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), a percentage that shows how much pace drops between the two distances.

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

The 6-step protocol (as seeded)

1. Warm-up (15 min): swim 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 effort. Record time accurately.
4. Rest (2 min): rest at wall.
5. 200 m all-out (~3 min): swim 200 m at maximum sustainable effort. Record time accurately.
6. Cool-down.

Inputs the form asks for

Grepiac's expected_inputs schema lists exactly two required fields:

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

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 athlete's 400 m and 200 m paces. A small drop (low SDI) means the athlete holds pace well across distances and is 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.

How to log

Open the athlete's profile, click New assessment, select CSS Test, fill in the two times, save. The linked calculator runs and stores both outputs. The CSS pace also feeds the MetricHistory record that drives swim zones.

See also: Grepiac swim zones, Logging an assessment overview.