Here’s a thing I ran on anonymized data I always wanted to answer myself.
Question: how many days fully off before you actually lose fitness?
TL;DR: a missed week is basically free, two weeks is a few percent you’ll get back quickly. Full blog article: We Measured 35,735 Sessions: How Long You Can Take Off Before You Lose Fitness | athletedata.health
I didn’t want to use CTL for this, because CTL dropping during a break is just the impulse-response model decaying on its time constant whether or not your physiology changed. So I used aerobic efficiency instead: NP/HR for rides, speed/HR for runs, on sessions >=20 min, IF < 1.05 where available. Higher = more output at the same HR.
Method: 35,735 sessions across 406 athletes. Each session z-scored against that athlete’s own efficiency baseline (>=15 sessions per athlete+sport), so we’re comparing each person to themselves, not pooling a Cat 2 with a beginner.
Then sorted by gap = days since the previous training day (any sport), looking at the session that ended the gap. 1 SD of efficiency works out to ~9% within-athlete, which is how I translated the z-scores below.
Comeback efficiency vs the athlete’s own normal, by days off:
1-2 days: ~0% (n=32,758)
3-5 days: -0.7% (n=2,438)
6-9 days: -2.0% (n=396)
10-14 days: -3.2% (n=69)
15-21 days: -5.1% (n=39)
Within-athlete paired check (the part I actually trust): 26 athletes had both trained-through sessions and at least one comeback after a 10+ day gap. 85% came back below their own baseline, median ~-0.57 SD (~5%). So it’s not composition.
Lines up with the detraining literature (Bosquet review, Sousa et al.): first clear VO2max drops ~10-14 days, ~4% by two weeks, 4-7% by three. Our ~5% by 2-3 weeks is right in that band.
Caveats:
- First session back is usually easy on purpose. Easy = lower HR = higher efficiency, so this biases toward UNDER-stating the loss. The real drop is probably a touch steeper.
- Long gaps sometimes mean illness/injury, which is its own hit, not pure detraining.
- Submaximal efficiency, not a VO2max or FTP test. It’s a proxy, just a continuously-available one.
