If you’re going to choose a place on the body to measure physical signals, Steven LeBoeuf says two places are far and away the best: the ear or the rear.

LeBoeuf knows it sounds like a joke, but he’s not kidding. He’s the president and cofounder of Valencell, a company based in Raleigh, North Carolina, that developed and licenses technology it says can accurately track vital signs like heart rate, temperature, and respiration rate from the same earbuds you’d use to listen to music. It does this with photoplethysmography, or PPG, which measures changes in blood flow by shining a light on the skin and measuring how it scatters off blood vessels (this is often done in hospitals with a device that fits over your fingertip).

At a time when many companies are betting on the wrist as the body part most likely to hold devices that measure activity and other signals from the body, Valencell may seem like something of an outlier: the market research firm Canalys predicts that more than 17 million wristband gadgets will ship this year, including eight million smart watches. And as LeBoeuf readily admits, it’s a lot harder to build a device that goes in the ear than something that wraps around the arm. The electronics need to fit into a smaller package, and rather than simply dealing with different wrist sizes you’ve got to contend with countless ear shapes, as many earbud makers have found in the past.

But the ear, LeBoeuf maintains, is a much better source of data than the wrist because it offers an area where blood flows neatly in and out, providing a much stronger signal and less noise. Blood also flows to different parts of the ear at different rates, which can be used to measure different metrics. And because we don’t move our ears as much as our arms, it can be easier to sort out intentional motions from unintentional ones.

Additionally, many of us are already wearing earbuds and headphones on and off throughout the day, and LeBoeuf contends it’s not necessary to keep them on constantly to collect useful data. With the right price and features, it could soon be easier to convince people to pick up a pair of earbuds that also happen to monitor your heart rate than a smart watch that does the same.

Valencell licenses its PerformTek technology to companies for biometric measurement on many parts of the body (Scosche, for instance, uses it in forearm-worn heart-rate monitors). But it’s also being used for a growing number of ear-worn devices, including LG’s Heart Rate Monitor Earphone and iRiver’s iRiverON Heart Rate Monitoring Bluetooth Headset (available to consumers for $180 and $200, respectively) and a pair of earbuds from Intel, which are still under development. And it could go far beyond health and fitness tracking, helping to monitor the condition of soldiers and firefighters and changing the way we play video games. “You’ll see games where your emotional state changes the character you’re playing,” LeBoeuf says.

To make this kind of thing work, PerformTek fits an optical emitter, photodetector, and accelerometer into an earbud. The emitter shines an infrared light on a part of the ear between the concha and antitragus—essentially, the lower part of the bowl of your ear, just above your earlobe—and the photodetector picks up the light that scatters off nearby blood vessels. The accelerometer, meanwhile, measures your movement. A digital signal processor (which can be housed inside or outside the earbud) analyzes the data, removing “noise” like skin movement or sunlight and extracting information like heart and respiration rates. With accelerometer and blood-flow data, LeBoeuf says, Valencell’s algorithms can also estimate things like the number of calories you’ve burned. The data is then sent on to your smartphone.

LeBoeuf says Valencell’s technology has been validated by groups outside the company; a paper he coauthored with researchers at Duke University’s medical school indicated that the company’s earbud sensor was able to accurately estimate total energy expenditure and maximum oxygen consumption (the latter is often referred to as VO₂ max) during exercise.

Kevin Bowyer, chair of the University of Notre Dame’s computer science and engineering department, who has studied iris and ear biometrics, thinks it’s certainly possible to get good physiological measurements this way. “I think that if you had the right, good-quality earbuds, you could actually do a lot in terms of reading biological signals related to the health of a person,” he says.

Like Bluetooth headsets and some noise-cancelling headphones, the PerformTek technology needs its own power source to work. For earbuds currently on the market, that looks kind of clunky: the LG earbuds connect to a wearable clip that holds the battery and Bluetooth device, while the iRiver earbuds are attached to a sort of collar that sits on the neck and includes a battery and device controls.

But a pair of PerformTek-using earbuds that Intel showed off this year at the International Consumer Electronics Show in Las Vegas—meant to be a reference design for manufacturers—avoids this kind of bulk by harvesting power from the microphone jack. Steve Holmes, who leads Intel’s New Devices Group, says this could also make it possible to add features like noise reduction. Accelerometer data already being collected by the earbuds could even be used to create a sort of 3-D stereoscopic audio experience, without needing an additional battery or adding much cost.