Heart-rate basics
Can You Trust Your Running Heart Rate?
Heart-rate training only works when the number is believable. Before changing training, learn when to trust the sensor and when to question it.
Wrist sensors are convenient, not perfect
Most watches use optical sensors at the wrist. Movement, cold skin, loose fit, sweat, and fast running can make the reading unstable.
Cadence lock
Sometimes a wrist sensor follows step rhythm instead of heart rhythm. Be suspicious when heart rate suddenly matches cadence while effort does not match the number.
Chest strap vs watch
Chest straps are usually more reliable for intervals, threshold runs, and max-heart-rate testing. A watch is often enough for easy runs if the trace is smooth and believable.
Sanity check
Compare heart rate with breathing, talk test, pace, heat, hills, and fatigue before changing zones.