Who Could Be Watching You Watching Your Figure? Your Boss

Those of us trying to lose some pounds after overindulging this holiday season can get help from a slew of smartphone apps that count steps climbed and calories burned. Self-tracking has also become a way for companies to make money using your fitness data. And some experts worry that the data collected could be used against users in the long run.

At a recent Quantified Self Meetup in downtown San Francisco, technology lovers are testing homemade do-it-yourself devices on people eager to measure their mind and body.

Charles Wong straps a belt to me that vibrates when I slouch. Jonathan Toomim slaps a Velcro headband on me, to measure my concentration according to prefrontal cortex activity. Heather Heine pokes my finger for a blood test.