NEW YORK • Sweatcoin, a fitness app, awards points - "sweatcoins" - for walking or running outside.
Its popularity began to spike last summer.
By early September, it had become the most downloaded health and fitness app in the United States on the Apple App and Google Play stores combined, according to data from App Annie, an apps analytics firm.
It has retained that top ranking nearly every week since.
The app, developed by a start-up in London called Sweatco, has raised about US$1.6 million (S$2.13 million) in financing and expects to close another round soon.
It taps the boom for health-tracking apps and devices, joining products from Apple, Fitbit, Garmin and Samsung that monitor statistics such as heart rate and physical activity.
Nearly 350 million wearable tech devices are expected to be sold globally this year, according to forecasts from Gartner, a market research company.
Where Sweatcoin's approach differs from others, however, is in taking its ideas from behavioural economics, the study of how psychology and emotions influence our decision-making, to motivate people to exercise.
Its rise comes as insurers and corporate wellness programmes are furiously trying incentives - including passing out fitness monitors - to nudge people to improve their behaviour.
Sweatcoin's "first premise is that physical movement has economic value", Mr Anton Derlyatka, a Sweatcoin co-founder, said.
He added that the app aims to overcome one's human tendency to choose immediate gratification - such as doughnuts or binge-watching videos - over activities such as daily exercise that offer long-term benefits.
By giving consumers points that can buy goodies, Sweatcoin hopes to incentivise couch potatoes to become more active.
"It's a tool, a hack, to help all of us, not just the select few, to remain fit," said Mr Oleg Fomenko, another Sweatcoin co-founder.
In theory, the app awards one sweatcoin for every 1,000 outdoor steps.
But users often get credit for fewer steps than they expected.
For one thing, Sweatcoin immediately shaves your sweatcoin earnings by 5 per cent - in the fine print, the app calls it a commission "to keep our lights on".
For another thing, the app counts only the steps it thinks the user took outside. Users get no credit for Zumba, spinning - or listening to music playlists and dancing around the kitchen while cooking.
To help it distinguish outdoor steps, Sweatcoin collects and analyses GPS signals and motion data from the user's phone.
To improve accuracy, it instructs users to turn off the app's battery-saver mode and keep the app running in the background.
In reviews on Reddit, Twitter and app stores, missing points are a common theme.
"Where are my missing 3,000 steps!" Mr Ian Wynne, a sports performance consultant in Britain, wrote on Twitter.
"This is happening a little too often."
"It's a little frustrating when you are outside and you are sure that you have taken 1,000 more steps than it claims," Mr Wynne, who won a bronze medal in kayaking at the Athens Olympics in 2004, said in an interview.
Nevertheless, he planned to continue using Sweatcoin. He regularly exercises, he said, and liked the idea of getting extra credit for the outdoor activities he was already doing.
Mr Fomenko said that the app's "draconian algorithm" is meant to deter cheating techniques such as, say, strapping a phone to a pet dog. And that fraud-deterrence system may verify fewer steps than users expect, he said.
In practice, according to the app's help section, that means the Sweatcoin algorithm typically verifies 65 per cent of a user's total steps.
It also means that some frequent consumer behaviours - using a protective smartphone cover, walking while connected to public Wi-Fi, walking slowly with small children - could reduce the step count.
In a follow-up e-mail, Sweatcoin's co-founders said they would soon hire a data science team "to help us improve the speed and accuracy of this algorithm".
They also said they were working on a verification algorithm to count indoor activity.
Sweatcoin generates revenue by marketing other companies' products to its users. When a user redeems reward points for a product through Sweatcoin, the maker of that product pays Sweatcoin a fee. Now the reward choices include the iPhone X for a whopping 20,000 sweatcoins.