Question
What is the mechanism of action with functional electrical stimulation?
Answer
Now that we have looked at our patient and decided that they will be appropriate for at least a trial of electrical stimulation, it is also important to know how electrical stimulation works. If we do not explain electrical stimulation properly, patients may think we put the electrode on them, shock them, and make the muscle move. I have had patients refer to it as shock therapy, so I try to educate them that this is a treatment designed to target the nerve of the muscle, to promote nerve and muscle recovery. When we place the electrodes on a patient, we want to identify the muscle group. For consistency sake, let's say we are working with the biceps. The electrode pads are placed over the bicepital body and electrical stimulation activates the nerve of the targeted muscle. We are activating the nerve with an action potential that travels through the tissue at the level of the synapse, and it goes all the way from the synapse in the muscles to the ventral horn of the spinal cord. That is where it interacts with the motor neurons. One of the important things to note is that following any sort of neurological injury, the connections of motor neurons are altered. We are trying to reestablish connections in the spinal column and then back out to the peripheral nerves where we are trying to ultimately strengthen a muscle group. We use this in combination with neuroplasticity, our brain and nervous system’s natural ability to change, reform, and reconnect.
Finally, we want to consider, with a muscle contraction, is it physiologically or electrically stimulated? There is an important difference. When you and I are going about our everyday lives, our muscles contractions are physiologically stimulated. They can be automatic or conscious reactions. Our bodies initiate the stimulation of the muscle utilizing what are called type I or slow twitch muscle fibers. They have a gradual onset with smooth motion. There is going to be a better recovery time for the motor units. The motor units can then, because of the gradual onset and gradual decrease, have a smoother activation and generally more recovery time. When we are utilizing electrical stimulation, the fiber type that is recruited is the type II muscle fibers which are the fast twitch. These muscle fibers have a sudden onset in their contraction. There tends to be kind of a jerky motion. You will all of a sudden see twitching or jerking in the muscle belly. This is considered the point of tetany, which is when all the individual twitches of the individual muscle fibers come together. You may have a muscle twitch here, a muscle twitch here, and one over here. The point of tetany is when all three come together to form a consistent solid muscle contraction.
Often with electrical stimulation and recruitment of Type II fibers, your patients are going to experience fatigue faster and require greater recovery time. That is because there is a stronger contraction and a more sudden onset and jerkier motion. It recruits the type II fibers because they are the more excitable. They have a lower threshold for stimulation and those are the ones that fire first when you are using electrical stimulation. For the best results or the ideal muscle contraction, you want to have a combination of physiological and electrical stimulation. If you have someone who can physiologically initiate a movement and then use electrical stimulation to complete the range of motion, that is most ideal and will ultimately build the most strength.
