muscle activity of the core musculature compared to the peak emg variable." therefore, these researchers would suggest that "arv emg data should be recorded alongside the peak emg measure when assessing core exercises." providing the reader with both sets of data would result in enhanced validity of the study and potentially eradicate the contradictions within the research. other measurements emg can also be used for indicating the amount of fatigue in a muscle. the following changes in the emg signal can signify muscle fatigue: an increase in the mean absolute value of the signal, increase in the amplitude and duration of the muscle action potential and an overall shift to lower frequencies. monitoring the changes of different frequency changes the most common way of using emg to determine levels of fatigue. the lower conduction velocities enable the slower motor neurons to remain active. a motor unit is defined as one motor neuron and all of the muscle fibers it innervates. when a motor unit fires, the impulse (called an action potential) is carried down the motor neuron to the muscle. the area where the nerve contacts the muscle is called the neuromuscular junction, or the motor end plate. after the action potential is transmitted across the neuromuscular junction, an action potential is elicited in all of the innervated muscle fibers of that particular motor unit. the sum of all this electrical activity is known as a motor unit action potential (muap). this electrophysiologic activity from multiple motor units is the signal typically evaluated during an emg. the composition of the motor unit, the number of muscle fibres per motor unit, the metabolic type of muscle fibres and many other factors affect the shape of the motor unit potentials in the myogram. nerve conduction testing is also often done at the same time as an emg to diagnose neurological diseases. some patients can find the procedure somewhat painful, whereas others experience only a small amount of discomfort when the needle is inserted. the muscle or muscles being tested may be slightly sore for a day or two after the procedure. emg signal decomposition emg signals are essentially made up of superimposed motor unit action potentials (muaps) from several motor units. for a thorough analysis, the measured emg signals can be decomposed into their constituent muaps. muaps from different motor units tend to have different characteristic shapes, while muaps recorded by the same electrode from the same motor unit are typically similar. notably muap size and shape depend on where the electrode is located with respect to the fibers and so can appear to be different if the electrode moves position. emg decomposition is non-trivial, although many methods have been proposed. emg signal processing rectification is the translation of the raw emg signal to a signal with a single polarity, usually positive. the purpose of rectifying the signal is to ensure the signal does not average to zero, due to the raw emg signal having positive and negative components. two types of rectification are used: full-wave and half-wave rectification. full-wave rectification adds the emg signal below the baseline to the signal above the baseline to make a conditioned signal that is all positive. if the baseline is zero, this is equivalent to taking the absolute value of the signal. this is the preferred method of rectification because it conserves all of the signal energy for analysis. half-wave rectification discards the portion of the emg signal that is below the baseline. in doing so, the average of the data is no longer zero therefore it can be used in statistical analyses. limitations needle emg used in clinical settings has practical applications such as helping to discover disease. needle emg has limitations, however, in that it does involve voluntary activation of muscle, and as such is less informative in patients unwilling or unable to cooperate, children and infants, and in individuals with paralysis. surface emg can have limited applications due to inherent problems associated with surface emg. adipose tissue (fat) can affect emg recordings. studies show that as adipose tissue increased the active muscle directly below the surface decreased. as adipose tissue increased, the amplitude of the surface emg signal directly above the center of the active muscle decreased. emg signal recordings are typically more accurate with individuals who have lower body fat, and more compliant skin, such as young people when compared to old. muscle cross talk occurs when the emg signal from one muscle interferes with that of another limiting reliability of the signal of the muscle being tested. surface emg is limited due to lack of deep muscles reliability. deep muscles require intramuscular wires that are intrusive and painful in order to achieve an emg signal. surface emg can measure only superficial muscles and even then it is hard to narrow down the signal to a single muscle. electrical characteristics the electrical source is the muscle membrane potential of about –90 mv. measured emg potentials range between less than 50 μv and up to 30 mv, depending on the muscle under observation. typical repetition rate of muscle motor unit firing is about 7–20 hz, depending on the size of the muscle (eye muscles versus seat (gluteal) muscles), previous axonal damage and other factors. damage to motor units can be expected at ranges between 450 and 780 mv. procedure outcomes normal results muscle tissue at rest is normally electrically inactive. after the electrical activity caused by the irritation of needle insertion subsides, the electromyograph should detect no abnormal spontaneous activity (i.e., a muscle at rest should be electrically silent, with the exception of the area of the neuromuscular junction, which is, under normal circumstances, very spontaneously active). 