I keep reading that you should keep your heart rate around 120 when doing cardio, because that's "optimal" in some sense.
So what exactly happens if you train cardio at a heart rate of, say, 150? Or 160? Why is it bad?
Does it not train the cardio system as effectively? Why?
Does it have negative chronic side effects? If so, what are they?
Is it acutely dangerous, in terms of heart attacks or similar bad things?