I enjoy running. On a good week, I might get out and run 3 or 4 times, and run between 10 and 20 miles total. I collect a few t-shirts by running in races, but I have made a clear decision not to run a marathon. Four hours of running just seems like too much wear-and-tear on the body. The stress of running a marathon also has an effect on your mind.

A paper in the June, 2009 issue of Psychonomic Bulletin and Review by Teal Eich and Janet Metcalfe (a daughter-mother research team) looked at the effects of marathon running on memory. They tested marathon runners either a couple of days before the race (in a public setting near other runners) or within 30 minutes of completing the race (in a public setting near other finishers).

Runners were given both tests of explicit and implicit memory. Explicit memory is what we usually think of as memory. That is, the ability to remember a specific thing you saw in the past. Implicit memory involves some evidence that you encountered something in the past without necessarily being able to say when and where you encountered it.

To start off, the runners were given a list of 26 words, and they rated the words for how pleasant they were. They also did two types of memory tests. They did an implicit memory test first and then they did an explicit memory test.

Next, they did an implicit memory test. Runners saw the first few letters of words (called word stems) and were asked to write down the first word that came to mind that started with those letters. Thirteen of these word stems were the start of words that people had seen on the list of words rated for pleasantness, though the test didn't mention anything about the words that people had rated for their pleasantness. If people fill in the word stem with words from the previous list, they are showing an influence of having seen that list before.

Finally, they did an explicit memory test. They saw another list of word stems (different from the stems in the implicit memory test). This time, they were told that the stems came from words that were part of the list that they saw earlier. They were told to write down the word from the list that started with that stem. This is a test of explicit memory, because the runners have to remember that the word appeared on the list they had seen.

So what happened?

As you might expect given the stress of running a marathon, runners who had just completed the marathon were worse on the explicit memory test than runners who had not yet run the marathon. The difference was not huge, but it was statistically reliable.

More surprisingly, the implicit memory test showed the opposite effect. That is, people who had just run a marathon were actually more likely to fill in a word stem with a word from the previous list than runners who had not yet run the marathon.

It is not at all clear why these two measures of memory led to different results. There are lots of reasons why memory ought to get worse after a marathon. The marathon is stressful to the body, and it leads to the release of hormones to deal with that stress. There are changes in levels of brain chemicals that help neurons pass along signals following a marathon. In addition, the body also has lots of waste products in it from breaking down sugars and using them to power muscles. However, none of these factors predict that runners would both get worse on an explicit memory test and at the same time would get better on an implicit memory test after a marathon. So, this paper presents one of those interesting studies that ends with a mystery. And hopefully researchers will continue to examine this interesting finding.

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