Barbell Medicine - From Bench to Bedside

We’ve previously published a number of articles on pain and the complexities of making sense of pain both as a human who personally experiences pain and being tasked with helping those clinically with their pain experiences. In this week’s article, we’ll try to further the conversation about the pain experience by examining how people with low back pain move in their environment versus those without.

As previously discussed, low back pain is a highly prevalent and debilitating experience throughout the world. There are many narratives on how to reduce the risk of low back pain, which is where many technique/movement discussions arise. We often hear statements such as “Don’t flex your back while lifting”, “Lifting with a straight back is safer”, “Don’t lift that way or you will injure your back”.

Key Points:

  1. Many beliefs about movement and pain are filtered through a kinesiopathological model of thinking. With respect to low back pain, we often hear narratives centered around not lifting or moving a particular way to avoid low back pain or “injury”. Although these narratives may be provided in an attempt to be helpful, they likely have negative consequences of their own.
  2. We do not have evidence demonstrating that a particular way of moving is guaranteed to lead to back pain and/or “injury”.
  3. People learn about pain in two ways: Direct experience (engaged in a behavior or event and experienced pain) and/or Indirect experience (language and/or observation of others).
  4. Verbal “rules” about low back pain and movement appear to be taken up by those experiencing low back pain. Nolan et al found “… people with LBP lift differently to pain-free controls. Specifically, people with LBP lifted more slowly, use their legs more than their back especially when initiating lifting, and jerk less during lifting.”
  5. The “safe” back narrative appears built on shaky ground, and we should provide helpful information about movement and pain without setting unnecessary limits / constraints. We should also avoid speaking confidently about causality. Instead, we should promote exploration of movement and trust in one’s body allowing for building of robustness and resilience. We should promote an understanding that pain is a part of life, and just because one experiences pain with a particular movement does not mean the movement should be demonized or avoided.

Confident, causality-based propositional statements predicated on “if-then” are put forth, but the evidence doesn’t appear overly supportive of this, and in fact is of low quality. This doesn’t negate the prevalence of these narratives and the negative social learning that takes place because of them. In fact, 89% of people reported receiving their beliefs from healthcare providers with the internet as the second source. Setchell 2017 This is problematic given that many healthcare providers erroneously believe a multitude of false narratives about pain, posture, and movement. Examples include: 

  • Lifting and bending with a round back is “dangerous.” Caneiro 2019
  • The back necessitates protecting via “safer” ways to complete movements. Nolan 2019
  • Optimal sitting and standing postures exist and should be a part of patient education. Korakakis 2019

Additionally, many internet-based resources are also providing suboptimal information regarding low back pain. According to Ferreira 2019,

“Websites from government agencies, consumer organizations, hospitals, nongovernmental organizations, professional associations, and universities demonstrated low credibility standards, provided mostly inaccurate information, and lacked comprehensiveness across all types of LPB. Our findings highlight the need for these organizations to reformulate their treatment recommendations to reflect current evidence in the management of LBP.” [emphasis added] 

At this point, some may be asking the question: What data do we have about risk factors for low back pain?

Recall that “risk factors” are things that increase the likelihood of the development of a particular condition. Typically, risk factors are discussed as either “modifiable” (i.e., things we can change like physical activity levels, smoking, or obesity) and “non-modifiable” (i.e., things we can’t change like age, prior history of low back pain, or genetics). It’s unlikely we will ever prevent the development of pain given that it’s a heavily conserved evolutionary adaptation, but there may be variables we can influence to mitigate the risk of developing symptoms.

