Loads on Spine
Forces acting on the spine are – body weight, tension of spinal ligaments, tension of supporting muscles, intra-abdominal pressure, applied external loads, if any.
IN upright posture, the main spinal loading is AXIAL (ie through the longitudinal axis of the body).
Erect standing: total body CG is anterior to spinal column, so that the spine is placed in a constant forward bending MOMENT. This TORQUE must be counteracted by tension in the back extensor muscles.
Trunk and arms flexed: These body segments get increasingly longer MOMENT ARMS, contributing to increased flexor torque and thus to increasing compensatory tension of back extensor muscles.
The spinal extensor muscles have small MOMENT ARMS compared to vertebral joints, and must generate large forces to counteract the torques produced by the large weights of body segments plus any external loads.
Compression increases (compared to standing) with sitting, spinal flexion, slouching while sitting.
Slouching involves backward rotation of sacrum compared to ilium, thus widening the posterior L5-S1 intervertebral disc space, and creating strain in ilio-lumbar ligaments