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Myoelectric Upper Limb Prosthetics 101

About Myoelectric

Myoelectric prostheses

For upper-limb amputees, myoelectric-controlled prostheses offer the ultimate combination of function and natural appearance. Designed to mimic human anatomy and motion, electronic components are the closest alternative to an anatomical hand or arm.

  • Thanks to constantly advancing technology, the latest prosthetic systems feature astonishing capabilities:

  • Elbows that flex and extend with muscle signals so you can reach for a beverage and bring it to your lips

  • Wrists that bend and rotate, allowing you to position objects for convenient viewing and handling

  • Hands that can lug a suitcase or hold an egg without cracking it

  • Thumbs that can change orientation to over multiple hand positions

What is a Myoelectric prosthesis?

“Myoelectric” is the term for electric properties of muscles. A myoelectric-controlled prosthesis is an externally powered artificial limb that you control with the electrical signals generated naturally by your own muscles.

Hand, wrist and elbow myoelectric components are available. With amputations above the elbow, a “hybrid prosthesis” may combine myoelectric-controlled components with body-powered components to control shoulder and/or elbow function. A skin-like glove covers the prosthesis for a natural appearance.

How does it work?

A myoelectric prosthesis uses the existing muscles in your residual limb to control its functions. One or more sensors fabricated into the prosthetic socket receive electrical signals when you intentionally engage specific muscles in your residual limb. Sensors relay information to a controller, which translates the data into commands for the electric motors and moves your joints. If muscle signals cannot be used to control the prosthesis, you may be able to use switches with a rocker or pull-push or touch pad.
Documentation of patient information.

The strength and speed of movements can be controlled by varying your muscle intensity. All of it happens in an instant. For those with damage to nerves or muscles in the residual limb or for people with no arms, muscles in the chest or back can be used to direct movements of a myoelectric prosthesis.

When a myoelectric prosthesis has several joints, each joint might need to be controlled by the same muscle. Sequential control allows positioning of one joint at a time. For example, you might use a muscle contraction to signal the elbow to bend, then use another contraction to signal the hand to close. With AxonBus technology, however, it will soon be possible to control multiple joints simultaneously.
Myoelectric above elbow socket.

The power behind a myoelectric system

Although a myoelectric prosthesis is controlled by natural electricity generated by your own muscles, the system needs a battery to power motors and electronics, and batteries need charging. Some batteries need to be removed from the prosthesis for charging. With others, a cord from a charging unit can be plugged into the prosthesis.

A myoelectric prosthesis is connected to the residual limb via a custom fabricated socket. The user ‘pulls in’ to the unique socket with a donning sock, which creates suction and helps suspend the socket.

Hybrid prostheses, which combine both myoelectric-controlled and body-powered components, is another option. A hybrid prosthesis may include a hand and wrist controlled that are myoelectrically along with an elbow that is controlled by a harness and cables worn on the torso and controlled by shoulder movements.

Compared to a traditional body-powered prosthesis, a myoelectric-controlled arm provides greater comfort, more range of motion, a larger functional area, and a more natural appearance. It may, however, cost and weigh more.

Prosthetic hands and terminal devices

Because upper-limb prostheses can end with a variety of artificial replacements for a hand, prosthetists refer to this broad assortment as “terminal devices”. With myoelectric prostheses, the options generally fall under these two categories:
Celina with children hand 2000 prosthesis playing with friends.

Prosthetic hands

The human hand is one of our most complex body parts. A perfect interplay of nerves, tendons, muscles and bones makes it a remarkably versatile, precision instrument. Recreating as many of its numerous functions as possible is one of the greatest challenges for medical technology.

Myoelectric hands may feature:

  • A variety of sizes for children as young as 18 months and adults
  • A cosmetic glove in a variety of skin colors or a transparent glove to show the hi-tech inner workings
  • A hand for amputation at or below the wrist
  • Ability to carry heavy loads (up to a set limit)
  • Aluminum construction for lighter weight
  • A manually adjustable thumb
  • A positionable thumb, driven by a separate motor, which allows for multiple grip patterns
  • A neutral position for a natural look and action while walking or at rest
  • Powerful grip force and grip speed
  • Grip force and grip speed proportional to the strength of the myoelectric signal
  • A “virtual switch” that requires a slightly stronger myoelectric signal to open the hand after applying maximum gripping force
  • A sensor in the thumb that detects friction caused by objects slipping and automatically tightens the grip to prevent dropping
  • A slip-clutch to quickly release the hand’s grip in an emergency
  • Passive functions to resist weight bearing and other loads
  • Multiple grip options, such as pinching with the thumb and index finger, power grip for handles, open palm for holding a platter, opposition power grip for grasping cylindrical object with a big diameter, three-point grip with thumb, index finger and middle finger, and holding flat objects between two fingers
  • A handshake function that automatically sets the grip force
  • A choice of multiple programs to tailor the speed and build-up of grip force to individual abilities and requirements
  • Separate fingers
  • Individually powered fingers with movable joints
  • Ability to extend the index finger to point or press a button
  • Fingers that spread when the hand opens and come together when the hand closes—so you can hold credit cards or bills between your fingers

Terminal devices: Greifer

A Greifer is a powerful, highly functional terminal device made by Ottobock. Designed to perform gripping applications, it excels at manual labor and other tasks. Features may include:

Control of grip speed and grip force

  • A flexible joint for convenient positioning
  • Adjustable gripping surfaces
  • Easy to swap out with other myo hands
  • Powerful grip force

Myoelectric wrists

Myoelectric-controlled wrists make it easier to grip and control objects close to the body. They also limit compensating movements you may make with your shoulder and the rest of your body—allowing you to move more naturally.

Capabilities may include:

  • Multiple positions of flexion and extension in set increments
  • Quick detachment for changing between terminal devices
  • A flexible mode that mimics natural movement with a spring-loaded mechanism that returns the wrist to a neutral position
  • Progressive resistance in flexible mode
  • A rigid mode that locks the flexion or extension in increments for holding and carrying objects
  • 360-degree rotation, with stops at multiple positions

Prosthetic Elbows

Myoelectric-controlled elbows, which typically include a forearm, flex and extend so you can do more without unnatural compensating movements. Capabilities may include:

  • Bending the elbow in set increments
  • Continuous adjustment—variable bending of the elbow—for more natural movement and exact positioning of the prosthetic hand
  • Locking and unlocking for reliable loading up to a certain weight limit
  • Automatic balance that creates a natural arm swing during walking
  • Patient-adjustable counterbalance that makes the arm feel lighter
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