Key takeaways
Well-trained athletes lost about 7% of VO2max in the first 3 weeks of stopping, mostly from lower blood volume
After about 8 weeks off, VO2max stabilised around 16% below the trained level, still above sedentary people
Cut volume but keep some hard efforts, two or three short sessions a week, to hold most fitness during a break
Injury, illness, a new baby, a busy month at work. Breaks happen. Here is what the research says about what you lose, how fast, and how to come back.
The first weeks
In a classic study, Coyle and colleagues (1984) had well-trained endurance athletes stop training completely for 12 weeks:
- VO2max fell about 7% in the first 3 weeks, mostly because of a drop in blood volume and stroke volume.
- It kept falling more slowly and stabilised about 16% below the trained level after around 8 weeks — still above sedentary people.
Endurance at submaximal speeds (the muscles' mitochondria and enzymes) fades too, which is why easy runs feel harder after a break.
The extreme case: bed rest
In the Dallas Bed Rest study, three weeks in bed cut VO2max by about a quarter in young men — more than they later lost in 30 years of ageing. See what limits VO2max.
Keeping fitness during a break
Reviews of detraining (Mujika and Padilla, 2000) found that you can maintain most of your fitness for weeks by cutting volume a lot while keeping some intensity:
- Two or three short sessions a week with some hard efforts go a long way.
- Cross-training (bike, rowing, swimming, elliptical) keeps the heart working when you can't run.
- Even partial training is far better than none.
Coming back after illness
- Don't train with a fever or with symptoms below the neck (chest tightness, coughing, aching muscles). Rest.
- After a viral illness, restart with easy sessions and watch your heart rate: higher than usual for the pace means you are not recovered yet.
- Expect resting heart rate and HRV to be off for several days. See resting heart rate, HRV and recovery.
- Rebuild over one to two weeks before hard intervals. If you have chest pain, palpitations, unusual breathlessness or feel faint, stop and see a doctor.
How fast it comes back
Fitness returns faster than it was first built, especially after short breaks. A 1–2 week break barely matters; after a month, plan two to four weeks to get back to your previous level. Your watch's VO2max estimate may lag or overshoot while heart rate settles — look at how accurate those estimates are.
Frequently asked questions
How fast do you lose fitness when you stop running?
In a classic study, trained athletes lost about 7% of their VO2max in the first three weeks without training. The loss then slowed and levelled off about 16% below their trained level after around eight weeks.
Does a 2-week break ruin my fitness?
No. A break of one to two weeks barely matters, and fitness comes back faster than it was first built. After a month off, plan two to four weeks to return to your previous level.
When can I exercise again after the flu?
Don't train with a fever or with chest symptoms such as coughing, tightness or aching muscles. Restart with easy sessions, watch for a higher heart rate than usual, and rebuild over one to two weeks before hard intervals.
Sources
- Coyle EF, et al. Time course of loss of adaptations after stopping prolonged intense endurance training. J Appl Physiol, 1984.
- Mujika I, Padilla S. Detraining: loss of training-induced physiological and performance adaptations. Part I. Sports Med, 2000.
- Saltin B, et al. Response to exercise after bed rest and after training. Circulation, 1968.