LogFAQs > #979971896

TC is very right, although I'd argue posture does affect your back. But what he said applies to most soreness in your back/shoulder/neck area (obviously not extraordinary genetic or medical conditions). When you "lift with your back" instead of your legs, it strains your back because it's literally too weak to bear the load you place on it. That's your legs that should be bearing the majority of the load.

Our backs often tighten up out of weakness because our muscles want to create that stability that isn't there. What poor posture can do is stretch the back muscles for long periods of time and thus weakening them. Think of a rubberband, if you stretch it out and keep it that way eventually it'll weaken and become slack or looser when you bring it back to it's rested state. If not snap.