Functional Electrical Stimulation (FES): Bridging the Gap Between Paralysis and Function

Functional Electrical Stimulation (FES) is not just "electric shock therapy." It is a sophisticated neuro-rehabilitation tool that uses low-level electrical currents to activate paralyzed muscles for a specific purpose—like lifting the foot while walking or grasping a cup. Unlike TENS (for pain), FES is for function. This guide covers the mechanism, parameters, and critical applications for Stroke, SCI, and MS.

1. Mechanism: How Does It Work?

FES works by stimulating the peripheral nerve (Lower Motor Neuron) that supplies the paralyzed muscle. It generates an action potential that travels to the muscle, causing a contraction.

The "Intact LMN" Rule:
FES ONLY works if the peripheral nerve is intact (UMN Lesions like Stroke, SCI, MS, CP).
It does NOT work for LMN lesions (Polio, GBS, Peripheral Nerve Injury) because the "wire" to the muscle is cut. Denervated muscles require long-duration Galvanic current, not FES.

Orthotic vs. Therapeutic Effect

• Orthotic Effect: Immediate improvement only while the device is ON (e.g., preventing foot drop during a walk).
• Therapeutic Effect: Long-term improvement even when the device is OFF. FES promotes neuroplasticity by providing repetitive, correct sensory-motor feedback to the brain.

2. Top 3 Clinical Applications

A. Foot Drop (The Most Common Use)

Used for hemiplegic gait to prevent tripping.

• Target: Common Peroneal Nerve.
• Action: Dorsiflexion + Eversion during the swing phase of gait.
• Timing: A heel-switch triggers the stimulator when the foot lifts off the ground and stops it when the heel strikes.

B. Shoulder Subluxation

Used in early stroke to prevent the humerus from dropping out of the glenoid fossa.

• Target: Supraspinatus and Posterior Deltoid.
• Action: Retracts the humeral head into the socket.
• Goal: Reduce pain and prevent stretching of the capsule.

C. Hand Function (Grasp & Release)

Used to train opening the hand (often difficult due to spasticity).

• Target: Extensor Digitorum Communis (to open) and Flexors (to grasp).
• NESS H200: A specialized FES orthosis that sequences these movements.

3. Setting the Parameters (The "Recipe")

Incorrect parameters lead to pain or rapid fatigue. Here is the standard setup for muscle contraction.

Parameter	Recommended Range	Reasoning
Waveform	Biphasic Rectangular	Balanced charge prevents skin irritation (chemical burns) under the electrode.
Pulse Frequency	20 – 50 Hz (pps)	Enough to cause tetany (smooth contraction). >50 Hz causes rapid fatigue.
Pulse Width	200 – 300 microseconds	Comfortable for recruiting motor nerves without stinging pain.
Duty Cycle	1:3 or 1:5 (e.g., 5s ON, 15s OFF)	Crucial: Muscles stimulated electrically fatigue much faster than physiological contractions. Rest is mandatory.
Ramp Up/Down	2 – 4 seconds	Avoids a jerky, startling start; mimics natural recruitment.

4. Contraindications & Precautions

"PACEMAKER" Alert: Never place electrodes over the chest of a patient with a cardiac pacemaker. Other key no-go areas:

• Over the carotid sinus (neck) – can drop blood pressure.
• Over areas of active cancer/malignancy.
• Over areas with broken skin or infection.
• Pregnancy (over the trunk/abdomen).
• Uncontrolled Epilepsy: Precaution (consult neurologist).

5. Revision Notes for Students

FES vs. NMES: FES implies the stimulation is used *during* a functional task (walking, reaching). NMES is often just strengthening in sitting.
Recruitment Order: Electrical stimulation recruits large, fast-twitch (Type II) fibers *first* (Reverse recruitment order). This is why FES causes fatigue quickly.
Biofeedback: Often combined with FES (EMG-triggered FES) to reward patient effort.
Cycling: FES Bikes allow paraplegics to cycle by stimulating quads/hamstrings/glutes in sequence.

6. FAQs

Q1. Can FES fix foot drop permanently?

For some, yes. Through the "Therapeutic Effect," consistent FES use can strengthen pathways so the patient eventually walks without it. For others with permanent damage, it remains an orthosis (must wear it to walk).

Q2. Does it hurt?

It creates a tingling or buzzing sensation and a strong muscle pull. It should not be painful. If it stings, check electrode contact or reduce pulse width.

Q3. Can I use FES on a Bell's Palsy patient?

Generally NO. Bell's Palsy is an LMN lesion (Facial Nerve). Standard FES won't work well and may interfere with regeneration. Tapping/massage and active exercises are preferred.

7. 10 Practice MCQs

Q1. FES is indicated for which type of lesion?

Answer: A) An intact peripheral nerve is required to transmit the signal.

Q2. To treat foot drop, electrodes are placed over the:

Answer: B) Stimulating the peroneal nerve activates the Tibialis Anterior for dorsiflexion.

Q3. Which fiber type is recruited FIRST during electrical stimulation?

Answer: B) This is the reverse of natural physiological recruitment (Henneman's size principle).

Q4. A Duty Cycle of 1:1 (e.g., 10s ON, 10s OFF) is likely to cause:

Answer: B) Muscles need adequate rest time (1:3 or 1:5) to recover ATP.

Q5. The optimal frequency for tetanic contraction without excessive fatigue is:

Answer: B) 35 Hz is a common "sweet spot" for smooth muscle tetany.

Q6. For shoulder subluxation, which muscles are stimulated?

Answer: B) These muscles align the humeral head into the glenoid.

Q7. Which of the following is a CONTRAINDICATION for FES?

Answer: A) Electrical signals can interfere with the pacemaker's rhythm.

Q8. Why is a "Ramp Up" time used?

Answer: B) A gradual onset mimics natural recruitment and prevents spastic startle response.

Q9. "Ramping Down" is important to:

Answer: A) Sudden cessation of current causes the limb to drop; ramping down allows controlled lowering.

Q10. The main goal of FES in C5-C6 SCI is often:

Answer: B) Enhancing grasp (using wrist extensors) significantly improves independence in quadriplegia.

References

• Marquez-Chin, C., & Popovic, M. R. (2020). Functional electrical stimulation therapy for severe hemiplegia. Biomed Eng Online.
• Knutson, J. S., et al. (2015). Contralaterally Controlled Functional Electrical Stimulation for Upper Extremity Hemiplegia. Stroke.
• Cameron, M. H. (2018). Physical Agents in Rehabilitation. Elsevier.