CSS from 3 points (200 / 400 / 800 m)

Three maximal swims at increasing distances. A linear regression between distance and time produces CSS, with R² as a goodness-of-fit measure.

Protocol

Swim 200 m, 400 m, and 800 m, each at maximal effort, with adequate recovery between them. Times must increase with distance (200 < 400 < 800). Select the pool type and record the three times.

Formula

Linear regression of (distance, time) over the three points; the slope coefficient (in s/m) is the inverse of critical speed:

pace/100 = slope × 100

The result includes the R² of the fit — the closer to 1, the more consistent the three swims are with each other; a poor fit signals that one of the times may not represent a genuinely maximal effort.

About the source

Same scientific basis as the 2-point protocol (Wakayoshi et al., 1992, 1993): using three distances instead of two improves the statistical robustness of the critical speed estimate.