Training for a 10M, and trying to use my Garmin305, but my HR is always higher than I expect. I used the formula to set the %s, so Max HR is 220-38 = 182. Catch is, my Garmin has my Max HR at 188 almost every run. I can see I am not sustaining that HR, but I am also not trying to get my HR up high. It makes me think that 182 is wrong (for me) but I'm at a loss as to how to determine what my training ranges might be. Should I just stick with the formula stop worrying about it?
If you have seen 188 then clearly 182 is not your max (MHR). You could run by "perceived exertion level" instead of heart rate, that is, by how hard you think the run feels. Compare it to something like your 5K race pace. This article discusses several tests to at least get close to your true maximum.
Determining HR Max can be tough. The consensus is that the 220-age formula is a good idea, but not nearly accurate enough to be a good indicator of one's HR Max. Take a look at this article for other methods of determining HR Max: http://www.beginnertriathlete.com/cms/article-detail.asp?articleid=53 You can also Google "determining HR Max" to get a bunch of useful article on the issue. Good luck.