Study Notes on Electrical Currents for Muscle Contraction

Electrical Currents for Muscle Contraction

Introduction by Andrew Keal PT, DPT

• Overview of the use of electrical currents specifically for muscle contraction.

Objectives

• Summarize the effects of electrically stimulated muscle contractions for innervated and denervated muscles. (Outcome 2)

• Explain the use of electrical stimulation for patients with various conditions through a case study. (Outcome 4)

• Develop a treatment plan for a patient lacking voluntary muscular contractions, utilizing the correct parameters based on best practice treatment guidelines. (Outcome 4)

Effects of Electrically Stimulated Muscle Contractions

• Strengthen muscles: Electrically stimulated contractions can increase muscle strength and promote hypertrophy.

• Improve endurance: Through repeated electrical activations, muscles may enhance their endurance capacity.

• Slow or prevent muscle atrophy: Especially beneficial in patients who cannot voluntarily contract their muscles, helping maintain muscle mass.

• Reduce spasticity: Can aid in reducing involuntary muscle spasms and improving overall muscle function.

• Restore function: Improves functional capabilities in patients who have lost some degree of muscle function.

Neuromuscular Electrical Stimulation (NMES)

• Definition: NMES is defined as the use of electrical currents to produce muscle contractions in innervated muscle.

• Peripheral Nervous System Requirement: NMES requires that the peripheral nervous system is intact and functioning properly.

• Functional Electrical Stimulation (FES): This technique applies electrical currents to generate muscle contractions while the patient actively engages in functional tasks. It uses the same parameters as NMES but emphasizes the active participation of the patient.

  • Contrast: FES (active) vs. NMES (passive).

Motor Unit Recruitment

• Action Potentials: The principles of action potentials are similar between physiologically stimulated and electrically stimulated muscle contractions.

• Physiologic Contractions: In physiological contractions, smaller, slow-twitch type I muscle fibers are activated before larger fast-twitch type II fibers.

  • This results in lower force contractions that are more resistant to fatigue and atrophy. Mechanism of Recruitment:

  • Smaller fibers (Type I) recruited initially.

  • Cause gradual escalation of force and energy consumption.

• Electrical Contractions: In contrast, electrically stimulated contractions activate larger, fast-twitch type II muscle fibers first.

  • These contractions are stronger and quicker but fatigue more rapidly and are more prone to atrophy with disuse. Mechanism of Recruitment:

  • Larger fibers (Type II) are activated first.

  • Results in rapid force production without the smooth, graded onset of physiological contractions.

Implications of Recruitment Differences

• Rest Requirements: Stimulated contractions necessitate longer rest times than physiological contractions.

• Recommendation: Whenever possible, patients should engage in physiological contractions alongside electrically stimulated contractions.

• Onset Differences: Electrically stimulated contractions exhibit a rapid, jerky onset versus the smoother, graded onset of physiological contractions.

• Asynchronous Recruitment: Physiological contractions tend to recruit motor units asynchronously, which contributes to smoother movement.

Electrically Stimulated Muscle Contractions: Denervated Muscle

• Challenge: Denervated muscle will not respond to NMES parameters designed for innervated muscles.

• Required Parameters: For effective stimulation, it requires direct current with a pulse duration of at least 10 msec; intensity/or amplitude must be gradually increased to achieve muscle contraction.

• **Research Findings: ** EMS (Electrical Muscle Stimulation) may delay reinnervation and effectively reduce the rate of muscle atrophy post motor nerve injury, with immediate application being more beneficial than delayed.

Mechanisms for Muscle Strengthening through Electrical Stimulation

1. Overload Principle: The greater the load placed on a muscle, the higher force contraction produced leads to an increase in muscle strength.

   • Physiological Exercise: Resistance can be increased using weights or resistance bands.

   • Electrically Stimulated Contractions: Contractile strength can be enhanced by increasing the current through methods such as raising the pulse duration, increasing current amplitude, or enlarging electrode size, which collectively recruit more muscle fibers.

2. Specificity: The electrical stimulation primarily strengthens the muscle fibers that are actively contracted.

Clinical Applications of Electrically Stimulated Muscle Contractions

1. Muscle Strengthening–Orthopedic Conditions:

   • ACL Injuries: NMES can slow early quadriceps strength decline post-surgery and aid recovery.

