077: What is EMG?

electromyography May 11, 2023

The act of physical movement, to which our conscious minds often pay no heed, is an intricate symphony of electrical and mechanical events. Electromyography, or EMG, is an investigative technique used to record and interpret the electrical activity generated by our skeletal muscles. The simplicity of a single flex or twist belies an undercurrent of complexity - a confluence of multiple muscles, each contributing to a singular action at a joint.

This phenomenon, known as muscle redundancy, poses a conundrum. Every muscle comprises hundreds, if not more, independently controlled motor units, each bearing its unique attachment points. Therefore, it becomes an exercise in futility to try and deduce the activity of individual muscles solely from the conditions of the joints, and the forces exerted on the body.

The solution to this riddle of biology lies in registering alterations in electric potential. (Electric potential is the difference in electric charge between two points within a cell or tissue.) Measuring electric potential can be either non-invasive, using surface EMG (sEMG), or invasive, entailing the insertion of a fine wire deep into the body, proximate to the muscle's membrane. Both methods, though starkly different in their execution, share the same purpose: to distil information specific to each muscle. This is the wonder of EMG - a lens through which we can view and appreciate the complex dynamics of our muscular system.

Performing electromyography (EMG) necessitates a specialized tool: the electromyograph. This instrument, like a divining rod for the electrical undercurrents of our musculature, generates a valuable artefact known as the electromyogram. An electromyogram serves as a roadmap to the electrical potential difference, the latent energy borne by muscle cells when they are roused into action, either electrically (using an external source) or neurologically.

The origin of this electrical activity is the muscle membrane potential, which can reach up to approximately -90 mV relative to the skin potential. The potentials we measure via EMG span a broad spectrum, from less than 50 μV to a substantial 20–30 mV. This range is dictated by a variety of factors, including the specific muscle in question and the chosen method of measurement - whether we pierce the skin's barrier in an invasive procedure or opt for a less intrusive surface measurement.

EMG systems are increasingly harnessing the power of wireless technology. This evolution not only streamlines the data collection process, but also ushers in an era of increased flexibility and reduced physical constraints in our quest to unravel the mysteries of the human body.

When considering muscle activation, imagine the muscular system as a finely tuned orchestra, with each muscle a musician awaiting its cue. Each time a muscle is sparked into action by a brief, electric stimulation, it embarks on a two-part performance: a period of contraction followed by a moment of rest and relaxation. However, when these stimulations arrive in rapid succession, the individual notes of contraction and relaxation start to blend together. If the time between stimuli is cut too short, the contractions fuse into a nearly continuous force, a steady hum of tension output. It's akin to a pianist's fingers dancing over the keys so quickly that individual notes blur into one sustained chord.

Consequently, the raw data we collect from electromyography (EMG) are inherently noisy and weak. The musical score of the body, when viewed under the microscope of an electromyograph, is a cacophony of overlapping notes and harmonies. Yet, even amidst this apparent chaos, we can discern the subtle melody of muscle activity, the song of our biology that plays on, tirelessly, beneath the surface of our conscious awareness.

Gleaning meaningful insights from the raw EMG signal is akin to distilling coherence from the chaos of a crowded room, a task demanding both finesse and precision. This endeavour begins with amplification, enhancing the whisper of the raw signal into a more discernible voice. Next, we apply filters at both ends of the frequency spectrum.

A high-frequency cutoff is implemented to discard those frequencies surpassing a critical threshold, frequencies incapable of impacting muscle dynamics. Conversely, a low-frequency cutoff is employed to eliminate minor frequencies, which might be the byproduct of mechanical conditions, such as the relative motion of muscle and skin or noise unrelated to the muscle's electrical activity and active force.

Emerging from this process of signal refinement, we are left with an EMG reading that is both meaningful and actionable. This polished data find applications across an impressive breadth of fields, from diagnostics to monitoring, from the active control of orthotics to the fine-tuning of prosthetics. Each application, a testament to the transformative potential of EMG, illustrates how marvels of technology shape our understanding and interaction with the human body.