I think you are correct: if you are struggling to hold a bow at full draw, you are less likely to be able to feel subtle changes within your body. And engagement of the back muscles is a subtle change from engagement of the arm muscles, because the transference of weight from the arm muscles to the back muscles is not something we commonly do in life apart from archery. I have noted a number of times in Arne’s commentaries on members’ videos where he has observed that the member does not appear to be using the back muscles at all, which was a surprise to the member and not that obvious to me, as I lack the trained eye that Arne has.
I think the best way to train for archery is to draw a bow that is completely within your control and hold it at full draw for an extended period of time. I like to do this with an actual bow, so I can release the arrow at the conclusion of the hold, which is sort of a reward for having to do this. However, if you don’t have a stable full of lighter weight bows to choose from, there are other low cost options available, such as the
Accubow The Accubow allows you to set any weight you want, and draw and shoot very similarly to a regular bow. I bring it on trips when it would be difficult to bring a regular bow. I like to use it with a Formaster or Shotrainer because it gives me instant feedback as to whether I’m using my back muscles, and avoids the annoying snap when you release the Accubow without it.
My routine is to hold the bow at full draw, both right and left handed, for 20 seconds. I do 5 reps of this. I use a bow weight that is not easy to hold for 20 seconds, but is not so strong that I shake and quiver either. During the 20 seconds I focus on relaxing my string arm and hand, and maintaining back tension. I also focus on my breathing. I have found that 3 slow breaths is about equal to 20 seconds, but I need to check myself on this from time to time, because the “slow” breaths tend to get faster over time, and the 20 seconds diminishes accordingly.
I also do a series of exercises I have developed to strengthen my rotator cuff muscles, that does not involve drawing the bow, but I think this post has gone on long enough and I will leave that for another time.