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Strengthening and building a muscle optimally requires the muscle to be stretched against resistance then contracted. Then why is it band to round the lower back,shouldn't it strengthen the spinal erectors?
Think of dumbbell side bents, they are safe and good for the quadratus lumborum ,never heard of anyone ever getting hurt with them, so why is it bad bending the spine forward?
In general, muscles should take more load than connective tissues. Even big strong ligaments and tendons (like the Achilles) do not have the absorption and range of the calf muscle they're connected to.
The connective tissues in your back are intervertebral discs, connecting your vertebrae together. They can absorb some impact, act as ligaments, and offer some flex and rotation. But they are not designed to allow you to pull 500 pounds off the floor.
When your spine is neutral, you are isometrically locking the erector spinae and abdominals, along with some other smaller helper-muscles.
When you then reach down to pick up that 500 lb weight for a deadlift, with that neutral spine, your intervertebral discs are receiving a load that is within their range to handle, but your back and ab muscles are where the real stabilizing force is coming from.
Rounded back lifts are important, and things like the straight-leg-deadlift (where your back will come out of neutral) are used by a lot of athletes. But it's also considered a potentially dangerous exercise:
Performing a movement with a weight or range of motion that is significantly beyond what they are accustomed greatly increases risk of injury, particularly with this movement, but that does not make [straight leg deadlifts] a dangerous exercise when performed with common sense guidelines.
Those "common sense guidelines" rely on the fact that you're relying on weaker muscles and putting load directly on connective tissues. Provided you use great form and light weights, the essay linked above indicates you'll have good results. But that's a far cry from the proper and safe lifting form for heavy deadlifts, lifting heavy things around the house or workplace, etc.
Another good reason for practicing rounded back lifts is that often the real world doesn't afford us perfect anatomical conditions. Lifting a child out of a carseat, as an example. There's a bend, a twist, and the weight is fairly far ahead of you.
Also, isometrically locking your spine and doing things like a conventional deadlift does strengthen those muscles, in much the same way that a plank strengthens the abdomen.
Strengthening and building a muscle optimally requires the muscle to be stretched against resistance then contracted. Then why is it band to round the lower back,shouldn't it strengthen the spinal erectors?
The thing that happens when your back rounds is that you're not actually engaging the erectors. You're not flexing them OR they're not strong enough to withstand the weight. Thus, they're not actually being trained. They're just being pulled apart by a force greater than they can withstand.
Think of dumbbell side bents, they are safe and good for the quadratus lumborum ,never heard of anyone ever getting hurt with them, so why is it bad bending the spine forward?
Bending the spine forward is not harmful. But during a deadlift with rounded back (as discussed above), the erectors are either not working, or not strong enough to prevent the rounding. This means that all the weight you're holding is loaded on your spine instead of the spinal erectors. This, combined with the fact that your pelvis is holding the bottom of the spine in place, means that your spine is "breaking" rather than simply bending.
It's important to note that the negative effects of this "breaking" isn't necessarily something that just snaps. Damage to the spine happens over time, and rounding the back is a sign that you're likely doing gradual damage to your spine. This is why you see some powerlifters lift with rounded backs a lot. They know that this isn't good, but they're willing to risk a little wear and tear for the glory of getting that huge PB and/or trophy. At some point, you just have to get that damn weight up. But don't let that mentality become a habit. Better safe than sorry.
