Young men who are considered healthy, but who watch more than 20 hours of television per week, have almost half the sperm count of men who do not watch much TV, a new study has found.

Furthermore, those who exercise for 15 hours or more per week have sperm counts that are over 70% higher than those who do not exercise much.

According to scientists from the Harvard School of Public Health in the US, semen quality has deteriorated in recent decades, but it is unclear why.

They set out to determine if leading a sedentary lifestyle may play a role. Semen samples from almost 200 men aged between 18 and 22 were analysed. The men were also asked about their lifestyle in the previous three months, including how much exercise they got and how much TV/DVDs they watched.

Other factors that can affect sperm quality, such as smoking, stress and diet, were also taken into account.

The scientists noted that reproductive problems were low among the group, three in four of the men did not smoke and over half were a healthy weight.

The study found that the amount of moderate to vigorous exercise undertaken each week ranged from five to 14 hours, while the amount of time spent watching TV ranged from four to 20 hours.

Not surprisingly, those who did more exercise and watched less TV tended to have healthier diets.

The scientists found that while exercise did not affect sperm motility (movement) or shape, those who exercised the most - more than 15 hours per week - had a 73% higher sperm count compared to those who did the least exercise.

However, this exercise had to be moderate or vigorous. Light exercise, no matter how often, made no difference to sperm count.

When it came to television, those who watched 20 hours or more per week had a 44% lower sperm count than those who watched the least amount. Again, there was no impact on sperm motility or shape.

The scientists acknowledged that a lower sperm count may not affect a man's ability to father a child, however they believe that the results indicate that a more active lifestyle may lead to improvements in semen quality.

"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," the team added.

Details of these findings are published in the British Journal of Sports Medicine.