Surface electromyographic biofeedback greatly simplifies the identification and training of surface EMG signals in patients with brachial plexus injuries whose muscle activity in the flail arm is extremely faint. With the use of surface EMG biofeedback, various motor commands and electrode positions may be tested and trained repeatedly as the technique is not invasive, easily applicable, and inexpensive. To set up a system for surface EMG biofeedback, place the device on a table in a quiet room and position the patient in front of the computer screen.
Ask the patient to think of hand movements while simultaneously attempting to contract the muscles intended to perform a specific action even if this will not result in real movement of the functionless hand, while palpating the forearm for muscle contraction. Place a surface EMG electrode on the exact skin position where muscle contraction can be felt and have the patient repeat the same motor command as just attempted to elicit contraction of the muscle. Observe the EMG signal on the computer screen to see if the amplitude consistently increases when the patient attempts to contract the muscle intended to perform a specific action.
If the amplitude is less than two to three times the background noise or the signal is inconsistent, try other motor commands with the same electrode position to see if higher amplitudes can be obtained. Then move the electrode to a new location on the forearm to assess the muscle contraction for a different gesture. Monitoring the signal amplitude on the computer screen while the patient thinks of making the gesture.
If no muscle activity is found in the forearm, repeat the procedure on the upper arm and shoulder girdle. When two or more EMG signals have been identified, encourage the patient to alternately activate the signals. To reliably drive a prosthesis, the independent EMG signals need to be controlled without interference.
Adjust the voltage gain of each signal independently to achieve a similar amplitude threshold for all of the signals during the training to make the signal separation and comprehension easier for the patient. Repeat and explain to the patient the mechanics of a prosthetic hand, that slight muscle contraction should be preferred over muscle strength to avoid signal coactivation. Observe the EMG signals on the computer screen and explain to the patient that the two signals are coactivated when attempting a specific movement.
Instruct the patient that the two signals should not be coactivated during the attempt of one specific action as each EMG signal is linked to a specific prosthetic action and that coactivated signals will therefore not result in the action desired by the patient. Instruct the patient to try slightly different movements and to observe which precise movement patterns are best in regard to signal separation. When appropriate signals have been identified, encourage the patient to practice performing these movements no more than 30 minutes per training session.
Instruct the patient that a perfect signal separation is unlikely at the beginning of the training but will improve with a high number of repetitions and that the signal separation might be easier in the beginning when performing slight contractions. As the signal consistency improves, instruct the patient to generate a higher signal amplitude to further strengthen the muscle and its signal. When a consistent EMG signal separation and solid control has been achieved, install a tabletop prosthesis connected to the corresponding EMG software and place the electrodes on the patient's arm to directly translate the EMG activity into mechanical prosthetic function.
Inform the patient that the myoelectric prostheses with direct control use the input of one electrode to control one prosthetic movement at a time. When a device with the proportional control of movement speed is used, instruct the patient about the correlation between the signal appearance on the computer screen and the speed and strength of the prosthetic movement. Then have the patient practice the co-contraction, allowing the patient to observe the EMG signals on the computer screen and explain that it is important that both signals simultaneously reach the peak.
If the prosthetic device does not move, the patient is performing the co-contraction correctly as both signals simultaneously reach the peak. When the patient has mastered the control of the tabletop prosthesis, introduce the concept of a hybrid prosthetic fitting that is individually tailored to the patient and attached above or below the impaired limb. The hybrid prosthetic fitting can then be used for additional training during rehabilitation before the elective amputation.
Before undertaking the procedure, ask the patient if they have any unresolved questions regarding the planned amputation and clearly communicate that it is possible at any time prior to amputation to revoke this decision which will otherwise result in an irreversible and life altering surgery. Next, perform a standardized assessment of the upper limb function using the functionless hand while videotaping the results. After four to six weeks of postoperative wound healing, determine the best hot spots for electric placement and have the patient practice the EMG signal as demonstrated Have an orthopedic technician designed the final prosthetic socket using the previously defined EMG electrode positions.
When the prosthesis is ready, have the patient practice simple prosthetic movements with the way that the prosthetic device being supported. Move on to simple prosthetic movements in different arm positions such as the elbow being extended flexed alternately and continue with simple grasping tasks such as picking up little boxes and manipulating small objects. Finally, have the patient practice performing activities of daily living starting with rather simple tasks and slowly adding complexity in tasks that the patient considers relevant for their specific life situation.
Three months after the prosthetic fitting, repeat the standardized assessment of the upper limb function using the prosthetic hand and recording video of the results. In this study, the demonstrated rehabilitation protocol using surface EMG biofeedback was successfully implemented in six patients with severe brachial plexus injuries, including multiple nerve root avulsions. The number of therapy sessions and the detailed results for each patient can be observed in the table.
In this assessment, the electrode on the volar aspect of the forearm sensed the EMG activity when the patient attempted to close their hand as indicated by the red wave. The signal separation in this patient is satisfying as the signal from the second electrode placed on the dorsal aspect of the forearm did not reach the threshold as indicated by the blue wave. Then when the patient thought of opening their hand, the amplitude of the second signal exceeded the threshold as indicated by the blue wave while the signal from the first electrode remained almost inactive as indicated by the red wave.
The neural input to muscles in the upper extremity of patients with severe brachial plexus injury is very sparse. So, various motor commands and electrode precisions need to be tested. To further enhance patient motivation and to increase engagement with training during the long-lasting rehabilitation process, surface EMG biofeedback can be embedded in game-based interventions.
