Middleground

I've been lifting weights more or less since the beginning of the year with the main goal of balancing myself out. See, my legs have always been quite strong and muscular, but I had a weak core and upper body that cause problems with my back. Anyway, the results have been really great so far. I feel quite strong and balanced, which was my main goal. Having said that, I'd like to look a bit bulker in the arms, shoulders and back and I'm not sure how to approach that with weight training/diet. Should I try to do more sets with less reps, or more reps, but less sets? Any foods that help with the building of muscles? How often should I lift? (At present, I do it about 2-3 times a week).

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