“Apple patents sensor-packed health monitoring headphones with ‘head gesture’ control” on Apple Insider:

In one of the earliest pieces of evidence pointing to Apple’s push into the fitness and health monitoring field, the U.S. Patent and Trademark Office on Tuesday granted the company a patent for a biometric headphone system that can sense a variety of metrics including temperature, heart rate and perspiration levels.

Claiming priority over a provisional patent application filed for in 2007, Apple’s U.S. Patent No. 8,655,004 for a “Sports monitoring system for headphones, earbuds and/or headsets” proves the company has been investigating integrated activity monitors for at least six years.

According to the patent, the fitness monitoring system is cleverly ensconced in a set of headphones, something users commonly wear to listen to music during workouts. By positioning the headset in or near the ear, the embedded activity sensor can pick up temperature, perspiration and heart rate data, among other metrics.

In addition to skin-based readings, an accelerometer may also be incorporated into the earbud chassis to facilitate the collection of accurate movement data. Some embodiments call for multiple accelerometers, each corresponding to a different axis.