If you have ever had difficulty activating a muscle after an injury, surgery, or period of inactivity, you may have heard your physical therapist mention neuromuscular electrical stimulation (NMES).
But how does electrical stimulation actually make a muscle contract?
Neuromuscular electrical stimulation uses controlled electrical impulses to stimulate nerves that cause muscles to contract. When appropriately incorporated into a physical therapy program, NMES can help patients produce muscle contractions when voluntary activation is difficult or limited.
Research has investigated NMES for improving muscle strength and function in people recovering from surgery, injury, neurological conditions, and periods of immobilization. However, electrical stimulation is generally most effective when it is used as part of a broader rehabilitation program rather than as a replacement for active exercise.
What Is Neuromuscular Electrical Stimulation?
Neuromuscular electrical stimulation (NMES) is a physical therapy technique that uses electrical currents delivered through electrodes placed on the skin to stimulate peripheral nerves and produce a muscle contraction.
The electrical signal essentially provides an external stimulus to the motor nerve. When the stimulus reaches an appropriate threshold, the nerve can activate the muscle fibers it controls.
In simple terms:
Electrical signal → nerve stimulation → muscle contraction
This is different from simply trying to make a muscle stronger by exercising it voluntarily. NMES can help create a contraction when the patient's ability to voluntarily activate the muscle is reduced.
This can be particularly relevant after surgery or injury, when pain, swelling, inhibition, weakness, or changes in the nervous system can make it difficult to generate a strong contraction.
How Does Electrical Stimulation Activate a Muscle?
Your nervous system normally controls your muscles by sending electrical signals from the brain and spinal cord through peripheral nerves.
When you decide to straighten your knee, for example, your nervous system sends signals to the motor nerves that control your quadriceps. Those nerves activate muscle fibers, producing a contraction.
NMES works through a similar basic pathway, but the initial electrical stimulus is delivered externally.
Electrodes are placed over or near the targeted muscle and its motor nerve. The electrical current travels through the skin and stimulates the underlying nerve. When the stimulation is strong enough, the nerve triggers an action potential that causes the muscle fibers to contract.
The patient may feel a tingling or pulsing sensation followed by a visible muscle contraction.
Why Would a Physical Therapist Use Electrical Stimulation?
One of the biggest reasons NMES may be used during rehabilitation is difficulty activating a muscle voluntarily.
This can happen for several reasons.
1. After Surgery
Muscle activation can be significantly affected following surgery.
The quadriceps are a common example. After knee surgery, patients can experience substantial quadriceps weakness and difficulty producing a strong voluntary contraction.
A systematic review and meta-analysis published in 2025 found that adding NMES to rehabilitation following ACL surgery improved quadriceps strength compared with standard rehabilitation alone. The researchers found benefits at both short- and longer-term follow-ups.
This does not mean every patient recovering from ACL surgery needs NMES. Instead, it suggests that NMES can be a useful adjunct when appropriate.
2. After Injury
Pain, swelling, and reduced activity can make it difficult for a patient to fully activate a muscle.
For example, someone recovering from a lower-extremity injury may understand how to contract a particular muscle but still be unable to generate a strong contraction.
NMES can provide an external stimulus to help produce the contraction while the patient works toward restoring normal voluntary activation.
3. After Periods of Immobilization
When a limb is immobilized or a patient significantly reduces activity, muscles can lose strength and size.
NMES has been studied as a way to help maintain or improve muscle characteristics during periods when normal exercise may be limited.
A systematic review examining NMES and lower-limb skeletal muscle found evidence of changes in certain metabolic and muscle characteristics, although the researchers emphasized that differences between studies made it difficult to draw definitive conclusions about specific stimulation protocols.
4. Neurological Conditions
NMES has also been studied in individuals with neurological conditions where voluntary muscle activation may be impaired.
Because NMES directly stimulates peripheral nerves and produces muscle contractions, it may provide an additional way to train or activate muscles when normal voluntary movement is limited.
Research has investigated NMES in conditions including stroke, spinal cord injury, multiple sclerosis, and other neurological disorders.
Does Electrical Stimulation Actually Make Muscles Stronger?
It can.
However, the answer depends on the individual, the condition being treated, the stimulation parameters, and how NMES is incorporated into rehabilitation.
A 2023 systematic review examining electrical stimulation and muscle strength in healthy adults found that all 10 included studies reported improvements in strength following electrical stimulation. However, the researchers also noted substantial differences between studies and were unable to determine one optimal combination of treatment duration, intensity, pulse characteristics, and frequency.
Another systematic review and meta-analysis comparing NMES training with conventional strength training found that, when training volume was matched, NMES produced similar strength gains to conventional strength training rather than clearly superior gains.