Parreira et al conducted an umbrella review of systematic reviews and meta analyses on risk factors for low back pain and sciatica. The authors examined prior published reviews from 2011-2016 and found:

“Overall, included systematic reviews varied from moderate to high methodological quality. Our results showed that exposure to a range of factors pertaining to the individual, poor general health; physical stress and psychological stress significantly increased the risk of [low back pain] and sciatica. Of the 54 risk factors investigated, this review identified a number of risk factors that are likely to be modifiable and therefore potential targets for prevention interventions. Examples include sleep problems (severe) (OR=3.2, 95% CI=1.9–5.5), time driving (OR= 4.8, 95% CI=1.4–16.4) and prolonged standing/ walking (OR= 2.9 95% CI=1.5–5.5).” [emphasis ours] Parreira 2018

What’s missing from these data is a particular movement technique being linked to low back pain. However, as mentioned previously, it is a commonly held belief at the societal level that lifting with a “straight back” is somehow “safer’ (reduces risk of low back pain) than with a stooped or flexed technique. We even have a recent review by Saraceni et al contradicting this stance and demonstrating lack of evidence to support the claim. However, many are now saying we need contradictory evidence to support the claim that straight back lifting is not not safer – which doesn’t make a lot of sense based on how science typically functions. This tends to follow the line of thinking of, “better safe than sorry” which should lead us to ask the question – What’s the harm in such a narrative? 

To begin, people appear to learn about pain through two primary pathways:

  1. Direct experience (engaged in a behavior or event and experienced pain)
  2. Indirect experience (language and/or observation of others)

According to Beeckman et al,

“…pain does not necessarily have to be directly experienced to influence what we think, feel, and do. Rather, humans can come to catastrophize, fear, and avoid a wide variety of stimuli and events based on what they observe, tell themselves, or are told by others.

Statements such as “Don’t move this way or else” have the potential to drive false beliefs about movement and pain which then lead to unnecessary conditioned behavioral responses. According to Vlaeyen and Crombez,

“Insight into the role of verbal language on human behavior is one of the major innovations in learning and conditioning and is discussed in relational frame theory (RFT).” 

In this context, think of our language as the linkage or building blocks connecting stimuli and events or by definition sense-making. Vlaeyen and Crombez again:

“RFT states that the ability to form equivalent relations between stimuli is necessary to understand behavior in response to language. For instance, the instruction ‘You will injure your back if you lift a heavy crate’ can only have an effect if there are equivalence relations between the word lift and actually lifting a crate and between the word crate and an actual crate. Furthermore, a notion of causality is implied in the instructions (“if…then”). Verbal rules provide information that otherwise might only have been learned over time by trial and error, and they can prevent catastrophic outcomes (e.g., ‘Do not cross the street when the traffic light is red’).”

At times, these “verbal rules” may have use without direct experience. One example, Erica and I recently had baby Lucy and we can pre-emptively explain to her that playing with fire is likely not a good idea as it may lead to burning herself and a painful experience unnecessarily. This approach can provide useful adaptive learning. However, these rules can be unnecessary in the context of movement and pain, as well as self-limiting, and even potentially perpetuating of a pain experience: 

“Rules allow one to behave without having to directly experience the contingencies. Nevertheless, rules can have negative effects when they are erroneous (e.g., ‘If you are not careful with your back, you will end up in a wheelchair’). Following rules may also make individuals less sensitive to actual changes in contexts or contingencies, a phenomenon called the “rule-based insensitivity effect.”Vlaeyen 2019

This hardline of thinking about movement and pain often drives a false dichotomy of good vs bad, injurious vs non-injurious, and right vs wrong. Beeckman et al state, 

Research suggests that rigidly relying on verbal information may play a central role in several types of psychopathology and may underpin impaired functioning in people with chronic pain as well. Indeed, when people persist in doing things based on beliefs about the right thing to do or how it should be, this pattern of thinking often has unwanted consequences (eg, see the literature on substance abuse, self-harm, delusions, or depression.Beeckman 2019

Which leads us to a recent article by Nolan et al,  titled “Are there differences in lifting technique between those with and without low back pain? A systematic review.”

Study Design

The authors stated, “…this systematic review aimed to compare kinematics and muscle activity of the trunk and lower limbs in people with and without LBP during freestyle lifting tasks.”

