"That pretty much says it all. The difference between a thru-hike and a high intensity day hike are vast. On the day hike, the opportunity to gain strength and muscle are quite good if the intensity is in the right range. On a thru-hike…??"
Sure. On a thru hike you do the same thing as a high intensity day hike at a lower level, day after day. Using poles for 4-5 hours, or more, has a number of benefits, one of which is building endurance. Do it a 2-3 times/week and you'll be able to "hit the deck running" on a thru hike, building ever more endurance as you go. It's not the only approach, but one that will work. I don't use it to build strength, BTW.
"Again, this is great for a training hike, but a disadvantage on a thru-hike where your cardio-pulmonary system is working overtime already."
If your cardio-pulmonary system is working overtime on a thru hike, you have a problem. Thru hiking is an endurance activity where you are functioning at well below VO2 max. The high intensity workout is meant to increase VO2 max. With increased VO2 max, comes the ability to hike faster without approaching VO2 max. I don't see this as a disadvantage. Also, a training hike is just that, training for a greater purpose, not an end in itself.
"Again it seems we agree. But again, I would consider those to be relatively high intensity moves rather than high rep low low weight moves. A good climber should be able to lower their bodyweight plus pack by their triceps alone (and still have plenty of strength in reserve). This type of strength is not generally needed or generated on a thru-hike by using poles.
Apples and oranges again, Lynn. They are definitely relatively high intensity moves. But you seem to constantly view strength as a one off proposition, throughout your posts. You mentioned in a previous post that you spent a lot more time building strength than stamina when you were climbing and, for the life of me, I cannot see how you did serious mountaineering following that regimen. Strength moves have to be repeatable, time after time after time, and that is what endurance is all about. Using poles is not the primary way to achieve climbing endurance. Indeed there are many much more effective ways, e.g. traversing back and forth on climbing wall, spending a day yo-yo-ing up and down sport routes, etc. My point was that hiking using poles on days when you are not climbing has a positive endurance effect on your upper body. As for lowering you body weight plus pack using only your triceps, hmmmmm….
I'll sum up my position on my way out the door: The OP asked if using poles on the AT would build muscle mass. Opinions vary but the concensus seems to be that they don't. I tried to introduce another beneficial effect of using poles, i.e. endurance. Maybe I shouldn't have, but it seemed to me to be too important to overlook, since thru hiking or any multi day backpacking trip, for that matter, requires considerable endurance from the entire muscular system. From there the discussion, at least as far as you and I participated, seemed to resolve into a strength/big muscles vs endurance question. I will readily admit that strength is very important in backpacking and even more so in climbing,and I have spent a lot of time down through the years trying to develop it. I have spent even more time developing endurance and skill sets, because I am certain that they are far more important, in the end, than brute strength. But I have yet to find in any of your posts the slightest recognition of the importance of endurance in general backpacking activities and climbing in particular. That being the case, there is not much left to discuss. I stand by what I have posted and invite comments from the rest of the community, hopefully to arrive at a concensus that will be of benefit to all.