Update: Today I managed to pull back the 83 lb bow. Last time I tried it was 3 weeks ago and I wasn't able to do it. Today I did it and did it without too much difficulty - no hard straining, could pull it back multiple times, no soreness afterwards. What I changed in the last 3 weeks was doing more chin ups/dips and a lot of trap bar deadlifts at around 75% of my 1RM. By a lot I mean 2-3 sets of 5 every day. I've also been pulling back my 70 lb bow religiously every night, usually 4 sets of 10-15 reps. My diet hasn't changed and I haven't gained any weight (actually lost about 3 lb) so it's purely a strength gain. Now I'm going to start pulling back the 83 lb bow and try work up to 90+, but even at this level I'm happy. There's no real mystery in this I guess (lift heavier weights = get stronger) but it's a great feeling to finally break through that plateau.