Our own calculation, 7 November 2026, for the article "Aerobic fitness declines noticeably after two weeks off". The declines come from a primary study; the runner and the pace rule are labelled assumptions; the times are computed two ways and the two agree.
Inputs
- Coyle et al. (1984), Journal of Applied Physiology: seven endurance-trained subjects studied 12, 21, 56 and 84 days after stopping training. VO2max declined 7% during the first 21 days of inactivity and stabilized after 56 days at a level 16% below the trained value; after 84 days it was still above never-trained controls (50.8 against 43.3 ml/kg/min); stroke volume fell early; muscle capillarization did not decline; citrate synthase and succinate dehydrogenase fell with a half-time of 12 days and stabilized 50% above controls.
- Example assumptions: a recreational runner with a 50:00 10 km; race pace falls in proportion to VO2max, a simplification (running economy and pacing also move), so a d% lower VO2max makes the time 1 / (1 - d) longer.
What we found
- With VO2max 7% down, as in Coyle's subjects at 21 days, the 50:00 runner's 10 km becomes 53:46: 3,000 seconds divided by 0.93; the second way, the added time is 3,000 x 0.07 / 0.93 = 226 seconds, the same 53:46.
- With VO2max 16% down, the plateau Coyle's subjects reached by 56 days, the 10 km becomes 59:31: 3,000 divided by 0.84, or 571 seconds added.
- The decline is front-loaded and then stops: 7 of the 16 points came in the first three weeks, and nothing further was lost between day 56 and day 84 in that study.
- Capillaries held and the oxidative enzymes settled at 50% above untrained levels, which is why 84 days of rest left the subjects fitter than people who had never trained.
Check it yourself
| VO2max change (Coyle et al. 1984) | When | 10 km time from 50:00 | Added |
|---|---|---|---|
| -7% | by day 21 | 53:46 | 226 s |
| -16% | day 56, then stable to day 84 | 59:31 | 571 s |
Second way: 3,000 / 0.93 = 3225.8 s and 3,000 + 3,000 x 0.07 / 0.93 = 3225.8 s; 3,000 / 0.84 = 3571.4 s and 3,000 + 3,000 x 0.16 / 0.84 = 3571.4 s. Same.
What this cannot tell you: seven trained subjects in one 1984 study; a recreational runner's decline may differ; the proportional pace rule is a simplification.