Functional Electrical Stimulation (FES) is a medical treatment that involves applying small electrical charges to impaired muscles of a patient to improve mobility, which has occurred due to brain or spinal cord damage.

Principle:

Electrical impulses are sent to the affected muscles or area via the nerves present in that area. It involves ‘units’ that are placed on the surface of the skin. Electric current is passed through the units, which are sometimes electrodes (active and common electrodes are used), which causes stimulation of the nerves in the region. Stimulation of the nerves causes movement of muscles, thereby movement of the limb. the current density of the electrical current is:

  • inversely proportional to the size of the electrode
  • higher at the region between the electrodes and the tissue and gets progressively weaker deeper into the body
  • distance between active and common electrodes depends on the depth of the stimulation.

Process of FES:

  1. The cell nucleus is unsure whether or not to produce signals and the motor neurons don’t receive sufficient input from the CNS.
  2. An FES (electrode) injects electrical stimulation into the cell.
  3. The dormant axon receives the stimulus and sends an action potential to the neuromuscular junction.
  4. The muscle fibers contract and the necessary muscle force is generated.

Muscle types targeted for FES:

  • Skeletal muscles (distal, axial)
  • Smooth muscles (bladder, bow, genitals)
  • Cardiac muscles (pacemaker)

Actions achieved by FES on various limbs:

  1. Upper limb – hand grasping & release, hand orientation.
  2. Lower limb – dorsiflexor, standing, cycling, walking, etc.

Some notable applications of FES:

Bionic glove:

It is a 3-channel stimulator. It preserves wrist movement (often a result of tenodesis). The conductive panels in the glove control the movements of the wrist. It is controlled by flexing and extending the wrist, which opens and closes the hand simultaneously. This can be used independently by the patient without assistance.

Odstock foot drop stimulator:

It is a one-channel stimulator that is used to correct dropped foot. It stimulates the common peroneal nerve or the popliteal fossa. The foot is controlled by heel rise and heel strike movements, which promptly triggers the stimulator to send electrical impulses and stop them. The Odstock 2 Channel Stimulator is used to aid people with a bilateral dropped foot.

Handmaster, developed by Neuromuscular Electrical Stimulation Systems Ltd. (NESS), is used to treat the impairment of the hand and shoulder from stroke and spinal cord injury.

Parameters involved in designing an FES system:

Impairment with the movement of the body occurs when the spinal cord is affected in its connection with the Central Nervous System (CNS). As a result, the motor neurons are affected. Even though sensory nerves are also affected, motor neurons get the brunt of it, unable to control the movement of the limbs. Some of the functions to be considered are:

  • functional types (kinematically open, kinematically closed)
  • functional requirements (state change, constant regulation)
  • functional joint involved (single joint, multiple joint)

The prominent parts in an FES are the stimulator, controller, and sensors.

Some parameters to be controlled by an FES are contact forces, acceleration, trajectories, contact slip reaction forces, joint moment, and joint stiffness.

Control strategies of the FES system:

  • functionally automatically triggered (triggered by movement in another part of the body)
  • functionally automatically regulated (regulated by continuous movement)
  • artificially triggered (triggered by voluntary movement)

Applications of FES:

  • epilepsy control
  • cochlear implant
  • tremor control
  • cardiac pacing
  • control of walking and standing
  • spasticity control
  • bowel emptying
  • wound healing
  • bowel emptying
  • pain suppression, etc.

Click to read about onam

Click to read about assistive systems