EMG-Reflex Device BIOE 385: BIOINSTRUMENTATION LABORATORY Erik Hansen, Ronal Infante
Table of Contents Introduction... 3 Summary... 3 EMG-Reflex Device... 4 Instructions... 6 Hardware Set-up... 6 Test Subject Preparation... 10 Recording a Measurement... 12 Safety Information... 14 Troubleshooting... 15 Device Limitations... 16 Technical Specifications... 17 DB9 Male Connector... 18 Hammer Inverting Amplifier... 20 Electrical Isolation Circuits... 21 EMG First Stage Instrumentation Amplifier... 22 EMG Passive Band-pass Filter... 23 EMG Second Stage Inverting Amplifier... 24 LabVIEW Code... 25 Front Panel... 25 Block Diagram... 25 Design Challenges... 27 References... 28 Appendix... 29 Troubleshooting Figures... 29 Bode Plot... 30 Front Panel Figures... 31 Block Diagram Figures... 34
1. Introduction Summary The human body has three types of muscles: cardiac muscle, smooth muscle, and skeletal muscle. While the first two types are found mostly in organs, skeletal muscles are attached to bone and facilitate movement. Fig. 1. A diagram depicting a reflex arc and sensory and motor neurons [1]. Afferent sensory neurons and efferent motor neurons are central to skeletal muscle function throughout the body. Sensory neurons transmit electrical impulses in the form of action potentials, or temporary membrane potentials along the length of cells, from a receptor (such as the skin) to the spinal cord. Motor neurons receive these impulses from the spinal cord and prompt skeletal muscle contraction (Fig. 1). It is not always required for the impulses to reach the brain. In areas of high sensitivity or when strong stimuli are detected [2], impulses travel straight from the sensory neurons to the motor neurons through an association neuron (also called an interneuron), forming what is known as a reflex arc, and a reflex is produced. A motor unit is comprised of a single motor neuron and all of the muscle fibers it controls [3]. When a motor neuron delivers an impulse, the muscle fibers in the motor unit respond by generating their own electrical signals that lead to contraction. These electrical signals, in the form of weak electrical potentials or voltages, may be detected on the overlying skin using surface electrodes. The process of detecting and amplifying skin voltages of underlying muscle contraction is called electromyography and the recordings are called electromyograms (EMGs). Group 127 Page 3 of 39
EMG-Reflex Device Our EMG-Reflex device records patient reflex time by measuring the difference in time between a hammer tap to the patellar tendon and the resulting knee-jerk reflex. When the hammer taps the patellar tendon, a resulting electrical signal is sent from the hammer to our device and the time of hammer impact is recorded. Upon tapping the patellar tendon, muscle spindles within the quadriceps act as receptors and send an electrical impulse to the spinal cord along afferent sensory neurons. Immediately, the efferent neurons respond by sending an electrical impulse back to the quadriceps, causing the muscle to contract and thus extend the lower leg. This reflexive contraction is commonly referred as the knee-jerk reflex (Fig. 2). Fig. 2. A visual representation of the knee-jerk reflex [4]. Then, two electrodes placed along the length of the quadriceps record the voltage produced by the returning efferent impulses along the muscle. The voltage is then amplified, filtered, and sampled past a threshold value. The resulting EMG time is recorded and the difference between the two times is calculated, recorded, and displayed. Group 127 Page 4 of 39
Clinical studies indicate that the average knee-jerk reflex time is about 21 milliseconds [5]. Significant deviation from this average could indicate a damaged central nervous system. Therefore, implementation of our device allows for detection of neurodegenerative diseases or neuromuscular disorders, such as Amyotrophic Lateral Sclerosis or Becker Muscular Dystrophy. Our device is built using a reflex hammer to produce the initial signal, alligator-banana cables and electrodes to detect EMGs, and simple electrical components, such as resistors, capacitors, diodes, and operational and instrumentation amplifiers, to amplify and filter the resulting signal. Therefore, it is quite cost-effective to manufacture. A LabVIEW virtual instrument (VI) provides an intuitive user interface and further signal processing to reduce noise. In addition to being costeffective, our device takes patient safety in consideration by including electrical isolation circuitry that prevents microshock. Because skin surface electromyography is non-invasive, it proves to be a painless alternative to needle-based approaches. By following the simple instructions detailed below, any user will be able to successfully set up and operate our device. The implementation of our device would be cost-effective, intuitive to use, and would improve patient outcomes. Group 127 Page 5 of 39
