Men who took up running and stuck with it had more "healthy swimmers", according to the research in the journal Reproduction.
The boost was only temporary, and began to wane within a month if the men stopped their treadmill training.
Experts say it is important to strike the right balance because too much exercise can harm sperm production.
Studies have shown that participation in competitive sports, like cycling, can lower sperm quality.
Keep your testicles cool - avoid tight underwear and hot baths