“A gym instructor told me that longer repetitions with small weights will build enduring muscles, which what I am looking for in boxing. Not big ones. And 39lbs is small weights”
Sounds like you already know the answers lol..why ask us?
Your arms and shoulders aren’t getting tired because of lack of strength..they are getting tired because of lack of endurance..gaining muscle won’t help that ,in fact it would hinder it in the long run.
I liked 5 pound weights in each hand, And shadow box with them in sprints .