"Performance" or in this case Workload at FTP has a few components:
1. VO2max
2. Efficiency / Economy
3. %VO2max at LT (largely influenced by training and I'm unsure of it's response to arm vs. leg cycling, so not discussed here).

If we start at the top:

V02Max is limited by the cardiorespiratory system to deliver oxygen to the muscles.
Support:
1. Oxygen delivery alterations have a profound effect on VO2 max (Blood doping, EPO, increase significantly and Beta-blockers decrease it significantly.)
2. Increases in VO2max with training are largely influenced by cardiac output improvements and less so at the local muscle.
3. Training one set of muscles (e.g. legs) also results in an increase in VO2max with an unrelated muscle group (e.g. arms).
4. When training a single muscle (e.g. leg extensions) oxygen uptake in that muscle is significantly higher than it is when whole-body exercise is being conducted.

Therefore, adding in the additional muscle mass of the arms does not net you more oxygen uptake. Working muscle is able to extract approximately 90% of the oxygen from arterial blood and as witnessed by point 5 above, working the whole body does not increase this extraction in a significant manner.

See:
Limitations: https://www.researchgate.net/...ndurance_performance
Arm vs. Leg Ergometry https://insights.ovid.com/...m-leg-air/7/00005768


Efficiency:
1. Gross efficiency with leg ergometry in a trained cyclist is typically in the 20 to 25% range
2. Gross efficiency for arm ergometry is typically in the 6 to 15% range.

Therefore, adding in the arms decreases the over-all efficiency of the system. This means that less workload is completed for the same amount of oxygen.

See:
https://pubmed.ncbi.nlm.nih.gov/6706760/

Given the interaction of little to no increase in VO2max with decreased efficiency, I'd say that your workload at threshold would decrease if you tested on the Airdyne vs. regular bicycle in a fit / habituated individual.

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I also give Training Advice via http://www.ForeverEndurance.com

The above poster has eschewed traditional employment and is currently undertaking the ill-conceived task of launching his own hardgoods company. Statements are not made on behalf of nor reflective of anything in any manner... unless they're good, then they count.
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I knew that Nagle study was coming out the second I saw your username.

For us joes......what's the cliff notes then on cyclists cross training rowing or cross country skiing that utilizes the arms? I can't read what was posted there and really comprehend what the answer would be. I'm having a slow morning.

Kind of like I have interest in converting my old Cycleops fluid that's just laying around into a poor man's rowing machine.

In regard to VO2max, activities that stress and increase your cardiac output are potentially worthwhile but likely no more worthwhile and often less worthwhile than your primary activity.

Ie. If you want to be good at cycling, ride a bike. If you want to do something else, Nordic skiing is better than knitting.

Edit to Add:
Beyond performance in a singular sport there are many reasons to "cross-train". E.g. Whole-body fitness, Potential for decreased injury, Fun, physique, etc. Just don't expect to be better at cycling for it.

---

I talk a lot - Give it a listen: http://www.fasttalklabs.com/category/fast-talk
I also give Training Advice via http://www.ForeverEndurance.com

The above poster has eschewed traditional employment and is currently undertaking the ill-conceived task of launching his own hardgoods company. Statements are not made on behalf of nor reflective of anything in any manner... unless they're good, then they count.
http://www.AGNCYINNOVATION.com