These days, everyone from the average Susan to the hardcore data-loving wellness enthusiast has the technology to track just about anything in the name of self-optimization. A ring wearable tells you ...

PRINTED ON TREADMILLS and exercise bikes in gyms around the world is a simple method for estimating the maximum rate at which your heart should safely beat, in beats per minute: 220 minus your age.

That tension becomes especially clear with heart rate zone training, a popular five-tier system that endurance athletes use to measure effort and fine-tune performance. The well-known formula is ...