Electrical Currents for Muscle Contraction (week 5)

NMES

•Neuromuscular electrical stimulation (NMES) is the use of electrical currents to produce muscle contractions in innervated muscles.

•NMES requires an intact and functioning nervous system

NMES can be used for the following activities:

•Muscle strengthening

•Muscle education or reeducation

•Edema control

Slow-twitch type I

•Innervated by smaller nerve fibers

•Activated first in physiological contractions

•Produce lower-force contractions

•Fatigue and atrophy resistant

Fast Twitch Type II

•Innervated by larger diameter nerve fibers

•Activated first when muscle is stimulated electrically

•Produce strongest contractions

•Fatigue rapidly and atrophy rapidly with disuse

How does ES strengthen muscles?

1) Overload Principle

2) Specificity Theory

2) Specificity Theory

Muscle contractions specifically strengthen the muscle fibers that contract

• ES targets fast twitch type II muscle fibers which produce a greater force

  • In patients with reduced muscle strength after surgery, immobilization or pathology and the resulting Type II fiber atrophy, the addition of ES with exercise amplifies and accelerates strength gains. (Kim, Croy, Hertel, et al)

1) Overload Principle

A principle of strengthening muscle that states the greater the load placed on a muscle and the higher force contraction it produces, the more strength that muscle will gain.  (assuming there is not damage to the muscle)

• With ES contractions, force is increased primarily by increasing the total amount of current, adjusting pulse duration and intensity or increasing externally applied resistance

Orthopedic Conditions

•ACL Reconstruction

•TKA (total knee arthroplasty)

•Nonsurgical management of orthopedic conditions

Neurological Conditions

•Occurs via direct muscle strengthening or increased excitability of the motor neuron produced by motor level ES

•Functional electrical stimulation (FES) is commonly used with neurological conditions

• Functional electrical stimulation – Application of an electrical current to produce muscle contractions applied during a functional activity.  Ex: Stimulation of the anterior tibialis to cause DF during swing phase of gait

Spinal Cord Injury

•NMES counteracts disuse muscle atrophy and improves circulation

•FES used to contract muscles to assist with locomotion and other bodily functions (grasp, voiding, respiration)

•Lower motor neuron, neuromuscular junction and muscle must be intact.

Stroke

Stimulation of weakened lower extremity agonist muscles in patients with hemiplegia due to stroke can achieve the following actions:

• Improve voluntary recruitment of motor units

• Improve gait

• Reduce agonist:antagonist co-contraction

NMES often triggered by voluntary contraction

•ES is also used to improve function in patients with:

•MS

•CP

•TBI

Sports Medicine / Performance

•ES can be used to increase strength in healthy subjects

•No clear evidence that the increased strength translates into improved performance.

Contraindications

•Pacemaker or unstable arrhythmias

•Over the carotid sinus

•Venous or arterial thrombosis or thrombophlebitis

•Pelvis abdomen, trunk or low back during pregnancy

Precautions

•Cardiac disease

•Impaired sensation or mentation

•Malignant tumors

•Skin irritation or open wounds

Electrodes place

on the muscle to be stimulated, parallel to the fiber direction.  One electrode is on the motor point.  Motor point is usually in the center of the muscle belly.

Limb can be secure to the table is movement is contraindicated

Pulse duration is between

150 and 350 microseconds

• As the pulse duration is shortened, higher amplitude current will be needed to achieve the same strength of contraction produced by a longer pulse duration

Pulse frequency is typically

35-50 pps

On: off ratio is typically

1:5 with 6-10 seconds on and 50-120 seconds off.  Ratio is decreased as patient gets stronger

• To relieve muscle spasm, the ration is 1:1 to cause muscle fatigue and force the muscle to relax

Ramp up/ down is

1-4 seconds

Amplitude is dependent on treatment goal

Strength without injury – 50% MVIC (maximum voluntary isometric contraction)

Post-op – 10% MVIC for initial treatments

Time

Duration is 10-20 minutes

Waveform

Pulsed biphasic waveform 

Russian protocol (Medium frequency alternating current, 2500 Hz frequency, 50 bursts per second, 10 ms burst)