backpack and adolescent back pain article by Joseph Muscolino.

Does Carrying a Backpack Cause Back Pain in Adolescents?

And perhaps the biggest problem I have with this study about backpacks and adolescent back pain is that an overuse syndrome usually does NOT cause pain in the early stages. When the body is physically stressed by something like carrying a backpack, it often requires many years before the overuse results in pain.

Permission Joseph E.Muscolino. Manual Therapy for the Low Back and Pelvis - A Clinical Orthopedic Approach (2015)

Low Back Pain: Time to Consider Massage and Exercise as Effective Treatments

So if we say that the cause of most low back pain is not understood, it should mean that we do not know what mechanism is causing the low back pain most of the time. But that is not true. We can often palpate tight myofascial tissue or a dysfunctional joint that is causing back pain.