Getting back after a break

Two weeks of vacation, a month of flu, six months of complicated life. The question is always the same: what have I lost, and how long to get it back? The answer is more encouraging than you might think on one point, and more severe on another.

What the data shows

Detraining has been measured quite precisely. The benchmarks that emerge:

• Up to 10 days: the drop in VO2max is negligible. A week off costs nothing — and after a loading period, it often pays dividends.
• 2 to 4 weeks: the drop becomes measurable, around 4 to 14% in highly trained subjects, with approximately 6% as a typical value.
• 4 weeks: approximately 6% loss in runners.
• 9 weeks: approximately 19%.
• 11 weeks and beyond: 20 to 25%.

The curve is therefore not linear: it's flat in the first few days, then accelerates sharply between the second and ninth week.

Why it drops, and why it comes back quickly

The initial drop in VO2max comes mainly from a decrease in maximum heart rate, which is itself linked to a decrease in stroke volume — which is largely explained by a loss of blood volume.

This is good news, because blood volume is reconstituted within a few sessions. The part of your fitness that disappears fastest is also the part that comes back fastest. On the other hand, slow adaptations — capillary density, mitochondrial enzymes, and especially the strength of tendons and bones — are lost more slowly but are rebuilt much more slowly.

Hence the central pitfall of returning to training, which must be stated clearly: cardio comes back before tendons. After three weeks of resuming, you'll feel capable of running as before, and your locomotor system won't be. It's in this window that return-to-running injuries concentrate.

How long to get back

A useful rule of thumb: it takes approximately half the duration of the break to get back to your level, if the break hasn't exceeded a few months. Two weeks off, one week of resuming. Two months off, one month.

Beyond six months, the benchmark no longer holds and it's better to think of it as rebuilding rather than returning. With a real advantage: training memory. A runner who has already built a base rebuilds it much faster than a beginner builds it.

How to resume, concretely

Resume at volume, not intensity. This is the opposite of what sensation dictates — after a break, you want to verify you're still capable. The first three weeks must be entirely in easy endurance, without exception.

Cut in half, then increase by 10% per week. Start from half the volume before the break, and increase by about 10% per week. Beyond that, injury risk increases much faster than fitness.

Alternate walking and running if the break exceeds six weeks. Without shame: this is what allows you to accumulate time under load without accumulating impacts. See getting started with running for the typical progression.

Reintroduce intensity in the fourth week, and just one session. First controlled paces, then interval training.

Add strength work right away. This is the most effective lever against return-to-running injuries, because it works precisely what cardio doesn't — see strength training for runners.

Forget your old paces

This is the hardest advice to apply and the most useful. Trying to maintain your pre-break paces is the number one cause of setbacks. Paces are a result, not a target.

The method that works: run for a few weeks by feel or by heart rate, without watching pace, and let the numbers come back on their own. They do come back. And simply following pace at the same heart rate week after week is the best progression indicator there is — see heart rate zones.

The special case of injury

After a break due to injury, everything above applies, plus one condition: the cause must have been treated. Resuming the same volume with the same mechanics that produced shin splints or Achilles tendinopathy gives the same result. See returning to running after injury.

Start from where you are

Return to training almost always fails the same way: you program based on memories of your fitness, not your current fitness. Sports Coach AI builds progression on your real data — load, paces at the same heart rate, recovery — and ramps up volume at the pace your body can handle, not the pace your memory demands.

References

• Coyle E.F. et al. — effects of detraining on physiological adaptations to aerobic training.
• Mujika I. & Padilla S., 2000, Sports Medicine — detraining, part II: insufficient training stimulus over the long term.
• Chen C. et al., 2022, European Journal of Sport Science — two weeks of detraining in endurance athletes.