Static effort occurs when a muscle is contracted and held in the contracted position for some time. Bloodflow is restricted in a contracted muscle. Dynamic effort, such as walking, is easy to sustain for long periods of time because fresh, oxygenated blood flows in a muscle every time it releases. Static effort, however, results in a continual deficit of blood supply to the contracted muscle, so a statically held muscle cannot rid itself of metabolic waste products. It is these waste products which produce the feelings of pain, tenderness and fatigue.
Most jobs have a combination of dynamic and static effort tasks.
It is the statically held muscles that eventually become painful and sore. A second factor contributing to occupational back pain concerns the orthopaedic aspects of the sitting posture. The human spine has a set of natural curves that become distorted when sitting. The lower back, or lumbar region, which normally curves forward, becomes bowed out. The backward tilting pelvis puts pressure on the intervertebral disks and nerves.