Apple’s U.S. Patent No. 8,749,380 is a “shoe wear-out sensor, body-bar sensing system, unitless activity assessment and associated methods.”
The patent describes a system where a sensor-laden device attaches to a weightlifting bar and counts and displays repetitions. A watch for remote readings is included. It is widely thought to be a part of an iWatch weightlifting tracker.
The device incorporates sensors, accelerometers, and processors with its display. Data can be sent wirelessly to a watch. Sensor hardware clips to fitness equipment, including bars, dumbbells and machines, and monitors movement and number of repetitions.