2. Instructions Hardware Set-up 1. Turn on the National Instruments Elvis (NI ELVIS). The system power button is found on the back (Fig. 3). Also make sure the power cable of the NI ELVIS unit is plugged in securely. Fig. 3. The back of the NI ELVIS unit showing the system power button (left) and the power cable (right). Group 127 Page 6 of 39
2. Turn on the prototype board power. The prototype board power button is found on the front (Fig. 4). The lights for the system power and the prototype board power should be shining. Fig. 4. The front of the NI ELVIS unit showing the prototype board power button and the two lights. Group 127 Page 7 of 39
3. The NI ELVIS unit s connector is found on the top of the unit towards the back (Fig. 5). Slide the prototype circuit board into the NI ELVIS unit s connector. The prototype board should snap tightly into the connector. Fig. 5. Placing the prototype circuit board into the NI ELVIS unit s connector. 4. Our device is built to function with Hammer G. Find Hammer G (Fig. 6). Fig. 6. Hammer G. Group 127 Page 8 of 39
5. Connect Hammer G s male pin connector to the female pin connector wired to the circuit board (Fig. 7). Fig. 7. Connecting Hammer G s male pin connector to its corresponding female pin receptor. 6. Collect two alligator-banana cables. Plug them into the Banana A and Banana B ports on the top right part of the circuit board (Fig. 8). Fig. 8. The Banana A and Banana B ports with cables plugged in. Group 127 Page 9 of 39
Test Subject Preparation 1. Collect two electrodes. Since electrical activity of muscle is to be measured, place them about 2 inches apart along the length of the quadriceps with significant muscle mass (Fig. 9). Attach the alligator ends of the Banana A and B cables to the electrodes. Since the device measures a potential difference along the length of the muscle, it does not matter which electrode you connect the banana cables to. Fig. 9. The two electrodes in position along the length of the quadriceps with banana cables attached. Group 127 Page 10 of 39
2. The subject should sit comfortably in a relaxed position. Any voluntary tension in the muscle group will produce inaccurate results. Raising the chair and letting the patient s legs hang off of the edge of their chair is a good way to prevent voluntary tension (Fig, 10). Fig. 10. Hanging legs off of your chair can help prevent voluntary tension. Group 127 Page 11 of 39
Recording a Measurement 1. Turn on the computer. Locate and open the VI file (Fig. 11). LabView should initialize. Run the VI (Fig. 12). Fig. 11. Selecting the VI file. Fig. 12. The run button (left) after it has been pressed to run the VI file. The stop button (right) stops the VI file from running. 2. On the VI s Record EMG-Reflex tab, select the Calibrate Device button and wait for the device to calibrate (Fig. 26). An error will appear if there is too much noise. If so, try recalibrating. To reduce noise, try braiding the alligator-banana cables. Group 127 Page 12 of 39
3. Tap the patellar tendon with the hammer (Fig. 14). The patient should be fully relaxed and not tapping their own tendon. Otherwise, voluntary tension might occur and reflex time will be inaccurate. You should observe a knee-jerk reflex. Fig. 14. The process of tapping the patellar tendon with Hammer G. Electrodes and alligator-banana cables are securely attached. A second individual should be tapping the patient s tendon. 4. The knee-jerk reflex must occur for accurate reflex time measurement. If the knee-jerk reflex does not occur, try tapping the patellar tendon with more force. Alternatively, engage in the Jendrassik maneuver (Fig. 23). 5. If the EMG signal passes the threshold value, reflex time is calculated and displayed. If the knee-jerk reflex occurred, press the Add Measurement to Table button (Fig. 26). This places the reflex measurement in the Table of Results. 6. To remove an erroneous measurement, enter the corresponding row number into the indicator. Press the Remove Measurement form Table button (Fig 26). Group 127 Page 13 of 39
