To understand how it works it would be best to explain how your body recognises pain. The ‘Pain Gate’ control theory of pain was put forward by two scientists called Melzack and Wall. They suggested that when body tissues are damaged, messages carrying information about the injury travel towards the brain along two quite separate sets of nerve fibres. The larger fibres carry messages about sensations other than pain, and the smaller fibres carry the pain messages. It is thought that messages which travel along the larger fibres arrive at the spinal cord before the messages travelling along the smaller fibres and, if there are enough non painful sensations travelling, the pain messages won't be able to get through to the brain. In other words the painful messages get ‘crowded out’ by other competing non-painful messages. This explains why rubbing a painful area helps, because what we are doing is increasing the number of non-pain messages travelling towards the spinal cord (and thence the brain). During the last ten years an enormous number of research projects have shown that TENS machines are convenient, safe and effective.TENS machines have been shown to be effective in the treatment of all kinds of pain. A Swedish study has even shown that TENS machines were the only painkillers required by 70 per cent of women in labour.