This is a simplified diagram of the spine. See that the spine consists of:

• the vertebrae, solid bones which have attachments for muscle tendons
• the discs, which are jelly-filled sacs between the vertebrae; these allow movement and shock-absorbtion
• muscles, and the tendons which connect them to the vertebrae
• the nervous column (also called the spinal cord), which connects the brain to the rest of the body and runs down through the vertebrae, between the discs and the muscles

In this first example, the load is carried on top of the spine. The blue arrow shows how the force tends only to compress the spine. This means that the discs are evenly loaded and the muscles and tendons are relaxed. The spine can take big loads safely when loaded like this.

This means that the discs are now compressed from below by the load, AND from above by the contracting muscles. Because the muscles are very close to the vertebrae, they exert a lot of compressive force in order to resist the bending. Conversely, even quite a light load creates enormous tensile forces in the muscles and tendons, because of the leverage.

So pressure in the discs is very high, because the weight of the load is magnified many times by the geometry of bending.

The strain on the muscles and tendons can be more than they can take without tearing, and stress damage can accumulate at a rate faster than it can be repaired. This can lay the foundations of an acute back-pain attack which may be triggered later by some unrelated and even trivial movement, disguising the real cause.

This last diagram shows how most of us lift things, and how so many of us get bad backs. The spine is leaning over and bent, stretching the muscles and tendons, and squeezing the discs.

The squeezing of the discs raises the pressure inside even higher, reducing their nutrition (which mainly comes through the walls not through a blood supply), and making them prone to bursting ( a herniated or 'slipped' disc). The escaping bubble of disc wall can then press on the nervous column creating pain that is often felt in some other part of the body ('referred pain' called sciatica). The discs themselves have few nerves, so you can't feel this damage happening until it's too late.

Herniated discs will normally repair themselves, as long as the stresses that caused the damage are not being repeated.