HOW DO I OBTAIN MOTOR UNIT DATA?

Transcription

1 HOW DO I OBTAIN MOTOR UNIT DATA? In order to obtain motor unit data, the user performs an experiment to collect the data and then uses the demg software algorithm to decompose the data into the constituent MUAPTs. The experiment consists of a subject tracking a trapezoidal force profile presented on a screen while semg signals are collected from a demg sensor placed on the muscle. The force profile is normalized to the subject s maximal voluntary contraction (MVC) level. The demg System provides a simple procedure to design the force profile and perform an experiment to collect semg signals for decomposition. The steps are outlined below. 1) Design the Force Profile a. Establish the characteristics of the force profile. b. Establish the modality of the force feedback. 2) Locate and Apply the demg Sensor a. Determine preferred location for the demg sensor. b. Prepare skin and apply sensor. 3) Collect and Decompose the semg signal a. Establish the MVC level for normalization of force. b. Perform a signal quality check to assure that the collected semg signal can be decomposed. c. Collect the semg signal during a force profile task. d. Decompose the semg signal. Step 1: Design the Force Profile a) Force Profile The demg software (v2.0) is designed to decompose semg signals from isometric contractions having trapezoidal force profiles defined by the parameters in the figure below.

2 Quiescent Region Up Ramp Constant-Force Region Down Ramp Quiescent Region Duration of Contraction Length of semg signal file Recommended Parameters Quiescent Region Up Ramp Constant-Force Region Down Ramp Maximum Duration of force profile At least 3 s duration required 5%MVC/s < slope < 20%MVC/s From >0% to 80% MVC, minimum duration 10 s -5%MVC/s < slope < -20%MVC/s < 45 s Detailed instructions for generating and customizing the force profile may be found in the demg User s Guide. All detected semg signals require a quiescent region of at least 3 s at the beginning and at the end of the signal. The 3 s epoch at the beginning is required for the algorithms to establish the magnitude of the baseline noise and to ascertain the presence of line noise contamination, that is, 50 or 60 Hz. The 3s epoch at the end is needed to ensure that the contraction has ended and no motor units are firing. The recommended slope of the up/down ramp region is designed to provide sufficient information for the decomposition algorithm to progressively identify the recruitment and derecruitment of motor units. The duration of the constant contraction region should be at least 10 seconds long to provide a sufficient number of motor unit firings for proper decomposition.

3 If the length of the semg signal file exceeds 45 s, the decomposition is likely to fail because the PC s virtual memory space is insufficient to store the intermediate results produced by the decomposition program. b) Select RMS or Force Feedback Force feedback is the preferred mode because the signal is relatively smooth. Using force feedback requires the following: 1) a force sensor (load cell) to measure the applied force, and 2) an apparatus to constrain the muscle to produce an isometric contraction. The apparatus should not touch or interfere with the sensor to avoid inducing movement artifacts in the semg signal. This approach is problematic in cases such the muscles of the face, neck, or back, where the generated muscle force cannot be easily measured. Alternatively, the root-mean-squared (RMS) value of the collected semg signal may also be used as a substitute to estimate the level of the contraction. However, due to the inherent variability the RMS signal (even at constant force), it is more difficult for the subject to smoothly track along the trajectory. Using RMS feedback requires only an apparatus to constrain the muscle to produce an isometric contraction. Examples of the experimental setup for using force sensor feedback and semg RMS feedback are show below. Force sensor feedback Secure the joint on which the tested muscle acts in a device so as to immobilize the joint. Place the force sensor against the body segment on which the force acts. The software will display the force output on a computer screen that also presents the force profile to be tracked. See figure for additional clarity. Force Sensor Biceps Force Sensor FDI F 90 F

4 semg Root Mean Squared (RMS) feedback This mode can be used to perform a tracking task when no force sensor is available. The joint on which the tested muscle acts is secured in a device so as to immobilize the joint, which performs a contraction against a fixed resistance. The RMS of the semg signal (either from the demg sensor or a DE-2.1 parallel-bar sensor) may be used for feedback and is displayed to the subject on the computer screen along with the tracking profile. See figure for additional clarity. Fixed Resistance Biceps 90 F

