Activity trackers like the Fitbit Ultra and Jawbone UP can track how many steps you take and miles you cover, but they don’t directly monitor your most intimate biological functions — and thus can’t report how many calories you’ve burned with unassailable accuracy. And then we have the BodyMedia LINK Armband. Armed with enough sensors to classify as an FDA-approved medical device, the LINK observes exactly what’s happening on the surface of your skin to verify the effectiveness of your calorie-killing exercise crusades.