Perhaps you’ve noticed you’re less likely to forget where you parked your car after a brisk tennis match than after a trip to the library. There’s a reason for that, says a new study: In healthy seniors and those with emerging memory problems, even a single brief bout of vigorous exercise and the release of norepinephrine that comes with it can enhance memory of what came just before it.

The phenomenon is one of evolution’s cleverest memory-enhancing tricks: When an event triggers high emotion — the unexpected sight of a snake, for instance, and the fear reaction that comes with it — we tend to remember longer and better the details surrounding that event. For the young and inexperienced, the ability to remember those details — where and when one saw that snake, and how exactly it behaved — increases the odds that one will live long enough to reproduce.

But do those who have already survived into old age also benefit from the norepinephrine effect, and can it help compensate for memory impairment? Researchers at University of California-Irvine set out to explore those questions.

To do so, they recruited 31 healthy older adults with an average age of 69, and 23 subjects who had been diagnosed with "amnestic" mild cognitive impairment — memory loss that is problematic but which falls short of Alzheimer’s disease. All were shown a series of 20 emotionally positive images — beautiful landscapes, baby animals, sports scenes. And then, half of those in each group were put on treadmills to exercise for six minutes at 70 percent of their aerobic maximum. Subjects in the other half of each group were allowed to sit quietly.

Sixty minutes after their brief workout (or their restful sit), the subjects were given a surprise free-recall test, in which they were asked to describe the photos they had seen and as many details as they could remember.

Among those with normal memory function, a single bout of exercise increased recall of photos and details by 30 percent. Among subjects with amnestic mild cognitive impairment, those in the brief-exercise group improved their recall even more: they remembered twice the number of photos and details than did those who did not exercise. They didn’t remember as much as the non-exercising adults with normal memory, but they did recall more than they would have.

Not only did those with cognitive impairment improve their recall, they released more norepinephrine in response to exercise, apparently compensating for their faulty memories.

Sabrina Segal, a post-doctoral fellow at Irvine’s Center for Stress and Health and the study’s lead author, said researchers have established that ongoing exercise regimens help support memory function in both healthy older adults and those with memory problems. But she and her colleagues were struck that for both groups, even a short, one-time bout of exercise — a brisk walk around the block, for instance — strengthened recall for information taken in just before.

The study was published this week in the Journal of Alzheimer’s Disease.