The authors primary premise is if we want to advise people about lifting then we need to gain a better understanding of how people with and without low back pain are moving during lifting tasks. 

The authors searched the following databases CINAHL, EMBASE, Pubmed, AMED, and SPORTDiscus for articles related to lifting, low back pain, kinematics and muscle activity. Study inclusion criteria consisted of:

  • Case-control comparisons
  • Participants reporting clinical low back pain (LBP), not experimentally induced, ongoing for at least six weeks
  • Participants with LBP and leg pain were included provided LBP was greater
  • Participants performed a freestyle lifting activity based on personal preference without postural or technique instructions
  • Pain-free control comparator group
  • Comparisons analyses between kinematics and/or electromyography (EMG) between pain-free and LBP groups

Studies were excluded if participants had potential underlying specific issues correlated to their low back pain, such as fracture, inflammation, infection, malignancy, or other issues such as disc prolapse with identified motor weakness or sensory changes). 

In total, the authors included 9 studies. The following information was extracted from each study: sex, age, eligibility criteira, pain/dsiability measured, lifting task, weight lifted, experiemtnal measures, and key findings. The authors combined the results data from included studies related to kinematic analyses (total spinal range of motion, speed of spinal motion, and coordination of spinal motion) & muscle activation (paraspinal activation and other trunk muscles activation).

The authors utilized the Critical Appraisals Skill Program (CASP) checklist to assess risk of bias for included studies. Overall included studies sample sizes ranged from seven to eighty one. Methodological concerns include:

  • 2 studies lack of reporting where participants were recruited from  
  • 1 study did not report how control group was recruited
  • 1 study stopped participants from completing a lift if it was considered “unsafe” without a definition of unsafe supplied. The study also failed to report how many participants were stopped due to “unsafe technique”. 
  • 2 studies failed to describe pain intensity amongst LBP group
  • 4 studies failed to report a disability measure for LBP group and large heterogeneity in disability measurement across included studies. 

Mean age of participants was 37.4 years (ranged from 33.4 – 43.1). Included participants had specific pathologies ruled out that may have been correlated to their LBP. Five of the studies included participants with predominantly LBP and four studies included participants with up to 50/50 presentation of LBP and leg pain but lacked any “hard” neurological findings, e.g., altered deep tendon reflexes or motor weakness. The length of time with LBP symptoms varied across studies but ranged from six weeks to nineteen years. Based on inclusion criteria, the participants were not provided any instructions on how to complete the lifting task.

Three studies had participants lift an item 33 centimeters from the floor for 20 minutes. Resistance was applied only to the concentric phase of the lift (pulling from ground); data was not recorded on the non-resisted eccentric phase (return to starting position). The six remaining studies had participants lift a weighted box and analyzed data from the concentric and eccentric phases, while assessing number of lifts rather than a time factor. Locations of items lifted varied between studies. Examples include: floor, waist height, shoulder height, loads placed at varying angles to the person completing the lift (45 degrees and 90 degrees) necessitating twisting and lifting techniques to be utilized bidirectionally (left and right). Loads lifted also varied between studies. Six studies utilized the same weight for both groups (control & LBP). Three studies utilized a load equivalent to 40% of particpant’s maximum isometric strength. Other examples of loads lifted include: 4.5 kg, 6.8 kg, 9.1 kg, 11.4 kg, 12 kg. 

Study Findings

The authors stated, 

“Most studies in this systematic review reported that people with LBP lift differently to pain-free controls. Specifically, people with LBP lifted more slowly, use their legs more than their back especially when initiating lifting, and jerk less during lifting. There were inconsistencies in whether differences exist in overall spinal ROM or muscle activation, but generally the larger studies involving people with more severe LBP show people with LBP lift with less spinal ROM and greater muscle activity.”

