Save Your Workouts From Mental Exhaustion

If your comfortable 10-minute-mile pace felt like an exhausting 7-minute-mile pace this morning, the problem could be your mind, not your muscles.

In a recent study published in Medicine & Science in Sport & Exercise, professional endurance runners ran for as long as they could—after completing a 45-minute computer task that involved quickly pressing a key when prompted. Afterwards, they tapped out earlier than they did when they completed the same running test after watching a documentary. Going into both tests, their heart rates, oxygen consumption and lactate levels were all the same—which means their bodies weren't at all compromised. Their brains were.

"Mental fatigue increased their perceived effort", otherwise known as the rate of perceived exertion (RPE), or how hard they thought they were working during their run, says Bruno Moreira Silva, PhD, an assistant professor in the department of physiology at the University of São Paulo, Brazil, who led the study. Because of that, they couldn't run as far.

Just as you can be physically fatigued after a 10-mile run or quick HIIT workout, you can be mentally fatigued after a long day of work or a tough task lasting just a few minutes, according to Kristy Martin, an assistant professor at the University of Canberra Research Institute for Sport and Exercise in Australia. A task that causes mental fatigue, or, more casually, "brain drain", is typically "something you'd prefer not to do, whether that's because it's really challenging or really boring", says Martin.

While studies have shown that brain drain can negatively affect your ability to maintain a certain power level on a stationary bike, hold an isometric exercise (like a plank) or keep up your speed for a 1,500-metre swim, it's less likely to impact, say, a 20-second dash. That's because endurance—"any activity where you need to pace yourself", according to Martin—can require conscious mental effort to decide to keep going. "And because mental fatigue increases perception of effort, this decision gets harder as you go on", says Martin. During a sprint, on the other hand, you have less time to think or lose motivation.

The symptoms of brain drain are easy to pinpoint: a lack of energy, no desire to push yourself, mood changes, delayed reactions, lapses in attention and even decreased accuracy. One theory for brain drain that Martin and her colleagues outlined in a recent review revolves around the build-up of a compound called adenosine in the part of the brain that controls perceived exertion, task perseverance and effort-versus-reward processing. (Adenosine is produced by intense mental and physical activity.) And researchers think that the more adenosine you have collecting in your brain, the stronger the signal your brain has to send to keep you pedalling, planking or swimming, which makes the task feel more challenging. Adenosine also blocks the release of dopamine, the chemical that regulates motivation, so it does a double whammy on your mindset.

The good news? If you make things easier on your head, your body can perform at its best. Here's how.

If you want to push hard in your workouts, avoiding mental fatigue beforehand is crucial, says Samuele Marcora, PhD, a professor in the department of biomedical and neuromotor sciences at the University of Bologna in Italy. To do that, prep as much as possible well before your workout (pick out your circuit, outfit and playlist the day prior) and think of if-then plans for problems that might come up, such as hitting snooze or having to stay late at work. Do all of this after getting seven to nine hours of sleep and both your workout and prepping for it will feel less taxing.

Sometimes—when you have back-to-back meetings all day long, for example—mental fatigue is unavoidable. There's little scientific evidence on exactly how long the fatigue lasts, but proper recovery "requires time and avoidance of triggers", says Silva. Martin recommends a 20- to 30-minute power nap so your brain can shut down and reboot, as well as an easy-to-stomach pre-workout snack, since your brain tends to store up adenosine when you're low on fuel.

Caffeine is similar in structure to adenosine, so when it binds to adenosine receptors, it can block the compound's effects. What's more, caffeine reduces activity in the pre-motor and motor areas of the brain during exercise, Marcora's research found, lowering your RPE in the moment. Down a mug about 30 minutes before your session and you might be able to go harder, for longer.

Research shows that you can lower your perceived exertion with music (the harder you want to go, the faster the beat should be), self-talk (think in the second person, so "You can keep going" instead of "I can keep going"), or even just smiling (which also improves how much energy you're putting into your session).

That voice in your head telling you that you can't go for one more minute is in your head—it's not a muscle or joint begging you to stop because it hurts and can't continue. "Several people have told me that knowing that the limit is perceptual has made them able to do their personal best", says Marcora. That little dose of perspective could be enough to help you do the same.