OK, by this point everyone has heard—and is tired of hearing—that they should exercise more. But here's another compelling reason from a new study: Exercise can defuse a deadly bomb in your brain.

Strokes—exploding or clogged brain blood vessels—hit 700,000 people per year and kill just over 160,000 of them. Those who survive can lose vision, speech, or memory, risk partial paralysis, and face long, arduous months of rehabilitation therapy. Avoiding all this, at the price of a little physical activity, sounds like a bargain.

Just how much activity, and of what kind, has been identified by Finnish researchers in the September issue of the journal Stroke. In one of the biggest research projects of its kind, they followed nearly 48,000 healthy women and men, ranging in age from 25 to 64, for an average of 19 years. During that time, the group had almost 3,000 strokes. But strokes did not strike everyone equally. Physical activity protected some of them. Which ones? To find out, the scientists divided the 48,000 into 3 activity groups: low, such as TV watchers and readers who did little to move their bodies; moderate, such as walkers and cyclists and gardeners who did these things for more than 4 hours per week; and high, runners or swimmers who pushed themselves for more than 3 hours per week.

Compared with the low group, the others had the following reductions in risk:

Moderate: 13 percent lower risk of a clogged blood vessel stroke, and 23 percent lower risk of one type of ruptured vessel stroke.

High: 20 percent less risk of stroke due to a clog, and 37 percent less risk of a ruptured vessel stroke.

Getting exercise while getting to work also helped. Again, the researchers sliced the whole group into three: no physical activity while commuting; moderate activity, up to 29 minutes of walking or cycling; and high, or more than 30 minutes of walking or cycling. Again, the reduction of stroke risk, when compared with the no-activity group, was profound:

The moderate group had a 7 percent lower risk of a clogged blood vessel stroke.

The high group had a 14 percent lower risk of a clog.

There was, however, no benefit for moderate or highly active commuters in avoiding ruptured vessel strokes, and the researchers don't really understand why. Nonetheless, the overall message is clear: A little movement means a much healthier brain.