I've been having a lot of trouble trying to comfortably hold the bar on my back for low bar squats. Right now, I'm out of commission for squatting as ordered by my doctor due to a knee injury. In the meantime, I've been going to the gym and just working on holding the loaded bar on my back in low bar position. High bar, I can hold a lot more weight comfortably, but trying to hold the bar in the low bar position takes a lot out of me just trying to hold it there, let alone trying to squat with it.

Is this something that takes time to build up strength for, or is it just me? I mean, I can hold 315 on my back fairly comfortably in the high bar position, but to hold 185, low-bar is a workout in itself. Is this all part of the strength that's gained from squatting?