Meet the WHOOP Strap 2.0: a smart wearable device that monitors strain, recovery, and sleep. This waterproof, screen-less wearable device can be worn on your wrist, forearm, or upper arm. It has 5 sensors to measure data 100 times per second.
The data can be used to optimize training and recovery. The device calculates your exertion based on lifestyle and workouts and even tells you how much sleep you need to recover. More info is available here.