Whoop 3.0 Performance Strap
I am obsessed with my Whoop 3.0 Performance Strap ($288 for a 12-month membership) and cannot leave my apartment without it. The Whoop strap measures your strain (how hard your body is working) during workouts and also tracks your sleep performance.
I love the sleep feature because it tells you how many hours of sleep you need based on your fitness goals, how you recovered, and how many hours you slept that night. It also lets you know the level of physical activity your body will be able to handle the following day. For example, if you only got five hours of sleep, it would recommend a light-impact workout or a recovery day over a high-intensity workout.
The strap also tracks how your body is adapting to the workouts you do and provides insight on whether or not you should scale back or progress the intensity of your workouts.