   • TKA (Total Knee Arthroplasty): Combining NMES with voluntary exercise increases quadriceps strength and speeds functional recovery.

   • Other conditions:

     • Osteoarthritis (OA): Enhances pain relief, strength, and function.

     • Rheumatoid Arthritis (RA): Improves strength and endurance.

     • Patellofemoral Pain Syndrome (PFPS): NMES increases vastus medialis oblique (VMO) force generation.

     • Upper Extremity Weakness: Enhances bicep and hand strength and endurance.

2. Cardiorespiratory and Functional Training:

   • Critical Illness: NMES of quadriceps improves O2 uptake, strength, 6-minute walking test results, overall functional outcomes, and quality of life in critically ill patients.

   • Research Evidence: Supports NMES efficacy in improving muscle strength and functional outcomes during reduced physical activity and muscle atrophy.

3. Muscle Strengthening for Athletic Performance:

   • Healthy Adults: Additional NMES may increase strength but correlation with function is not always established.

   • Athletic training: NMES may assist in strength gains, though benefits on agility and performance remain inconsistent.

   • Specialized Cases: Used in zero gravity conditions for astronauts to prevent muscle atrophy.

4. Motor Control – Neurological Conditions:

   • CVA (Cerebrovascular Accident): NMES increases strength and enhances recovery.

   • Additional Benefits: Reduces shoulder subluxation, spasticity, and improves range of motion, proprioception, and coordination.

   • Use with EMG: Enhances motor control and provides proprioceptive feedback.

   • FES in Stroke Rehabilitation: Addresses issues such as foot drop and upper extremity function.

   • SCI (Spinal Cord Injury): Needs to be strong yet comfortable; it should involve intact lower motor neurons, neuromuscular junction, and muscle integrity.

     • Applications: Hand grasp, breathing, bowel/bladder voiding, enhancing gait, strength, endurance, mitigating atrophy, and cardiovascular responses.

   • Other CNS conditions: Similar benefits reported for TBI (Traumatic Brain Injury), MS (Multiple Sclerosis), CP (Cerebral Palsy); NMES is used for dysphagia and urinary incontinence.

5. Edema Control and Improved Circulation:

   • Implementation: NMES for edema from poor circulation through lack of motion, utilized cautiously to avoid aggravation of inflammatory edema.

     • Mechanism: Targets the muscles around major draining veins with legs elevated, promoting blood flow.

     • Effectiveness: Combining with compression garments accelerates healing; NMES is more efficient than pneumatic compression in preventing DVT (Deep Vein Thrombosis).

6. Muscle Spasm Reduction:

   • Causes of Spasms: Muscle strain, injury, or disease.

   • Mechanism: Motor-level electrical stimulation reduces spasm by interrupting the pain-spasm-pain cycle and decreasing muscle contractility.

Contraindications

• General contraindications for all electrical stimulation include:

  • Demand-type cardiac pacemaker or implanted defibrillator.

  • Unstable arrhythmia.

  • Application over the carotid sinus.

  • Usage over venous/arteriothrombosis or thrombophlebitis areas.

  • Pregnant women (abdominal or back regions).

  • Conditions where muscle contraction could disrupt healing or worsen symptoms.

    • Muscle or tendon tears.

    • Overuse injuries.

• Additional Contraindications Include:

  • Cardiac disease.

  • Impaired mental function.

  • Reduced sensation.

  • Presence of malignant tumors.

  • Areas with skin irritation or open wounds.

  • Possible association with Delayed Onset Muscle Soreness (DOMS).

Documentation for NMES Treatment

• Essential elements to document include:

  • Area of body treated.

  • Patient positioning during treatment.

  • Specific stimulation parameters used.

  • Placement of electrodes.

  • Duration of treatment.

  • Patient’s response to treatment.

• Documentation Detail: Should enable another clinician to reproduce the treatment accurately based on provided notes.

Recommended Parameter Settings for Electrically Stimulated Muscle Contractions

Note: Each parameter setting contains specifications that must be adhered to for effective treatment outcomes.