A common place piece of advice thrown around in the PT and weight lifting world is to “never round your back when you lift”. More like a law than a piece of advice.
The understandable rationale behind this advice is that a flexed spine places our lumbar spine in a more vulnerable position. Funneling/bottle necking pressure in the lumbar disc rather than distributing across the whole disc. This can lead to a bulge or herniation.
But is it actually dangerous to round the back when you lift?
The main bulk of this advice is the biomechanics of the lift. Imagine the disc is like a water balloon. If I push on one edge of the balloon the opposite side will swell unevenly whereas if I put even pressure right in the middle of the balloon the entire balloon swells evenly in all directions. The biomechanical function of the discs is similar. When an area is chronically loaded in an uneven manner it can lead to delamination (like silk fraying with too much friction) of the annulus, a type of fraying where the robust “meat” of the disc begins to loosen allowing the disc contents (nucleus) to start pushing out.
Spinal flexion is most likely unavoidable. Research has confirmed that even when we are braced, wedged and attached to the bar our spine is still flexing in a fairly significant way. Even if it looks like it’s not moving at all & neutral, there’s still some motion occurring.
Challenges and Nuances
Loaded spinal flexion can be a difficult topic to navigate. Limiting spinal flexion WHILE LOADED is probably the easiest means of cueing.
Is spinal flexion bad? No, our spine is designed to move in all sorts of neat ways.
Should we flex the spine while attached to a barbell? It depends. If the flexion occurs and stays the same while performing the lift then I would argue no. If your spine moves more like Tina (below) while attached to the weight then I’d argue that’s potentially more injurious.
This is where the adaptation of the tissue and physiology comes into play. If you’ve perfected a slightly flexed spine position when you deadlift or squat and you are VERY good at bracing/creating spinal stiffness in that position is it bad? Absolutely not.