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Advanced Biomechanics of Physical Activity (KIN 831). Electromyography (EMG) Material included in this presentation is derived primarily from two sources: * http://www.delsys.com/library/tutorials.htm
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Advanced Biomechanics of Physical Activity (KIN 831) Electromyography (EMG) Material included in this presentation is derived primarily from two sources: * http://www.delsys.com/library/tutorials.htm * Nigg, B. M. & Herzog, W. (1994). Biomechanics of the musculo-skeletal system. New York: Wiley & Sons * Winter, D.A. (1990). Biomechanical and motor control of human movement. (2nd ed.). New York: Wiley & Sons
Electromyography (EMG) • Electro – electrical • Myo – muscle • Graphy – record -------------------------------------------------------- Electromyography – involves recording the electrical activity of muscle Electromyogram – electrical signal associated with the contraction of a muscle
Selected Historical Events Related to EMG • Andreas Vesalius, “father of modern anatomy”, appearance and geography of dead muscle, 1555
Selected Historical Events Related to EMG • William Croone, in De Ratione Motus Musculorum concluded from nerve section experiments that the brain must send a signal to the muscles to cause contraction, 1664 • Physiologists became excited over the phenomena produced by electrical stimulation of muscles, 1740
Selected Historical Events Related to EMG • Albrecht von Haller (1708-1777) summarized many of the earlier studies in his treatise on muscular irritability. • Robert Whyatt (1714-1766) reported clinical observations on a patient treated by electrotherapy. • “Animal electricity” was proposed as a substitute for the “animal spirits” which earlier experiments believed to be the activating force in muscular movement.
Selected Historical Events Related to EMG • Luigi Galvani (1737-1798) studied the effects of atmospheric electricity upon dissected frog muscles. He concluded that the movement of the muscle was the result of its exterior negative charge uniting with the positive electricity which proceeded along the nerve (1786). Galvani’s Commentary on the Effects of Electricity on Muscular Motion (1791 or 1792) is probably the earliest statement of the presence of electrical potentials in nerve and muscle. He showed that electrical stimulation of muscular tissues produced contraction and force. He is considered the “father of experimental neurology.”
Galvani’s demonstrations of the effects of electricity on muscles of frogs and sheep (De viribus electricitatis in motu musculari commentarius, 1792)
Selected Historical Events Related to EMG • “animal electricity” became the absorbing interest of the physiological world. The greatest name among the early students of the subject was Emil DuBois-Reymond (1818-1896). He laid the foundation of modern electrophysiology. He was probably the first to discover and describe that contraction and force production of skeletal muscle were associated with electrical signals originating from the muscles (1849).
Selected Historical Events Related to EMG Guillaume Benjamin Amand Duchenne (1806-1875) set out to classify the functions of individual muscles through electrical stimulation. He recognized the problem of attempting to isolate muscle contractions.
Duchenne’s book, Physiology des Movements (1865), has been acclaimed “one of the greatest books of all time.” He was probably the first to perform systematic investigations of muscular function using an electrical stimulation approach.
Guillaume Benjamin Amand Duchenne de Boulogne investigating the effect of electrical stimulation of the left frontalis muscle on one of his cooperative (prisoner) subjects
Selected Historical Events Related to EMG • Wedinski (1880) demonstrated the existence of action currents in human muscle. Practical use had to await the invention of a sensitive string galvanometer (W. Einthoven - 1906).
Selected Historical Events Related to EMG • The physiological aspects of EMG were first discussed (1910-1912) by H. Piper of Germany • E.D. Adrian, in an article in Lancet (1925, vol. 2, pp. 1229-1233) entitled “Interpretation of the Electromyogram” demonstrated for the first time that it was possible to determine the amount of activity in a human muscle at any stage of movement.
Selected Historical Events Related to EMG • Toward the end of WWII, with marked improvement of electronic apparatus; anatomists, kinesiologists, and orthopedic surgeons began to make increasing use of EMG. The first study that gained wide acceptance was that of Inman, Saunders, and Abbott who reported their work on the movements of the shoulder region in “Observation on the Function of the Shoulder Joint” in the Journal of Bone and Joint Surgery (1944, vol. 26, pp. 1-30).
Selected Historical Events Related to EMG • During the 1950’s and beyond , EMG for kinesiological studies became widespread.
Selected Historical Events Related to EMG • John Basmajian (1921- ) wrote the bible of electromyography entitled Muscles Alive. He and Carlo De Luca summarized the existing knowledge and research on muscle function as revealed by EMG studies.
