Great video visualizing why we shouldn't flex our back under load
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Alright, so this needs to be quashed right now. This kind of thinking is a god damned epidemic in fitness.
The spine is a beam designed to resist moment force applied at the shoulders, towards the front of the body. This means it experiences both compression (along the inside of the spine) and tension (along the ridge of the back). Is it possible to place a sufficient moment force on the spine that it fails? Yes, obviously, as with all things. That is what is being demonstrated here.
The beam of the spine resists compression via the disks, and tension via the musculature of the back. If there was no back musculature (or, as is often found, the back musculature is not being called into contraction), then yes, the spine would flex uncontrollably, the only thing resisting the flexion would be the disks, which would fail if the load was sufficiently heavy.
However, in a normal situation, where the back musculature is being engaged, even if the spine is pulled out of full extension, the spine is merely in one of many configurations which it is capable of resisting. Will the force on the disks be greater than if it was in total extension? Yes. Is it anything to worry about? No.
Indeed, we often find that a person is failing to engage the back musculature and "resting" on the interior arc of the vertebral discs to keep the spine from flexing uncontrollably. This is undesirable. But it's easily corrected in pretty much everyone. This is not a reason why if the back goes into flexion at all, with fully engaged back musculature, your disks will explode. If you have compromised disks, will you have to exercise greater caution? Yes. But you are not going to compromise your disks doing deadlifts.
And moreover, strength training at a reasonable volume hardly constitutes "repetitive flexion": you execute 45, *maybe* 50 reps of the squat, and between 5 and 15 reps of the deadlift. You probably "load your back under flexion" more times getting dressed in the morning than you do lifting weights all *week.*
The actual reason we don't flex out backs under load is that it makes the lift less efficient. Why this is is discussed in detail in the Blue Book. A deadlift with the back in excessive flexion will fail to break the floor before it becomes flexed to such a degree that it places the vertebral discs under an unsafe stress.
This reply is an attempt to stem the tide of people "deloading their deadlift" to "work on form" because they notice that 265 caused their vertebrae to deflect half a centimeter when the bar left the floor.
Originally Posted by tom33
