Preliminary
After 11 Forest Tests, How Much Speed the Treadmill Misses
- Paired tests
- 11
- R²
- 0.79
- Left unexplained
- 21%
The short version
Eleven athletes now have both tests, treadmill and forest, at 4 mmol/L. Names are codes A1–A11, shuffled so they do not follow speed.
Treadmill speed sets the broad level. It explains about 79% of the spread in forest speed, so roughly 21% is something else: terrain, technique, rough-ground running. That 21% is a point estimate. With only 11 people it could plausibly be a lot bigger or a lot smaller.
What the leftover looks like
Forest speeds in the group vary by 1.12 km/h (standard deviation). Once treadmill speed is accounted for, the leftover is 0.53 km/h. In the forest that is about ±15–17 s/km. Forest speed here runs from 58% to 68% of treadmill speed, averaging 63%.
Forest speed relative to the treadmill line
Click an athlete. Bars are km/h faster or slower in the forest than the treadmill line predicts.
The residuals run from +0.81 km/h (A5) to −1.01 km/h (A4).
How sure is 0.79?
Two usual methods, and they agree. Fisher's z assumes the speeds are roughly normal. The bootstrap redraws the 11 athletes over and over. One person can swing a redraw, so both are shown.
90% confidence interval for R²
At 90% confidence, treadmill speed explains somewhere between about half and a bit over 90% of forest speed. If R stayed near 0.89, the planned sample of 25 athletes would pull the lower bound up to about 0.6 which is much better.
Some of the leftover is just noise. Each 4 mmol/L speed comes from five points, and a few fits were not perfect, so those values probably carry ±0.2–0.4 km/h of uncertainty on their own.