Introduction

Facial electromyography (fEMG) is a psychophysiological technique used to measure electrical potential generated by facial muscles. As the activity of certain facial muscles has been associated with emotional expression, fEMG can be used to index variance in the valance of emotional expression. fEMG is measured in millivolt (mV). 

Availability, support and advice

For availability of EMG equipment within your unit, contact your lab coordinator. BIOPAC EMG equipment can be borrowed from SOLO through the Helpdesk. For fEMG, the use of the Biopac EL-CHECK device is recommended (also available through the Helpdesk).

Protocol

This protocol shows how to measure fEMG over the corrugator supercilii muscle ("frowning" muscle) and zygomaticus major muscle ("smiling" muscle) with BIOPAC equipment and BIOPAC reusable electrodes. Note that an adequate training is required before working with physiological data acquisition equipment and collecting such data from participants. If you are uncertain about the procedure, please contact your supervisor or lab coordinator. 

Setup

Equipment

fEMG can be measured with the BIOPAC EMG100C (wired) or EMG2-R (wireless) modules. With one EMG100C you can measure the activity of one muscle, with the EMG2-R you can measure the activity of two muscles. Thus, when measuring activity of both the corrugator supercilii and zygomaticus major, you need either two EMG100C modules or one EMG2-R module.

Leads and electrodes

EMG100C
The EL254S and EL254 leads can best be used when measuring fEMG with and EMG100C. An electrode is connected to each of these low-weight leads, which needs to be properly cleaned after use (see Cleaning up). Connect two EL254S leads to VIN+ and VIN- (and shields) of each module. Connect one EL254 lead (unshielded) to the GND (ground) on one of the modules (the module that is used to measure activity of the corrugator supercilii). Note that the ground is not required when the participant is already connected to a ground on another BIOPAC wired module (see Grounding guidelines | BIOPAC).

EMG2-R
The EMG2-R module comes with a transmitter and a velcro strap (BN-STRAP-76) with which the transmitter can be attached to the head. Connect one BN-EL15-Lead3 to the EMG channel that is used to measure activity of the corrugator supercilii and one BN-EL15-Lead2 to the other EMG channel. The ground electrode is always used when measuring with the wireless (BioNomadix) system. When measuring fEMG with EMG2-R, use 5 Ag-AgCl reusable snap electrodes (EL654). These electrodes need to be properly cleaned after use (see Cleaning up).

Materials

For SSH researchers, supplies can be picked up at SOLO 1B11 in PdlC or 2.5.01 in Sylvius, see lab supplies for more info.

• fEMG equipment (see Setup)
• Electrode sticker rings (13x5 mm)
• Electrode gel (Signa)
• Plastic syringe with curved tip
• Scrub gel (NuPrep)
• Non-woven swabs or Q-tips
• Gloves
• Incidin OxyWipe

Electrode preparation

Prepare the electrodes before the participant arrives:

• Fill the syringe with a little electrode (Signa) gel.
• Fix the electrode sticker rings to the electrodes, so that the hole in the sticker is above the electrode cavity.
• Squirt a little electrode gel into the cavity. Use a syringe that is made entirely of plastic for this. Ensure that the electrode cavity is filled entirely with gel, but that it does not overflow and make sure there are no air pockets.
• Just before applying the electrode on the skin, remove the protective plastic from the sticker (achieving this while wearing gloves requires some practice).

Skin preparation

Before applying the electrodes it is important to reduce the impedance (resistance) of the skin by removing oils and dead skin cells. It is recommended to use a scrub gel (NuPrep) for this. Do not use alcohol wipes as they are generally too aggressive for the skin.

• Put on disposable gloves.
• Put a little NuPrep scrub gel on a non-woven swab or Q-tip.
• Rub the gel gently onto the skin.
• When the skin remains wet, dry the skin with a clean swab or tissue.
• Remove the protective plastic from the electrode sticker ring and apply the electrodes directly after cleaning the skin.

Electrode placement

In general, five electrodes (when the ground is included, see Grounding guidelines | BIOPAC) are required when measuring activity of the corrugator supercilii muscle ("frowning" muscle) and zygomaticus major muscle ("smiling" muscle). 

Place two electrodes on the zygomaticus major. The zygomaticus major runs from the side of the mouth to the edge of the eye. To find the muscle, place your fingers on the participant's cheek and ask the participant to smile.
Place two electrodes on the corrugator supercilii, which is just above the eyebrow. Ask the participant to frown to find the correct location and place the electrodes over the 'belly' of the muscle. Usually, this means that 1 electrode is placed just above the start of the eyebrow and 1 electrode a little further along (around 2cm) the eyebrow.
The last grounding electrode can be placed in the middle of the forehead just below the hairline.
When you are using the wireless EMG2-R, attach one pair of leads (red/white) to the corrugator electrodes and one pair to the zygomaticus electrodes. Attach the black lead to the grounding electrode.   When you are using the wired EMG100C, the leads can be placed behind the participant's ear, to allow minimal tension on the electrodes. If necessary, medical tape can be used to keep the leads in place.

EL-CHECK

When measuring fEMG, the use of an EL-CHECK device before starting measurements is advised. With the EL-CHECK, the impedance between two electrodes can be measured. For the best signal, the impedance should be as low as possible. Use the EL-CHECK after the electrodes have been allowed to set for 5-10 minutes.

