Men who watch more than 20 hours of television a week have almost half the sperm count of those who watch hardly any, according to a study.

Exercise, however, appears to improve men's sperm count. Those who took part in the research by the Harvard School of Public Health who did 15 or more hours of moderate to vigorous exercise every week were found to have sperm counts that were 73% higher than the least active.

The study is a relatively small one, and a low sperm count does not mean that a man will be unable to father a child but, say the authors, it could be that semen quality can be improved by a more active lifestyle.

There has been much discussion about whether semen quality has been declining over recent decades, say the authors in their paper, published online in the British Journal of Sports Medicine.

"Despite the inconsistent findings, the majority of data supports a decline in sperm concentration in most western countries, and the causes of the observed decline are still debated," the authors write. "One possible explanation could be the concurrent decrease in physical activity and increase in sedentary behaviour over the same time frame."

Some studies have suggested that strenuous exercise may damage male fertility, based mainly on the results of studies involving cyclists and long-distance runners. Although there has been plenty of evidence of the dangers of "couch potato syndrome" – usually too much food and too little movement contributing to obesity – an association between television watching and lower semen quality has never been investigated, they say.

The 222 participants surveyed were among those recruited for the Rochester Young Men's Study in 2009-10 in New York. All were healthy and aged between 18 and 22.

The men were asked about their exercise over the previous three months, and how much time they spent watching television, DVDs, or videos over the same period. They were also asked about medical or reproductive health problems, diet, stress levels, and smoking.

More than half the men were within the normal range for weight for their height, and three out of four were non-smokers – those obvious differences were all taken into account in the analysis of the results.

The men took between five and 14 hours of moderate to vigorous physical activity per week. They spent between four and 20 hours each week watching television. The study found that the men who were most physically active had a sperm count that was 73% higher than the least active. It also found that those who watched the most TV (more than 20 hours a week) had a sperm count 44% lower than those who watched the least. There was no impact, however, on sperm motility, shape or sample volume.

Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said the authors' conclusions were plausible. It was possible that testicular heating – through sitting on a sofa for a long time – could decrease sperm counts.

"However, it remains to be seen if coaxing a TV-watching couch potato into doing some regular exercise could actually improve his sperm count," said Pacey, "or whether there exists an unknown fundamental difference between men who like exercise and those who do not, which might account for the findings. This should be a relatively easy study to perform but, before all worried men hunt for their sports bags, it's important to note that other research suggests that doing too much exercise can be harmful to sperm production, and this study did not examine the type and intensity of exercise their participants were undertaking."

This article originally appeared on guardian.co.uk