Neuromuscular electrical stimulation (NMES) is a therapeutic treatment that uses electrical impulses to produce muscle contractions. The device sends signals to nerves, causing muscles to contract in a way that mimics the body’s natural process. NMES is designed to supplement voluntary muscle activity, not replace it.
Mechanism of Neuromuscular Electrical Stimulation
NMES functions by delivering a controlled electrical current through electrodes placed on the skin over a targeted muscle group. This current is specifically intended to stimulate the motor nerves, which are the nerves responsible for carrying signals from the brain and spinal cord to the muscles. When the impulse reaches the motor nerve, it causes the nerve to depolarize, which is a change in its electrical charge.
This depolarization generates an action potential, the same type of signal the nervous system uses. The action potential travels along the nerve to the neuromuscular junction, where the nerve and muscle connect. The signal then triggers the release of neurotransmitters that command the muscle fibers to contract, resulting in a controlled, involuntary contraction.
While the resulting contraction is similar to a voluntary one, the signal’s origin differs. A voluntary contraction command originates in the brain and travels down the spinal cord. With NMES, the stimulation is initiated externally by the device, bypassing the central nervous system to activate the nerve directly. This external activation allows NMES to be used for individuals who have difficulty generating voluntary contractions.
Therapeutic Applications of NMES
NMES is commonly used for muscle strengthening and re-education after orthopedic surgery, such as an ACL reconstruction or total knee replacement. Patients often experience weakness in surrounding muscles, and NMES induces contractions to help restore strength and improve motor control more efficiently than voluntary exercise alone. This is beneficial in early rehabilitation when pain or swelling may limit a patient’s ability to exercise.
NMES is also used to prevent muscle atrophy, the loss of muscle tissue, in immobilized limbs. When a limb is in a cast or a patient is on bed rest, lack of use causes a decline in muscle mass. By artificially contracting the muscles, NMES provides the stimulus to maintain muscle integrity, making the subsequent recovery process shorter and more effective.
NMES aids in the rehabilitation of individuals with neurological conditions like stroke or spinal cord injury. These conditions can impair neural pathways, leading to weakness or paralysis. NMES can help to retrain these pathways and facilitate motor recovery by providing sensory feedback and promoting muscle activation, sometimes improving walking ability or the functional use of a limb.
A specialized application of NMES is treating dysphagia, or difficulty swallowing. Electrodes placed on the front of the neck stimulate the muscles involved in swallowing. The goal is to strengthen these muscles and improve their coordination, making it safer for individuals to eat and drink.
Other applications include increasing local blood circulation, relaxing muscle spasms, and maintaining joint range of motion. In some post-surgical cases, it is used on calf muscles to help prevent venous thrombosis.
NMES Treatment Parameters and Procedures
Electrode Placement
Self-adhesive electrodes are placed on the skin over the belly of the target muscle. Proper placement ensures the electrical current reaches the motor nerves, maximizing the contraction’s quality while minimizing discomfort.
Intensity (Amplitude)
The intensity of the electrical current, measured in milliamperes (mA), is gradually increased until a strong, visible muscle contraction is achieved. The level should be effective for the therapeutic goal but remain within the patient’s comfort tolerance. The amount of fatty tissue can influence the intensity needed to get a response.
Frequency
Frequency, measured in Hertz (Hz), determines the quality of the muscle contraction. Lower frequencies produce individual twitches, while higher frequencies (20-50 Hz) create a smooth, sustained contraction called tetany. The choice of frequency depends on the treatment goal, such as endurance or strength.
Pulse Duration
Pulse duration, or pulse width, is the length of time each electrical pulse lasts and is measured in microseconds (µs). This parameter influences which nerve fibers are stimulated. Smaller muscles often respond to a shorter pulse duration (150-200 µs), while larger muscles may require a longer one (200-300 µs) for an effective contraction.
On/Off Cycle and Ramp Time
To prevent muscle fatigue, NMES is applied with an on/off cycle. The “on time” is when the muscle is contracting, and the “off time” is a rest period. A common ratio is an off time at least three times longer than the on time, such as 10 seconds of contraction followed by 30-50 seconds of rest. The ramp-up and ramp-down time, often lasting at least 2 seconds, gradually increases and decreases the intensity for patient comfort.
Contraindications
There are specific situations where the use of NMES is not appropriate. NMES is contraindicated in the following circumstances:
- On individuals with demand-type cardiac pacemakers, as the electrical impulses could interfere with the device’s function.
- Over areas with malignant tumors, as it could potentially promote the growth or spread of cancerous cells.
- Across the chest, over the front of the neck (carotid sinus), or directly over the eyes or mouth.
- During pregnancy, especially over the abdominal or lumbar regions.
- On individuals with epilepsy or other seizure disorders.