Does Lumbar Flexion Actually Increase Shear Forces During Lifting?
Among healthcare professionals and Strength and Conditioning coaches, the avoidance of lumbar spine flexion during lifting is commonly believed to be a strategy for reducing injury risk. As part of this reasoning, the attenuation of anterior shear forces during lifting has been used as part of the argument for maintaining a ‘neutral’ spine alignment by avoiding significant lumbar flexion during lifting exercises. In this blog, we will highlight some of the evidence that therapists and coaches may want to consider for managing shear forces when coaching lifting exercises.
What is shear?
When performing a lifting task, a single ‘resultant’ forces acts on the spine. However, to help us analyse the loading on the spine, the resultant force can be divided into separate components. During lifting, a lot of attention is devoted to the compressive and shear forces acting on the spine due to the association with tissue damage (Adams et al., 2006). While compression is defined as the force acting perpendicular to the mid-plane of the intervertebral disc, shear forces act parallel to the disc and 90º to the compressive forces.
When we lean forward, as we do during lifting exercises, gravity pulls the trunk downwards towards the ground in a vertical direction, resulting in an anterior shear force that must be resisted by the spine so to prevent pathology. Of note, the greater the forward inclination of the spine (all other variables, such as mass, being equal), the higher the anterior shear forces that will need to be supported at the spine in order to avoid tissue failure.
Can shear forces cause injury during lifting?
The tolerance of the spine to anterior shear forces is much (much!) lower than what the spine can endure for compression before tissue damage occurs. For example, cadaver studies have shown that the ultimate shear strength of an intact motion segment of the lumbar spine is approximately 2,000 N (Gallagher and Marras, 2012), while the ultimate compressive strength may exceed 10,000 N (Hutton and Adams, 1982). This feature of the spine is by design, as the compressive forces acting on the spine are much larger than the shear forces when performing lifting activities (Cholewicki et al., 1991; Edington et al., 2018).
The 2,000 N threshold for anterior shear force may appear to be problematic for us if we want to prescribe lifting exercises in the weight-room, with the shear forces during movements like deadlifting approaching (Cholewicki et al., 1991) or even exceeding (Edington et al., 2018) the structural limits of the spine established within cadaver studies. When the shear forces during a movement are greater than the spine’s capacity for loading, damage of the pars interarticularis (Yingling and McGill, 1999), pedicles (Gallagher and Marras, 2012) or apophyseal joints (Adams et al., 2006) can occur. As such, it appears the risk of heavy lifting resulting in pathology is high, unless we can implement strategies to minimise this risk. This is why changing lifting technique is an attractive tool for moderating anterior shear forces during lifting activities.
Does lumbar flexion increase anterior shear forces during lifting?
Adopting a technique that includes the maintenance of a lordotic spine posture when lifting is commonly suggested as a strategy to minimise anterior shear force (McGill, 2015). This recommendation is often based on a study showing an individual (n = 1) holding a 22 kg load in a static forward lean position was able to reduce anterior shear forces significantly by moving their lumbar spine from a fully flexed position to a lordotic posture (Potvin et al., 1991). One potential reason for this finding is that tension developed in the spine ligaments (in particular, the supraspinous and interspinous ligaments) during full flexion increases the anterior shear forces the lumbar spine is exposed to (Potvin et al., 1991).
Another contributing factor may also be the change in fibre orientation of the lumbar portion of the erector spinae muscles (longissimus thoracis and iliocostalis lumborum). When leaning forward whilst maintaining a lordotic posture, McGill et al. (2000) showed that at L3, the lumbar erector spinae muscles were angled in a posterior-caudal oblique direction (approximately 28º), providing a line of action that would allow them to develop a posterior shear force to support the anterior shear forces associated with lifting. However, when the spine was fully flexed whilst leaning forward, this oblique orientation was drastically reduced to approximately 11º. Mechanically, this would diminish the muscles’ ability to prevent anterior shear forces from exceeding the tolerance threshold of the neural arch during lifting.
Whilst this evidence appears to suggest that avoiding lumbar flexion is favourable during lifting, this is not the complete picture and is certainly not supported by in vivo biomechanical modelling studies. Importantly, lifting with a flexed spine may actually decrease anterior shear forces, particularly at the lower lumbar spine where shear forces are highest during lifting. For example, Khoddam-Khorasani et al. (2020) showed that the anterior shear forces at L4/L5 and L5/S1 were actually greater when adopting a lordotic posture compared to lifting with a flexed spine. As lumbar flexion during lifting reduces the forward inclination of the lumbar spine relative to the ground, anterior shear forces caused by gravity are less relative to a lordotic posture that inclines the lumbar spine further forwards. Similar findings were reported by both Arjmand and Shirazi-Adl (2005) and Kingma et al. (2007), further contradicting the popular belief that lifting with a flexed lumbar spine increases anterior shear forces.
Additionally, the function of the erector spinae muscles is different at each level of the lumbar spine (Arjmand and Shirazi-Adl, 2005; Khoddam-Khorasani et al., 2020). Whilst these muscles are able to produce a posterior shear force at the mid-region of the lumbar spine, the lumbar extensor muscles actually generate an anterior shear force at L5/S1 that compounds the loading on the apophyseal joints from gravity (Arjmand and Shirazi-Adl, 2005). As a result, any benefits of a lordotic posture for reducing anterior shear forces during lifting by optimising the fibre angle of the lumbar extensors are not present at L5/S1, where shear forces are greatest during lifting (Khoddam-Khorasani et al., 2020).
Therefore, the available research does not appear to support the notion that we should avoid lumbar flexion during the performance of lifting tasks in an attempt to reduce anterior shear forces. On the contrary, the evidence presented here suggests that encouraging some flexion during lifting could be used as a strategy to reduce anterior shear forces at the lower segments of the lumbar spine.
How can we help people manage anterior shear forces during lifting?
Whilst the anterior shear forces associated with lifting may be reduced by incorporating some lumbar flexion as part of the technique, focusing our attention on programme design may be more beneficial for minimising injury risk. Resistance to anterior shear forces during lifting derives predominantly from the bony structures of the neural arch (Skrzypiec et al., 2013). Importantly, the ultimate shear strength of the human lumbar spine is positively associated with the bone mineral density of the vertebrae (Skrzypiec et al., 2013), indicating that developing the structural architecture of the lumbar vertebrae will likely allow lifters to better tolerate the shear forces accompanying heavy lifting. As bone mineral density of the lumbar vertebrae can be increased with resistance training (Almstedt et al., 2011; Hinton et al., 2015; Mosti et al., 2014), gradually exposing lifters to greater loads over time in order to drive structural adaptations that allow them to tolerate higher shear forces would likely be a very effective strategy for minimising injury risk.
The anterior shear forces associated with lifting are certainly something that should be considered by practitioners when prescribing lifting exercises. The evidence presented in this blog suggests that contrary to popular belief, lifting with a flexed lumbar spine may decrease anterior shear forces. However, adjusting lumbar spine posture appears to be only part of the puzzle in reducing the risk the anterior shear forces expose the spine to during lifting. As the tolerance of the lumbar spine is positively correlated with the bone mineral density of the lumbar vertebrae, a well-designed training programme will likely drive adaptations that allow passive structures to better tolerate greater shear forces. Consequently, while exercise technique should be carefully considered to improve performance and attenuate injury risk, programming may play a more important role in helping lifters cope with the anterior shear forces associated with heavy lifting.
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