Revolutionizing ProstheticsWhat is EMG and how can we use it in our daily lives? With about 185,000 amputations occurring per year, about 2 million US citizens are living with limb loss and severe limitations in daily routines. In order to help these patients regain regular body function, research teams all over the world have been working hard on making artificial anthropomorphic prosthetics a reality. Mind-blowing projects such as DARPA’s “Revolutionizing Prosthetics” as well as the EU-funded NEBIAS project have been developing intricate robotic limbs which not only resemble human limbs in appearance but also in function. In 2014, the US Food and Drug Administration (FDA) approved the first myoelectric prosthetic arm that performs multiple simultaneous powered movements of hand, wrist or elbow based on electrical signals recorded from the residual neuro-muscular system of a patient using electromyographic (EMG) electrodes.

This fascinating and cutting-edge technology of steering robotic devices with muscular signals is based on the simple fact that whenever a muscle (or its residual) contracts a burst of electric activity is generated which propagates through adjacent tissue and bone and can be recorded from neighboring skin areas.

EMG actionpotentialIn detail, the process is a bit more complex: A voluntary action command such as “I’d like to grab my cell phone on the table” triggers brain processes in motor cortex (Brodmann Areas 4 and 6) which travel along spinal cord and limbs via motor neurons. Eventually, the elicited action potential arrives at the motor end plate of the respective muscle. Its arrival causes a release of the neurotransmitter Acetylcholine (ACh) at the synaptic cleft [1] inducing a depolarization which is propagated downward from the muscular surface in a transverse tubule [2]. As a result, Calcium ions (Ca2+) are released [3], which results in cross-bridge binding [4] and muscular contraction [5].

Importantly, EMG activity (as measured in microvolt) is linearly related to the amount of muscle contraction as well as the number of contracted muscles – or in other words, the stronger the muscle contraction and the higher the number of activated muscles, the higher the recorded voltage amplitude will be. Since EMG activity is even measurable when we do not display obvious actions or control our body to not perform certain behaviors, EMG recordings represent an additional source of insights into cognitive-behavioral processing which would be hidden based on pure observation techniques. Previous research indicates a close coupling between muscular EMG and motor-cortex EEG as reflected by significant correlations in signal features such as frequency power and phase in the (12 – 25 Hz) beta band. This emphasizes the power of EMG recordings for monitoring the interaction of cortical and motor systems.

So what should you keep in mind when getting started with EMG?

  • Use surface electrodes. Surface EMG is a completely non-invasive technology that allows you to easily place EMG electrodes with stickers to the skin. No muscle innervation is necessary, which renders EMG an ideal method for monitoring physiological processes without interfering established routines and movement patterns. In order to obtain high-quality data, keep in mind to always clean the recording sites and remove make-up using alcohol rubs.
  • Place EMG electrodes over muscle groups of interest. Admittedly, this requires a certain level of anatomic knowledge. Only if you know the muscle regimes involved in a specific action you will be able to get valid and reliable signals of high quality. Facial EMG recordings, for example, are complicated by the fact that there are 43 muscles in the face. Most of these are controlled by the seventh cranial nerve (the “facial nerve”), which routes from cerebral cortex to five primary branches (temporal, zygomatic, buccal, mandibular and cervical). Each branch innervates muscles in different face areas, allowing for intricate facial twists and contortions.Facial Muscles Huang et al 2005
  • Select an appropriate reference site. EMG data is collected in a bipolar fashion – in fact, the EMG signal is the voltage difference between recording site and reference site. Therefore, selecting an appropriate reference site is as important as the actual recording site. We recommend to place EMG reference channels at bony body parts such as elbow, hip or collar bones.
  • Use short electrode cables/leads. In order to minimize the amount of electrical noise picked up from surrounding power sources, keep the length of the cables that connect the recording electrodes with the amplifier/recording device as short as possible.

What is EMG and how can you use it?

With iMotions Biometric Research Platform you can wirelessly collect EMG data using Shimmer devices in synchronized fashion with eye tracking, EEG, GSR sensors as well as camera-based automatic facial expressions in order to analyze the interaction between external stimuli, sensory perception, neural processing and muscular actions. And using our Application Program Interface (API), you can connect iMotions Biometric Research Platform with third-party soft- and hardware – even robotic arms and prostheses! Our team is looking forward to assisting you with digging deeper into biometric and physiological processes associated with human behavior!

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