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). abnormal results emg findings vary with the type of disorder, the duration of the problem, the age of the patient, the degree to which the patient can be cooperative, the type of needle electrode used to study the patient, and sampling error in terms of the number of areas studied within a single muscle and the number of muscles studied overall. interpreting emg findings is usually best done by an individual informed by a focused history and physical examination of the patient, and in conjunction with the results of other relevant diagnostic studies performed including most importantly, nerve conduction studies, but also, where appropriate, imaging studies such as mri and ultrasound, muscle and nerve biopsy, muscle enzymes, and serologic studies. abnormal results may be caused by the following medical conditions (please note this is not an exhaustive list of conditions that can result in abnormal emg studies): disorders of muscle: inflammatory myopathies polymyositis dermatomyositis inclusion body myopathy myotonia congenita muscular dystrophies: duchenne muscular dystrophy becker muscular dystrophy facioscapulohumeral dystrophy limb girdle muscular dystrophy centronuclear myopathy myotonic dystrophy mitochondrial myopathies disorders of the neuromuscular junction: myasthenia gravis lambert–eaton myasthenic syndrome botulism poisoning organophosphate poisoning hypermagnesemia hypocalcemia disorders of nerves: carpal tunnel syndrome ulnar neuropathy at the elbow radial nerve palsy (saturday night palsy) peroneal (fibular) nerve palsy diabetic neuropathy alcohol related neuropathy nutritional neuropathy amyloid neuropathy aidp cidp bell's palsy laryngeal neuropathy pudendal neuropathy femoral neuropathy sciatic neuropathy tibial neuropathy tarsal tunnel syndrome charcot-marie-tooth syndrome zoster neuropathy oculomotor, facial, vagal, trigeminal, glossopharyngeal, spinal accessory neuropathies hemifacial spasm multifocal motor neuropathy axillary neuropathy long thoracic neuropathy suprascapular neuropathy toxic neuropathies drug-induced neuropathies complex regional pain syndrome (crps) plexus disorders: neuralgic amyotrophy (idiopathic brachial plexitis) traumatic brachial plexopathy lumbosacral radiculopathy hirayama disease root disorders: cervical, thoracic, lumbar, sacral radiculopathy spinal stenosis arachnoiditis leptomeningeal disorders motor neuron disease amyotrophic lateral sclerosis west nile virus poliomyelitis kennedy's syndrome (spinobulbar muscular atrophy). history the first documented experiments dealing with emg started with francesco redi's works in 1666. redi discovered a highly specialized muscle of the electric ray fish (electric eel) generated electricity. by 1773, walsh had been able to demonstrate that the eel fish's muscle tissue could generate a spark of electricity. in 1792, a publication entitled de viribus electricitatis in motu musculari commentarius appeared, written by luigi galvani, in which the author demonstrated that electricity could initiate muscle contraction. six decades later, in 1849, emil du bois-reymond discovered that it was also possible to record electrical activity during a voluntary muscle contraction. the first actual recording of this activity was made by marey in 1890, who also introduced the term electromyography. in 1922, gasser and erlanger used an oscilloscope to show the electrical signals from muscles. because of the stochastic nature of the myoelectric signal, only rough information could be obtained from its observation. the capability of detecting electromyographic signals improved steadily from the 1930s through the 1950s, and researchers began to use improved electrodes more widely for the study of muscles. the aanem was formed in 1953 as one of several currently active medical societies with a special interest in advancing the science and clinical use of the technique. clinical use of surface emg (semg) for the treatment of more specific disorders began in the 1960s. hardyck and his researchers were the first (1966) practitioners to use semg. in the early 1980s, cram and steger introduced a clinical method for scanning a variety of muscles using an emg sensing device. 7/12/1954 mayo clinic medical sciences emg lab. ervin l schmidt in the chair, mildred windesheim's arm holding the electrode. research began at the mayo clinic in rochester, minnesota under the guidance of edward h. lambert, md, phd (1915–2003) in the early 1950s. lambert, known as the "father of emg," with the assistance of his research technician, ervin l schmidt, a self taught electrical engineer, developed a machine that could be moved from the emg lab, and was relatively easy to use. as oscilloscopes had no "store" or "print" features at the time, a polaroid camera was affixed to the front on a hinge. it was synchronized to photo the scan. fellows studying at mayo soon learned that this was a tool they wanted, too. as mayo has no interest in marketing their inventions, schmidt went on to continue to develop them in his basement for decades, selling them under the name ermel inc. it was not until the middle of the 1980s that integration techniques in electrodes had sufficiently advanced to allow batch production of the required small and lightweight instrumentation and amplifiers. at present, a number of suitable amplifiers are commercially available. in the early 1980s, cables that produced signals in the desired microvolt range became available. recent research has resulted in a better understanding of the properties of surface emg recording. surface electromyography is increasingly used for recording from superficial muscles in clinical or kinesiological protocols, where intramuscular electrodes are used for investigating deep muscles or localized muscle activity. there are many applications for the use of emg. emg is used clinically for the diagnosis of neurological and neuromuscular problems. it is used diagnostically by gait laboratories and by clinicians trained in the use of biofeedback or ergonomic assessment. emg is also used in many types of research laboratories, including those involved in biomechanics, motor control, neuromuscular physiology, movement disorders, postural control, and physical therapy. research emg can be used to sense isometric muscular activity where no movement is produced. this enables definition of a class of subtle motionless gestures to control interfaces without being noticed and without disrupting the surrounding environment. these signals can be used to control a prosthesis or as a control signal for an electronic device such as a mobile phone or pda . emg signals have been targeted as control for flight systems. the human senses group at the nasa ames research center at moffett field, ca seeks to advance man-machine interfaces by directly connecting a person to a computer. in this project, an emg signal is used to substitute for mechanical joysticks and keyboards. emg has also been used in research towards a "wearable cockpit," which employs emg-based gestures to manipulate switches and control sticks necessary for flight in conjunction with a goggle-based display. unvoiced or silent speech recognition recognizes speech by observing the emg