The vertebrae, along with intervertebral discs and ligaments are part of a strong, resistant and very elastic axis of the body – the spine. Its structure is adjusted to its static and dynamic functions. Previous studies have revealed the most common sites of chronic back pain: intervertebral discs, apophyseal joints and sacroiliacal joints. The force that acts upon the spine can be divided into two components, the first one acting perpendicularly to the surface of the vertebrae and causing compression, and the second one acting directly onto the surface of the vertebrae and causing shear. The body of the vertebrae and the intervertebral discs together resist most of the compression force, while the apophyseal joints resist the horizontal forces. The ligaments of the vertebral arcs and spinal muscles prevent flexion of the lumbar spine. Continuous (static) and repetitive (cyclic) lumbar flexion are the causes of different degrees of spinal injuries. Researchers have established that the flexed position of the lumbar spine produces less pressure on the apophyseal joints, less compression on the posterior part of the annulus fibrosus, the diffusion of the metabolites into the intervertebral disc is better and the compressive power of the spine is stronger. Therefore, a slightly flexed position of the lumbar spine should protect against spinal injuries during sitting or weight lifting.