You are correct about the increased oxygen demand for engaging the upper body.
For the last few years, I have been primarily mountain running in the summer and XC skiing in the winter. Skate skiing (freestyle xc) is a significantly higher intensity than running and you are generating a lot of power through your poling. I am skate skiing in zone 3 or 4 almost all the time. The only time I can stay in zone 2 is skiing at a casual pace on a flat or downhill trail.
I need to retest my AeT now that the snow is melting away, so these numbers are probably not totally accurate. After skate skiing all winter I did a 40k race and spent 1:26:11 in zone 3 and 53:52 in zone 4 (per chest strap HR monitor). I didn’t think that was possible for me. I imagine it was a combination of being pretty well trained for high intensity and that my AeT has also changed since I last tested it.
Now that I am running again, I struggle to get my legs to move fast enough to get into zone 2. I am mostly running in zone 1. I suppose if you are going to have a problem, that’s a good one to have.
I know that doesn’t answer your questions, but you are not the only one to experience this. I often run with poles. On a training run, I usually slow my pace to stay within the target HR. In a race, I will push it harder, and feel pretty comfortable with my ability to maintain a higher HR for a longer time period. For your case, I think your HR is likely accurate because you are utilizing more oxygen. I generally feel less fatigue running uphill with poles, which I imagine is because I am able to utilize those other muscle groups to help out.
I live in Anchorage, and things are just melting out this spring. I will try to update this after I can retest my HR (I’m waiting until I can do it on a surface that isn’t snow). I’m interested in how much my HR zones may have changed after all the skiing I have done this winter. I’m also hoping to keep some of this higher-intensity aerobic fitness as the running season progresses.
Brandon