Anatomy and Biomechanics of the Spine and Musculoskeletal System
Structure
The spine has 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral and 4 coccygeal. Between the vertebrae are discs with a gel-like nucleus pulposus and a tough annulus fibrosus. The lumbar spine carries much of the body's load and allows flexion, extension and rotation. Back muscles, abdominal muscles and ligaments stabilise the spine; intra-abdominal pressure adds support during effort.
Biomechanics
The spine acts as a lever. The load is held away from the spine, while the back muscles pull close to it, so the muscles must generate a much larger force than the load. Moment = force × distance from the pivot. Doubling the distance of the load from the spine roughly doubles the moment the muscles must resist. Flexion increases disc pressure; twisting adds shear and torsion, which the disc tolerates poorly.
Worked example
A 10 kg box held at 50 cm from the lumbar spine creates a larger moment than the same box held at 25 cm. At 50 cm the moment is about 5 kg·m; at 25 cm it is about 2.5 kg·m. Holding the box close and avoiding bending and twisting therefore reduces the load on the discs and muscles far more than simply 'keeping a straight back'.
Traps
Do not say the spine is straight; it has natural curves. Do not claim a single bend 'slips a disc'. Discs are largely avascular and degenerate over time. Do not ignore twisting: it is a major risk factor. Do not confuse the number of lumbar vertebrae (5) with the total number of vertebrae (33).