How can I avoid overworking lower back?
Everything I do hits lower back in some way... squats, deadlifts, rows.
How can everyone do all these exercises that work the lower back, every one of them without getting injured?
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To build on Eric's comment; the best advice is to follow a workout program that is tried and true.
If you're trying to make your own program, and you find yourself having to ask this question, you should probably not be making your own program in the first place.
To make your own program is something you do when you know your body very well, and you know how much it can handle. This very question indicates that maybe you haven't trained so much that this is true for you.
For the record, expect it to take years - even a decade - to develop this kind of intuition for your own body, and more generally, knowledge of the human anatomy.
For the time being, there are dozens of programs that are already developed for both the beginner, the intermediate, and the advanced lifter. You should seek these out instead.
If you're working under one of these programs, and you're wondering if you're overtraining your back, let's just say you'll know it when you feel it. Chronic fatigue and general tiredness are two of the symptoms of overtraining.
Lower Back Injuries / Pain (LBI or LBP) whether it's upper back pain or low back pain frequently is caused by weak abdominal muscles. Since your abs are the front anchor of your spine, if they are weak, then the other structures supporting your spine (your deep back muscles - such as your quadratus lumborum, multifidus etc.. for example) will have to work harder.
By developing innercore stability, you'll be less likely to injure or your back or spinal column.
If you're more powerful global muscles are firing before your deep stabilizers this results in large forces applied to an unstabilized spinal column. This is why the trA is so important. It activates first before movements in any direction.
The local muscles promote segmental stabilization. Inclusion of global muscles too early may be deleterious. Local muscles are superior to global muscles in controlling shear loads.
Unnecessary activation of global muscles may impose excessive compressive loads to the spine.
It's important to incorporate exercises that the cylinder shaped "wrapping" of muscles called your innercore. Exercises should involve the major that make up this cylinder and protect and brace your vital organs and spinal column in place. Some of the key muscles are -- internal and external obliques, quadratus lumborum and by far the most important the transverse abdominis (tRA).
Core Strengthening Exercises for Back Pain: Prehab
Below are some exercises that can help you develop a strong innercore and prevent back pain.
Any variation or progression of the above is fine as well (i.e. change the stability of the exercise surface, single leg vs double, unilateral vs contralateral.)
Your back is not supposed to work during these exercises. Your stabilizers (core, ...) stabilize your trunk and then the movers do the work (hamstrings, glutes, ...)
If your back gets sore during squats or deadlift, you are most probably not performing the movement correctly and using your back muscles as movers instead of stabilizers