Do you know the feeling of clarity you get after a good run or a good ride? Feeling like your synapses are firing, your mind is a laser, and if someone flashes a series of random clues on a computer screen, could you hit the right button in a split second? No? Trust me. There is a number of indications that brief bouts of moderate exercise improve performance on cognitive tasks immediately afterwards. It’s not necessarily what you can feel, but it’s a very repeatable result.
But there are also many unanswered questions about this phenomenon, as a recent study in the Journal of Sports Sciences makes clear. How much exercise is enough to trigger this effect? How much is too much Does it matter how fit you are? 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 test protocol on two different days: 15 minutes of moderate cycling; a pair of cognitive tests that last about four minutes; another 30 minutes of moderate cycling; repeated the same cognitive tests; a gradual drive to exhaustion that takes an average of 11 to 12 minutes; one final round of cognitive tests. The moderate cycling was 50 to 55 percent of the peak performance from a previous test, which resulted in them hitting an average of about 75 percent of maximum heart rate after 15 minutes and 80 percent after 45 minutes.
The first cognitive test assessed reaction time: four black boxes were displayed on a screen, and when one of them turned red, the subject had to press a button that corresponded to that box as quickly as possible. The second cognitive test, known as the Stroop test, assessed more complex elements of executive function, such as the ability to override your instinctive response. It included a series of words (red, green, blue, black) that appeared in random colors (red, green, blue, black). The subjects had to press a button that matched the color of the letters, not the meaning of the word. Sometimes the color matched the word; sometimes not. (Believe me, it’s harder than it sounds!)
A 2015 study by researchers from Taiwan found that 20 minutes of moderate exercise produced the greatest cognitive boost, while 45 minutes was not as good. There were a number of differences between this study and the new Australian one, but the most important one is that the earlier study used healthy but non-athletic university students. For this population, 45 minutes of exercise can be quite unusual and stressful, which could affect cognitive performance. In the new study, trained endurance athletes were used, who may benefit better from a longer period of training.
In fact, 45 minutes of exercise resulted in better cognitive performance than 15 minutes. This is what the results looked like. Each bar shows an “effect size” that shows how the test subjects compared their pre-training baseline values in their cognitive tests (higher is better in all cases):
(Photo: Journal of Sports Science)
There are three points in time (after 15 minutes, 45 minutes and exhaustion). For each point in time there are three bars that represent three different cognitive results. Light gray is easy response time. Medium gray is Stroop test answers if the color and word match; Dark gray is when the color and word do not match.
The first point is that 45 minutes was better than 15 minutes in all cases (although the difference for the dark gray bars was not statistically significant). There’s probably a point for everyone where, if you exercise long enough, your cognitive performance will decline. For these trained endurance athletes, this was not the case for 45 minutes, even for the more complex parts of the test.
And contrary to what the researchers had expected, this did not happen after complete exhaustion. Some previous studies have indicated that extensive exercise affects cognitive performance, possibly because your system is flooded with stress hormones that block the higher processing power. However, other studies have yielded opposite results, and this again does not support this idea. It’s worth noting that there was about a two-minute delay from the moment of exhaustion to the start of the cognitive tests, so that delay might just be enough to get you out of this fight-or-flight mode.
Another detail is that the subjects were not allowed to drink liquids during the study. The male participants lost an average of 2.3 percent of their starting weight and the female participants lost 1.7 percent – well above the threshold that is sometimes suspected of cognitive impairment. Given that cognitive performance improved after all phases of the training protocol, it seems unlikely that this would be a serious problem.
Another nuance: the subjects repeated the entire protocol twice. During one of the tests, they were given two capsules after the 15-minute mark and said they were “designed to improve cognitive (mental) function during exercise”. They were just placebos, but the researchers wanted to test whether better or worse performance on the tests was affected by whether the subjects believed exercise would harm or help their brain performance. In the end, the placebos had no significant effect, which supports the case that it is a physiological effect – for example a result of improved blood flow to the brain or an increased concentration of neurotransmitters.
Of course, a quick boost in brain function is nice, but we can’t always jog all day long before every important decision, meeting, or deadline. For practical purposes, the more telling results are those that deal with long-term gains in cognitive function (or at least gentler deteriorations for those of us after puberty). To that end, too, we can have endless debates about the right dose and type of exercise – but the detail that stuck with me from a 2015 University of Kansas study is that the best predictor of cognitive Progress over an extended period of time is VO2 max gains. In other words, whether you are a beginner or a seasoned athlete, do whatever it takes to get your body fitter and the brain will follow.
Hat tip to Chris Yates for additional research. For more sweat science, visit me on Twitter and Facebook, subscribe to the email newsletter, and read my book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.
Main photo: Matthew Smith / Stocksy