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The One

The Whoop 4.0 Is the Best, Most Comprehensive Fitness Tracker

It’s smaller and thinner, so you can be more productive.

The Whoop Strap 4.0.

The Whoop Strap 4.0.

Photographer: Sarah Anne Ward for Bloomberg Businessweek

Fitness wearables have been around for only 20 years, but the market is filled with so many Fitbits, Garmins, and others, it can feel as if they’ve never left us alone. Whoop, founded by former Harvard squash captain Will Ahmed a decade ago, packs a lot into its unassuming device—readings of your heart rate, respiratory rate, and sleep patterns—to diagnose strain and help you rebound faster. Its new 4.0 sensor, which is free with a $30 monthly subscription, is a third smaller than the last version but adds blood oxygen and skin temperature measurements.