Conversely, men who do 15 or more hours of moderate to vigorous exercise every week have sperm counts that are 73% higher than those who exercise little, the findings show.

Semen quality seems to have deteriorated over the past few decades, although it's not clear why, say the authors.

To find out if an increasingly sedentary lifestyle might be a contributory factor, they analysed the semen quality of 189 men between the ages of 18 to 22 in 2009-10, all of whom were from Rochester in New York State, USA.

The men were asked about the quantity and intensity of weekly exercise they had had over the preceding three months, and how much time they spent watching television, DVDs, or videos over the same period.

And they were asked about factors that might affect sperm quality, including medical or reproductive health problems, diet, stress levels, and smoking.

Over half the men were within the normal range for weight for their height, and three out of four were non-smokers. The prevalence of reproductive health problems was low.

The amount of moderate to vigorous physical activity taken every week ranged from 5 to 14 hours, while weekly TV screen time varied from 4 to 20 hours. Men who were more physically active tended to have a healthier diet than those who watched a lot of TV every week.

The analysis showed that those who were the most physically active -- 15+ hours a week -- had a 73% higher sperm count than the least physically active. Exercise did not affect sperm motility, shape, or sample volume.

When analysed by exercise intensity, the results showed that light physical exercise made no difference to the sperm count, no matter how frequent it was.

TV viewing had the opposite effect. Those who watched the most -- 20 or more hours a week -- had a sperm count that was 44% lower than those who watched the least. It had no impact on sperm motility, shape, or sample volume.

And unlike smoking or weight, the amount of TV viewing seemed to counteract the beneficial effects of exercise, although this may be a chance finding, say the authors.

The authors caution that a reduced sperm count does not necessarily curb a man's fertility or his chances of being able to father a child, but the findings do suggest that a more physically active lifestyle may improve semen quality.

The type of exercise might also be important, say the authors, who conclude: "Future studies should also evaluate the extent to which different exercise types affect semen quality as previous studies suggest that there might be opposing effects of different types of activity on semen characteristics."