From the Coach guide
Why a watch can stop recording heart rate, and what Coach does then.
A watch reads your heart rate with light through your skin. When the sensor loses contact, it records nothing until contact comes back. The watch can still record your pace and distance.
A loose band. Cold skin, which has less blood flow. Some tattoos. Movement or sweat that shifts the watch on your wrist.
Wear the watch snug, above your wrist bone. On cold days, keep it under your sleeve. If your watch often loses your heart rate, you can pair a heart-rate band with it. A band on your upper arm is comfortable, and in studies it read almost the same as a chest strap.
Coach never fills a gap with a guess. Your pace and distance still count. Heart-rate scores use only the minutes your watch recorded. When most of a run has no heart rate, Coach does not score its heart rate.
Your pace still sets your threshold pace and your hard-day paces. Your zones change only after 30 min of hard running with heart rate. Until then, they stay as they were.
The causes and the heart-rate bands come from Apple's guidance and studies of heart-rate monitors. How Coach handles gaps is an app choice.
Apple's support guidance for its watch
Low blood flow in the skin, as in the cold, can stop the sensor from getting a reading. Some tattoos block its light. Rhythmic movement such as running reads better than irregular movement. Apple suggests a Bluetooth chest strap when readings stay unreliable.
Guidance from the maker, not a study. It does not say how often gaps happen.
50 adults on a treadmill, bike and elliptical, against ECG
The chest strap agreed with ECG most closely. Wrist monitors were less accurate, and their accuracy changed with the activity. The forearm band in the study was less accurate than the best wrist monitor.
One centre, healthy adults, devices from 2016. It measured accuracy, not gaps. Newer arm bands were not tested.
16 adults, 9 activities from lying down to hard intervals
An optical band on the upper arm read within about 1.4 bpm of a chest strap on average. The authors suggest the upper arm moves less than the wrist or forearm, so it reads more steadily.
A small study of healthy adults, with one band. It compared heart rate, not gaps.
28 adults at rest, in a warm-up, in burpees and on a graded treadmill test, against a chest strap
Three identical sensors were worn at once. The one on the upper arm agreed with the chest strap most closely, and the one on the wrist least. All placements were accurate at rest and in steady running.
Young, healthy adults. Accuracy fell in short, full-body efforts such as burpees, at every placement. It compared accuracy, not gaps.
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