Smiling while running could boost efficiency and help you save energy over long distances, according to a new study.
Researchers discovered being happy helped runners consume 2.2 per cent less oxygen than those who showed no emotion.
Smiling provided an even bigger boost compared to frowning, the scientists from Ulster and Swansea universities found.
Oxygen is often used to measure how much energy runners use, with higher amounts signalling they have used more of their energy.
Some 24 club-level runners were involved in the research, published in the journal Psychology of Sport and Exercise.
They all had breathing masks attached to them to monitor how much oxygen they used during four six-minute running blocks on a treadmill.
During each run, the participants either smiled, frowned, attempted to relax their hands or adopted their normal focus of attention.
It is believed that smiling helps runners to relax, which in turn improves their running economy.