During electrical stimulation, the body's excitable tissues always react to an electrical current by depolarizing in a predictable order: Sensory nerves Motor nerves Pain nerves Muscle fiber Since directly depolarizing muscle fiber also causes the depolarization of pain nerves, this technique is not commonly used in athletic therapy. So, we'll limit our discussion to the remaining three nerves. But first, let's consider stimulation intensity levels. Then we'll talk about the effect of pulse duration. If you yourself have experienced electrical stimulation, think about how it felt. As the unit was turned on and gradually turned up, you passed through periods of feeling nothing (the subsensory level), "pins and needles" (the sensory level), muscle contractions (the motor level), and eventually, pain (the noxious level). Each of these four stages represents a general stimulation intensity level.  Level  Sensation  Physiological Event(s)  Subsensory  None None. The current has not yet reached the threshold for nerve depolarization. The effects of microcurrent stimulation are said to occur at this level.   Sensory  "Pins and needles" Depolarization of a-beta sensory nerves.  Motor  Muscle contractions Depolarization of motor nerves.  Noxious  Pain (not discomfort, which a person may experience at the lower levels of stimulation) Depolarization of a-delta pain fibers (this may also occur during the upper portion of the motor level) progressing on to depolarization of c-fibers.   Now, let's consider what happens when the duration of the pulse varies. Click Next when you're ready to continue.