Detraining anxiety is one of the most common reasons people train through illness, injury and holidays that they should have taken off. The actual numbers are considerably more forgiving than the fear.
Week by week
Days 1 to 7. Essentially nothing is lost. Plasma volume starts to fall, which can make a run feel harder within a week despite no loss of actual capacity. Muscle glycogen drops, which changes how you look before it changes what you can do.
Weeks 2 to 3. Aerobic decline becomes measurable. VO₂max starts falling, and this is the first genuine loss. Strength is still broadly intact.
Weeks 4 to 8. Aerobic losses accumulate meaningfully. Strength begins to decline, slowly, and muscle size follows behind that. Neural qualities go before tissue does, so the bar feels heavy before the muscle is smaller.
Months 2 to 6. Substantial detraining across both. Not back to baseline for most people, and the older the training history the more is retained.
Why the order surprises people
Most people worry about muscle and are surprised to lose endurance first. The reason is that aerobic fitness rests on things that adjust quickly, blood volume and enzyme concentrations, while muscle protein turns over slowly.
Practically: if you have two weeks off, come back expecting your conditioning to be down and your strength to be roughly intact. Program accordingly rather than deloading everything.
Maintaining costs far less than building
This is the genuinely useful finding. Maintenance requires substantially less volume than development, provided intensity is preserved.
For strength, a single session per muscle group per week at a decent intensity maintains most of what you have for a long time. For endurance, cutting volume hard while keeping some intensity preserves far more than the reverse.
Which means a busy month does not have to be a lost one. It has to be a smaller one, with the hard bits kept. That is also the logic behind a deload week.
Coming back
Faster than the first time, reliably. Muscle memory has a genuine cellular basis: myonuclei acquired during previous training appear to persist, so regaining is a different process from building.
The mistake on return is resuming at your old volume immediately, which is exactly the load spike most associated with injury. Rebuild across two or three weeks even though it feels unnecessary.
The other direction is how long it takes to improve VO₂max.