Typical multifactorial gait-recording showing: • Angular accelerometer on the left leg • Vertical accelerometer • Horizontal accelerometer • Strain gauge tensiometer on left gastrocnemius • EMG of left gastrocnemius
Electromyography is a seductive muse because it provides an easy access to physiological processes that cause the muscle to generate force, produce movement and accomplish the countless functions which allow us to interact with the world around us. The current state of Surface Electromyography is enigmatic. It provides many important and useful applications, but it has many limitations which must be understood, considered and eventually removed so that the discipline is more scientifically based and less reliant on the art of use. To its detriment, electromyography is too easy to use and consequently too easy to abuse.C. J. De Luca, 1993
Schematic Representation of a Recording an EMG Signal from a Single Muscle Fiber • Measure of changes in electrical potential across the muscle fiber • At rest, potential ≈ -90mv • With sufficient stimulation potential inside cell rises to ≈ 30-40mv • Change in potential (fiber action potential) can be recorded • Action potentials from multiple fibers in a motor unit are simultaneously recorded • Signal from depolarization of a motor unit is called motor unit action potential
Electrophysiology of Muscle Contraction • Motor unit action potential (muap) – change in electrical potential across the muscle fiber membranes when a motor unit is stimulated beyond a critical threshold • Electrodes placed inside (indwelling) or on the surface of a muscle record the algebraic sum of all muap’s transmitted along muscle fibers that reach the electrodes • Motor units far away from the electrode have their muap attenuated (i.e., are smaller) • Motor units of a muscle are controlled by motor neurons activating them at their motor end plates
Electrophysiology of Muscle Contraction • End plate potential (EPP) – depolarization of post synaptic membrane • EPP that reach a threshold initiate action potential in muscle fiber membrane • Depolarization of the transverse tubular system and sarcoplasmic reticulum results in a depolarization wavealong the direction of the muscle fibers • EMG records the depolarization and subsequent repolarization
Two Categories of Electrodes 1. By placement of electrode: • Surface • Indwelling (needle)
Indwelling (needle) Steps in making a bipolar fine-wire electrode (Basmajian and Stecko, 1962)
Surface Non-invasive Detect average activity of superficial muscles and give more reproducible results Metal (silver/silver chloride) disk or bar May be subject to cross-talk (EMG signals from motor units of other muscles near by Indwelling Invasive Used to detect EMG signal from small muscles and deep muscles Fine hypodermic needle with insulated wire conductors May be subject to cross-talk Surface vs. Indwelling Electrodes
Preparation of Skin for Surface Electrodes • Reduce electrical impedance of skin • Shave the area • Apply rubbing alcohol or abrasives to remove dead skin and oils • Use electrode gel and pressure, adhesive tapes and/or elastic bands to affix electrode to skin
Categories of Electrodes 2. By electrode configuration: • Monopolar – records difference in voltage relative to ground • Bipolar – two contacts to measure electrical potential, each relative to a common ground, most common electrode type • Multipolar
Biphasic Signal Signal associated with single electrode and ground
Triphasic Signal Signal associated with voltage difference when two electrodes are used at one site
Factors Affecting EMG Signal • Propagation velocity of wave front (≈ 4m/s) • Fatigue results in decreased propagation velocity • Distance between electrodes • Depth of muscle fibers being recorded • Electrode surface area • Larger surface area longer duration of muap • surface electrodes record longer muap than indwelling electrodes (≈ 3-20ms) • Size of muscle fibers being recorded • Larger fibers have larger signals
Preferred electrode location is between motor point (innervation zone) and the tendonous insertion.
Amplitude and frequency spectrum of EMG signal affected by electrode placement with respect to: A Myotendonous junction B, C Edge of muscle D B C Preferred location: D Midline of belly between innervation zone and myotendonous junction - greatest amplitude detected A
Factors to Consider in Recording EMG Signals • EMG signal is summation of muap’s • Goal is to have signals that are undistorted (linear amplification) and free of noise (biological – ECG, other muscles; man-made – power lines, machinery) and artifacts (false signals from electrodes and cabling – movement artifacts from touching electrodes or moving cables) • Large signals 5-10 mV; small signals 100 V
Factors to Consider in Amplifying EMG Signals • Amplifier gain – ratio of output voltage to input voltage (gain of 1000: 2 mV 2 V) • Linear amplification over entire band width • Do not overdrive the amplifier system (large signals clipped off) • Full range frequency response for amplifier should be fast enough to handle highest EMG frequencies • Amplifier input impedance –resistance • High so as not to attenuate the EMG signal *Report magnitudes of voltage as they are sensed at the electrodes; not amplified signal
Factors to Consider in Amplifying EMG Signals • Frequency response • Amplify without attenuation all frequencies • Frequency spectrum of EMG signals from 5 to 2000 Hz • Recommended range for surface electrodes – 10 to 1000 Hz • Recommended range for indwelling electrodes – 20 to 2000 Hz • Bandwidth of amplifier difference between upper and lower cutoff frequencies • Possible filtering of signals to avoid unwanted noise
Want frequencies of EMG signals to fall within range where all frequencies are linearly influenced by gain 2000 Hz 5 Hz
Power density spectrum – mathematical conversion of EMG signals from time to frequency domain for analysis of the frequency content of the signal • Higher frequency content of indwelling electrodes because of closer spacing of electrodes and their closer proximity to active muscle fibers • Most of EMG signal concentrated in band width between 20 and 200 Hz • Problem with power lines because frequency is in middle of band width • Movement artifact (0-10 Hz) can be filtered without adversely affecting desired EMG signal
Factors to Consider in Amplifying EMG Signals • Common mode rejection • Human body good conductor; acts as antenna to electromagnetic radiation • Want to eliminate extraneous signals • Unwanted signals picked up simultaneously at two locations can be eliminated resulting in amplification of only difference in voltage associated with EMG signal • Desired amplified signal = A[(Vhum + emg1) - (Vhum + emg2)] = A[emg1 – emg2]