Kinematic Findings

Kinematic findings included total spinal range of motion, speed of spinal motion, and coordination patterns. 

  • Total Spinal Range of Motion: The authors found 2 studies (n=84) demonstrating, “…LBP patients flexed the lumbar spine less than pain-free controls when lifting.” One study reported a 30% difference and another found a significant difference but didn’t provide quantification. 2 other studies (n=17) found no range of motion differences between groups. 
  • Speed of Spinal Motion: 6 total studies (287, 2 studies used same participants leaving a total of 225 original participants) found “…people with LBP lifted slower, or reached peak acceleration later when lifting.”
  • Coordination Patterns: 4 total studies (n = 151).
    • 2 studies found LBP patients led the lifting task by initially straightening their legs and finishing the lift by using their back. 
    • 1 study found, “…LBP patients started with a deeper knee bend and finished more vertically than pain-free controls.”
    • Another study found, “…LBP participants used a deeper squat style of lifting compared to pain-free controls.” and “…46 out of 81 LBP participants lifted with a ‘low jerk style’. 

Paraspinal muscle activation

Anatomy of the paraspinal muscles.

5 studies (n = 177) examined EMG activity (magnitude, duration, or timing) of erector spinae (ES) between LBP and pain-free controls.

  • 2 studies demonstrated no between group changes in lumbar ES 
  • 3 studies denoted between group differences but with some nuance:
    • 1 study found higher lumbar ES EMG activity in LBP patients
    • 1 study found lumbar ES activation earlier and longer in LBP patients
    • 1 study found lumbar ES activation LOWER in LBP patients during eccentric phase of the lifting task (lowering weight from waist to floor) but not during the concentric phase (lifting weight from floor to waist). An added plot twist, this particular study also found higher thoracic ES EMG during both concentric and eccentric phases of the lifting task. 

Activation of other trunk muscles:

3 studies (n = 84) examined EMG activity (magnitude, duration, or timing) of other trunk muscles between LBP and pain-free controls.

  • 2 studies demonstrated abdominal and latissimus dorsi muscles were more active in LBP patients. 
  • 1 study showed the right latissimus dorsi and external obliques were activated longer in LBP patients. The authors reported inconsistencies in abdominal (internal and external obliques) and latissimus dorsi muscles timing showing either no difference or LBP patients having early activation of these muscles during the lifting task.

Why does this article matter?

I want to lead this discussion with a concession – we are biological creatures who must interact with our environment based on physical constraints. Said differently, we must obey the rules of physics and biology, as we currently know them, and thus are inherently constrained at some level to our body’s abilities and interactions with our environment. We have examples of biological limitations, such as what occurs with an acute bone fracture. On the surface level we can see this example as an overload to bone which results in a break, however, as we examine the topic a bit more closely we can learn of the multifactorial nature at which bone “fails” at the micro → macro level, which may be related to issues such as osteoporosis, altering the strength of bone.Wolfram 2016 

But we also can’t forget that we are a dynamic system interacting with an ever-changing environment, and because of this, we have evolutionary affordances such as adaptability. In the context of our discussion, we must consider Wolf’s Law. In essence, the law postulates our bones adapt to the stress applied to them over time. Fortunately, we have evidence that imposing external stress via resistance training can improve osteopenia and osteoporosis (e.g., LIFTMOR trial). How does this fit with our current discussion about biomechanics of lifting an item, pain, and injury?

If you’ve been following BBM content for a while now, then you know how much we enjoy searching for the origins of a presupposition. As it relates to a ‘safer’ way to lift a load, much of the argument appears centralized to a fearful belief of lifting a particular way will result in pain which later becomes attributed to spinal disc alterations. There are layers of missteps in this discussion that I will do my best to elucidate. Let’s begin with the “disc damage” narrative. One of the originating pieces appears to be a 5-page article by Adams and Hutton in which the authors examined cadaveric lumbar spines (keep in mind this inherently limits the adaptability of the system under examination):

“In our attempts to produce disc prolapse in the laboratory, cadaveric lumbar intervertebral joints were first wedged and then compressed, either once (to simulate trauma) or repetitively (to simulate fatigue processes). The discs were then examined to see if prolapse or other structural lesions had been produced. We present the results for 134 intervertebral joints.”

