This complete Manual Handling Powerpoint presentation can be purchased here
manual handling tasks - lifting and carrying
Whilst the weight of the load is obviously important the effect of this can be exacerbated if the load is not kept close to the body. The effect of moving the load away from the body can actually be calculated and it can be shown that it increases the strain on the back considerably. This calculation is covered on a later slide.
It's also important to know that the effect of twisting the body can have undesirable effects on the disks in the spine that are between the vertebrae. In order to minimise this, turning around should be done with the feet and whole body and not the waist.