Now let's turn to polarity, another aspect of the output protocol. When we talked about electricity on Day 2, I mentioned that electrical currents flow from a negative pole, or cathode, to a positive pole, or anode. You'll also recall that the cathode is an area of high electron concentration and the anode is an area of low electron concentration. When you use electrical stimulation to depolarize cells, you'll use electrodes of opposite polarity to create a closed electrical circuit between the stimulator and the body. Direct currents and monophasic currents are characterized by the flow of electrons from the cathode to the anode. When the current flow is prolonged, the ionic structure under the anode and cathode change in relationship to the electrical charge of the electrodes. Under the anode, positive ions are repelled and negative ions are attracted. The reverse occurs under the cathode. This effect is most pronounced when a direct current is used (although it doesn't come into play with most of the monophasic generators we have here at the clinic). Under the cathode, sodium ions (Na+) gain an electron, and the sodium becomes uncharged (Na). Proteins are liquefied, softening the tissues and decreasing the sensitivity of nerve endings. Under the anode, protein coagulates and the tissues harden. Remember that this information only applies to direct currents (the iontophoresor is the only DC generator that we use here) and monophasic generators. Most of the time the polarity used over the injured area has little effect on the treatment. Some patients, however, do find that for an acute injury, having the positive electrode over the injured site is more comfortable, whereas for chronic pain, having the negative electrode over the painful area is better. If the goal of treatment is to re-educate muscle, it's important not to overly fatigue the fibers. That's where the duty cycle comes in. Take a break if you'd like to, and then we'll talk about the duty cycle.