I was hoping some of you experts can help me out. I'm new to heart rate training and have been working on it for about four weeks now. I know there is a direct correlation to hot temperatures and an elevated heart rate, but I have a question about colder temperatures. Today I was attempting to do some power interval training and had trouble getting my heart up to the necessary level. It was cold outside (about 45 degrees), and I seemed to be hittting my lactate threshold at about 7-9 bpm less than I should. So, does cold weather have anything to do with this, or is it more likely fatigue from a long MTB ride the day before, or cold weather issues with the Garmin heart strap?