How trustworthy is your fitness tracker score?

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Millions of people now start their day with a number — a “readiness” score, a “body battery”“ level or a measure of "strain”“ — delivered by the wearable device on their wrist or finger. But how much trust should we place in these scores?

Composite health scores are increasingly used by digital fitness trackers to offer a single, daily number that reflects how your body is coping with recent demands. Whether it’s marketed as a measure of energy, recovery or resilience...