E- Stim for Muscle Contraction

Neuromuscular electrical stimulation

Repeated application of current to produce contraction of innervated muscle by depolarizing local motor nerves

NMES vs TENS

NMES

• lower frequency

• larger pulse duration

• amplitude to tolerance

• strengthening

TENS

• higher frequency

• lower pulse duration

• intensity submotor

• pain control

How does FES work

1. electricity is applied across the skin over an intact peripheral nerve

2. the flow of electrons from a stimulator is converted to a flow of ions in the motor nerve

3. if adequate to depolarize the motor nerve, an action potential results

4. a muscle contraction occurs

*therapeutic benefit is enhanced with pt’s voluntary intent of movement

Electrically Driven ≠ Physiologic Contractions

Differ in

• Order of motor unit recruitment

• Smoothness of contraction

• Time to fatigue

NMES vs Volitional Muscle Contraction

• Voluntary: smaller recruited first followed by larger motor units (Henneman size principle)

• Voluntary: allows for alteration/selective activation of motor units and firing frequency

• NMES activation: random activation pattern (does not recruit according to size principle); no predictable order

• NMES: non-selective activation of motor units

• Slow or fast fibers can be recruited at lower contraction intensities

• NMES activation: increased fatiguability

Voluntary Contraction

• the action potential travels down to the NMJ

• Recruitment of motor units:

  • slow, small fatigue resistant type I before larger, faster type II

• quality of movement

  • smooth and graded movement due to asynchronous activation of motor units

• more fatigue resistant

Electrically driven contraction

• the AP travels anterograde (to the NMJ) and retrograde (to the anterior horn cell)

• recruitment of motor units:

  • larger, faster type II before smaller type I

  • also dependent on proximity to simulation electrode

• quality of movement

  • graded and isolated movement not possible

• fatigue occurs more rapidly

NMES Waveform

• Typically- biphasic pulsed alternating current

• Bipolar set up (2 electrodes)

• Channel placement (red vs black, cathode vs anode) doesn’t matter

• Electrode placement is key

A Note About Russian Stimulation?

• Similar goals as NMES (strengthening, hypertrophy) but with different waveform

• Aims to elicit supra-max contractions with minimal pain

• 2500 Hz carrier frequency. Burst modulated at 50 Hz

• 50-200 pulse width

• Dantas, et al compared 4 different waveforms on isometric knee extension torque and perceived discomfort in healthy women

  • Results showed similar discomfort levels between Russian and PC but with Russian demonstrating less evoked torque

• Bellew, 2012 compared muscle force production of biphasic pulsed current, IFC, and Russian.

  • Results showed greater knee extensor %MVIF with burst modulated biphasic pulsed vs conventional Russian

Strength and Muscle Size after NMES aren’t always related

• Muscles become stronger from factors other than just hypertrophy

  • Increased muscle size can take weeks

  • Improved motor unit recruitment (non muscle adaptations) is more rapid

• Initial strength gain observations following NMES are often due to changes in motor unit recruitment, with or without changes in muscle fiber size

  • NMES is very beneficial when motor recruitment is impaired

  • NMES within 1-4 wks post surgery is best

Yet Prolonged Use Leads to Improved Morphology

• Mastropietro, et al studied 4 individuals with chronic SCI following 6 months of FES cycling

• Muscle hypertrophy was observed

  • Average increase in muscle volume of 22.3% at 3 months and 36.7% at 6 months.

  • Remained 23.2% higher than baseline one month post training

• Evidence supports NMES use in increasing muscle volume and decreasing fat infiltration in SCI.

For whom is NMES Appropriate?

• Indicated for upper motor neuron injuries

• Use is predicated on the pt having an intact peripheral nerve

  • NMES implies muscle activation through peripheral nerve stimulation due to its lower resting membrane potential

  • Technically, can stimulate a denervated muscle directly but much harder

    • Has no effect on rate of nerve regeneration

• LMN injuries, therefore, don’t respond well to NMES

  • Could consider its use as a screen for injury such as cauda equina syndrome or peripheral nerve damage

Optimizing Muscle Response to E-Stim

• Parameter adjustment

• Electrode size

• Electrode placement

• Pt education

Three Setting Triad

Frequency, Intensity, Pulse Width

Frequency

• Pulses per second

• As frequency increases, the tension summates

• Tetany is achieved at 30-50 PPS

• Higher frequencies = more force but faster fatigue

You need enough frequency

You need enough frequency in order to reach twitch summation and tetany because the generated tension doesn’t have time to decrease between action potentials

Pulse Duration

• The length of time each group of pulses lasts

• Microseconds

• Typical (starting point) = 200-300 (lots of variability based on many factors)

• Longer pulse width = recruit more motor neurons and improve muscle contraction

• Short pulse width = more comfortable

Intensity (aka Amplitude)

• = magnitude of current

• Measured in milliamperes

• Varies with pulse width

• As intensity increases, force of contraction increases

• Ideal current = tolerable to the patient and produces the desired motor response

Waveforms and Parameters- What Does It All Mean?

