Yesterday I went to the track in the morning to do some fast repeats. The scheduled called for 5 x 2:30 at 92% MHR with 1 minute rests. I ran a faster half mile to get my legs ready and proceeded to launch into the first repeat. I was running hard, breathing hard, but my HR was dawdling along at 86%. So I ran faster and harder, cutting the first repeat short at 1:44 when my lungs gave out. The HRM said I was at 88%.
Mystified, I ran my 1 minute jog and then tried again. Same thing. I jogged some more and thought about it. What was going on? Were my legs too tired to get myself going fast enough to get my HR up into the low 90%s? Was I glycogen depleted? Was the dreaded iron deficiency issue back? Why were my lungs giving out before my heart and legs?
I tried another and the same thing happened again. This was getting silly. Finally, on the fourth lap it occurred to me to look at my pace. I was running 6:18 pace. Aha. The HRM was bullshitting me again. Now I figured I’d been running these in the mid-90%s (which would explain why I was getting breathless before the 2+ minute mark). So I ran a few more quick ones and learned another lesson: don’t over rely on technology. I knew I was working hard and running fast. I should have trusted those perceptions over what the watch was telling me.