Sport Science Reads · Detraining
Every athlete on your roster will stop training at some point this year. A postseason break, a holiday week, an ankle sprain, a bout of flu. The question isn’t whether it happens. It’s how much ground they lose, how fast, and what it takes to get it back.
Sport science has a name for this: training reversibility, or as most coaches put it, use it or lose it. One of the most cited papers on the topic is Detraining: Loss of Training-Induced Physiological and Performance Adaptations. Part I by Iñigo Mujika and Sabino Padilla (Sports Medicine, 2000), written from Athletic Club Bilbao’s medical department. It pulls together the research on what happens in the first four weeks after training stops or drops off sharply.
4–14%VO2max lost in under four weeks off
10–17%Drop in stroke volume within three weeks
2 daysBefore plasma volume starts to fall
Here’s what it found, and what it means for your staff.
01Detraining isn’t the same as taking a break
Mujika and Padilla define detraining as the partial or complete loss of training-induced adaptations because the training stimulus is no longer enough. That’s different from a few related terms that often get mixed up:
- Training cessationStopping a conditioning program entirely, whether temporarily or for good.
- Reduced trainingA planned, steady cut in training volume. Done right, it can maintain or even improve adaptations. It’s sometimes called a step taper.
- TaperingA progressive, gradual reduction in load before competition, designed to shed fatigue and peak.
- Detraining syndromeA separate clinical condition seen when athletes with a long endurance background suddenly stop all activity. Symptoms include dizziness, heart palpitations, headaches, insomnia and anxiety.
The distinction matters.
Doing less is not the same as doing nothing.
A well-planned reduction protects fitness. A full stop starts the clock on losing it.
02What fades, and when
The paper looks at short-term detraining: less than four weeks of insufficient training. That’s the length of a typical postseason break, and longer than most minor injuries. Here’s roughly how the losses stack up in trained athletes:
The detraining clock Trained athletes
First 2 days
Plasma volume starts to drop
About 5 days
Glycogen synthase activity down 42%
About 1 week
Muscle glycogen down about 20%. In swimmers, muscle oxidative capacity fell by as much as 50%
6 to 10 days
GLUT-4 (the muscle’s glucose transporter) down 17 to 33%
2 weeks
Strength roughly holds, but muscle activation (EMG) is down 8 to 13%
12 to 21 days
Stroke volume down 10 to 17%
3 weeks
Maximal cardiac output down about 8%
Under 4 weeks
VO2max down 4 to 14%. Oxidative enzymes down 25 to 45%. Swimmers’ in-water power down 13.6%
Not every athlete loses everything at once, and individual results vary. But the pattern is clear:
The engine goes first, and it goes fast.
03The engine: why aerobic fitness fades first
The fastest loss is cardiorespiratory. In highly trained athletes, VO2max dropped by 4 to 14% in under four weeks, and the fitter the athlete, the bigger the drop tends to be.
The main driver isn’t the heart muscle itself. It’s blood volume. Total blood and plasma volume fall by 5 to 12%, starting within the first two days. With less blood coming back to the heart, it fills less on each beat, so stroke volume drops by 10 to 17%.
The heart tries to make up for it by beating faster. Exercise heart rates rise by about 5 to 10%. It’s not enough. Maximal cardiac output still fell 8% after three weeks off, and time to exhaustion in endurance-trained athletes dropped by 4 to 25%.
4–25%Drop in time to exhaustion for endurance-trained athletes. This is the athlete who comes back from a break and says the conditioning test feels harder than it should. Their heart rate data will show it before their legs do.
04The fuel system: more carbs, less fat
Detraining also changes how the body fuels exercise. The respiratory exchange ratio rises, a sign that working muscles lean harder on carbohydrate and less on fat.
- Insulin sensitivity drops quickly. Whole-body glucose uptake falls, linked to a 17 to 33% drop in GLUT-4 after 6 to 10 days off.
- Lactate shows up earlier. At the same submaximal workload, athletes produce more blood lactate, and the lactate threshold arrives at a lower percentage of VO2max.
- Glycogen stores shrink. Muscle glycogen fell about 20% in a week in competitive swimmers.
The result: the same session costs more. Paces and loads that felt routine a month ago now push athletes closer to their limit.
05The muscle: strength holds, power and endurance don’t
Here’s the good news for strength coaches. Raw strength is surprisingly durable. After two weeks without training, strength-trained athletes showed only small, non-significant drops in bench press, squat, isometric and isokinetic strength, and vertical jump.
But look closer and the cracks show:
- Muscle activation falls first. EMG activity in the vastus lateralis dropped 8 to 13%, along with eccentric knee extension force.
- Sport-specific power fades. Swimmers kept their dry-land strength over four weeks but lost 13.6% of their power in the water.
- Fibre size shrinks. Gains in muscle fibre cross-sectional area reversed, mostly in fast-twitch fibres in strength and sprint athletes.
- The aerobic machinery goes. Oxidative enzymes such as citrate synthase fell 25 to 45%. Capillary density declined, though it stayed about 50% higher than in sedentary people.
Old numbers in the weight room. Still short of game-ready.
06Elite athletes have more to lose
One of the paper’s clearest findings: highly trained athletes lose more, and faster, than recently trained people. Recently trained individuals saw VO2max drop by only 3.6 to 6% over two to four weeks off, and their endurance gains held for at least two weeks.
3.6–6%VO2max lost by recently trained people over two to four weeks. Highly trained athletes lost up to 14%. The athletes you’ve spent years building are the ones most exposed during time away.
07What this means for your staff
Mujika and Padilla’s conclusion is simple: reduced training is far better than complete cessation. Their Part II goes deeper into how to limit losses. For now, here’s how to put Part I to work:
Plan the break, don’t just give one
A postseason or holiday week with a minimum training dose protects far more than a full stop. The paper is clear that a reduced dose beats a full stop.
Keep injured athletes moving where you can
An ankle sprain doesn’t have to mean zero conditioning. Upper-body work, the bike or the pool keep some training stimulus going while the injury heals, instead of starting the detraining clock.
Expect conditioning to lag strength
An athlete who hits their old squat numbers after a layoff isn’t necessarily ready. Test aerobic fitness and sport-specific power too.
Ramp the return
After time away, the same session costs more. Bring load back gradually instead of jumping straight to where they left off.
Measure it
Heart rate at a standard workload, conditioning test results and training load before and after the break show you exactly how much ground each athlete lost.
Where Whistle fits
See detraining before it costs you.
Whistle helps performance staff see detraining before it shows up as a slow first week or a soft-tissue injury. Track every athlete’s training load through breaks and layoffs, compare their numbers to where they were before, and plan the ramp back with Whistle Return to Play, from day one of an injury to fully back.
PDF
Read the full paper
Want the details behind the numbers?
Mujika I, Padilla S. Detraining: Loss of Training-Induced Physiological and Performance Adaptations. Part I: Short Term Insufficient Training Stimulus. Sports Medicine. 2000;30(2):79-87.
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