The fundamental problem I have is that I seem to be incapable of running in zone 2 according to my (Garmin) watch. My maximum (observed) heart rate (MHR) is 175 bpm, and Garmin appears to be just using fixed percentages of my maximum heart rate:
Zone 1: 88-104 (50-60%) Zone 2: 105-122 (60-70%) Zone 3: 123-139 (70-80%) Zone 4: 140-157 (80-90%) Zone 5: 158+ (> 90%)
However, I have a low resting heart rate. My sleeping heart rate is around 33-36 bpm, but my resting heart rate while awake is more like high 30s or low 40s, so for the sake of argument I'll take my resting heart rate (RHR) to be 40 bpm.
Yesterday I found out about the Karvonen formula for target heart rate (THR) for a given PERCENTAGE effort:
THR = RHR + PERCENTAGE*(MHR - RHR)
Which gives a very different set of heart rate zones:
Zone 1: 108-120 (50-60%) Zone 2: 121-134 (60-70%) Zone 3: 135-147 (70-80%) Zone 4: 148-162 (80-90%) Zone 5: 163+ (> 90%)
So the Karvonen zone 2 largely overlaps with Garmin zone 3, which means that using the Karvonen formula I can run in zone 2.
So my question is, does my low resting heart rate mean that the Garmin method is not appropriate and I should use Karvonen's method (or something else?)?
Additional info: I never seem to get problems with lactic acid when running in Garmin zone 4 and I have had runs where I have run in Garmin zone 5 for over 45 minutes. Both of those things seem inconsistent with the usual textual descriptions of the zones.
Update: Have switched to the Karvonen/HRR approach and have found it difficult to maintain zone 1 when walking and zone 2 using the elliptical trainer. Will hopefully test it when running on Saturday if my Achilles is up to it.
Update: Ran a slow (6:49/km) 10k this morning and had no difficulty keeping to zone 2 (97% of the time). Felt very easy, would not have had a problem holding a conversation, breathing slowly but deeply (in for four strides, out for four strides). Found it easier than zone 2 on the elliptical trainer, but I suppose that just means I am better adapted to running as I do a lot more of that these days. I suppose the question is whether it is cheating to become able to run in zone 2 by changing the formula for computing the zones! ;o)
Update: Ran a zone 3 10k this morning, felt mildly uncomfortable, but could have continued for another hour without difficulty. Not particularly fast (6.09/km) but more enjoyable than zone 2 running. Breathing in for three strides, out for three. Could have held a conversation, but only just. That seems consistent with the usual textual description.
Update: Ran a zone 4 10k yesterday, which felt much more like being at a "threshold" pace than before (5:26/km). Breathing hard (in 2 strides, out for three), would not be able to hold a conversation. Legs beginning to get a bit tired at 7-8km. Again, much more consistent with textual description. Seems like the Karvonen zones may indeed be better for me.
Probably last update: Max effort 5k this morning, 23:58, which was my third fastest time, not bad considering recent injury issues and I did a 3k swim yesterday. In zone 5 for the last 3k, max HR 173. Wanted to rest my hands on my knees at the end, but could stay standing upright, but very out of breath. Again that seems appropriate for zone 5.