Dear Heavenly Glute Man,
I try to keep a good arch when I deadlift, but when the weight gets really heavy, say around 95% of my 1RM and higher, I find myself rounding my back in order to complete the lift? Why am I stronger when I round?
Mark, you’re in luck. I feel that I’m very qualified to answer this question for several reasons.
First, I live by the deadlift and die by the deadlift. If all goes well I’ll leave this world immediately after performing a heavy set of deadlifts (hopefully when I’m around 88). That’s how much I love deadlifting. Second, I’ve thought about this phenomenon for many years. And third, I’ve spoken to researchers and powerlifters regarding this concept.
Many beginners round their back because they have crappy hamstring flexibility or poor glute strength so they’re forced to deadlift with a combination of hip and spinal extension. However, since you hinted that you only round when you go heavy, this indicates that you possess the hip mobility and core stability to lift properly.
I can think of two main reasons why one would be able to lift heavier when rounding the low back:
1) First, powerlifters often use their glutes and hip extensors to “roll” their pelvis which allows them to hang on their ligamentous structures in the spine as well as the thoracolumbar fascia. This gives them a tremendous boost in strength. This is probably the most important factor.
2) Second, by rounding the back you change the kinematics and kinetics of the lift. You slightly decrease the angle of hip flexion when initiating the lift (your hips aren’t bent forward as much), the hips are moved closer to the bar in the sagittal plane (your butt doesn’t stick out as far), the barbell’s range of motion is slightly diminished (when you lockout the bar hangs lower with a rounded upper back in comparison to an arched back), and you use the erector spinae as prime movers rather than stabilizers. These factors definitely add up and allow for heavier lifting.
Now let’s address the safety aspect.
First, it’s never a good idea to round the low back in a deadlift. Under such heavy loading it’s very easy to herniate a disc or damage other structures in the spine when the lumbar spine is flexed forward significantly.
It’s not as big of a deal to round the upper back in a deadlift. It is quite possible to control the spinal segments and allow the thoracic spine to flex while keeping the lumbar spine in neutral (or better yet slightly arched). Obviously the safest way to deadlift is to keep a slight arch in the lumbar and thoracic spines, while keeping the cervical spine in neutral with the chin tucked.
So powerlifters and strongmen can round all they want, as can recreational lifters who understand the risks versus the rewards. But if you’re a strength coach or a personal trainer, it’s best to avoid round-back deadlifts (or squats, good mornings, bent over rows, t-bar rows, and bent over rear-delt raises) and keep an arch when deadlifting.