A study in the British Journal of Sports Medicine, shows falling sperm quality is linked to lower levels of physical activity.
It found that young men who watch television for just three hours a day have half the sperm count of men rarely found in front of the box. Yet 15 hours or more of exercise a week boosts semen quality, according to the results in nearly 200 college students.
Experts say the effect could be due to lack of exercise, or to overheating of the testicles caused by prolonged sitting. And some experts say men wanting to father children need to be selective about the sport they do, as well as avoid wearing tight underpants as these may harm sperm.
In the latest study from the Harvard School of Public Health (HSPH), healthy young men who watched more than 20 hours of television each week had a 44 per cent lower sperm count than those who almost did not watch.
Men who exercised for 15 or more hours weekly at a moderate to vigorous rate had a 73 per cent higher sperm count than those who exercised less than five hours per week. Mild exercise did not affect sperm quality.
In the study, the researchers asked 189 young men who were students at a university in New York to record how many hours they had been spending doing physical activity and watching television in a typical week.
The volunteers, all aged between 18 and 22, were also asked to provide a sperm sample for laboratory analysis.
When the researchers compared the survey findings with the sperm test results, they found the link between sedentary lifestyle and low sperm count.