7. Repeat steps 2 and 3 to acquire as much data samples as required. The Save Table button exports the table to an Excel file (Fig. 26). Safety Information When operating the device, please consider the following safety procedures: Our device features two electrical isolation circuits, one for each incoming lead, as barriers to prevent high currents from flowing back into the body from the device, causing macroshock. If high currents attempt to flow through the diodes, they will burn out and serve as circuit breakers, saving you from dangerous shock. If these were to burn out, immediately turn off all power to the device. If you smell burning or experience any form of shock when handling the device, a component might have malfunctioned. Turn off all of power and seek technical help. Refrain from hitting the hammer with excessive force as it may break. Do not tamper with the prototype board s circuitry as it may expose you to shock hazards, produce inaccurate results, or compromise the isolation circuits. Do not consume food or beverages near the device to prevent spills. Wet environments lead to equipment failure. Always handle the device with dry hands. Moisture reduces the resistivity of the skin and increases the risk of microshock. Group 127 Page 14 of 39
Troubleshooting If there seems to be an issue with the device, consider the following common solutions: Make sure the ELVIS system and the prototyping board are both powered on. Lights on the front of the unit will indicate if both are powered (Fig. 4). Double check to make sure the prototype circuit board is securely connected to the NI ELVIS unit (Fig. 5). Make sure all three green lights on the bottom left of the prototype circuit board are turned on (Fig. 24). If not, the NI ELVIS unit has blown a fuse and it must be replaced in order to proceed. Make sure you are using Hammer G (Fig. 6). Not all Hammers are guaranteed to work with our device. Make sure the male and female pin connector/receptor pair for Hammer G is connected securely (Fig. 7). Examine the prototype circuit board to confirm that the female pin receptor has not unwired itself from the board. Confirm that the electrodes are in contact with the quadriceps muscle and that the alligator-banana cables are securely connected to the electrodes (Fig. 9). Use tape to secure the electrodes if necessary. If you are acquiring too much noise, try the following tricks: o Performing the Jendrassik maneuver (Fig. 23) by cupping and pulling on your hands will enhance the patellar reflex by inhibiting descending brainstem inputs that would inhibit reflex arc interneurons [6]. o Additionally, braiding the alligator-banana cables reduces noise by canceling out the interference produced by the magnetic fields created by running current through wires. Group 127 Page 15 of 39
3. Device Limitations While our device measures consistent reflex times that agree with clinically determined values, it is still limited and has the potential for improvement. Our design only implements 2 electrodes and uses the machine ground as reference. Machine ground is not ideal when dealing with biological noise as it is out of context. Therefore, it is possible that our device is limited by additional noise that can be eliminated via adding a third electrode placed on the patellar bone. Future versions of our device would benefit from the addition of a third electrode. This addition has the potential to remove unwanted noise by providing a biological ground that we may reference to. Since it experiences similar biological noise to our other electrodes, the measured voltage will be free from this noise due to the relative measurement. Additionally, our device is only optimized to detect EMG in the quadriceps in response to patellar tendon stimulation. For a better diagnosis of neurodegenerative diseases or neuromuscular disorders, a whole body assessment of reflex times could be employed. Currently, our device is not that sophisticated. Future versions of our device would benefit from the addition of a feature that allowed you to choose the reflex you want to measure. Perhaps the user could be given a list of reflexes to choose from. Upon choosing, optimized filtration and amplification settings could be applied to the VI for accurate and consistent measurements of a range of reflex types. Group 127 Page 16 of 39
4. Technical Specifications The following block diagram describes the general mechanics of our device: Fig. 15. Block diagram of our device. Ultimately, two channels of data will feed into LabVIEW for analysis and recording, one from the reflex hammer and another from the electrodes. Group 127 Page 17 of 39
