CSS Test (swim assessment, 6 steps, ~41 min)

What this assessment is

The CSS Test is the seeded swim assessment that pairs with the CSS calculator to produce your Critical Swim Speed (your sustainable threshold pace per 100 m) and your Speed Drop-off Index (SDI).

Domain: swim. Estimated duration: 41 minutes. Linked calculator: css.

The seeded 6-step protocol (standard 400 + 200)

1. Warm-up (15 min): 800 m easy with mix of drills and build efforts.
2. Rest (5 min): rest at wall. Stay loose.
3. 400 m all-out (~6 min): swim 400 m at maximum sustainable pace. Record time accurately.
4. Rest (2 min): rest at wall.
5. 200 m all-out (~3 min): swim 200 m at maximum effort. Record time accurately.
6. Cool-down.

Inputs the form asks for

The seeded form lists two required fields named after the default protocol:

• 400 m Time in mm:ss.
• 200 m Time in mm:ss.

The names are just labels — the math is (d2 - d1) / (t2 - t1), so any pair of distances works.

Other protocols that fit this assessment

The CSS calculator and the math behind it accept any two-distance pair, plus single-TT and 3-rep modes. Run whichever protocol suits the session — record the actual distances in the assessment notes so the result is interpretable later.

Canonical: 400 + 200 (above) and 100 + 500 (60 s rest)

100 + 500 is a session-friendly compressed variant:

• Warm-up 600-1000 m + 4 x 50 build.
• 100 m all-out (fresh).
• 60 seconds at the wall.
• 500 m all-out.
• Cool-down.

To log a 100+500 in the seeded form, enter the 100 m time into the "200 m Time" field and the 500 m time into the "400 m Time" field — the slope math doesn't care which label is on each input. Add a note ("100/500 protocol with 60 s rest") so the seasonal trend is interpretable.

Other valid protocols

The calculator doesn't constrain you to either canonical pair:

• 1000 m all-out single TT — open the calculator directly (not via the seeded assessment) and use input mode 3.
• 3 x 400 with full recovery — input mode 2 (3-rep set); enter time + SR + DPS for each rep, get averaged CSS + SDI.
• Any two distances — 800 + 200, 600 + 300, etc. As long as the distances differ enough to give a stable slope, the estimate is fine.

The seeded 6-step assessment template is just one supported shape; coaches who run a different protocol can either reuse the seeded form (enter distances in either field, add a note) or open the calculator directly.

What to keep consistent

Pick a protocol for the athlete and stick with it across the season — different protocols give slightly different CSS estimates and SDI readings, so retesting with a different protocol on top of a 12-week training block confounds the trend. Athlete's relative improvement is the signal.

Outputs

• CSS Pace in seconds per 100 m (formatted mm:ss per 100 m).
• Speed Drop-off Index (SDI) as a percentage.

What the SDI tells you

SDI compares the two paces. A small drop (low SDI) means the athlete holds pace well across distances — more economy-limited than VO2-limited. A large drop (high SDI) means the athlete falls off sharply above threshold, suggesting room for top-end work.

Caveat: SDI varies by protocol. The 100+500 variant tends to read higher than 400+200 for the same swimmer because the 100 m is more anaerobically loaded. Compare like with like — within a protocol, across retests, not across protocols.

How to log

Open the athlete's profile, click New assessment, select the CSS Test, enter the two times (or for non-standard protocols, note which distances you ran), and save.