EL-CHECK usage (see also an instruction video here: EL-CHECK | BIOPAC);

• Make sure the leads are attached to the electrodes (when using the wireless EMG2-R), but not to the EMG2-R transmitter or EMG100C module. Instead, insert the leads into the appropriate connectors on the EL-CHECK.
• Us the switch to select the desired position. When the ground is used, check all three positions. When the ground is not used, only check the Vin+/Vin- position. 
• Press and hold the Test button to check the impedance between the two selected electrodes. 
• For fEMG, ideally the impedance should be <5 kΩ or 5-10 kΩ. When impedance is too high, consider reapplying the electrodes. That is:
  • Detach the leads and electrodes.
  • Fix new electrode sticker rings to the electrodes.
  • If necessary, add a bit more electrode gel to the electrodes.
  • Prepare the participant's skin again by rubbing a little NuPrep scrub gel on the skin using a Q-tip or non-woven swab. 
  • Remove the protective plastic from the electrode sticker ring and apply the electrodes and if necessary attach the leads.
  • Check the impedance again with the EL-CHECK.
• When the impedance is sufficient, attach the leads to the BIOPAC EMG100C module or EMG2-R receiver:
  • When using the EMG100C, connect the two EL254S leads to VIN+ and VIN- (and shields) on each module. Connect the EL254 lead (unshielded) to the GND (ground) on one of the modules (the module that is used to measure activity of the corrugator supercilii). Make sure you connect the leads to the correct module and that you know which module records the zygomaticus and which the corrugator. 
  • When using the wireless EMG2-R system, you can attach the strap with the transmitter to the participant’s head. Ensure that the strap is between the Biopac transmitter and the skin, so that the transmitter does not make contact with the skin. Attach the leads to the receiver. Make sure you connect the leads to the correct channel (A or B) and you know which channel records the zygomaticus and which the corrugator.

Cleaning up

Remove the equipment from the participant:

• Put on disposable gloves.
• When using the wireless EMG2-R, first remove the leads and the transmitter unit.
• Carefully remove the electrodes. Removing the electrodes can be painful for some people. Depending on the sensitivity of the participant’s skin, there may be red marks visible. These will normally fade within a few hours.
• Give the participant a tissue to remove any excess gel, or allow the participant to wash their face.

Clean the reusable electrodes as soon as possible after use. Dried-up gel is more difficult to remove and diminishes the electrical contact with the skin. At PdlC, use the kitchen in SB12 or SB02. at Sylvius, use disinfection room 2.4.23.

• Remove the electrode sticker rings from the electrodes and dispose of stickers.
• Run the electrodes (both the reusable snap electrodes EL654 and EL254S/EL254) under a lukewarm tap and use a soft toothbrush to gently remove the gel out of the cavity. Ensure that all the gel has been removed. Be careful when cleaning the EL254S/EL254 electrodes to only clean the electrode part of the leads with water and not immerse the full leads under water.
• After cleaning, dry the electrodes thoroughly with a paper towel before tidying them away.
• If necessary, the leads can be cleaned carefully with Incidin OxyWipe.
• When a strap has been used for the wireless EMG2-R transmitter, the strap can be washed if necessary (use multiple straps to allow for drying), please contact labsupport@fsw.leidenuniv.nl when straps need to be washed.

Tips & Tricks

• Ask the participant beforehand to not wear make-up on the skin (such as foundation). Make-up can make cleaning the skin and reducing impedance more difficult.
• Make sure the electrodes and the leads are properly attached.
• Use the EL-CHECK device to check the impedance of the skin.
• Before you start the experiment, ask the participant to smile and frown briefly. This will allow you to check if you get a signal and whether the electrodes are placed correctly.
• Practice applying the electrodes at least a few times before starting actual data collection.

More information

• BIOPAC webinars:
  • How to Record Great Facial EMG Data | BIOPAC
  • Understand Human Emotions with Facial EMG | BIOPAC
  • Emotion and Objective Physiological Measures | BIOPAC
  • EMG Webinar | How to get Great EMG (Electromyography) Data | BIOPAC
• Papers:
  • Facial EMG: 
    • van Boxtel, A. (2010). Facial EMG as a Tool for Inferring Affective States. Proceedings of Measuring Behavior 2010 (Eindhoven, The Netherlands, August 24-27, 2010), Spink, A.J., Grieco, F., Krips, O.E., Loijens, L.W. S., Nodlus, L.P.J.J., and Zimmerman, P.H., Eds.
  • Startle eye blink reflex: 
    • Blumenthal, T.D., Cuthbert, B.N., Filion, D.L., Hackley, S., Lipp, O.V., and van Boxtel, A. (2005). “Committee Report: Guidelines for Human Startle Eyeblink Electromyographic Studies”, Psychophysiology, 42:1-15.
  • EMG:
    • Hermens, H.J., Freriks, B., Disselhorst-Klug, C., and Rau, G. (2000). “Development of Recommendations for SEMG Sensors and Sensor Placement Procedures”, Journal of Electromyography and Kinesiology, 10(5):361-374.
    • Stegeman, D.F., Hermens, H.J. (2007). “Standards for Surface Electromyography: The European Project ‘Surface EMG for Non-Invasive Assessment of Muscles (SENIAM)’.”, 108-112.
    • ​​Fridland, A.J., and Cacioppo, J.T.  (1986). “Guidelines for Human Electromyographic Research”, Psychophysiology, 23(5):567-589.