What does NMES do?
- Uses electrical currents to create or facilitate muscular contraction
Weak or physiologically inable to do it themselves. Ex: SCI.
Neuromuscular Physiology
Normal Voluntary Contraction:
- Brain signal send to anterior horn (alpha motor neuron) - Gusses # of motor units and frequency they should fire (lifting something heavy/light, speed of movement).
- Signal from horn goes down A Motor Neuron to Muscle (travels to periphery is called orthodromic conduction - heading out) Signal causes depolarization and contraction.
- Muscle contraction (Tension (GTO) may cause movement, may cause change in length of muscle (muscle spindle))
- Signal send back via Alpha Beta motor neuron into dorsal and back through spinal cord to brain
- Brain makes adjustments (loop is continuous to make changes, feed back loop)
- Asychronous muscle recruitment
- Firing frequency is 20-50 Hz (up to 100)
Not all muscle fibers fire at the same time - asynchronous
Most often recruit slow twitch fibers first (if needed recruit fast twitch)
With NMES
- Cause depolarization of nerve, AP goes down nerve going in orthodromic direction but signal also send antidromic direction (Not fully understood, adds to stimulation). Conduction in both directions.
-** Signal comes to muscle and contracts (Tension - GTO and Change in Length - Muscle Spindles) ** - Sensory coming back into the nervous system is the same.** Put lots of sensory feedback into the system.**
- Firing Frequency 35-50 pps
Differences between voluntary and NMES
- NMES sends signal othodromic and antidromically (Voluntary is only orthodromic)
- Synchronous muscle firing in NMES (Asynchronous voluntarily)
- Activate slow twitch and fast twitch with NMES - induce fatigue quicker (Voluntary is slow and possibly recruit fast)
- NMES increased Fatigue factor due to recruitment of fast twitch which fatigue faster
- NMES causes more neurological adaptations
Strength Duration Curves
- Large diameter fibers recruited first
- Anatomical location of the nerve matters – motor are usually deeper than sensory
- In humans: Sensory before motor
- Proximity to charge matters (Sensory is closer to skin surface, easier to stimulate than alpha motor neurons; very rare place sensory and motor next to one another results in motor being activated first)
Nerve Depolarization
- Also applies to TENS and IFC
- Depolarization more pronounced at Cathode (-), only need to worry if using an unbalanced curve
- Outward Capacitive Current: Current that causes depolarization
Lipid bilayer has protein. Is within axon. Extracellular Na+, Intracellular K+ (-70). Negative portion is connected, positive flaps. In the negative position of the biphasic current, positive is attracted to negative and allows for Na (+40) to come in -> Results in an Action Potential.
Indications for NMES
- Muscle strengthening/neuromuscular re-education
- Fatigue of spastic muscle (Allows for less resistance to perform movement; Ex: Biceps reach)
- Facilitation of functional activities for neuromuscularly compromised patients (Ex: SCI)
- Pain relief from shoulder subluxation s/p CVA (Ex: Stroke, flasid shoulder muscles; Facilitate muscle contraction - remove pain of subluxation)
- Pressure ulcer prevention (Contraction muscles near pressure ulcers creates blood flow to the area prevents ulcers)
- Chronic edema reduction (Ex: gastroc is limited in muscle pump, use NMES for strong contraction)
- Maintenance of denervated muscle (Don’t need a nerve but takes a lot of muscle activity)
Additional Contraindications - NMES
Doing it where active movement is contraindicated
Seizure disorder
Damaged skin or over a scar (increased impedance)
Pregnancy – local, ANYWHERE
Areas of impaired circulation
Lower abdomen
Chest or intercostal muscles
Any area unstable due to recent surgery, fracture, or osteoporosis
Contraindications - NMES
-
Where to apply electrodes in NMES?
- Electrode Placement
- On the motor point (where periphral nerve ends muscle and innervates)
- Surrounding the motor point
- Ideally over motor point and muscle belly.
- Does not have to be in line with fiber direction.
Motor Point
- Area where terminal nerve branches enter muscle
- Low Resistance, Low Capacitance
How many channels? - NMES
- For small muscles, one channel
- For large muscles, two or more channels
- Ex: If going for grip need electrodes on both flexors and extensors
What waveform do you use? - NMES
Pulsed, Square (Most Common)
Russian (Bursted AC - 2500 Hz AC burst at 50 bursts/sec with a burst duration of 10 milliseconds)
NMES Parameters - Pulsed
- Amplitude - Vigorous but tolerable tetanic (smooth) contraction; Tolerated better with volitional contraction
- Pulse Duration: 200-400 microsec
- Frequency: 35-50 pps (If fatigue is the goal, increase frequency, otherwise lowest frequency to achieve tetany)
- Duty Cycle: In Seconds; Lower Duty Cycle for Weaker Patients (Longer Rest), Higher Duty Cycle for Stronger Patient Ex: 5 seconds on, 5 seconds off = 50% Duty Cycle (5/10 = 50%) (Less Rest, more work)
- Ramping: A ramp time of 1-2 seconds is usually tolerated better (Lower ramp for more instance reaction would be good for foot drop)
Is Pulsed or Russian better?
Doesn’t matter! Both work just as well.
Picture is Russian Current
Muscle Production and Comfort
- Short duration bursts (2 ms), 50 Hz burst frequency, best for max torque
- 4 ms bursts at 50 Hz results in optimal comfort
When doing Russian Current how do you set up settings for work?
- DO NOT CONSIDER DUTY CYCLE (Waveform, length of burst over number of bursts) Duration setting for Russian
- It is CYCLE TIME (Work vs Rest)
Russian Stimulation Math Examples
Best: Frequency 50 Hz and 20% Duty Cycle
Effects of NMES (Healthy)
Healthy subjects
- Versus no intervention, conflicting evidence of effects on strength and endurance
Versus exercise, exercise generally better
Combination exercise + NMES is somewhat effective
BUT, Generally requires intolerable levels of stimulation (Only in Elite athletes to get type 2 fast twitch muscles)
Effects of NMES (Weak)
- Consistent evidence that NMES will increase strength
- Fast twitch fibers become more fatigue resistant
- Neuroplastic adaptations
- Cortex and spinal cord
NMES for Muscle Re-Education
- Combine this with therapeutic exercise to get cortical feedback (isometric, isotonic).
- Make sure they are cleared for muscle activation.
NMES for Muscle Re-Education Common Examples
- Quad stimulation after ACL reconstruction (and meniscus procedures)
- Pelvic floor stimulation
- Shoulder (post-op, other)
NMES For Spasticity
Decrease spasticity for 2-3 hours (2 options):
1. Agonist (the spastic muscle)
- Duty cycle >50% because you want to fatigue
- Frequency 80 pps or bursts/sec or higher
- Treatment time about 1 hour
2. Antagonist
- Duty cycle 33% to facilitate reciprocal inhibition
- Treatment time 1 hour
- Frequency 35-50 pps or bursts/sec
1 is more effective but if not tolerable do 2.
Functional Electrical Stimulation (FES)
NMES for direct facilitation of functional activity
* Example: Lower Extremity Orthosis
* Used in patients lacking strength in anterior tib or fibularis m.
* AFO (Ankle Foot Orthosis) is typically used with this, but NMES offers “active” option
* Goal is dorsiflexion, not inversion.
