Heart-rate monitors
Evidence: strong
The chest strap is the practical reference standard. Optical wrist sensors are fine at rest and easy efforts but lag and underread during intervals; use a strap when the number drives a decision.
Keep it in proportion
This is one of the last few per cent, at most. It pays off only once the basics are already in place: consistent volume, sleep and adequate fuelling. Most runners gain far more from those than from anything on this page.
Heart rate is one of the more useful numbers a runner can track, but how it is measured decides how far to trust it. Two technologies dominate, and they fail in different ways.
Electrical (chest strap)
A chest strap measures the heart’s electrical signal, the same principle as a clinical ECG, from electrodes held against the skin by the band. It is the practical reference standard. Against a true ECG, a good strap agrees closely, with small bias and tight limits of agreement at rest and across exercise intensities (Schaffarczyk et al. 2022). It tracks fast changes in rate without lag, which is exactly what interval training needs. The drawbacks are comfort, the need to wet the electrodes, and occasional spikes from a dry strap or a cold start.
Optical (wrist and armband)
Optical sensors use photoplethysmography: green LEDs shine into the skin and a detector reads the pulsing of blood with each beat. This is the technology in wrist watches and in optical armbands. It is convenient and, at rest and at steady effort, reasonably accurate, typically within about 5% of an ECG (Shcherbina et al. 2017). Its weaknesses are systematic and worth knowing:
- It lags and underreads during fast changes and hard efforts. A meta-analysis of 44 studies found optical heart rate close to accurate at rest but systematically underestimating under high motion, worst during cycling and resistance work (Zhang et al. 2020). During a short surge the watch “catches up” only as the rep ends, so it is a poor guide to interval intensity.
- Motion artefact, cold and skin tone all degrade it. Wrist movement, cold-induced reduced blood flow to the wrist, and darker skin tones at high intensity all widen the error (Hung et al. 2025).
- Position matters. An optical sensor on the upper arm or forearm, where there is less movement and better blood flow, is more accurate than one on the wrist, and approaches chest-strap quality in controlled conditions.
Which to use
For easy runs and all-day trends, a wrist optical sensor is fine, and its convenience means it actually gets worn. For anything where the heart-rate number drives a decision, intervals, threshold work, or any metric derived from heart rate such as estimated VO₂max or training load, use a chest strap or an optical armband. The same rule applies as to the other metrics: match the precision of the tool to the precision the decision needs, and do not let a convenient but noisy reading drive a session.