This is an important distinction.
Electrical stimulation is not necessarily "better" than exercise.
Instead, it can be a useful tool when a patient needs additional assistance producing a muscle contraction.
Electrical Stimulation and Voluntary Exercise
One of the most interesting applications of NMES is combining electrical stimulation with voluntary muscle contraction.
Instead of simply allowing the machine to contract the muscle, the patient actively attempts to contract the muscle while NMES provides additional stimulation.
This approach is sometimes called NMES superimposed on voluntary contraction.
A systematic review of 24 studies found that combining NMES with voluntary contractions can improve muscle strength and may produce strength improvements equal to or greater than voluntary training alone in some populations. However, the authors also emphasized that differences in stimulation protocols and exercise types contributed to inconsistent results.
This approach can be particularly useful in rehabilitation because it encourages the patient to participate in the movement rather than relying completely on the electrical stimulation.
Why Can Electrical Stimulation Feel Different From Normal Exercise?
NMES does not necessarily recruit muscle fibers in exactly the same way as a normal voluntary contraction.
During voluntary movement, your nervous system controls motor unit recruitment according to the demands of the task.
Electrical stimulation can alter the recruitment pattern and may activate larger motor units at relatively low levels of voluntary effort.
This is one proposed reason NMES can complement conventional exercise rather than simply duplicating it.
However, the exact physiological response depends on factors such as electrode placement, current intensity, pulse duration, stimulation frequency, muscle size, and the individual's nervous system and tissue characteristics.
What Does Electrical Stimulation Feel Like?
Patients commonly describe NMES as:
- Tingling
- Pulsing
- Twitching
- Muscle contractions
- Tightening or pulling of the muscle
The sensation can vary depending on the treatment settings and individual sensitivity.
The goal is generally to produce an effective muscle contraction while keeping the treatment tolerable.
Your physical therapist can adjust the stimulation parameters based on your response and treatment goals.
What Determines How Effective NMES Is?
Electrical stimulation is not simply a matter of "turning up the intensity."
Several factors influence the response.
Electrode Placement
Where electrodes are placed can affect which nerves and muscles are stimulated.
Intensity
The stimulation needs to be strong enough to produce the intended response, but the appropriate intensity depends on the individual and treatment goal.
Frequency
Different frequencies can produce different muscle responses and levels of fatigue.
Pulse Duration
The duration of each electrical pulse can influence how nerves and muscles respond.
Treatment Duration
The length and frequency of treatment sessions can affect the overall training stimulus.
Voluntary Contraction
Combining NMES with active muscle contraction may produce different outcomes than passive stimulation alone.
Because these variables interact with one another, there is no single NMES protocol that is appropriate for every patient.
Is More Electrical Stimulation Better?
Not necessarily.
One of the biggest misconceptions about electrical stimulation is that increasing the intensity or duration will automatically produce better results.
Research has not established one universal "best" NMES dosage for every condition.
A systematic review of electrical stimulation in healthy adults found substantial differences in treatment protocols and could not determine a universal optimal threshold for treatment duration, intensity, pulse, or frequency.
This is why NMES should be individualized rather than treated like a one-size-fits-all intervention.
Can NMES Help After ACL Surgery?
The quadriceps are one of the most commonly discussed muscles when it comes to NMES after knee surgery.
Following ACL reconstruction, patients may experience significant quadriceps weakness. Restoring quadriceps strength is an important component of rehabilitation and return to activity.
A 2025 systematic review and meta-analysis of 11 studies involving 202 patients found that postoperative rehabilitation using NMES resulted in greater quadriceps strength improvements compared with standard physical therapy alone. The researchers reported benefits at both short- and longer-term follow-ups.
NMES may therefore be considered as an adjunct during certain stages of ACL rehabilitation, particularly when quadriceps activation is limited.
However, restoring strength ultimately requires progressive rehabilitation and functional loading.
Can NMES Help With Knee Osteoarthritis?
Researchers have also investigated NMES in people with knee osteoarthritis.
A 2026 systematic review and meta-analysis examined NMES combined with therapeutic exercise compared with exercise alone in adults with knee osteoarthritis. The study evaluated outcomes including pain, function, and muscle strength, using GRADE recommendations to assess the certainty of evidence.
This type of research supports an important principle in physical therapy:
Electrical stimulation may be most useful when it is combined with active rehabilitation.
Instead of using NMES as the entire treatment, a physical therapist can integrate it with strengthening, mobility, movement retraining, and functional exercises.
Can Electrical Stimulation Replace Strength Training?
No.
This is one of the most important things patients should understand.
