot durability: 3 x 20 min at 88–94% FTP/CP to build TTE and muscular endurance.
Pacing with real examples
Assume CP = 280 W, W′ = 20 kJ, FTP ≈ 285 W.
- 20‑minute climb (TT‑style): Target 95–100% FTP/CP (265–285 W). Avoid surges above 110% early. Settle, then lift slightly in the final 5 minutes if breathing stays controlled.
- Punchy climb with a 45‑second ramp: If you surge to 450 W for 30 s, you use (450 − 280) × 30 s = 5.1 kJ of W′—about 25% of your “battery.” Plan no more than 3–4 such surges before you’ll need meaningful recovery.
- Rolling TT with short rises: Climb at CP + 20–40 W for 30–60 s and descend slightly below CP to recharge W′. Average power stays high, but you avoid emptying the tank.
Rule of thumb: each hard surge above CP spends W′; each minute a bit below CP helps recharge it. Use that trade‑off to keep your average watts high without cracking.
Which should you use?
- Crits and road races with lots of surges: CP + W′ gives superior pacing insight for attacks, hills, and breakaways.
- Time trials, steady climbs, gran fondos: Either FTP or CP works well. Pick one, validate it with TTE, and pace at or just under that value.
- Indoor‑only or single‑test convenience: FTP is quick to set, but verify with a steady effort so your zones match reality.
The best choice is the one you can measure well and apply consistently. Re‑test every 4–8 weeks, track your TTE at threshold, and adjust zones so that hard days are hard and easy days are easy.