5 Step 2: Locate and Apply the demg Sensor After the subject has been positioned in the apparatus, the location of the demg sensor is determined and the site prepared so that the sensor can be applied to the skin. The following sections describe where to place the sensor on the surface of the muscle and how to prepare the skin before affixing the sensor. a) Preferred Sensor Location Locate the demg sensor in the middle of the muscle surface area. It is well known that the characteristics of the semg signal vary in amplitude and in frequency components along the surface of the muscle (Basmajian and De Luca 1985 ; Roy et al ; De Luca 1997 ). The MUAPs have greater amplitude in the middle of the muscle where the diameter of the muscles fibers is generally the greatest. The amplitude decreases when the sensor is placed near the tendon zones. The innervation zones should be avoided because in these regions the action potential propagates in opposing directions, rather than across the demg sensor s detection pins. It follows that the middle of the muscle is the preferred location. A study by Zaheer et al. (2012) supports this logic and presents data showing that the motor unit decomposition yield is greatest from the recommended location. The study also recommends avoiding locations where large amounts of adipose tissue are present beneath the skin. This problem is likely to be encountered in regions such as the Quadriceps and the Biceps Femoris where there may be excessive (> 1.5 cm) fatty tissue between the surface of the muscle and the sensor. The additional tissue attenuates the signal amplitude.

6 b) Skin Preparation and Sensor Application The demg sensor is specifically designed to work with the demg algorithms. It should not be treated as an ordinary semg signal sensor. The small electrode contacts (pins) of the demg sensor, by their nature, generate more electrical skin/interface noise than sensors with larger electrodes. Their performance is affected by small quantities of debris and oils on the surface of the skin. It is important that the skin where the sensor is to be attached should be clean and moist. Consequently, it is necessary to prepare the skin surface prior to attaching the demg sensor. The quality of the recorded data is only as good as the quality of the skin preparation. For Best Practice adhere the following steps: Shave site to remove hair. To remove skin oils, swab the identified sensor location site twice with a commonly available alcohol prep pad saturated with 70% isopropyl alcohol and allow 30 s to dry. Remove the surface dead layers of the skin by applying hypo-allergenic tape (3M TM Transpore or equivalent) to the site, then peel back to remove contaminants. Repeat 3-5 times. Allow 1-2 minutes for the skin to re-hydrate. Moisten the tip of the 5 contact pins on the bottom of the sensor by dabbing with a wet alcohol prep pad, making sure not to wet the surface in-between the contacts. Next, use a fresh alcohol prep pad to lightly dab the sensor application site so that the skin is also moist. Do not soak the skin, as it will cause the semg signal to be attenuated. Immediately apply the sensor to the skin while it is still moist. Use hypo-allergenic tape (3M TM Transpore or equivalent) to firmly hold the sensor in place. The sensor pins should push into the skin, but not break the skin. If the skin is Dry Without using the alcohol pads, perform the tape peels as described in the normal skin preparation. Gently wash the sensor application site with a mild solution of soap and water and wipe dry. Use a fresh alcohol prep pad to pre-moisten the sensor pins, also dab the sensor application site with the pad so that the skin is moist, and immediately apply the sensor to the skin. Re-application of demg Sensor When re-applying the demg sensor to address baseline noise or line interference, use a fresh alcohol prep pad to re-moisten the sensor pins. Next, dab the sensor application

