Which makes you smarter: running or lifting weights? Meathead jokes aside, there’s new evidence that moderate aerobic exercise helps increase new brain cells, while strength training may not be much healthier for the brain than sitting on the couch.

In a new study, researchers at the University of Jyvaskyla in Finland gave rats three different workout programs–running, weight lifting, and high-intensity interval training–to see if some types of exercise are better for the brain.

“I think the training regimes were quite close to what humans might do,” says study lead Miriam Nokia. The rats doing endurance training, for example, ran on a treadmill three times a week for half an hour. The rat version of weight lifting was a little different–tiny weights were tied to the rats’ tails while they climbed up a ladder–but like weight-lifting humans, the rats were noticeably buffer after eight weeks of working out.

But the weight-lifters didn’t show signs of neurogenesis, or new brain cells. By the end of the study, the rats that jogged on a treadmill had significantly more new brain cells than those that lifted weights or were sedentary. They also fared much better than those that did high-intensity training–spurts of sprinting, then jogging, repeated for 20 minutes.

“[High intensity training] is in nature more stressful which could have dampened its effects on adult hippocampal neurogenesis,” says Nokia. “Stress is in general considered to decrease adult neurogenesis.” On the other hand, the rats on the treadmills were prodded on by tiny shocks–also stressful–and the weight-training rats weren’t, so stress alone doesn’t explain the differences. Nokia also points out that mild stress may actually increase the number of new brain cells.

[Photo: Lake Mead Flickr

The study also found that some rats were more likely to benefit from running than others. Those who were selectively bred for their ability to get fit–and who jogged on their running wheels voluntarily, often for miles every day–ended up also seeing the biggest increase in neurogenesis.

None of this proves that the same thing would happen in humans. But a 2014 study of elderly women suggested something similar: aerobic exercise made the part of the brain responsible for memory grow, and weight lifting didn’t have any effect.