The human body is able to function because of its ability to communicate with the brain. Without good communication, the body begins to falter and disease results.
While good communication between the body is the brain is necessary for health, there are times when the constant transfer of pain signals from a point of injury to the brain results in stresses which cause disease of its own.
Electrotherapy stimulates the nerves and muscles surrounding an injury. The result of that stimulation is the release of what are commonly called endorphins. Endorphins are naturally-occurring pain killers that block pain pulses from reaching the brain.