Mountain biker Trevor Shepherd was feeling triumphant after reaching the top of a steep, rocky trail near his home in Manchester, England—posting a personal-best time of 3½ minutes.

He took out his phone and quickly recorded the accomplishment on his Strava app, which lets cyclists see how their times stack up against others.

On the app later that evening, the 50-year-old advertising executive was stunned to see another rider had handily beat his time a few months earlier. Upon looking at her profile picture, he noticed she was sitting atop an electric-powered bike on the same gnarly section of trail.

“The lady in question is up there with a time of 1:55. It’s obvious that was on an e-bike,” said an exasperated Mr. Shepherd.

The growing popularity of health and exercise apps has led to more people fudging on their fitness. They’re hacking fitness trackers like Fitbit, inflating their step counts and “forgetting” to record that afternoon bag of sour cream and onion potato chips.