Is It Better To Bend Your Back Or Knees When Picking Something Off The Ground?

Thanks to Tasneem and Joakim for the photo

I can't find any similar statistics in the local Singapore context, but more than 40% of employees in European countries suffer from work related musculoskeletal disorders (WMSD) yearly. This results in 3-6% absence from work, affecting 2.5% of the gross domestic product across Europe. 52% of injuries are attributed to overloading during lifting tasks, of which 65% affect the lower back.

When picking up something off the floor, we generally use two standard lifting techniques, the stoop or squat technique, which have been well described in many articles. The squatting technique is usually advised when lifting heavy objects since this technique is thought to result in lowering intervertebral disc (IVD) compression and shear forces compared to the stoop technique. 

Squat (a) versus Stoop (b)

Stoop lifting is usually restricted to lifting light objects since it is thought that stoop lifting causes higher IVD compression and shear forces compared to the squat.

However, there is no agreement or sound evidence for lifting ergonomic guidelines (or good lifting techniques). Hence the following study to investigate if squat or stoop lifting imposes more load on the person lifting.

Different weights were tested for both lifting techniques using a full body musculoskeletal model, 3D marker and ground reaction forces. They were used to calculate joint angles, forces and power. Muscle activity of nine different muscles were also measured to calculate effort.

Ready for the results? For all lifting conditions and both techniques, the highest joint forces during lifting and lowering were at the L5S1 spine levels, followed by the hip and knee. The hip joint generated the most power while lifting for both techniques.

Squat lifting was mainly executed by additional work on the knee extensors (Quadriceps), shoulder and elbow, potentially explaining similar prevalence of WMSD in the shoulder, elbow to LBP in people who do frequent lifting. 

The authors concluded that based on their findings, squat lifting is not favored over stoop lifting (this is in contrast to current recommendations) although forces were slightly higher in the L5,S1, hip and knee during the stoop technique. This is also supported by Van Dieen et al (1999) who concluded that there is no evidence to suggest squat over stoop lifting after reviewing 27 studies.

Now, many of you reading this may already not agree with what the authors concluded. There are so many variables that can change the results. How low the object is will influence the load on the back, making comparisons with other studies difficult. 

Even a small adjustment like turning the knees out (not controlled in this study) while squatting already reduces back loading. Likewise, healthy volunteers in this study will likely have different lifting dynamics compared to real blue collar/ laborers since the latter would have efficient strategies for all the lifting done over the years. Lifting in the research setting may not totally replicate real world situations.

Weight (10kg in this study) and dimensions of the load lifted will definitely affect the lifting technique as well since the box (length 36 cm, width 14 cm and height 21 cm) used in this study was relatively small. 

For those of you who do not have low back pain, this post may possibly challenge or even change the way you lift a box off the ground or while picking a pen that you've dropped. You have always been told that when you're picking up something or lifting that you should bend your knees, keep your back straight before you lift. That is supposed to be good form or good ergonomics.

Well, especially for those of you who are pain free, you can say to anyone who insist you bend your knees and not your back while lifting that it is not totally true. 

Reference 

Van Der Have A, Van Rossom S and Jonkers I (2019). Squat Lifting Imposes Higher Peak Joint And Muscle Loading Compared to Stoop Lifting. Ap Sci 9(18): 3794. DOI: 10.3390/app9183794

Van Dieen JH, Hoozemans MJM and Toussaint HM (1999). Stoop Or Squat: A Review Of Biomechanical Studies On Lifting Technique. Clin Biomech. 14: 686-696

Thank you for reading this long article. If you're interested, another study by Mawston et al, (2021) pictured above and referenced below found that during a maximal lift in pain free individuals, a flexed lumbar spine (picture C) is more efficient and stronger as opposed to a straight spine! So much for having a 'good' posture while weight training. Perhaps that shall be another post.

Mawston G, Holder L, O' Sullivan P et al (2021). Flexed Lumbar Spine Postures Are Associated With Greater Strength And Efficiency Than Lordotic Postures During A Maximal Lift In Pain-free Individuals. Gait and Posture. DOI: 10.1016/j.gaitpost.2021.02.029