7 site with the pad so that the skin is moist, and then immediately re-apply the sensor while the skin is still moist. Over-wetting will greatly attenuate the semg signal. Ensure that the bottom surface of the sensor is dry before re-moistening the pins. Excessive, repeated rubbing with alcohol wipes will overly dry the skin resulting in a noisy electrical interface. The Reference Electrode Use a large-area (~20cm 2 ) reference electrode such as Dermatrode ( or similar reference electrode. Before application clean the site with an alcohol wipe. Place reference electrode on top of a bony region. CAUTION Use care when routing the demg sensor, input module, and reference electrode cables to avoid interference with power line noise. Use of devices containing RF transmitters (including cell phones, and laptops with wireless internet) may cause interference with the sensitive signal recording equipment. Be sure to remove these sources from the recording environment. Line interference from nearby wires connected to power sources may interfere with the proper working of the system. Step 3: Collect and Decompose the Data After the tracking profile has been defined and the sensor has been correctly positioned on the muscle, semg signals can be acquired. The demg System provides a simple graphical interface to help the operator perform the sequence of tasks required for data acquisition. A series of icons representing each type of task are displayed on the screen in the order in which they are to be performed. The operator can click on an icon to execute that task, or the program can automatically advance to the next task when the current task is completed. A screenshot showing the tasks required for acquiring the semg signal in a typical experiment using force feedback is shown below:

8 First, the MVC task is performed to normalize the force feedback. Then the Signal Quality Check task is performed to ensure that the semg signals are suitable for decomposition. Finally, the Profile Tracking task is performed to acquire the semg and force signals. Clicking on the Analyze task icon transfers the resultant data to a workspace for viewing and decomposing the signal. Detailed instructions for creating each of these tasks may be found in the demg User s Guide. Each of the tasks performed during data acquisition is described in the following sections: a) Establish MVC The software automatically calculates the MVC value from either the force or the semg RMS signal, depending on which feedback modality was chosen. The MVC is determined by instructing the subject to contract as forcefully as possible for about 3 s. We suggest that at least three attempts be performed, each spaced at least 3 min apart to reduce the effect of muscle fatigue. The program automatically selects the highest reading of the 3 attempts as the MVC value, and uses this value to normalize the tracking profile. The following plot shows an example of a successful attempt at collecting the MVC value. MVC trial 1 trial 2 trial 3 Force or RMS 3 minute rest 3 minute rest

9 b) Verify Signal Quality Note: This is a PATENT PENDING procedure. It consists of Proprietary Material. The signal to noise ratio (SNR) of the semg signal is the critical factor that determines the ability of the decomposition algorithm to decompose the collected signal as well as the accuracy of the decomposition. The signal amplitude is influenced by: the level of muscle contraction, the location of the sensor on the surface area of the muscle, and the amount of fatty tissue between the muscle and the skin. The factors that influence SNR are: 1) the amplitude of the semg signal, 2) the level of baseline noise, 3) the level of 50/60 Hz line interference, and 4) signal clipping. The signal quality factors are automatically measured by the program and are provided to the user as a Pass/Fail display to indicate if all signal conditions for a successful decomposition have been met. If not, the contraction must be repeated. The program provides suggested remedies for correcting failure conditions which should be performed before repeating the contraction. A description of each measurement and acceptable values are provided below. Signal to Noise Ratio (SNR) The SNR value must be > 2 in order to perform decomposition. Refer to Preferred sensor Location link for locating the sensor for best SNR. The SNR is defined by the following equation: semg signal amplitude (RMS) SNR = Baseline signal amplitude (RMS) The following data plots acquired from one of the sensor channels show the baseline noise region and the signal region where the muscle is contracting. The upper plot is the amplitude of the semg signal and the lower plot is its RMS value.

10 +500 µv µvrms 0 0 Baseline signal amplitude (RMS) time (s) semg signal amplitude (RMS) 20 Baseline Noise Must be < 4.8 µv RMS The baseline noise level reflects the stability of the skin-sensor interface. The baseline noise component of the signal should be minimized to ensure the detection of low-threshold motor units. With proper skin preparation the typical baseline noise value is 3 µv RMS. The following is a plot of the baseline noise acquired from one of the sensor channels with appropriate skin preparation. +10 µv 0-10 Baseline Noise.05s Line Interference Must be < 1.0 (normalized magnitude of Power Spectrum at 50/60 Hz) Interference from power lines and other sources may adversely affect motor unit yield and accuracy. There may be excessive line interference if the demg sensor and reference electrode

