Electrical Stimulation for Muscle Contraction Notes

Unit 5: Electrical Stimulation for Muscle Contraction

Introduction to Electrical Stimulation for Muscle Contraction

  • The lecture focuses on neuromuscular electrical stimulation (NMES) for muscle disorders caused by musculoskeletal conditions such as:

    • Weakness

    • Atrophy

    • Decreased range of motion

  • For more details, refer to Chapter 13 in the textbook: Electrotherapy for Musculoskeletal Disorders.

Objectives of the Lecture

  • Explain the Physical Principles:

    • Cover NMES and Functional Electrical Stimulation (FES), abbreviations for neuromuscular electrical stimulation and functional electrical stimulation, respectively.

  • Differentiate Types of Muscle Contractions:

    • Understand the differences between physiological contractions and electrically induced contractions.

  • Parameters and Techniques:

    • Study application techniques for muscle strengthening.

Indications for NMES

  • Principal indications include:

    • Strengthening muscles

    • Treating disuse atrophy

    • Managing spasticity

    • Reducing edema

    • Improving range of movement

Differences Between Types of Muscle Contractions

  • Voluntary Contractions:

    • Motor unit recruitment is progressive:

    • Smaller, slower motor units are activated first.

    • Larger, faster motor units are activated subsequently.

    • Recruitment is asynchronous, which helps in maintaining smooth movements.

  • Electrically Induced Contractions via NMES:

    • Recruitment is non-selective:

    • Small and large motor units are recruited together.

    • Recruitment is synchronous and spatially fixed:

    • All muscle fibers contract at the same time, not following physiological norms.

Currents Used in NMES

  • Main currents for muscle contractions include:

    • Russian Current:

    • Burst-modulated alternating current

    • Carrier frequency: 2500 Hertz

    • Burst frequency: 50 Hertz

    • Pulsed Currents:

    • Low-frequency currents that allow the practitioner to set pulse duration and frequency to achieve optimal muscle contraction.

Effects of Electrical Stimulation on Muscle

  • Muscle Mass:

    • Takes weeks to months to observe changes in muscle mass.

  • Neural Factors (non-muscle mass):

    • Enhancements occurring earlier (within days to weeks):

    • Increased number and frequency of motor unit recruitment,

    • Synchronized recruitment of motor units.

    • Both effects may improve gradually over a year or more.

Parameters for Muscle Strengthening

  • Common parameters (varying values due to different studies):

    • Pulse Duration:

    • Typical value: 200 microseconds

    • Frequency:

    • Most common: 50 Hertz

    • Ramp Up/Down:

    • Standard values: 2 seconds for both ramp-up and ramp-down

    • Intensity/Amplitude:

    • Typically set above 50% of Maximal Voluntary Contraction (MVC) to ensure strong contractions.

    • Duty Cycle:

    • Contraction time of 10 seconds with relaxation of either 30 seconds or 50 seconds.

    • Burst-modulated currents have adjusted parameters instead of pulse durations.

Important Considerations for NMES

  • High pulse duration and amplitude are needed to achieve strong muscle contractions for muscle strength development.

    • Recommended pulse duration: 200 microseconds.

    • Intensities above 50% of MVC are typically used.

    • Frequencies of 30-50 Hertz are ideal for smooth or tetanic contractions, while cautions should be taken against high frequencies (greater than 100 Hertz) due to the potential for muscle fatigue.

  • Consideration of fatigue:

    • Reduce frequency and increase resting time to prevent fatigue.

    • Ramp times should be comfortable, typically set between 1-2 seconds.

  • Higher intensity should be close to patient's maximum tolerable levels, often cited as 50-70% of the MVC of the opposite leg.

MRI Observations in Electrical Stimulation

  • MRI images show that electrical stimulation can activate:

    • Skeletal muscle fibers near the femur even at low stimulation intensities.

    • Activation of deep muscle fibers occurs at approximately 25% of maximum voluntary isometric contraction.

    • Stimulation applied superficially can still recruit deeply located muscle fibers.

Electrical Stimulation for Denervated Muscles

  • NMES does not apply to denervated muscles:

    • Electrical Stimulation (ES) directly activates the sarcolemma without motor nerve fibers due to disruption.

  • Parameters for denervated muscles require:

    • Longer pulse duration

    • Greater stimulation amplitude

    • Specific waveforms, e.g., direct current, which increases risk of skin burns.

Pulse Duration in Denervated Muscles

  • Emphasis on the necessity for longer pulse durations in denervated muscles due to higher chronaxie:

    • Chronaxie for denervated muscles: approximately 10 milliseconds.

    • Chronaxie for innervated muscles: approximately 0.2 milliseconds.

  • For further understanding, refer to the Electrophysics lecture covering chronaxie and rheobase.

Electrode Placement

  • Different electrode placements are illustrated and can vary based on patient size:

    • For example, larger patients may require four electrodes for effectively stimulating the quadriceps muscles.

Additional Resources

  • Video links regarding NMES application, focusing on various muscles, electrode placements, and parameter settings.