The authors conducted three experiments on the intervertebral joints (IVJs):

  • “Excessive compression with moderate flexion”
  • “Moderate compression and excessive flexion”
  • “Moderate compression and moderate flexion: Fatigue loading”

For test A, thirty two intervertebral joints (as depicted in the image below)  were tested. Of these joints, thirty failed via fracture and only two demonstrated disc prolapses. The authors reported,

“The high compressive forces required to damage a slightly flexed lumbar intervertebral joint (test a) are beyond the scope of the back muscles so could not be generated by a single heavy lift. They could only occur in life in a fall on the buttocks or similar trauma.” 

Test B, the authors utilized sixty one intervertebral joints. All of the joints underwent a complete laminectomy according to the authors to, “…allow a clear view of the posterior annulus fibrosus.” This is probably not the best idea if we are wishing to test the integrity of a joint under load in vitro. The authors continued altering the angle and amount of force applied to the IVJs until failure was reached; in other words, their intention was to evoke failure to find in vitro limits of the IVJs. The authors found 26 of the 61 IVJs “failed” by disc prolapse. However, they don’t make mention of the 32 fractures that also occurred (see Table 2 in original article). We also can view this data from the perspective that 35 IVJs did not demonstrate a disc prolapse “failure” based on the biomechanical demands placed on them.

Test C included 41 IVJs and were subjected to four hours of repetitive loading at 40 repetitions / minute or until failure reached. The authors report 11 of the IVJs failed during the four hours of testing due to the “…crushing of the vertebral body”. They go on to find that the main site of “failure” was typically the vertebral body and not the disc. However, they did find three annular protrusions. The authors conducted post-testing examination and went on to make claims to “distortions” found in the discs to further support their operational premise. The authors considered Test C as a representation of a day in a manual laborer’s life, but it, understandably so, fails to encompass a broader lens of the topic which includes labor laws today as well as complex individual topics of fatigue constrained activity. We’ve previously discussed the complexities of fatigue in a prior issue of BMR. 

When discussing test C, the authors go on to admit that their testing would likely not be reproduced in real life (in vivo) – 

“It is unlikely that anyone would load their spine as relentlessly as we have loaded our specimens but, because the collagen turnover of the disc is very low and the repair processes consequently slow, the same activity performed over a longer period of time could have a similar effect on the disc.”

Note the highlighted conjecture in their claims. This is but one study demonstrating the beginnings of the “safe” back lifting belief that’s often shrouded in fear-mongering and a severe misunderstanding of the multifactorial experience of pain.

With that said, based on this article, people with low back pain do indeed appear to move differently than their pain-free counterparts. The next question is why do those with low back pain move differently? The authors propose three possibilities:

  1. Spine protection from “excessive” loading 
  2. Pain behavior, meaning in response to coping with pain the person alters their movement (often coined antalgic movement/posture). 
  3. Cultural acceptance” of a particular lifting style as safer (squat/stopped vs flexed vs hinged) 

Interestingly, those with low back pain who moved differently appeared to adopt a movement pattern/posture similar to what is often recommended by clinicians in order to “protect the back” (recall the citation above). It appears that people don’t trust their backs and wish to protect them, and in doing so actually do the opposite by not using them. Even those without low back pain have adopted such beliefs about movement and their low back, as Caneiro 2018 found

Pain-free participants displayed an implicit association between images of “round-back” bending and lifting postures and words representing ‘danger’. Importantly, our findings support the idea that pain-free people may have a pre-existing belief that the back is in danger when rounded during bending and lifting.”