DB9 Male Connector The reflex hammer is connected to our device via a DB9 Male Connector. The male pin connector has 9 total pins, only five of which are used by our device. The pin connector was soldered onto colored wires. Each pin number its corresponding data and wire color are listed in the table in Figure 17. The input power supplies are wired to pins 6 and 9. The +5V is wired from the DC supply of the prototype board. The -5V supply is provided by the LabVIEW code. Since the hammer is not in contact with the body for a prolonged period, it requires neither electrical isolation circuits, nor filters to remove biological and ambient noise. Pins 2 and 4 are the outputs of the hammer and the inputs to the OP07 hammer amplifier. To reference the output signal of Hammer G, pin 4 is grounded. Pin 3 is wired to ground and the other four pins are unused. We opted not to use shielding because we are inexperienced solderers. While shielding can provide more noise reduction, doing it incorrectly would have introduced much more noise. After getting a working prototype we decided that noise levels were low enough to justify not adding shielding. Group 127 Page 18 of 39
Fig. 16. Pin diagram of the male DB9 connector. Pin Number Data Carried Wire Color 1 Shield Not Used 2 Vin+ Green 3 Ground White 4 Vin- (ground) Orange 5 Shield Not Used 6 +5V (ref) Red 7 None Not Used 8 None Not Used 9-5V (ref) (DAQ0) Blue Fig. 17. A table listing the DB9 male pin connector s pins and their function in circuitry. Group 127 Page 19 of 39
Hammer Inverting Amplifier The signal produced by the hammer undergoes amplification (Fig. 18). The hammer voltage signal comes from the pins 2 and 4 of the male pin connector. The amplifier is composed of an OP07 differential amplifier in the inverting configuration with 110Ω and 220kΩ resistors and a gain of -2000. Gain = Rf/Rin = 220000/110 = 2000. The amplifier is powered by +15V and -15V DC supplies on the prototype board. The output of the amplifier is then connected to LabVIEW through an ACH port on the prototype board. Fig. 18. Reflex hammer OP07 amplifier configuration. Group 127 Page 20 of 39
Electrical Isolation Circuits The two electrical isolation circuits, one for each incoming lead, protect the test subject from macroshock by acting as barriers to prevent high currents from flowing back into the body from the device (Fig. 19). First, the inputs from the electrodes enter the prototype board through both banana cables. Both electrical isolation circuits are comprised of 10kΩ resistors and 2 (MFG# 1N4148) diodes. The 10kΩ resistor forces the current down to 60μA, well below the current safety threshold. If the device sends voltages over 0.6V and high currents attempt to flow through the diodes, they will burn out and serve as low impedance circuit breakers, directing harmful current to ground and saving you from dangerous shock. Fig. 19. One of the two electrical isolation circuits. Both are identical. Group 127 Page 21 of 39
EMG First Stage Instrumentation Amplifier Both isolation circuits then feed into an AD620 instrumentation amplifier (Fig. 20). The amplifier is powered with +15V and -15V DC supplies from the prototype board. The difference in voltage between the two terminals is amplified when the signal from the Banana B is arbitrarily lead into the positive terminal and the signal from Banana A is lead into the negative terminal. Using a 1kΩ resistor, the gain of this amplifier is 495. We used the more sophisticated instrumentation amplifier for the first stage amplification of the EMG signal because it amplified a foundational signal with less noise than the OP07 operational amplifier. G = 1 + (49400/Rg) = 1 + (49400/100) = 495. Fig. 20. The AD620 Instrumentation Amplifier Group 127 Page 22 of 39
EMG Passive Band-pass Filter After the first stage amplifier, the EMG signal is filtered to remove both low and frequency components (Fig. 21). According to De Luca, Carlo J., et al., surface EMG signals should be filtered between 20 Hz and 450 Hz [7]. Using two 33μF capacitors and 10Ω and 220Ω resistors, the approximate cutoff frequencies for our passive band-pass filter were 22Hz and 482Hz. Frequencies above and below these frequencies were attenuated, as shown by bode plot analysis (Fig. 25). Cutoff frequency = fc = 1/(2 pi R C) Fig. 21. The passive band-pass filter with cutoff frequencies of approximately 22 and 482Hz. Group 127 Page 23 of 39