• Magnitude of the pt’s muscle response is influenced by the total amount of current delivered to the pt

• The total charge is the area under the curve, which is the sum of amplitude and duration

  • You can increase the amplitude or duration or both

How do I get a muscle to contract /c e-stim?

• Stimuli must be of sufficient amplitude and duration and adequate frequency

• Many combinations exist that could lead to depolarization

  • As intensity is decreased, you must increase the duration

  • As duration is decreased, you must increase the amplitude

  • Frequency must be enough to reach tetany

*must have sufficient amount of current in order to depolarize nerve

Why does a shorter pulse duration activate nerve fibers with less pain?

strength duration curves, chronaxie, and rheobase

Rheobase

minimum intensity required to excite the nerve given long current duration

Chronaxie

minimum duration required for a current of twice the intensity of the rheobase to produce an action potential

Strength-Duration Curves

• Represents threshold of depolarization for a given nerve

• Non-linear

• Shorter chronaxie = greater excitability

• Sensory nerves have a shorter chronaxie than motor nerves

• Shorter pulse durations require higher current amplitude to reach the motor threshold

Short vs long pulse

• Short pulses: more comfortable b/c they deliver less total charge and preferentially stimulate low threshold sensory fibers

• Longer pulses deliver more charge and recruit higher threshold motor units

Goal = increase contractility

• Intensity: high

• Pulse Width: high

• Frequency: high

Goal - to increase patient comfort

• Intensity: low

• Pulse Width: low

• Frequency: high

Goal - to decrease spasticity

• Intensity: low

• Pulse Width: low

• Frequency: high

In Addition to Nerve Type and Size, Depolarization Depends on Electrode Location

• Nerve fibers closer to the electrode are recruited before more distant fibers

• Closer dermal sensory nerves just under the surface are activated before deeper lying, larger diameter motor nerves

  • Helps explain why you feel stim before you see a contraction

Electrode Placement

• It’s all about anatomy!

• Place over muscle belly at the motor point or over nerve trunk

• Palpate muscle to determine placement

Electrode Size

Current spread across a larger area is perceived as less strong and more tolerable

Electrode size ultimately depends on the size of the muscle to be stimulated

Electrode Size and Current Density

Use the largest electrode possible based on the muscle size in order to optimize motor response and improve comfort

Electrode Spacing and Depth of Penetration

• Electrodes close together produce high density current in superficial tissues.

• Increased distance increases current density in deeper tissues

• High current density close to the neural structure to be stimulated makes success more certain with the least amount of current.

• Electrotrode placement is likely one of the biggest causes of poor results from electrical therapy

Ramp Time

Positives:

• Can prevent abrupt or uncomfortable contraction

• Can minimize triggering spasticity

• Less alarming to the pt

• Can prevent sudden “flopping” of the muscle (ramp down specifically)

Negatives:

• Can mean less time at max stimulation intensity

Limitations of NMES

• Muscle soreness can be greater

• Sufficient training intensities may not be reached due to pt tolerance

• Difficulty incorporating into multi-joint exercises

• Clinically difficult to achieve required dosage given multifactorial demands of a POC

Dosage Matters

• Appropriate training dose must be applied!

• Efficacy of NMES depends on the intensity of training

• This means your parameters must be optimized!

• Most studies report “maximally tolerated”

  • Up to 70% of max voluntary contraction is ideal dosage

• Dosage example:

  • High training frequency (1-2 hrs/day for 5 days/wk for 6-8 wks) in the ICU led to improved strength, function, and muscle mass

Optimizing Dosage

• Choose correct stimulator

  • Consider if functional tasks are concurrently occuring

• Ensure optimal electrode placement over motor points

• Number of reps

• On/off times and rest times

• Frequency of application (days/wk)

• Tx duration

• Stimulation parameters

  • “ease in” but adjust as pt apprehension wanes

Action Steps

• Position pt

• Clean skin

• Electrodes go on the motor nerve of the target muscle

• Ask pt when they first start to feel it (if sensation is intact)

• Increase current until you see a motor response

• If motor response does not occur before intolerance, adjust parameters and/or electrodes

• Make sure you have pre and post measures so you can document and objectively evaluate the response

• Check skin post treatment