Glossary: FTP, CSS, threshold pace

Three threshold anchors, one per sport. Each is your sustainable maximum for that discipline — the ceiling you can hold for roughly an hour. Zones and pacing prescriptions all hang off these numbers.

FTP, Functional Threshold Power (bike)

The bike power you can sustain for approximately 1 hour. The most common bike threshold metric, anchored in watts. Used to compute the Yousuli 8-band bike zones.

Common ways to determine FTP:

• 20-minute time trial: 95% of your average 20-min power is a reasonable FTP estimate for many athletes.
• Full 60-minute time trial: the gold standard. If you have a recent solid hour on the bike (e.g. a hilly time trial), use that average.
• Ramp test: a graded test that estimates FTP from your point of failure.

Retest FTP every 4 to 8 weeks during a build phase, less often in base.

CSS, Critical Swim Speed

Your best steady swim pace sustainable for roughly 30 minutes in a pool. Used as the swim equivalent of FTP, anchoring swim training zones and pace prescriptions.

CSS is estimated from a two-distance time trial — any pair of distances works, since the math is (d2 - d1) / (t2 - t1). The CSS calculator also supports a 3-rep set with stroke rate / DPS (for stroke economy via the SDI score) and a single 1000 m TT as a fallback.

Two canonical protocols

Most coaches use one of these:

Standard 400 + 200 (long rest)

• Warm-up.
• 400 m all-out.
• 5 to 10 min easy.
• 200 m all-out.

This is the protocol seeded in the CSS Test assessment and step 1 of the race planning wizard.

100 + 500 (60 s rest)

• Warm-up.
• 100 m all-out, fresh.
• 60 seconds rest at the wall.
• 500 m all-out.

A faster session-friendly variant (~11 minutes after warm-up vs 25-30 for the standard 400+200). The 60-second rest leaves the 500 partially recovered, which is intentional and consistent across retests.

Other valid protocols

Coaches aren't locked into either canonical pair:

• 1000 m all-out single TT — feeds the calculator's single-TT input as a quick baseline.
• 3 x 400 with full recovery — feeds the 3-rep input; also reads stroke rate and DPS to compute SDI.
• Any two distances — 800+200, 600+300, etc. The math is the same as long as the pair is far enough apart to give a stable slope (don't pair 200+250).

Whichever protocol you pick, retest with the same protocol so the season trend is meaningful. Cross-protocol comparisons add noise.

Retest every 6 to 12 weeks.

Threshold pace or LT pace (run)

The fastest running pace you can hold for roughly 45 to 60 minutes. Similar to FTP and CSS but for running. Anchors run training zones.

Common ways to determine:

• 10K race result: average pace from a recent 10K is close to threshold.
• 30 minute time trial.
• Lab lactate test for accuracy.

Retest every 8 to 12 weeks, or implied from race results.

Why three different anchors

Each sport has a different limiting system:

• Bike: power is independent of terrain when normalised.
• Swim: pace depends on stroke and drag.
• Run: pace varies with surface, grade, and wind.

A single "threshold heart rate" doesn't translate cleanly across all three. Anchor each discipline to its own metric and trust the zones built from there.