"Bend your knees!?"
There is a bit more to lifting heavy than simply bending your knees if you want to remain injury free.
Our lumbar spine (lower back) is a very strong, capable structure that can withstand significant forces for completing daily tasks and activities, including lifting heavy objects. We become at risk of injury when forces placed on the spine are greater than what it can tolerate, either from a single event (eg lifting heavy object), or repetitive load over time. It must be said that more than 85% of low back pain is non specific in nature and you can not reliably tell what the cause of your pain really is. Despite this, there are certain loads placed on the spine when performing movement and other tasks, and these can be measured, and from there can give an indication of the best way to prevent injury.
If we look at performing a single heavy lift of an object, there are different forces acting on the spine (compression, sheer and torsion forces). Below are two vertebrae, divided by an intervertebral disc.
As this picture illustrates, the vertebrae can handle much higher compressive forces compared to sheer and torsion. Compressive forces have been shown to be protective for the spine. In a particular group of young rowers studied, high compressive forces on the spine during rowing were indicative of significantly better bone mineral density of the vertebrae in the lumbar spine compared to non rowers, even at the age of 19 (2). Running has also been shown to benefit to the discs between the vertebrae due to compressive loads and the result is a stronger disc, and has the opposite effect of "wear and tear" (1).
In contrast, the spine is less able to tolerate high load when it comes to sheer or torsion movements. These involve rotation movement (torsion) and load far from our centre of mass (sheer). Common activities would include starting a lawn mower, lifting a heavy box from the ground and twisting to place it on a shelf to your left without moving your feet, vacuuming, hitting a golf ball or using a brush cutter.
How To Lift Safely
If we look at the best way to lift heavy, it is reasonable that we reduce sheer and torsion loads by reducing how far away the object is from our centre of mass (in other words bring it in close to your body), and preventing rotation during the lift. We welcome compressive loads, which is why we get told to bend our knees. Keeping a straight line position and keeping the object close to your body is a good way to remember it.
There is a caveat though. If the object is big and bulky, and is not able to fit between your knees during a lift, you are better off performing a bent over lift, bending more at the hips and less at the knees. This is due to the high sheer forces placed on the spine when we bend our knees and squat down to lift objects that are far out in front of us (see picture on left side below). Additionally, the muscles of the lumbar spine and gluteals are able to function really well when we bend from the hips to lift bulky objects (see middle picture below). If the object is small (even if heavy!), and can be fit close to our centre of mass, bending our knees and using our legs is a great way to lift (see picture on right side below). You will notice in the middle picture performing a bent over lift, the spine is still relatively straight and the bend is coming from the hips, not excessive flexion through the lower back.
On a finishing note, the best way to make sure your back stays healthy and injury free, is to give it the right nutrition, exercise and staying strong. No perfect lifting technique will prevent injury if you simply do not have the load tolerance or strength to complete the task safely. Simple ways to keep it happy and strong include regular cardiovascular exercise such as walking, running or rowing, do a variety of daily tasks (and limit sitting) and perform three times a week resistance training.
If you want to get some personalised advice, a tailored (safe) resistance training program or require assistance with regaining function after a lifting injury, get in touch with Monty Physio today!
Belavý, D., Quittner, M., Ridgers, N. et al. Running exercise strengthens the intervertebral disc. Sci Rep 7, 45975 (2017). https://doi.org/10.1038/srep45975
Morris. L et al Compressive and sheer force generated in the lumbar spine of female rowers. Int J Sports Med 2000; 21(7): 518-523