Millions of us use fitness tracker watches to count how many steps we take each day - they also claim to tell us how we're sleeping and how stressed we are, as well as a host of other metrics. But how accurate are they - and are they the best thing since Sliced Bread? We've had loads of requests from you to look into fitness trackers. So with multiple watches on each arm, Greg's being put through his paces at Loughborough University under the watchful eye of Dr James Sanders, senior research associate in digital health for lifestyle. PRESENTER: GREG FOOT PRODUCER: TOM MOSELEY