Intensity Factor

Definition

“Intensity Factor” (IF) is the ratio of a training session’s normalized power (NP) to the athlete’s stored threshold power. The value is dimensionless and describes at what fraction of threshold a session or segment took place.

IF = NP / threshold power

In running, normalized speed or pace is often used instead of power. IF is also one of the inputs for the Training Stress Score (TSS).

Why it matters

Average power underestimates the load of sessions with strongly varying intensity. Normalized power corrects for this, but only the reference to the athlete’s own threshold makes the value comparable: 250 W is base for one athlete and threshold training for another.

IF places every session on a scale that means the same thing for all athletes. At a glance it shows whether a session hit the planned intensity.

An IF above 1.05 sustained over a full hour is not physiologically plausible. When it occurs, the stored threshold power is almost always too low.

How obseed measures it

obseed calculates IF per session and per segment from normalized power and the threshold stored in the performance parameters. A complete set of performance parameters for the sport must be available.

The quality of IF depends entirely on the quality of the threshold. An FTP estimated from a 20-minute test shifts every IF by the same error. obseed therefore recommends an objectively measured threshold from a metabolic or lactate test as the reference, not a field-test estimate.

Example

An athlete with a diagnostically measured threshold power of 280 W rides 90 minutes at a normalized power of 230 W.

IF = 230 / 280 = 0.82

The session sat in the base-to-tempo range. From this follows the Training Stress Score:

TSS = IF² × duration in hours × 100
    = 0.82² × 1.5 × 100
    = 0.672 × 1.5 × 100
    = 101 TSS

Had the threshold been stored as 260 W because a 20-minute test estimated it too low, the same ride would show IF 0.88 and 117 TSS. The workout would suddenly count as near-threshold although nothing changed physiologically.