NMES can create a muscle contraction, but rehabilitation is about much more than creating a contraction.
Patients also need to develop:
- Strength
- Coordination
- Balance
- Motor control
- Power
- Endurance
- Functional movement
- Confidence with activity
For these reasons, electrical stimulation is generally best viewed as a complement to active rehabilitation.
Research comparing NMES and conventional strength training has found that NMES can produce strength improvements, but it does not consistently demonstrate superiority over traditional strength training.
The goal is to use the right tool at the right stage of rehabilitation.
How Electrical Stimulation Fits Into Physical Therapy
A physical therapist may use NMES as part of a larger treatment session.
For example, a patient recovering from knee surgery might receive electrical stimulation while performing a quadriceps exercise.
A session could include:
NMES + quadriceps activation → strengthening exercises → functional movement → progressive loading
As the patient's voluntary muscle activation improves, the therapist can progressively increase the demands of the exercise program.
The ultimate goal is not to rely on the machine.
The goal is to help the patient regain the ability to activate, strengthen, and use the muscle independently.
What Is the Difference Between NMES and TENS?
Although both use electrical stimulation, NMES and TENS have different primary purposes.
NMES is designed to stimulate motor nerves and produce muscle contractions.
TENS, or transcutaneous electrical nerve stimulation, is primarily used to influence pain perception through sensory nerve stimulation.
In simple terms:
NMES: primarily used to create or enhance muscle contraction.
TENS: primarily used for pain modulation.
The two should not be considered interchangeable.
What About High-Powered Electrical Stimulation?
Some physical therapy technologies use higher-powered electrical stimulation systems designed to produce strong neuromuscular contractions.
These systems may be incorporated into rehabilitation when a clinician determines that stronger stimulation is appropriate.
The technology itself is only one part of the treatment. Electrode placement, stimulation parameters, patient positioning, exercise selection, and clinical reasoning all influence how the treatment is used.
At Core Performance Physical Therapy, technologies such as NerveOTX/ARPNeuro may be incorporated into individualized rehabilitation when clinically appropriate. Treatment should be based on the patient's specific presentation and goals rather than the technology alone.
Is Electrical Stimulation Safe?
NMES is generally well tolerated when appropriately administered, but it is not appropriate for every patient.
A physical therapist or other qualified healthcare professional should determine whether electrical stimulation is appropriate based on the patient's medical history, diagnosis, medications, implanted devices, skin condition, sensation, and treatment goals.
Patients should also communicate any unusual pain, excessive discomfort, skin irritation, or other unexpected response during treatment.
The Bottom Line
Neuromuscular electrical stimulation helps activate muscles by using controlled electrical impulses to stimulate motor nerves and produce muscle contractions.
It may be particularly useful when a patient has difficulty voluntarily activating a muscle because of surgery, injury, immobilization, weakness, or certain neurological conditions.
Research supports NMES as a potentially useful tool for improving muscle strength in several populations, but the evidence does not suggest that electrical stimulation should replace conventional exercise.
Instead, the strongest clinical approach is often to use NMES alongside active rehabilitation.
At Core Performance Physical Therapy, electrical stimulation can be incorporated into an individualized treatment program alongside therapeutic exercise, strength training, movement assessment, and functional rehabilitation.
The goal is not simply to make a muscle contract. The goal is to help you regain the strength, control, and function needed to move confidently again.
Frequently Asked Questions About Electrical Stimulation
Does Electrical Stimulation Actually Activate Muscles?
Yes. NMES delivers electrical impulses that stimulate motor nerves and can produce an involuntary muscle contraction.
Does Electrical Stimulation Build Muscle?
NMES can contribute to improvements in muscle strength and may influence muscle characteristics. However, it should generally complement progressive exercise rather than replace strength training.
Is NMES Painful?
NMES can feel like tingling, pulsing, or muscle tightening. The sensation varies between individuals and depends on the stimulation settings.
Can Electrical Stimulation Help After Surgery?
Research supports the use of NMES as an adjunct to rehabilitation after certain surgeries. For example, studies have found improvements in quadriceps strength when NMES is incorporated into rehabilitation following ACL surgery.
How Long Does an Electrical Stimulation Session Take?
Treatment time varies depending on the condition, muscle being treated, stimulation parameters, and rehabilitation goals. There is no universal treatment duration.
Can Electrical Stimulation Replace Exercise?
No. NMES is generally best used as a complement to active rehabilitation, particularly when voluntary muscle activation is difficult.
What Is NMES Used for in Physical Therapy?
NMES may be used to facilitate muscle activation, address weakness, support strength development, and complement rehabilitation following injury, surgery, immobilization, or certain neurological conditions.
Comments