By chance I found this interesting article on the web. I decided to collect some useful information about muscle contraction and EMG analysis. Voilà 🙂
As God Wikipedia reports, “Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles”. Skeletal muscles are, as their name suggests, attached to the bones of the skeleton by means of tendons. Such muscles are controlled by the nervous system (precisely, the somatic nervous system, which is part of the peripheral nervous system). Thus, they can be activated voluntarily.
This image clearly shows that muscle fibers are the basic cellular units of skeletal muscles. They are long, cylindrical and multinucleated cells also called myofibers or myocytes.
If we zoom in on a single muscle fiber, we find that it contains myofibrils, whose diameter is about 1-2 micrometers! Myofibrils are very long chains of sarcomeres, that are the contractile units of the cell.
The sarcomeres are composed of protein filaments, some of them are thin (actin) and some others are thick (myosin). During the muscle contraction process (video explanation here), the thick filaments pull the thin ones towards the center of the sarcomeres, thus shortening (contracting) the length of the myofibrils, of the myofibers and, as a consequence, of the muscle itself.
Muscle contraction can be seen as the production of mechanical energy caused by either a chemical imbalance or an electrical impulse (motor neurons). The electrical activity related to this process can be measured by means of EMG analysis.
An electromyograph is needed to detect the electrical potential (up to 30 mV) generated by muscle cells at the moment of their electrical or neurological activation. Electrodes (or needles) are positioned on the patient’s skin (or inserted inside the muscle) and the electrical activity is recorded. The result is a signal called electromyogram, that can be analyzed for diagnostics (detection of medical abnormalities and measure of useful parameters) or to analyze the biomechanics of the considered movement. Muscle tissue at rest is normally electrically inactive. When the muscle is voluntarily contracted, action potentials begin to appear. As the strength of the muscle contraction is increased, more and more muscle fibers produce action potentials. When the muscle is fully contracted, there should appear a disorderly group of action potentials of varying rates and amplitudes (a complete recruitment and interference pattern).