Hopefully, this explanation will be of use to those of you who perform this exercise as part of your routine.
So after I got done doing some weighted 45 degree back extension, an older gentlemen came up to me saying that he had never seen anyone used that much weight on that exercise before and if it was safe. It was a 100 lb dumbbell which to me wasn’t that heavy since I had slowly worked up to it over time. He was concerned that using that much weight would be dangerous since it involved the lower back. I explained to him how it was possible to use heavy loads with exercises that involved the lower back while still being safe.
As you can see from the figure below, number 1 is in neutral position, number 2 is in an extended position, and number 3 is in a flexed position.
In my video, it’s hard to see from that angle but my lower back remained pretty neutral during the entire range of motion for each repetition. Being able to keep my lower back in this neutral position prevents it from hyperextending which can potentially pinch a nerve since the formina decreases during this motion. It also prevents it from hyperflexing which can potentially compress the disc and cause a disc herniation. You’ll also notice that I don’t go down as low as some people do. The reason for this is that I only go down to the point in which I start feeling like my lower back would start to round (aka flex).
This is about the range of motion that I use and you’ll notice that there is no flexion of the low back which means that the low back is still in a more neutral position.
This is the range of motion that some people use during this exercise and you’ll notice that the low back is flexing.
So does this mean that you should never flex your low back? The answer to that is it depends. Under no load, it might be acceptable depending on the health of your discs and other soft tissue in that area but you dramatically increase the risk of a disc herniation under heavy load in a flexed position. It also depends on one’s flexibility since some people can go much lower without putting too much flexion on their lower back.
So the whole point to this ramble is that any movement where you’re bending over is much safer if you can maintain a neutral low back position even if you’re doing it under heavy loads but you just have to make sure that your low back muscles are strong enough to maintain that position.