Watch accuracy

Wrist heart rate and GPS are reliable at a steady effort, less so when things change fast; a chest strap and open sky give the most precise data.

• Heart rate: the wrist’s optical sensor is usually within a few beats at a steady pace, but it can lag 10 to 20 seconds on short intervals and sprints, and misread in the cold or when loose.
• A chest strap reads the heart’s electrical signal: it is the reference for intervals.
• Wear the watch snug, a finger’s width above the wrist bone, a notch tighter while running.
• GPS: expect 1 to 3 % distance error; tall buildings, dense trees and tunnels make it drift, so the pace jumps there. Wait for a GPS fix before you start.