Anyone use them? I bought a Polar one a few weeks ago. Now apparently the way to work out your max heart rate is 220 less your age so mine is 179. The other night my heart rate was just touching 190 on a fast uphill run. I wasn't in any discomfort, pain or anything. My question therefore is it dangerous to hit that kind of heart rate? Also is it risky to sustain it for any length of time?