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Normalized power, intensity factor and training stress

Normalized Power raises a 30-second rolling average of your watts to the fourth power before averaging, which weights surges the way your physiology does — a ride of 200 W with hard punches costs more than a flat 200 W. Intensity Factor is that number divided by your FTP, and TSS is IF squared times hours times 100, so one hour at threshold is 100 TSS by definition.

All three lean on FTP being a real physiological threshold. Borszcz and colleagues tested that assumption directly: in trained cyclists the 20-minute-test FTP tracked the maximal lactate steady state closely on average but with individual differences of several percent in either direction ^[1]. That spread is why the audit shows an estimated FTP as a band, not a point, and why a tested FTP tightens every number on the page.

Decoupling: heart rate against power over time

On a long steady ride, stroke volume falls and heart rate climbs to compensate while the work stays the same — cardiovascular drift ^[2]. Heat makes it worse: in the heat, drift between 15 and 45 minutes corresponded to a proportional loss in VO2max, so a fixed heart-rate target quietly becomes a harder effort as the ride goes on ^[3].

The audit compares the heart-rate-for-power relationship in the first and second halves of the ride at a shared reference power, which removes the pacing confound a plain ratio carries. It is shown only on rides that were steady and aerobic enough for the comparison to mean something, because on tempo and threshold rides the drift reads pacing, not durability. Indoors the same physiology runs warmer: at matched effort, skin temperature was higher and airflow near zero in the lab, which is why a fan changes this number more than fitness does over one winter ^[4].

Time in zone and session balance

The seven power zones are fixed fractions of FTP; the seven heart-rate zones are fractions of maximum heart rate, used only when the file has no power. The session-balance bar collapses them into easy, moderate and hard, the three-zone split Seiler's work on intensity distribution uses ^[5]. Across sports, the athletes who improve most spend most of their time easy and a small share genuinely hard; the moderate middle is where training load accumulates without a matching return.

Critical power and the work above it

When a ride contains hard efforts between three and twenty minutes, the audit fits a hyperbola through them. The asymptote is critical power — the highest intensity at which physiological steady state is still reachable — and the curvature is W′, the finite reserve of work you can do above it ^[6]. Critical power sits a little above the one-hour threshold FTP approximates, and the two-parameter model predicts time to exhaustion above it ^[7].

One ride rarely contains a full set of maximal efforts, so the fit is shown only when the curve supports it with medium or high confidence, and never when it had to be propped up by your FTP.

Durability: the power that survives fatigue

Durability is the resistance of your physiological profile to deterioration over a long ride — how much of your fresh power is still there after hours of work ^[8]. The audit measures it within the ride you uploaded: your best effort at a matched duration after 1,500 kJ of work, divided by the best fresh effort earlier in the same ride. A ratio near one means the engine held; a ratio well below it is the gap long events are decided on.