Men's sperm quality may be significantly affected by their levels of physical activity, according to a new study led by researchers at Harvard School of Public Health (HSPH).

The researchers found that healthy young men who were sedentary, as measured by hours of TV viewing, had lower sperm counts than those who were the most physically active. The study were published online Monday in the British Journal of Sports Medicine.

The researchers analyzed the semen quality of 189 men between the ages of 18 to 22 participating in the Rochester Young Men's Study during 2009 at the University of Rochester. The men were asked about their physical activity and TV-watching habits, in addition to health issues that may affect their sperm quality, such as diet, stress levels, and smoking.

Results showed that men who watched more than 20 hours of TV weekly had a 44 percent lower sperm count than those who watched almost no TV. Men who exercised for 15 or more hours weekly at a moderate to vigorous rate had a 73 percent higher sperm count than those who exercised less than 5 hours per week. Mild exercise did not affect sperm quality.

"The majority of the previous studies on physical activity and semen quality had focused on professional marathon runners and cyclists, who reach physical activity levels that most people in the world cannot match. We were able to examine a range of physical activity that is more relevant to men in the general population," said Jorge Chavarro, senior author of the study and assistant professor at HSPH.