Do you know that feeling of clarity you get after a good run or a good run? The feeling that your synapses are going off, your mind is a laser, and if someone showed a series of random signals on a computer screen, would you be able to press the right button in a fraction of a second? No? Well, trust me. There is ample evidence that short periods of moderate exercise improve performance in cognitive tasks immediately afterward. It’s not necessarily the kind of thing you can Feel, but it is a highly reproducible finding.

But there are also a lot of unanswered questions about this phenomenon, like a recent study in the Sports Science Journal makes it clear. How much exercise is enough to trigger this effect? How much is too much? Are you in good shape? Or what kind of cognitive task are you doing? A group of researchers from the University of Sydney and Griffith University in Australia, led by Danielle McCartney, are trying to fill some of these gaps.

The study involved 21 trained cyclists and triathletes (11 men, 10 women), who repeated the following testing protocol on two separate days: 15 minutes of moderate cycling; a pair of cognitive tests lasting about four minutes; another 30 minutes of moderate cycling; the same repeated cognitive tests; a gradual journey to exhaustion taking about 11 to 12 minutes on average; a final round of cognitive tests. Moderate cycling was 50-55% of peak power from a previous test, which ended up causing them to average around 75% of maximum heart rate after 15 minutes and 80% after 45 minutes.

The first cognitive test assessed reaction time: four black boxes appeared on a screen, and each time one of them turned red, the subject had to press as quickly as possible a button corresponding to that box. The second cognitive test, known as the Stroop test, assessed more complex elements of executive function, such as the ability to override your gut response. It was a series of words (red, green, blue, black) appearing in random colors (red, green, blue, black). The subjects had to press a button corresponding to the color of the letters, do not the meaning of a word. Sometimes the color matched the word; sometimes not. (Believe me, it’s more complicated than it looks!)

A 2015 study researchers from Taiwan found that 20 minutes of moderate exercise produced the biggest cognitive boost, while 45 minutes was not as good. There were a bunch of differences between this study and the new Australian study, but more importantly, the previous study used healthy, but non-athletic college students. For this population, 45 minutes of exercise could be quite unusual and exhausting, which could adversely affect cognitive performance. The new study used trained endurance athletes, who may be better able to benefit from longer training.

Indeed, 45 minutes of exercise produced better cognitive performance than 15 minutes. Here’s what the results looked like. Each bar shows an ‘effect size’, representing how subjects performed their cognitive tests against their baseline values ​​before exercise (a high value is better in all cases):

(Photo: Sports Science Journal)

There are three moments (after 15 minutes, 45 minutes and exhaustion). For each time point, there are three bars representing three different cognitive outcomes. Light gray is a simple reaction time. Medium gray corresponds to responses to the Stroop test when the color and the word match; dark gray is when the color and the word do not match.

The first point to note is that in all cases 45 minutes was better than 15 minutes (although the difference for the dark gray bars is not statistically significant). There is probably a point for everyone where, if you exercise for long enough, your cognitive performance will start to drop. For these trained endurance athletes, this did not happen at 45 minutes, even for the most complex parts of the test.

And contrary to what the researchers expected, this also did not happen after complete exhaustion. Some previous studies have suggested that full exercise impairs cognitive performance, possibly because your system is inundated with stress hormones that stop higher processing. But other studies have produced opposite results, and this one does not support this idea either. It should be noted that there was a delay of about two minutes between the moment of exhaustion and the start of cognitive tests, so this delay could have been just enough to get out of this mode of combat or of flight.

Another detail is that the subjects were not allowed to consume liquids during the test. Male participants lost an average of 2.3 percent of their starting weight, and female participants lost 1.7 percent – well above the threshold sometimes suggested to cause cognitive impairment. Since cognitive performance improved after all stages of the exercise protocol, this seems unlikely to be a serious concern.

Another nuance: the subjects all repeated the entire protocol twice. During one of the tests, after the 15 minutes, they were given two capsules and told they were “designed to improve cognitive (mental) function” during exercise. They were just placebos, but the researchers wanted to test whether better or worse test performance was influenced by whether subjects believed exercise could hurt or help their brain. Placebos did not ultimately have a significant effect, which reinforces the fact that this is a physiological effect – a result, for example, of improved blood flow to the brain or elevated levels of neurotransmitters.

Sure, a brief boost in brain function is fine, but we can’t always go out for a jog before every important decision or meeting or deadline of the day. For practical reasons, the most powerful findings are those that deal with long-term gains in cognitive function (or at least milder declines for those of us after puberty). To this end too, we can have endless debates about the right dose and the best type of exercise, but the detail attached to me, from a study from the University of Kansas in 2015, is that the best predictor of cognitive gains over a long period of time is gains in VO2 max. In other words, whether you are a new exerciser or a veteran athlete, do whatever it takes to get your body fitter, and the brain will follow suit.


Hat tip to Chris Yates for further research. For more Sweat Science, join me on Twitter and Facebook, subscribe to Email, and look at my book Endure: mind, body, and the oddly elastic limits of human performance.

Main photo: Matthew Smith / Stocksy

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