This video and article explain what is a muscle stimulator and why the term itself is incorrect in practical application. The correct term is Functional Electrical Stimulator, FES unit) and a main use of a FES unit is to aid in stroke rehab. This video explains how to use a FES unit and what is important.
A muscle stimulator is not the proper name for what this type device is designed for. The proper name is a Functional Electrical Stimulator or FES unit for short. Tens units are not muscle stimulators, tens units are used for pain. Muscle stim. units are used to regain function such as post stroke rehabilitation, or to retard disuse atrophy, and in some situations to reduce swelling and edema. Basically any electrical device, microamperage unit excluded, can be powered up enough to stimulate motor, muscle, nerves. That is a function of the amount of energy necessary to "fire" a motor nerve.
That is generally not the goal when one refers to muscle stimulation. It's very misleading for a tens machine distributor or manufacturer to refer to their tens units as muscle stimulators.
The purpose of a FES unit, functional electrical stimulator, is to help a patient regain control of the major and minor muscle functions so the patient can resume daily activities themselves and not be dependent on others to achieve simple tasks like walking, drinking from a cup, or turning knobs and buttons. The movement process generally involves the primary activation of small motor nerves for fine control, and then progresses to major activation of motor nerves for movement of larger muscle groups. In many situations the goal of the use of a FES unit is to help the patient's brain learn new ways to achieve tasks. When a patient uses a F.E.S. unit it is imperative that the patient work with the unit as the learning process is a voluntary process and requires patient involvement in accomplishing tasks.
For example, if one wanted to pick up a cup and all the motor nerves, small and large were stimulated at once, then there would be no function. What one would have is a catastrophe as the muscles would not work together, but separately, and the function of picking up the cup would be lost. The brain would not be re-educated as to how to accomplish the task of picking up a cup, but all the muscles would be spastically stimulated. A true FES unit slowly activates certain muscle nerves and then slowly activates the larger nerves to achieve a synchronous movement. The process repeated over and over, with the patient's active participation helps the patient relearn and gain self control. As previously stated this application is generally indicative of helping a stroke patient regain function.
Of course there are other applications for a functional electrical stimulator such as edema or swelling reduction, retarding disuse atrophy and also for increasing blood flow. All of these functions are appropriate for what is called a muscle stimulator or functional electric stimulator. Tens units are not appropriate for the major functions of muscle stimulation.