Was I doing something wrong? I'm doing Chad's Big Boy Basics program. Hack squats are prescribed on lower body day. But my back and shoulders are sore today and lower body isn't very sore at all.
Well lets see you are grabbing the bar with your arm which hook up to your uppes body at your shoulders. This inturn lifts all the weight and put a LOT of tension on your core including your back.
So yes it will make your upper body sore this is why movements such as the hack squat, DL's and anything where you are lifting a considerable weight from the ground are GREAT movements to build overall mass.
Hope that helps,
My traps were absolutely trashed after doing 10x3 hack squats. I don't think they've ever been so sore. I think either Cressey or Robertson said that the upper back responds extremely well to isometric tension so it makes sense. You have constant tension on that area throughout the duration of the set.