The discs sit in between each vertebra to make up your spine. These discs act as a spacer and shock absorber for the spine, and are made up of a gel like substance surrounded by very tough rings of tissue.
As you age beyond 30, fissures start to develop in the outer shell of the disc. If engaged in manual labour, the fissures develop at a quicker rate, with manual activity placing pressure on the disc and pushing the gel centre backwards along one of the fissures. If this happens it results in a bulge, which can come into contact with surrounding nerve roots.
This is particularly common in men who work in hard labour jobs which involve bending and lifting, usually between the ages of 30 and 50.