Those who exercise with a friend post better workout time and intensity by as much as 200%, says a study by Brandon Irwin, assistant professor of kinesiology at the Kansas State University.

He and his colleagues tested whether individuals engage in more intense physical activity when alone, or with a virtual partner or while competing against a teammate.

"People like to exercise with others and make it a social activity," Irwin said. "We found that when you're performing with someone who you perceive as little better than you, you tend to give more effort than you normally would alone."