New Critical Power and W′ metrics on your Performance page

What's new

We've added Critical Power (CP) and W′ (W-prime) to the Performance page---the two parameters of the critical-power model rooted in endurance physiology (Monod & Scherrer, 1965). CP is the highest power you can hold in something close to steady state: the line above which fatigue ramps quickly. It often tracks FTP within a few watts, but unlike a fixed 60-minute test target, CP is inferred from the shape of your power--duration curve over the dates you select.

W′ is the finite amount of work you can spend above CP before you have to settle back down---your anaerobic "matchbook." Hard efforts above CP spend it; time below CP lets it refill. Larger W′ usually means sharper attacks, longer surges, and bigger breakaways. Both tiles sit in the Power section of Performance, only for cycling, next to FTP, power-to-weight, and power bests. CP is in watts with a Coggan-style tier badge, same spirit as FTP. W′ is shown in kilojoules; its badge combines mass-normalized capacity (for example 220 J/kg) with tier labels from Developing through Recreational, Trained, Strong, and Elite.

We fit the Monod--Scherrer two-parameter model (with Morton's three-parameter formulation as a fallback) to your combined power--duration curve in the chosen window. The fit leans on a strong short effort (~3--5 min) and a strong longer effort (~10--30 min) as its natural anchor points---without those extremes, estimates get shaky.

W′ accuracy deserves a straight note: if you haven't actually gone very hard briefly lately, fits can shrink W′---not because your anaerobic capacity vanished, but because your data hasn't demonstrated it yet. Watch for weak fits when your best ~3 minute barely beats your best ~20 minute; W′ can collapse there. A maximal ~3-minute effort from a rested start ("CP3") is usually the clearest thing you can feed the model; after one of those, W′ tends to stabilize at something credible.

Why it helps

You get a sharper read on the sustainable ceiling (CP) and how hard you can go above it before paying the bill (W′)---so FTP and bests sit in fuller context for pacing, interval design, and understanding why certain efforts feel manageable vs. draining.

How to try it

Open Performance, scroll to Power, and pick your date range like usual---the new Critical Power and W′ tiles appear alongside your other cycling power summaries.

Note: Applies to cycling activities only. Estimates reflect the workouts in your selected range and underlying power data quality.

Example