this post was submitted on 08 Jun 2026
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Those posts about your heart and shins exploding if you go above zone 2 are just memes.
Unless you have some congenital heart defect, there is nothing unsafe about sprinting around in z5. You'll just get tired quickly, but not unsafe. Your brain will shut you down before you hurt yourself.
Anyway, your original question... as you get fitter, you'll just get faster for a given heart rate... i wouldnt think of it as lowering your heart rate. (6m/km @ 150bpm, vs 6m/km @ 140bpm)
Think of it as getting faster at a given effort (6m/km @ 150 bpm then running 5m/km @ 150bpm)
you get faster by putting in more miles, mainly. Sprinkle in some fast days, but mostly just lots of slow volume.