Anyone who has watched CrossFit instructional videos and read CrossFit Journal articles focusing on lifting technique will know the importance of maintaining a straight torso with normal lumbar curvatures. This month I want to briefly discuss lumbar spinal anatomy and mechanics.
I believe that being able to express mechanical concepts (such as different postures during lifting) in numbers provides the strongest possible support for coaching points. Therefore, I have also included a quantitative analysis of the deadlift using a biomechanical computer model.
The three directions in which forces are applied to human tissues are compression, tension, and shear (shown in figure 1). In case you are wondering, bending places one side of the object in compression and the other in shear, and twisting (torsion) is just a type of shear.
For this discussion on lumbar mechanics we do not need to focus on tension as it is as a force that tends to pull a tissue apart and is not relevant to our purposes. Our focus will be on compression and shear. Shear is defined as a force that acts parallel to a surface; in the spine, it can create sliding of one vertebra with respect to another.