I second this: "2nd hand Garmin Forerunner 55 and with a chest based HR monitor".
For heart rate training in general, the gold standard is an ECG-based chest strap. Avoid an optical-based monitor because you get some lag between readings or it even misses readings (this is especially evident during short intense intervals).
For a chest strap HR monitor, the main one is Polar H10, for a cheaper monitor you can get a Coospo H808S.
Saying that, I mainly use an Apple Watch Ultra 1 because it is convenient, it is good enough and I do not have to hassle with putting on a chest strap - I just have to settle with it taking a few seconds to register the correct heart rate after an intense set.
Comments
I second this: "2nd hand Garmin Forerunner 55 and with a chest based HR monitor".
For heart rate training in general, the gold standard is an ECG-based chest strap. Avoid an optical-based monitor because you get some lag between readings or it even misses readings (this is especially evident during short intense intervals).
For a chest strap HR monitor, the main one is Polar H10, for a cheaper monitor you can get a Coospo H808S.
Saying that, I mainly use an Apple Watch Ultra 1 because it is convenient, it is good enough and I do not have to hassle with putting on a chest strap - I just have to settle with it taking a few seconds to register the correct heart rate after an intense set.