TENS machines work in two ways:
1. On a high pulse rate of 90-130 Hz (the normal method of use), the electrical impulses generated by the TENS machine interfere with and block pain messages sent to the brain. This is due to the gate control theory of pain. This proposes that there is a gate mechanism in the brain and spinal cord nerves (the central nervous system). When the gate is open, pain messages get through to the brain and we feel pain. When the gate is closed, these pain messages are blocked and we do not feel pain. TENS machines are thought to stimulate certain non-pain-carrying nerves and close the gate. In effect, the brain is busy dealing with the messages it receives quickly from the TENS machine, rather than the slower (more painful) pain signals that the body is receiving from elsewhere. It explains why, if you injure yourself, rubbing that area can temporarily reduce the pain.
2. When the machine is set on a low pulse rate (2-5 Hz) it stimulates the body to make its own pain-easing chemicals called endorphins. These act a bit like morphine to block pain signals.
If you'd like to read what our mums thought about using a TENS machine during labour, click here.