EMG Second Stage Inverting Amplifier After undergoing filtration, the EMG signal is attenuated. A second round of amplification using an OP07 operational amplifier and 100Ω and 3kΩ resistors provides -30 gain to the EMG signal (Fig. 22). Gain = Rf/Rin = 3000/110 = 30. Fig. 22. The second OP07 Inverting Operational Amplifier. Group 127 Page 24 of 39
5. LabVIEW Code Front Panel The front panel of our VI features 3 tabs each with very intuitive navigation: a welcome tab, the reflex time recording tab, and a troubleshooting tab (Figs. 26-28). There are buttons to move between tabs and to stop the VI. On the Record EMG-Reflex tab, calibration is visually confirmed by a progress bar as well as a bright button. Both the EMG signal and Hammer G s signal are visualized on a large graph. Numerical indicators show impact time for each signal type in exact real time. The reflex time is calculated from these variables and is displayed with units. Adding values to the table of results is very intuitive. A save table button allows the user to export their saved data into an Excel file. On the Advanced tab, various troubleshooting graphs and indicators are available to view. There are graphs showing unfiltered signals and the filtering intervals in both time and frequency domains. The calibrated threshold is displayed to the user as well as a confirmation of the DC input to the Hammer for pin 9. Block Diagram The block diagram codes for all of the code block functions of the VI: Outside the main while loop, many values are reset in case the VI is stopped, the Table of Results is initialized, and within its own independent while loop, -5V is sent to pin 9 of the Hammer (Fig. 29). Just inside the while loop, before the large case structure that holds much of the tab-dependent code, the stop conditions are coded, taking in booleans from the Stop VI buttons on each front panel tab (Fig. 30). The EMG signal and the Hammer signal are read and converted from analog data to digital data through the use of the DAQ function block. The signals are separated and the EMG signal runs through a SubVI titled Notch Filters that holds the code for two 60 Hz notch filters, the original noise and one harmonic at 120 Hz (Fig. 31). Both dynamic data types are converted into waveform data types. The time data is separated and sent to the tab case structure. Gains are given to each signal based on experimental values. Group 127 Page 25 of 39
When sent into the Record EMG-Reflex tab, the data is stored into an array and visualized on the large graph along with the threshold if it s been stored (Fig. 32). The threshold is calculated when the Calibrate button is pressed (Fig. 33). Before calibration much of the front panel is disabled. During calibration the progress bar moves and resting noise data is stored into an array. After 10,000 values are stored, a maximum value is determined and 1 is added to establish a solid EMG threshold. If the threshold is greater than 5V (indicating max noise at 4V), then calibration will fail due to large amounts of noise. A message is displayed when calibration has either successfully been completed or when it has failed. If it fails, the user is prompted recalibrate and much of the front panel remains disabled. The EMG signal is compared to this new threshold value and the Hammer signal is compared to a threshold value of 1V (Fig. 34). The indices where the amplitude data crosses each threshold is calculated and if the default value of 99 is not returned, these indices are used to index the time data that was previously separated. If the indexed Hammer time is not larger than the indexed EMG time (which would result in a negative time difference), then the respective times are stored in numeric indicators. Once stored, reflex time is immediately calculated by taking the difference between the two times. Since reflex time is displayed in milliseconds, the difference is multiplied by 1000. The remaining code on the block diagram produces the aesthetics of the front panel (Fig. 35). These include concatenating new values onto the Table of Results when the Add Measurement to Table button is pressed, the removal of indices from the Table of Results when the Remove Measurement from Table button is pressed, and array building for Excel report generation. Code on the remaining case structure tabs, such as the Advanced tab, provides for the storage of data into numerical indicators and for the function of buttons (Fig. 36). Group 127 Page 26 of 39
