To find out what genes controlled high activity levels, Lightfoot and his colleagues bred a super-active strain of mice with a super-sedentary strain of mice. Then, after measuring speed, endurance, and distance for the mice on a hamster wheel, they were able to correlate the mouse behavior with genetic information, identifying 23 genes involved in activity during two studies. The findings are contrary to what Lightfoot originally thought caused the difference between the marathon and couch potato mice. In previous experiments he tried to show that genetic differences in muscle performance were led to lazier mice, but he was unable to find a gene that differentiated muscle performance. That's what led him to believe its all happening in their furry little heads.