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Fitness sometimes feels like an enigma. What makes you fast or slow or strong or weak? A lot of the time, runners aren’t exactly sure. When faced with unanswered questions, it’s easy to default to human nature and fall back on superstition. It’s like when baseball players don’t understand why they hit home runs sometimes and strikeout others, so they wear the same underwear for a 162-game season.
As a coach, I think the most dangerous superstition of all is one that lurks in the back of almost every runner’s mind. I must not take time off or I will get slow.
The detraining superstition may cause a runner to push too far, not take time off when they are faced with an injury scare or run through extreme stress. One scenario that arises over and over is when an athlete has a little niggle in their leg but has a race upcoming. Should the athlete take time off, or push through it?
That question is not a hypothetical. On Saturday, March 3, Keely Henninger, a biomechanics expert at Nike and top trail runner, felt some pain in her lower leg. Her big early season race, the Chuckanut 50K, was just two weeks away. She should push through it, right?
Keely is an expert in this area, so she didn’t rely on fitness superstition. Instead, she knew she could take time off without losing much at all. So she rested all the way to the following Friday, five days without running. She built back carefully over the following week, then won Chuckanut this past Saturday in a blazing-fast time.
Getting over the training superstition and getting comfortable with needed breaks requires an understanding of what changes with time off. There are three general things to think about.
Even if you feel like a whale with asthma after a few days off, there is little change in your aerobic capacity with short breaks. Studies show VO2 max doesn’t really decrease until seven to 10 days off. There are some losses after (anywhere from five percent at two weeks to close to 20 percent at eight to 10 weeks, with minor variation across studies), but those can be reversed pretty quickly with training.
The general rule is that it’s easier to get back to where you were than it was to get there the first time, which likely has lots of fun physiological explanations ranging from epigenetics (changes in gene expression) to neuromuscular efficiency to capillary/mitochondrial density.
The takeaway is that your engine will still be ready after you spend some time idling in the parking lot.
What explains the changes after seven to 10 days? The likely culprit is a reduction in blood-plasma volume, essentially from the liquid content in your blood going down, as explained in this 2007 review from the American Journal of the Medical Sciences. A seminal study from the 1980s in the Journal of Applied Physiology found blood-volume reductions of five to 12 percent after just a few days of inactivity. So the heart has less blood to pump, cardiac stroke output decreases and working muscles have less fuel to snarf up. Rats!
There is a silver lining—blood volume bounces back quickly with training. That explains part of why you feel like a bucket of wet noodles after a few days off, but back to having delicious al dente legs soon after. The takeaway is that your blood volume will be back in action quickly, even though you probably don’t want to jump right back into hard training or racing immediately after a few days or more off.
Metabolic and Muscle Changes
With time off, respiratory-exchange ratio (oxygen consumed over carbon dioxide produced) rises, corresponding with less-efficient fat burning. At the same time, glycogen uptake is impaired. Slow-twitch muscle fibers become less predominant and lactate accumulates more rapidly. Capillary density and oxidative enzymes are reduced. As Ghostbusters said, it’s “dogs and cats living together, mass hysteria!” (broken down in this 2000 study from the journal Sports Medicine).
So your gas mileage goes from Tesla to semi-truck. But take heart! Most metabolic adaptations don’t start until around a week and respond quickly to training.
That’s not all, folks. There could be some other factors that decrease performance that aren’t reflected in these studies. For example, the good type of muscle tension that makes your legs feel like coiled springs could go down, making you feel flat. You could drown your sorrows in 10 pints of ice cream a day, leading to weight gain. There may be neuromuscular changes that reduce running economy for some athletes.
Legendary coach Jack Daniels summarizes all these physiological variables in his helpful VDOT metric, providing a proxy for fitness change with time off.
Up to 5 days off: no change
7 days off: 0.6% change
14 days off: 2.7% change
28 days off: 6.9% change
So short breaks don’t change fitness much at all, plus there could be big benefits. A few days on your butt could heal muscles, balance hormonal fluctuations from hard training and stoke the motivation fire. After a short time off, you may even be stronger than you were in ancient times (a few days prior) when you were able to run.
Don’t immediately jump back into training, but ease back with an emphasis on reversing any minor detraining. If you’re out for weeks, start with lots of easy running. If it’s a short time, like the five days Keely took off, you can come back a bit more quickly. Keely’s training looked like this:
Saturday: 20 miles
Friday: 6 miles easy
Saturday: 11 miles easy
Sunday: 14 miles easy with a 20-minute moderate climb
Monday: 8 miles easy with 4 x 30-second hill strides
Tuesday: 3 miles easy with a few strides, 3 miles moderate, 3 miles easy with 4 x 30 seconds fast/30 seconds easy
Wednesday: 6 miles easy
Friday: 5 miles easy with 4 x 30 seconds fast/30 seconds easy
Saturday: Chuckanut 50K for the win!
Keely started with some easy running to rebuild blood volume, did some short efforts to get the feel again and regain any minor reductions in aerobic capacity, then tapered for the race (relative to her normal high-volume training).
So when in doubt, be like Keely (a good general rule). It’s OK to take time off. You won’t lose anything, and what you do lose bounces back really quickly. Some rest now may lead to your best later.
David Roche runs for HOKA One One and NATHAN, and works with runners of all abilities through his coaching service, Some Work, All Play.