11 are not properly applied. Use care when routing the sensor, input module, and reference electrode cables to avoid interference with power line noise. The following is a plot of the baseline noise acquired from one of the sensor channels showing an additional 60Hz line interference component with an unacceptable level of contamination. In this situation the demg sensor should be re-applied. Baseline Noise + 60 Hz line interference +10 µv s Signal Clipping (saturation) No signal clipping should be present in the acquired data. demg sensor detachment, reference electrode detachment, or excessive signal amplitude, may cause saturation of the amplifier output commonly referred to as Clipping. If this occurs, check the demg sensor, reference electrode, and cable. Re-position the demg sensor to reduce signal level, or reduce the peak contraction level to a lower percentage of the MVC level. The following is a plot of the semg signal acquired from one of the sensor channels showing regions of the signal where the excessive signal amplitude is clipped as indicated by the arrows. Signal Saturation Clipping +5 mv s

12 Performing a Signal Quality Check The Signal Quality Check is automatically performed and the Pass/Fail results are displayed at the completion of every tracking task to ensure that the acquired semg signal is suitable for decomposition. The Signal Quality Check may also be configured as a separate task, typically performed at the outset of the experiment in order to evaluate if the location site and skin interface of the applied sensor are suitable for decomposition analysis. The following figure shows a sample display of a successful Signal Quality Check result, expanded to see its details. The values indicate a stable, low noise, skin preparation with suitably high SNR value. The operator is instructed that the signal check is successful and that it is OK to proceed.

13 A description and usages of the Signal Quality Check features may be found in the demg User s Guide. c) Force Profile Task The tracking profile is presented to the subject on a computer screen along with the force feedback signal. The subject is instructed to contract the muscle so that their force feedback signal matches the tracking profile, while the operator monitors the display to ensure that the subject s force follows the force profile. It is recommended that the subject practice the tracking task several times to become familiar with producing a smooth force that closely follows the force profile. The screen shot below shows a 30% MVC experiment using force feedback with the target force profile in red and the subject s force trace in blue. The subject was able to smoothly follow the force profile with only small deviations from the target. 30 Force (% MVC) Time (s) At the end of the task, the demg acquisition software automatically performs an additional signal quality check on the four channels of semg data to ensure that they meet the criteria for decomposition analysis, and suggest remedies to any problems encountered.

14 d) Analysis The analyze function transfers the collected signals into the EMGworks Analysis software where the user creates a workspace to view and decompose the collected data file into its constituent MUAPs). Viewing the collected data The four semg signals and force signal collected from the tracking task can be previewed using the EMGworks analysis software plotting functions. An example of the force trace and four channel semg signals recorded from the tracking task is shown in the figure below. Force (N) 10 0 Ch 1 semg signal Amplitude (+/- 2mV) Ch 2 Ch 3 Ch semg signal Amplitude (+/- 1mV) Ch 1 Ch 2 Ch 3 Ch Time (s) 5.55

15 Amplitude (mv) The yellow highlighted regions in expanded segment of data show the occasional occurrences of individual MUAPs whose wave shapes are uncontaminated by superposition. These MUAP s can be identified by simple visual observation of their different wave shape signatures in the four channels. However, the remaining areas of the signal are typically more complex, consisting of numerous superpositions of MUAPs. The demg sensor is designed to provide this more complex signal that is representative of a large population of MUAPs, unlike the design of indwelling needle sensors that only capture a limited number of MUAPs. The demg sensor also retains the ability to discriminate between numerous MUAP wave shapes that would be lost when using sensors with electrodes having much larger surface area and 1 to 2 cm inter-electrode spacing. A comparison with between the demg sensor signal and the iemg sensor signal is provided in the following figure showing their difference in complexity. Surface EMG Signal Indwelling EMG Signal ms ms Decomposing the collected data The demg algorithms simultaneously analyze the four semg signal detected by the sensor. All four channels are needed for the decomposition algorithms to work effectively. Each MUAP has a different electrical signature in each of the four channels; this property assists the decomposition algorithms in identifying the contributory MUAPs by providing four representations of the same physiological event. The automatic decomposition algorithms use this additional information to resolve the complex signal superpositions of MUAPTs, and determine the firing times for the individual motor units. Decomposition of the data is initiated from EMGworks Analysis, and the results of the decomposition process are then made available in the workspace for further exploration.