Caneiro et al cites prior in vitro studies like the one discussed above and in vivo studies on disc pressure (like these – Nachemson 1963, Nachemson 1965, Nachemson 1966) as the building blocks for these beliefs. The authors go on to outline their counter argument to this belief and I highly recommend going to read it. 

It would appear that based on the current evidence, these verbal rules are more harmful than useful. However, changing beliefs on any topic is often not as easy as it may seem. We amass a network of influential sources for our various beliefs, each likely weighted differently with trust – healthcare professionals, internet, family, co-workers, significant others, social media, personal experience, etc. A paradigm shift will certainly take time and effort and isn’t able to be accomplished by a single individual or company (albeit we at Barbell Medicine will continue to give it our best efforts). 

Limitations

This study was not without limitations. First, it was based on cross-sectional studies so causation can not be inferred but rather levels of association. Second, there were some methodological concerns with recruitment and reporting of data related to pain and disability, along with heterogeneity in disability reporting. Third, these studies were often of small sample sizes, ranging from 7 – 81 participants. Finally, and likely one of my biggest complaints, one of the larger studies (Rudy et al) stopped participants for “unsafe” lifting technique but failed to define what unsafe meant or how many were stopped prematurely. 

What’s the Take-Home Message?

We have sufficient and legitimate counter arguments for the “safe” back lifting technique being presented in the research world. The next step is ensuring this information is being disseminated to those often looked to the most for advice about low back pain, healthcare professionals. Healthcare professionals are often tasked with making sense out of why pain is occuring (enter possible pathoanatomical or kinesiopathological narratives) and how to address the issue in the future (enter future behavioral movement responses). We can see the verbal linkages occurring where these narratives are trickling down to others and spreading – see the articles cited in the beginning of this discussion about what healthcare professionals believe about movement, posture, and pain and then review articles like Caneiro et al and their findings from those not experiencing low back pain about their beliefs regarding movement and pain. Even social media is a great example of how much a kinesiopathological model has taken hold in our world and influencing our beliefs and environmental interactions (behaviors). 

To combat this misinformation, hopefully we can continue to maintain focus on the information being presented in the research world while integrating appropriately to clinical practice and life. The spread of misinformation also falls on others in positions of perceived authority and trust – coaches. Being cautious of the why behind our movement cues can help stifle some of these misinformed beliefs. Moving ourselves away from narratives such as – “Don’t lift like that our you will hurt your back” or “If you flatten your back that’ll ensure you don’t get injured” may not be the best explanations for providing feedback for learning a new movement to fit the predetermined constraints we’ve created. Finally, we have an entire industry not really touched on in this article – workplace ergonomics. Somehow we need to find balance in our messaging based on risk vs. benefits, as Nolan et al discussed, “Teaching people to move, lift and bend the back less cautiously has been shown to help people with severe back pain; how these messages are integrated into workplace training while fulfilling legal requirements of a risk assessment require wider consultation with key stakeholders.” 

