The stresses most likely to produce trauma in the spine are bending and torsion (twisting). These actions reflect combinations of compression, tension and shearing forces.
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Between 85-95% of disc herniations occur either at L4/L5 or L5/S1. The disc between L5/S1 has the potential to produce to greatest moment and is one of the most vulnerable tissues to force-induced injuries.

Lifting tasks develop compression and tension in the vertebral column. Poor lifting form exacerbates these forces and the mechanism of injury.

Disc compression is thought to be largely responsible for vertebral disc herniation and compressive force is generally a good predictor of lower back overexertion injury.

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(FM in the diagram is represented by FES in the below calculation)

The compressive force at the L5/S1 intervertebral disc can be calculated by:

Sum of Forces = 0

(sin(theta)*FW+ sin(theta)*FP) + FES - FS = 0

Where theta is the alignment of the trunk to the horizontal and FC is the compressive force.

Proper Lordotic Squat Lift Compression at 80 degree trunk angle:

FC = (sin(80)*450 + sin(80)*200) + FES
FC = (640.1) + (3020)
FC = 3660 N

Improper Kyphotic Squat Lift Compression at 65 degree trunk angle:

FC = (sin(65)*450 + sin(65)*200) + FES
FC = (589.1) + (4250)
FC = 4839 N

The compressive force in each lifting scenario would be attenuated slightly by an intraabdominal pressure force not shown in the above calculations. This internal force acts parallel to the line of action of the compressive force and is developed through the contraction of the abdominal wall muscles and the contraction of the diaphragm during inhalation.

For a given lifting task, the recommended maximum intervertebral disc compressive force cut-off is ~3.4 kN (NIOSH), which closely coincides with the proper lordotic lifting technique for this individual and load. The lordotic lifting technique produces less compression on the L5/S1 intervertebral disc and should be adopted to avoid lower back pain and injury.

Vertebral and intervertebral disc strength weakens over time and should be a factor in job design and job designation for labouring workers as they age.

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The lumbar vertebrae have a larger cross-sectional area to support more body weight and to accommodate greater compressive forces. Prevalence of lower back pain is higher when a worker is subject to greater compressive forces throughout the workday.

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The shear force at the L5/S1 intervertebral disc can be calculated by:

Sum of Forces = 0

(cos(theta)*FW + cos(theta)*FP) - FS = 0

Where FS is the shear force.

Proper Lordotic Squat Lift Shear at 80 degree trunk angle:

FS = cos(80)*450 + cos(80)*200
FS = (78.1) + (34.7)
FS = 113 N

Improper Kyphotic Squat Lift Shear at 65 degree trunk angle:

FS = cos(65)*450 + cos(65)*200
FS = (190.2) + (84.5)
FS = 275 N

The proper lifting technique allows for a neutral, lordotic spine posture. This posture lessens the shear force on the spine by allowing the errector spinae to be active. This muscular force reduces the shear force on the discs by disabling the interspinous ligaments, which contribute to shear.

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Anterior shear forces act parallel to the vertebral end plate. This force’s line of action causes relative sliding between the vertebrae. To lessen the shear force, avoid twisting when carrying a load and always remember to balance the load evenly.