6. Design Challenges The most important challenge to consider when dealing with living systems is electrical safety. Our device requires the use of 2 electrodes placed against the human body, contact points for macroshock. The electrical isolation circuits protect the patient from this threat. Blocking out biological and ambient noise was the next big design challenge. The body has various electrical signaling pathways that produce undesirable electrical noise. Therefore, filtration is required to isolate the desired signal. We decided to address this issue with the passive band-pass filter displayed in the Technical Specifications that features ideal cutoff frequencies supported by literature. Interactions with non-living systems, such as the noise from 60 Hz household electric power supply and its harmonics, can be also be eliminated by filtering. We addressed these issues with two high-ordered notch filters with cutoff frequencies at 60 Hz and the 120 Hz harmonic. This was enough to reduce noise. Differences in test subject are another issue that must be taken into account. Humans have many characteristics that make us physiologically unique. These characteristics make sampling data challenging because fixed parameters don t work for everyone. An ideal device must adapt to these differences and produce accurate and consistent results for every test subject. We addressed this issue by adding a calibration feature to our device. For 10 seconds, our device stores resting EMG data. Then, the data is analyzed for a max resting noise value and the EMG threshold value is adjusted. This prevents noise from triggering the recording and successfully allows us to personalize the device to the individual s noise level. Whether or not to allow the device to adapt to different locations in the body is another design challenge that we addressed. Different parts of the body have varying levels of biological noise and muscle mass. Our device is optimized for use on the patellar tendon, taking measurements from the activated muscle fibers in the quadriceps muscle. If we were to allow our device to be compatible with multiple sites in the body, we would need to develop a method of optimizing the filtering and amplification of EMG signals for a dynamic location. Since this proves too challenging, we did not add this feature. Group 127 Page 27 of 39
7. References [1] TheFreeDictionary.com. "Achilles Reflex." Farlex, n.d. <http://medical-dictionary.thefreedictionary.com/achilles+reflex>. [2] The Columbia Electronic Encyclopedia, 6th ed. (Columbia University Press, New York, 2000). [3] Biopac Systems Inc. "Biopac Student Lab Manual." (1998). [4] "Knee Jerk Reflex Pathway." Edoctoronline.com. EDoctorOnline.com, n.d. <http://www.edoctoronline.com/medical-atlas.asp?c=4&id=21800>. [5] Frijns, C. J. M., et al. "Normal values of patellar and ankle tendon reflex latencies." Clinical neurology and neurosurgery 99.1 (1997): 31-36. <http://www.sciencedirect.com/science/article/pii/s0303846796005938>. [6] Biopac Systems Inc. BSL PRO Lesson H28: Reflex Response (Patellar Tendon) Using BIOPAC Refex Hammer Transducer SS36L. (2006). [7] Luca, Carlo J. De, Gilmore, L. Donald, Kuznetsov, Mikhail, Roy, Serge H., Filtering the surface EMG signal: Movement artifact and baseline noise contamination. Journal of Biomechanics 43.8 (2010): 1573-1579. Group 127 Page 28 of 39
8. Appendix Troubleshooting Figures Fig. 23. The Jendrassik maneuver. Fig. 24. The prototype board s 3 fuse lights. Group 127 Page 29 of 39
Bode Plot Fig. 25. Bode plot showing the frequency range of our band-pass filter with cutoff frequencies of 10.6 Hz and 530 Hz. Group 127 Page 30 of 39
Front Panel Figures Fig. 26. The Welcome screen of the front panel. Group 127 Page 31 of 39
Fig. 27. The Record EMG-Reflex screen of the front panel. Group 127 Page 32 of 39
Fig. 28. The Advanced screen of the front panel. Group 127 Page 33 of 39
Block Diagram Figures Fig. 29. Initializing code. Group 127 Page 34 of 39
Fig. 30. The code before the tab case structure. Group 127 Page 35 of 39
Fig. 31. The code within the subvi. Fig. 32. Visualizing the two signals and threshold. Group 127 Page 36 of 39
Fig. 33. Calibration and the calculation and storage of a threshold. Fig. 34. The calculation of reflex time. Group 127 Page 37 of 39
Fig. 35. The remaining code on the Record EMG-Reflex tab that codes for the table and the buttons. Fig. 35. The remaining code on the Advanced tab. Group 127 Page 38 of 39
Group 127 Page 39 of 39 Fig. 36. The entire block diagram as a reference.