Motor Unit Recruitment in EMG

5/26/13Motor Unit Recruitment in EMG Medscape Reference Reference News Reference Education MEDLINE Motor Unit Recruitment in EMG Definition of Motor Unit Recruitment and Overview Author: Friedhelm Sandbrink, MD; Chief Editor: Nicholas Lorenzo, MD more... Updated: Jun 26, 2012 Definition of Motor Unit Recruitment and Overview Motor unit recruitment may be defined as "the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction."[1] The central nervous system can increase the strength of muscle contraction by the following: Increasing the number of active motor units (ie, spatial recruitment) Increasing the firing rate (firing frequency) at which individual motor units fire to optimize the summated tension generated (ie, temporal recruitment) Both mechanisms occur concurrently. The primary mechanism at lower levels of muscle contraction strength is the addition of more motor units, but the firing rate of the initially recruited motor units also increases. When nearly all motor units are recruited, increase in firing frequency becomes the predominating mechanism to increase motor strength. At this level and beyond, motor units may be driven to fire in their secondary range to rates greater than 50 Hz. The next section of this article discusses the physiology of motor unit recruitment in detail. Subsequent sections look at ways of examining recruitment during an electromyography (EMG) study. Assessment is made at different levels of innervation—minimal muscle contraction to determine the onset and recruitment firing rates (ie, recruitment pattern); maximal voluntary contraction to provide information about the interference pattern; and moderate voluntary contraction at various levels for assessment of the turns/amplitude analysis. Order of Recruitment As a general rule, motor units are recruited in order of their size. When the muscle is activated initially, the first motor units to fire are small in size and weak in the degree of tension they can generate. Starting with the smallest motor units, progressively larger units are recruited with increasing strength of muscle contraction. The result is an orderly addition of sequentially larger and stronger motor units resulting in a smooth increase in muscle strength.[2] emedicine.medscape.com/article/1141359-overview#showall 1/8 the later recruited type II fibers. The size principle is also true to a limited extent for the EMG study.[3] After nerve injury. (A) With minimal effort of muscle contraction. or simply "size principle. Henneman et al concluded that motor axon diameter. assesses type I motor units predominantly. contain oxidative enzymes Type IIa or type FR (fast. however. corresponding to a firing rate of 6 Hz (ie. w hich is 85 ms. See the image below. Motor unit recruitment patterns vary for different movement tasks.Fast twitch.com/article/1141359-overview#showall 2/8 . In rather general terms. the reciprocal). which have different physiologic and staining properties.[4] The 3 main types of motor units. In this example. and central control. therefore. which are active only at relatively high force output. have larger diameter muscle fibers generating higher potentials than the smaller.[4] With time. Type II motor units are recruited later and are not analyzed in this way. especially the FF type. 3. The goal is to identify the recruitment pattern by measuring the firing rate of the first few recruited MUAPs. and to end with units analogous to type FF (type IIb).[5] Assessment of Recruitment at Low Level of Muscle Contraction An essential part of an EMG study is the assessment of motor unit recruitment at low levels of muscle contraction. slow twitch type I units. to progress to type II units that first include type FR (type IIa). In an EMG study.Fast twitch. however. the first recruited motor units arise from the small and relatively slowconducting type I motor units exclusively. As described in the previous section. the reciprocal of the recruitment interval. (B) Gradual increase in muscle strength results in recruitment of a second motor unit. the term "size" of a motor unit usually refers to the amplitude of the motor unit action potential (MUAP). it is 12 Hz. fatigue-resistant units with smallest force or twitch tension and slowest contraction. sensory feedback. 4] Recording from the ventral rootlets in cats and measuring the amplitudes of motor axon spikes. The time betw een 2 discharges is approximately 166 milliseconds (ms). including the mechanical function of the muscle. fatigable) . contain glycolytic enzymes The recruitment sequence is thought to begin with type I motor units analogous to type S units. a size-dependent branching of axons accounts for the rematching of motor neuron size and muscle unit size.5/26/13 Motor Unit Recruitment in EMG This orderly recruitment of sequentially larger motor units is referred to as the "Henneman size principle". The needle EMG examination cannot assess anatomic size or degree of tension of a motor unit. (C) With further increase in muscle strength.[2] There are exceptions to the size-ordered activation of motor units.Slow twitch.medscape. a single motor unit is seen firing at 6 Hz. fatigue-resistant units with larger forces and faster contraction times."[2. Recruitment frequency is defined as the firing frequency of the first motor unit w hen a second motor unit is recruited. the relationship between motoneuron size and the number and size of muscle fibers that the motoneuron reinnervates is initially lost. Recruitment analysis at low levels of muscle contraction. resistant) . the size of consecutively recruited MUAPs during an EMG study varies considerably. Because of the small uptake area of standard EMG needle electrodes. contain oxidative and glycolytic enzymes Type IIb or type FF (fast. include the following: Type I or type S (slow) . easily fatigable units with largest force and fastest contraction. and the size-ordered organization of motor units properties is restored. by further inference. conduction velocity and. Normal recruitment pattern. emedicine. a third motor unit is recruited. depending on many factors. motor neuron cell size all increase with functional threshold. the reciprocal of the recruitment interval. Assuming that the sweep speed is 10 milliseconds (ms)/cm. This is the "onset frequency. The time difference between 2 sequential potentials of MUAP A is the recruitment