Resources

Myo-Testing

Myo-testing identifies the optimal control site through an assessment process where the strongest and most independent signal is identified. 

The selected control site must be in a location which maintains contact with the socket throughout the range of motion of the arm.

In the initial stages it can be useful to close their eyes and imagine motion of the absent hand, while also moving their sound hand.

Typically, the Prosthetist and Therapist work together to select the ideal control sites.


Muscle Site Selection

The muscles used for control are dependent on the users’ level of absence. Typically, the muscles used should closely match the natural anatomy as the user will find this more instinctive rather than using muscles whose natural action is not related to the motion being assigned.

Common myo-sites include…..

Trans-radial

  • Close: wrist flexors
  • Open: wrist extensors

Trans-humeral

  • Close: biceps
  • Open: triceps

If an electronic elbow is also within the prosthesis, these muscles will have a dual action to also control flexing and extending of the elbow.

Partial hand

  • Thenar eminence
  • Hypothenar eminence
  • Dorsal interossei

Alternatively, if these muscles are not present, then muscles on the forearm similar to those used for trans-radial level absence can be utilised.


Preparation for Myo-testing

  1. Lightly clean the skin surface with an isopropyl alcohol swab, to remove any oils that could interfere with testing.
  2. Describe to the user the action which they should adopt by picturing an image in their mind of the movement of their absent limb.
    Trans-radial level
    For closing: wrist flexion, closing fingers, making a fist
    For opening: wrist extension, finger extension, abducting fingers
    Trans-humeral level
    For closing: elbow flexion
    For opening: elbow extension
    It can also be beneficial to mirror the action in their mind by also moving the sound limb. The user should not apply maximum force, it should be a comfortable, repeatable level of effort, which does not excessively tire the muscles.
  3. Ask the user to contract their residual limb and palpate antagonistic muscle groups for two-site control, or a single usable muscle group for single-site control.
  4. Once you have identified the middle of the muscle belly, make a small mark on the area with an indelible pencil or a permanent marker.
  5. Moisten the skin with a wet paper towel to improve electrode conductivity, if required.

Virtu-Limb

The Virtu-Limb is a useful tool for myo-testing and assessment of the user. It connects to the Biosim App and provides a similar user interface as the i-Limb and i-Digits, providing continuity to the user as they progress from evaluation through prosthesis fitting and functional therapy.

The Virtu-Limb can be used with or without an i-Limb hand attached. This allows progression of the user's visual feedback; from muscle signals on the myo-graph to virtual on-screen hand, to training games through to controlling the i-Limb hand in front of them.

Myo-testing Process

Connect Virtu-Limb or i-Limb hand to Biosim App and open the myo-graph screen.

Begin by assessing one muscle group at a time.

Set the electrode gain dial (on the back of the electrode) between 4 and 5.

  1. Place the electrode over one of the marked locations (established in preparation stage). Ensure the electrode runs longitudinally with the muscle belly.
  2. Instruct the client to contract their muscles, aiming to produce a signal between 30% and 80% height on the myograph.
  3. Once a sufficiently strong signal has been established, ensure the optimal site has been identified by moving the electrode proximal to the initial site while instructing the client to continue to contract and relax the muscle. Once the signal strength begins to decrease, mark a line along the proximal edge of the electrode.
  4. Next, move the electrode distally from the initial site and, when the signal begins to decrease, mark the client’s skin along the distal border of the electrode.
  5. Repeat these steps in the radial and ulnar directions from the initial mark creating a border around the proposed electrode site. The ideal electrode site should be in the centre of this outlined area.
  6. If two electrode sites are to be used, repeat 1 through 6 for both sites.

Verifying Isolated Control

Once the optimum electrode placement has been identified, ensure the user can independently isolate each muscle contraction to establish good separation between the open and close signals.

  1. Instruct user to contract the “open” muscle group, relax, and then contract the “close” muscle group. Verify that signal separation can be achieved and the opposing muscle group remains relaxed or, at least does not overtake the active muscle group.
  2. Ensure the electrodes are not picking up interference from other muscle groups or from general motion of remaining joints.

Hints and tips:

Coach user to relax all muscle activity while moving their residual limb in various planes of motion. Review Myo-graph to ensure muscle signals stay at a minimal level (less than 10%).

Ensure user is able to activate opening and closing actions across various planes of motion. This will enable them to pick items up from the floor or out of high cupboards with their definitive prosthesis.

Electrode gain adjustments can be made to assist in providing separation of signals.

Note: The red lines on the Myo-graph relate to the open signals and the blue lines refer to the close signals.

Ideal gain settings are between 3 and 5 on the dial on the back of the electrode. Higher gains settings may indicate too weak of a muscle. It is expected that the gain setting can be reduced as the individual's signals improve in control and strength from the training process.

Optimal graph

Red (open signal) and blue (close signal) remain isolated with one always stronger than the other.

Example of poor control

When the red (open signal) and blue (close signal) mix, the hand will jump back and forth between opening and closing, causing frustration to the user. Clear signals are required, therefore further myo-training should be conducted.

Video Tutorial

Training and Games is accessible when using Biosim on an iPad (not available if using an iPhone or iPod).

  • When connected to a Virtu-Limb: select Training and Games on the home screen
  • When connected to an i-Limb hand: select Training and Games on settings screen

The virtual hand can be rotated around on the screen to the user's preference by dragging it on the screen.

Strength bars show the strength of each muscle contraction while the user works to open and close the virtual hand.

Once consistent opening and closing have been achieved, the user can progress to demonstrating use of muscle triggers to engage the various grips such as the tripod and precision pinch modes.