As a devoted Garmin-ite for two years or so, I have learned to take the heart rate output with a grain of salt. Heart rates vary day-to-day for reasons that are both known and unknown. Things that affect heart rate during a workout include ambient temperature; time of day; sleep; caffeine intake; weight; pace; state of hungover-ness and general workout fatigue, i.e., if you workout three straight days, heart rate will be higher on third day; it will also be lower after a rest day.
With all that, I have learned to accept variations (plus or minus 5 heart beats per minute) in heart rates during training without freaking out. Of course, I love it when I have a workout that shows a strong pace and a surprisingly low heart rate. I immediately chalk that up to my improving training and fitness level. And a bad day I chalk up to the vagaries of the heart rate gods. But now and then a workout will come along that you just have to say is an aberration and move on without trying to figure it out.
On Monday February 15 (after a two day break, but with a twinge of hangover from Valentine's Day), I ran in my gym at about 5 p.m. I ran 6.2 miles at 8:27 pace. My heart rate was pleasantly low: in the 140's with a high of 145. I ended with the feeling that I had just had a GREAT workout.
Yesterday, I ran in the same gym, at the same time, at the same pace, on the same treadmill. No alcohol remnants in my system, and after a day on which I had not run (although I did a weight workout that left me sore). heart rate -- high 150's, with a top of 164. On average, about 10% higher than 3 days earlier. I was stunned; I actually had planned to run faster, but held back because of my high heart rate.
For heart rate purists, this is a dramatic difference. A 75% heart rate workout is easy; an 85% is approaching lactate threshold or tempo run pace, and carefully managed. Through the genius of SportTracks, here is a graphic comparison of the two workouts:*
So what do I make of all this? Not much, actually. A vivid demonstration of the reason not to take any one workout or data point too seriously. And, evidence of the reason not to focus on the heart rate to the exclusion of all other things, such as how I feel on a run.
*Also note the spike in heart rate at the beginning of the Feb 15 workout. My heart rate usually spikes in first 5 minutes or so, even if I warm up. Without the spike, the difference would be even more dramatic. Also note the effect on my heart rate on the red line when I had to stop and tie my shoe.