Work in upper back exercises too along with ab/lower back. They strengthen your whole upper body so your shoulders don't slouch forward.

Simplest and easiest way to build up your upper body muscles at home is pushups and pull ups. Get one of those pull up bars that attach to door frames. I have an "Iron Gym" one I got at Target for 20 bucks. No screws or anything needed. Pull ups will also strengthen your lower back and abs too. If you can't do a pull up just start with negatives (google it) and you will be able to do them in no time.

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1985 CA 300D Turbo , 213K mi