As exercise buffs and reformed couch potatoes count steps on a Fitbit or chart cycling splits on the Strava app, they are helping turn fitness into a data-driven pursuit.

The latest obsession, at least among some in the tech industry, is tracking body composition—the proportion of fat versus lean.

In the parking lot of Google’s Mountain View, Calif., campus, there’s a semitrailer truck emblazoned with the words “Real Raw Results.” Go inside, and come out 10 minutes later with a breakdown of how much body fat versus lean mass you are carrying around.

Inside the truck, Google employees take turns lying still on a table as an X-ray scanner moves slowly down the body. Bone, muscle and fat each have different densities, so the rays are absorbed at different rates. That allows for a reading of their volumes.

Also popular on tech corporate campuses are traveling hydrostatic weighing tanks, also known as “dunk tanks,” which assess muscle mass as an individual is submerged in a pool of warm water.

Body Spec, of Los Angeles, has offered dual-energy X-ray absorptiometry scans, or DXA testing, at Google since early 2015. Employees pay $45 for each scan, or $140 for a four-pack; the truck fills up for the three days a month it is there performing scans. “It’s like somebody picked up a rock and they all came running,” says Ariana Strickland, a Body Spec technician.

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