Jogging with a friend could make you run faster, further and burn more calories, scientists claim.
Exercise is "socially contagious" with our activity levels strongly influenced by those around us, they found. Experts looked at 1.1 million adults around the world belonging to running groups on social networking sites, such as Facebook.
They wore electronic fitness devices which recorded their speed and length of runs, and shared them with the group. The results showed that if they saw their friend had run an extra kilometre, that would motivate them to go a third of a kilometre further next time they went on a run.
Dr Christos Nicolaides, of the Massachusetts Institute of Technology in Boston, said: "These results suggest social contagion will spread behaviour change more effectively."
Women tend only to be motivated to exercise more with other women but men are inspired by either sex, the US researchers found. The study in Nature Communications journal put the contrast between men and women down to "gender differences in the motivations for exercise and competition".
Dr Nicolaides said the findings could be used to combat obesity by encouraging friends to exercise together.