If you’ve got a small kid in daycare, you already know how this story goes. They come home on Monday afternoon with a runny nose. By Thursday, you’re the one sniffling on the couch with a tissue box, watching your carefully-planned training week fall apart. The Saturday long ride? Cancelled. Sunday’s group ride? Also cancelled. By the time you’re back on the bike, you’ve missed a week (or two, if it was a really bad one) and you’re convinced all your hard-earned fitness has evaporated.
This topic has been on my mind lately. I’ve been dealing with a lingering infection for the past few weeks that’s really cut into my time on the bike. Like most cyclists, my first instinct was to start worrying about how much fitness I’m bleeding away with every missed ride. So I figured I’d dig a bit into the research and see what the evidence actually says.
I’ve been there more times than I’d like to admit since becoming a dad. And every time, the same worry creeps across my mind: how much have I lost? It turns out, the research on this question is both more nuanced and more reassuring than I expected. So this month I want to dig into what actually happens to your fitness when you take time off and maybe save you some of the panic the next time illness strikes.
The first two weeks: mostly fine, actually!
Here’s the good news first. For the typical 3-to-7 day cold that knocks you off the bike for a week, the fitness losses are surprisingly small. Most of what you “lose” in the first week or two isn’t actually fitness, it’s plasma volume. Your blood volume drops within days of stopping training, which is a big part of why your VO2max might take an early hit. The good news is that plasma volume rebounds fast once you start riding again, usually within a few days.
The classic reference here is a 2000 review by Iñigo Mujika & Sabino Padilla, which pulled together decades of detraining studies. The short version: in trained athletes, VO2max drops about 4–7% in the first 2–3 weeks of complete training cessation. That sounds like a lot, but most of that early drop comes from plasma volume changes that reverse quickly. Endurance performance often holds even better than VO2max suggests. One study they referenced (Houmard et al., 1990) had endurance runners cut their training volume by 70% for three weeks, & their 5km time trial performance didn’t change at all!
So if you’re stuck on the couch for a week with a cold, take a breath! The evidence suggests you’ll be back to normal within a few sessions of getting back on the bike.
Two-to-four weeks: the picture gets more honest
So far, a small break off the bike doesn’t sound too bad for your fitness! But what happens if a short break gradually turns into a longer time off the bike?
Mujika & Padilla followed up their initial review with a more detailed 2001 paper specifically on cardiorespiratory and metabolic detraining (published in Medicine & Science in Sports & Exercise), and it’s worth a read if you want the full picture of what’s happening at this stage. They report that VO2max in highly trained athletes can drop somewhere in the range of 6–20% between 2 & 4 weeks of complete inactivity. Lactate threshold drifts downward. Muscle glycogen storage decreases. Cardiac stroke volume drops as the heart adapts to the lower exercise demand. Unlike the early plasma-volume losses, these changes take longer to reverse once you do come back.
The practical takeaway: if you’ve been off the bike for 2+ weeks, plan to ease back in. Don’t try to make up for lost time with a hero week of training the moment you’re back – that’s a great way to dig yourself into a hole.
The really good news: your body remembers
When digging into the topic of detraining a bit further, I found some evidence that was interesting and also encouraging. There is some evidence that previously-trained muscle has a kind of “memory” at the molecular level, at least in resistance training. A 2018 study by Robert Seaborne & colleagues at Keele University in the UK showed that human skeletal muscle retains epigenetic markers (specifically, DNA methylation patterns) from previous training, even after a complete detraining period. When subjects retrained, those markers became even more pronounced, and the muscle responded more strongly to the same training stimulus than it had the first time around.
To me, this suggests that when you come back to training after time off, you’re not starting from scratch. Your muscles remember the work they’ve done. Retraining is faster than initial training, sometimes substantially so. So even if you’ve taken a meaningful chunk of time off – say a few weeks with a real illness – the path back to fitness is likely shorter than the path you took to build it in the first place.
A small caveat: most of the referenced studies I encountered during my research were based on runners or resistance-trained subjects, not cyclists. The cycling-specific detraining literature is a bit thinner, but the underlying physiology is likely general enough that the broad picture likely applies on the bike too.
Putting it all together
The way I think about it now, especially for myself as a dad in the thick of the daycare season of life:
- A few days to a week off the bike (typical cold)? Don’t sweat it. You haven’t lost meaningful fitness, and you’ll feel normal within a session or two of returning.
- One-to-two weeks off (a worse bug, or a stretch of bad sleep with a sick kid)? Some real losses, but mostly plasma-volume stuff that rebounds quickly. Ease back in with a couple of easy rides before doing anything hard.
- Two-plus weeks off? More substantial losses and a longer ramp back. But your body remembers and you’ll get there faster than you think.
Cycling fitness is built over consistent weeks, months, & years, not a few days. A bad week, or even a couple bad weeks, doesn’t necessarily undo that! The hard part isn’t the fitness loss. It’s the patience to come back gradually instead of trying to make up for lost time. That’s the part I’m still working on.
Thanks for reading. That’s all for this month – stay safe, ride fast, and I’ll see you next month!
References:
Mujika I, Padilla S. Detraining: loss of training-induced physiological and performance adaptations. Part II: Long term insufficient training stimulus. Sports Med. 2000 Sep;30(3):145-54. doi: 10.2165/00007256-200030030-00001. PMID: 10999420.
Houmard JA, Costill DL, Mitchell JB, Park SH, Hickner RC, Roemmich JN. Reduced training maintains performance in distance runners. Int J Sports Med. 1990 Feb;11(1):46-52. doi: 10.1055/s-2007-1024761. PMID: 2318562.
Mujika I, Padilla S. Cardiorespiratory and metabolic characteristics of detraining in humans. Med Sci Sports Exerc. 2001 Mar;33(3):413-21. doi: 10.1097/00005768-200103000-00013. PMID: 11252068.
Seaborne RA, Strauss J, Cocks M, Shepherd S, O’Brien TD, van Someren KA, Bell PG, Murgatroyd C, Morton JP, Stewart CE, Sharples AP. Human Skeletal Muscle Possesses an Epigenetic Memory of Hypertrophy. Sci Rep. 2018 Jan 30;8(1):1898. doi: 10.1038/s41598-018-20287-3. PMID: 29382913; PMCID: PMC5789890.
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