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THREAD: Effects of lumbar spine posture on shear forces during lifting.
I’ve recently seen a lot of discussion on this so below is a thread on evidence highlighting what coaches may want to consider when coaching lifting technique.
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I’ve recently seen a lot of discussion on this so below is a thread on evidence highlighting what coaches may want to consider when coaching lifting technique.
1/20
Firstly, managing anterior shear forces during lifting exercises is considered important so to minimise injury risk, as tolerance for the anterior shear forces associated with lifting is much lower than compressive loading at the lumbar spine.
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In human cadaver studies, failure occurs at approximately 2kN of anterior shear forces when testing intact motion segments.
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While joint shear forces at L4/L5 during heavy deadlifting can approach these values.
https://pubmed.ncbi.nlm.nih.gov/1758295/
4/20
https://pubmed.ncbi.nlm.nih.gov/1758295/
4/20
And some exercises may potentially exceed this 2kN threshold…
(This study tested lifters during a maximal isometric pull from a deadlift setup - CBDL is just a conventional deadlift technique)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6162543/
5/20
(This study tested lifters during a maximal isometric pull from a deadlift setup - CBDL is just a conventional deadlift technique)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6162543/
5/20
As a consequence of heavy lifting exercises resulting in ant shear forces that approach the failure threshold of a motion segment, coaches may choose to encourage strategies that prevent excessive shear forces from occurring.
But is lumbar spine posture one such strategy?
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But is lumbar spine posture one such strategy?
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It is often suggested that a neutral spine position reduces ant shear forces during lifting, while a flexed posture increases ant shear forces. This is usually based on the work of Potvin et al and can be seen in this figure from their work.
https://pubmed.ncbi.nlm.nih.gov/1948399/
7/20
https://pubmed.ncbi.nlm.nih.gov/1948399/
7/20
One reason for this finding may be that lumbar spine flexion tensions the ligaments of the neural arch that increase anterior shear forces (in particular the interspinous and supraspinous ligaments).
https://pubmed.ncbi.nlm.nih.gov/1948399/
8/20
https://pubmed.ncbi.nlm.nih.gov/1948399/
8/20
This may also occur as the lumbar portion of longissimus thoracis and iliocostalis lumborum are aligned better to resist anterior shear forces in a neutral alignment relative to a flexed lumbar spine posture.
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This work by McGill et al showed the when flexed at the hips 30°, the fibre angle of these muscles at L3 ran in an oblique direction (approx. 28°) that would allow them to produce posterior shear forces.
https://pubmed.ncbi.nlm.nih.gov/11050362/
10/20
https://pubmed.ncbi.nlm.nih.gov/11050362/
10/20
These values are similar to those reported here by Macintosh et al.
https://pubmed.ncbi.nlm.nih.gov/8316889/
11/20
https://pubmed.ncbi.nlm.nih.gov/8316889/
11/20
However, does this change in fibre angle decrease anterior shear forces at all levels?
More on this further down …
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More on this further down …
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Importantly, not all models support the findings of Potvin et al.
This work by Khoddam-Khorasani et al found ant shear forces were actually lower at L4/L5 and L5/S1 (and comparable between T12/L1-L3/L4) in a flexed lumbar spine posture relative to a lordotic position.
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This work by Khoddam-Khorasani et al found ant shear forces were actually lower at L4/L5 and L5/S1 (and comparable between T12/L1-L3/L4) in a flexed lumbar spine posture relative to a lordotic position.
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This disparity in findings is likely due to different biomech models used to estimate shear forces.
Regardless, what is certain is greater forward inclination of the lumbar spine during lifting increases the ant shear forces caused by gravity.
https://europepmc.org/article/med/32147242
14/20
Regardless, what is certain is greater forward inclination of the lumbar spine during lifting increases the ant shear forces caused by gravity.
https://europepmc.org/article/med/32147242
14/20
As a flexed lumbar spine posture during lifting reduces the fwd inclination of the lumbar spine relative to the ground, ant shear forces caused by gravity are reduced relative to a lordotic posture that inclines the lumbar spine further forwards.
https://pubmed.ncbi.nlm.nih.gov/16319750/
15a/20
https://pubmed.ncbi.nlm.nih.gov/16319750/
15a/20
This is nicely illustrated in this image from Arjmand et al.
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15b/20
Therefore, it may be that a flexed lumbar spine decreases ant shear forces during lifting.
Either way, there doesn’t appear to be enough evidence to suggest that some flexion is inherently more dangerous than a neutral spine when considering shear forces during lifting
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Either way, there doesn’t appear to be enough evidence to suggest that some flexion is inherently more dangerous than a neutral spine when considering shear forces during lifting
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Additionally, Arjmand et al and Kingma et al both showed that the shear forces created by the lumbar erector spinae muscles was specific to the motion segment.
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From T12/L1 to L3/L4, these muscles produced a posterior shear force, while at L5/S1, these muscles actually created an increase anterior shear force.
https://pubmed.ncbi.nlm.nih.gov/16531071/
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https://pubmed.ncbi.nlm.nih.gov/16531071/
17b/20
As such, the effect of posture on the capacity of the erector spinae muscles to buttress anterior shear forces may be difficult to use in the decision-making process for choosing specific techniques for lifting.
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Lastly, as the shear strength of the lumbar spine is positively associated with the bone mineral density of the vertebral body, it may be that tolerance thresholds are a driven by loading history.
https://pubmed.ncbi.nlm.nih.gov/22578739/
19a/20
https://pubmed.ncbi.nlm.nih.gov/22578739/
19a/20
And bone mineral content in the lumbar spine has been shown to be positively associated with annual training volumes.
https://pubmed.ncbi.nlm.nih.gov/3589805/
19b/20
https://pubmed.ncbi.nlm.nih.gov/3589805/
19b/20
Consequently, a well-designed training programme may result in adaptations that allow passive structures to tolerate the high shear forces present during lifting exercises.
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