References

  1. Nolan D, O’Sullivan K, Newton C, Singh G, Smith BE. Are there differences in lifting technique between those with and without low back pain? A systematic review. Scandinavian journal of pain. 2019.
  2. Setchell J, Costa N, Ferreira M, Makovey J, Nielsen M, Hodges PW. Individuals’ explanations for their persistent or recurrent low back pain: a cross-sectional survey BMC Musculoskelet Disord. 2017; 18(1).
  3. Caneiro JP, O’Sullivan P, Smith A, et al. Physiotherapists implicitly evaluate bending and lifting with a round back as dangerous. Musculoskeletal science & practice. 2019; 39:107-114. 
  4. Nolan D, O’Sullivan K, Stephenson J, O’Sullivan P, Lucock M. How do manual handling advisors and physiotherapists construct their back beliefs, and do safe lifting posture beliefs influence them? Musculoskeletal science & practice. 2019; 39:101-106. 
  5. Korakakis V, O’Sullivan K, O’Sullivan PB, et al. Physiotherapist perceptions of optimal sitting and standing posture. Musculoskeletal science & practice. 2019; 39:24-31. 
  6. Ferreira G, Traeger AC, Machado G, O’Keeffe M, Maher CG. Credibility, Accuracy, and Comprehensiveness of Internet-Based Information About Low Back Pain: A Systematic Review J Med Internet Res. 2019; 21(5):e13357-.
  7. Parreira P, Maher CG, Steffens D, Hancock MJ, Ferreira ML. Risk factors for low back pain and sciatica: an umbrella review. The spine journal : official journal of the North American Spine Society. 2018; 18(9):1715-1721. 
  8. Saraceni N, Kent P, Ng L, Campbell A, Straker L, O’Sullivan P. To Flex or Not to Flex? Is There a Relationship Between Lumbar Spine Flexion During Lifting and Low Back Pain? A Systematic Review With Meta-Analysis J Orthop Sports Phys Ther. 2019.
  9. Beeckman M, Hughes S, Kissi A, Simons LE, Goubert L. How an Understanding of Our Ability to Adhere to Verbal Rules Can Increase Insight Into (Mal)adaptive Functioning in Chronic Pain. The journal of pain : official journal of the American Pain Society. 2019; 20(10):1141-1154. 
  10. Vlaeyen JW, Crombez G. Behavioral Conceptualization and Treatment of Chronic Pain Annu. Rev. Clin. Psychol.. 2019; 16(1).
  11. Wolfram U, Schwiedrzik J. Post-yield and failure properties of cortical bone . 2016; 5.
  12. “Wolff’s Law.” Wikipedia, Wikimedia Foundation, 25 Dec. 2019, https://en.wikipedia.org/wiki/Wolff’s_law.
  13. Watson SL, Weeks BK, Weis LJ, Harding AT, Horan SA, Beck BR. High-Intensity Resistance and Impact Training Improves Bone Mineral Density and Physical Function in Postmenopausal Women With Osteopenia and Osteoporosis: The LIFTMOR Randomized Controlled Trial. Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research. 2018; 33(2):211-220. 
  14. Beaver, David I., and Bart Geurts. “Presupposition.” Stanford Encyclopedia of Philosophy, Stanford University, 1 Apr. 2011, https://plato.stanford.edu/entries/presupposition/.
  15. Adams M, Hutton W. The mechanics of prolapsed intervertebral disc International Orthopaedics. 1983; 6(4).
  16. Caneiro J, O’Sullivan P, Lipp OV, et al. Evaluation of implicit associations between back posture and safety of bending and lifting in people without pain . 2018; 18(4):719-728.
  17. NACHEMSON A. THE INFLUENCE OF SPINAL MOVEMENTS ON THE LUMBAR INTRADISCAL PRESSURE   AND ON THE TENSIL STRESSES IN THE ANNULUS FIBROSUS. Acta orthopaedica Scandinavica. 1963; 33:183-207. 
  18. NACHEMSON A. THE EFFECT OF FORWARD LEANING ON LUMBAR INTRADISCAL PRESSURE. Acta orthopaedica Scandinavica. 1965; 35:314-28. 
  19. Nachemson A. The load on lumbar disks in different positions of the body. Clinical orthopaedics and related research. ; 45:107-22. 
  20. Rudy TE, Boston JR, Lieber SJ, Kubinski JA, Stacey BR. Body motion during repetitive isodynamic lifting: a comparative study of normal subjects and low-back pain patients. Pain. 2003; 105(1-2):319-26. 

 

About Michael Ray

Dr. Ray is the founder of Shenandoah Valley Performance Clinic in Harrisonburg, VA. He obtained a M.S. in Exercise Science from the University of South Carolina and graduated Magna Cum Laude with his Doctorate of Chiropractic (D.C.) from Sherman College of Chiropractic.

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