I want to work my upper back to help fix my hunched "computer posture" shoulders. I'm sitting with my legs extended, wrapping the band around my feet, and pulling the handles of the band to my torso. I did 65 reps yesterday, some with my elbows bending downward, some with them bending out. Today, I'm not sore! What gives? Is there a more efficient exercise for this?
From your statement I understand that you did some sort of rowing motion. Since rowing is a motion where the lats are involved as major muscles, probably the resistence was not big enough.
To your posture fix I would rather recommend band pull aparts: Picture showing the motion
and rotator cuff exercises.
I would also recommend working in multiple sets of 15-20 reps, instead of one all-out set.
Soreness is not an indicator of effectiveness.
Bands do not provide the most strenuous resistance.
A more efficient exercise to work the upper back would involve pulling and bracing exercises using dumbbells, a barbell, or a pull-up bar, or something that can be jury-rigged into a pull-up bar. For instance, rows, pull-ups, or deadlifts. Proper shoulder retraction and depression during certain yoga poses (e.g. down dog, or its more vigorous relative, the Hindu push-up) could assist as well, but I find best return on time/effort investment from heavier exercises.