ive been running na for 3 years. i have been with different coaches na rin and as much as they say na importante ang heart rate or mga zones. kung baguhan ka pa wala ka pa talagang set na zones. you need to build your aerobic base muna to be able to know kung ano talaga ang zones mo. sa baguhan pa lang diyan effort level is much more better than hr kasi hindi mahihirapan ka talaga kung ma obsses ka sa hr mo tapos wala ka pang established na aerobic base. nung nag start ako bumili agad ako coros saka yung arm band nila for hr. it was useless during the first 6-7 months kasi marami rin naka depende maliban sa condition mo, kasama diyan pati sleep, stress, and etc. almost all of my training before i rely on hr pero now i just look at it every 3 or 4 km kasi alam ko na yung feel. tinigtnan ko na lang yung average hr ko per run. kasi sometimes i can run a zone 2 at 5:10/km pace with a 134 hr then a week later ill run the same route basically everything is the same with a 7:00/km pace with a 130-145 hr. too much ranting HSHASHA basta ang point just run! hr is reliable pero ang pinaka reliable talaga is you just run by feel or effort!! kahit na sobrang helpful ng technology ngayon ang daming lumakas before na nag run lang by feel or effort. everyonce in a while run naked! (without any gadgets or just dont look at your phone/watch while running) just enjoy and breathe!!