Most trainees are taught to train with a flat back although many will notice that most of the world’s elite lifters use a slightly round back.
Like all exercises there’s a Risk/Reward profile in each lift.
The lumber spine is susceptible to flexion/extension injuries so lifting with a flat back decreases the likelihood of injury. (In other words…avoid both flexion and hyperextension).
In studies analyzing the electrical activity of the spinal erectors, Dr. Stuart McGill has shown conclusively that as the lumbar spine becomes fully flexed (rounded forward), the contribution of the muscles to the required torque decreases, and the supportive force generated by the ligaments increases.
This means you start to switch off your muscles and allow your ligaments to take the weight.
Although the ligaments counteract the torque load (often allowing you to actually lift more with a flexed back), the line of action of the ligament force adds to the joint shear, which increases likelihood of injury.
So why do athletes sometimes lift with a rounded back (and why can they lift more)?
Competitive powerlifters may flex their spines considerably but they can stay (serious) injury free if they avoid the end ranges of lumbar flexion. Dr. McGill’s research showed that the back can tolerate some flexion just not end range flexion. With a rounded back the athletes gain some advantage mechanically, through both the active and passive tissues, and increased stabilization.
With a slightly rounded back the hips will be higher and closer to the bar reducing torque on the hip (and therefore mechanical load) and also be closer to a 1/4 squat position than 1/2 squat position (which is biomechanically stronger).
The back extensors can produce more force when flexed (even though their leverage is diminished). The thoracolumbar fascia (TFL) provides a spinal extension torque when stretched (as the back rounds), and this contribution increases as the muscles attached to it are activated.
Stabilization can be improved as the body can produce more intra-abdominal pressure (IAP) in spinal flexion than extension, and co-contraction strength is increased as all the anterior core (except internal oblique) are stronger in a slightly flexed position.
The (slight) increase in risk for these athletes is worth the increase in weight they can lift… and they manage the risk by becoming skilled in lifting this way and very conditioned to it!
However if back pain issues are something you are not interested in your best bet is to lift with a flat back as the risk of serious injury is less than if round back deadlifting.
The 2 techniques (Round and Flat), showcase the body’s different strategies for dealing with force, in this case, muscular vs ligamentous support.
There are inherent pro’s and con’s of both techniques, and your biomechanics will often decide which is better for you. Good athletes often train (and are good) at both! Closing notes: It’s important to note that WHATEVER lift you perform… once the lift has started…the spine does not move either flex or extend until you let go of the bar!.