interval. and so on. The recruitment interval may be measured by placing 2 time markers on the 2 sequential MUAPs A.. MUAP A). equivalent to 1x per 100 ms.. Normal recruitment pattern. Recruitment interval is the time difference between 2 motor unit potentials belonging to the first firing motor unit when the second unit first appears. and the monitor of a typical EMG machine has 10 cm (10 divisions) across the entire screen. The time betw een 2 discharges is approximately 166 milliseconds (ms). it results in 200 ms across the entire screen. In this setting. if the unit is seen only once per screen but successively closer to the beginning of the trace with each new sweep. This event can be recognized by observing the screen and by listening for a change in sound associated with firing of 2 motor units. therefore. MUAP A then may increase its firing frequency and at one point a second motor unit (MUAP B) appears." Just 1 motor unit firing regularly should be identified (ie. Newer EMG equipment often has a 20-cm across the screen. A fast estimate of the firing rate of MUAPs is obtained by looking at the screen of the EMG machine. the first motor unit usually begins to fire irregularly at 2-3 Hz and then achieves a stable and fairly regular firing rate at 5-7 Hz. (B) Gradual increase in muscle strength results in recruitment of a second motor unit.5/26/13 Motor Unit Recruitment in EMG The patient is instructed to make only a very gentle contraction of the muscle under investigation. The patient then is asked to very gradually increase the force of muscle contraction. therefore. The recruitment interval is the reciprocal of the recruitment frequency. A unit seen only once per screen is firing at 5 Hz. The same multiplication factor is used for the 10-cm screen. has a firing frequency of 20 Hz. A MUAP firing twice per screen. therefore. In this example. Recruitment frequency is the firing frequency of the first motor unit when the second unit just begins to fire regularly." When the patient minimally increases the force of contraction. the width of a single screen represents 200 ms (see images below). simply multiplying the number of times the MUAP is present on the screen by 10 yields an estimate of the firing frequency. therefore.com/article/1141359-overview#showall 3/8 . This is a precise but cumbersome way of determining the recruitment frequency. emedicine.. the second unit is recruited once the first unit achieves a firing rate of about 10 Hz.just think about contracting the muscle. Recruitment frequency is defined as the firing frequency of the first motor unit w hen a second motor unit is recruited. In practice (see image below). A MUAP firing at 10 Hz means that it is firing 10x per 1000 ms. As long as the sweep speed is 10 ms/cm and the screen is 10 cm across the screen. A MUAP firing at 10 Hz appears twice on the screen. it is 12 Hz. the first unit increases the rate of firing to 6-10 Hz. the patient is instructed to make only a minimal contraction of the target muscle.. in practice. The recruitment frequency may be calculated as the reciprocal of the measured recruitment interval. a third motor unit is recruited.medscape. the reciprocal). The term "recruitment rate" is used interchangeably. firing frequency is between 10 and 20 Hz. often by using a phrase such as ". the multiplication factor of 5 is used to arrive at an estimate of the firing frequency. With further increase of muscle contraction. if the sweep speed is increased to 20 ms/cm. most examiners use an estimate of the recruitment frequency instead. Once this event is recognized. once on the screen. corresponding to a firing rate of 6 Hz (ie. It appears. therefore. w hich is 85 ms. In the normal situation. At the same sweep speed of 10 ms/cm. one screen represents 100 ms. (A) With minimal effort of muscle contraction. (C) With further increase in muscle strength. a unit seen 3 times is firing at 15 Hz. the examiner should "freeze" the screen. a single motor unit is seen firing at 6 Hz. corresponding to a firing rate of 6 Hz (ie. This ratio should be close to 5.medscape. w hich is 85 ms. Decreased recruitment in neurogenic conditions.com/article/1141359-overview#showall 4/8 . many motor units are activated simultaneously at a low level of muscle contraction. At this point. until MUAP A reaches about 15 Hz and MUAP B about 10 Hz. then this suggests an abnormality. The recruitment ratio is calculated from the firing frequency of the fastest firing MUAP divided by the number of different MUAPs on the screen. motor units are too few for the greatest firing frequency and force produced (ie. MUAP A and B increase their firing frequencies. MUAP C is activated. (A) With minimal effort of muscle contraction. Note the low amplitude and short duration of individual units. With further increase in muscle contraction force. Each time a motor unit is recruited. Facial muscles are an exception to this rough guide. Despite maximal voluntary effort. The time betw een 2 discharges is approximately 166 milliseconds (ms).5/26/13 Motor Unit Recruitment in EMG Normal recruitment pattern. MUAPs of facial muscles have shorter recruitment intervals (around 40 ms) and higher recruitment frequencies (about 25 Hz)." Motor units begin firing at stable rates at 5 Hz. Decreased Recruitment in Neurogenic Conditions emedicine. (B) Gradual increase in muscle strength results in recruitment of a second motor unit. the underlying condition in this patient is amyotrophic lateral sclerosis. Abnormal recruitment patterns such as these are discussed in the next sections. This single motor unit is firing at 15 Hz. Incomplete interference pattern. a single motor unit firing should not be seen more than twice. MUAP C is activated when the firing frequency of the fastest firing MUAP (ie. The firing rate is calculated from the presence of 3 MUAPs on a screen of 200 milliseconds. MUAP A) is 15 Hz. In this electromyographic (EMG) study of a patient w ith inclusion body myositis. (C) With further increase in muscle strength. early recruitment). In the example of an EMG screen set at 200 ms across the entire screen and recording from an extremity muscle. Most extremity muscles have a recruitment interval of about 90-100 ms. 5 Hz is serially added to the firing frequency of each MUAP already present. In this example. Early recruitment in myopathic conditions. the second motor unit (MUAP B) is activated and fires at 5 Hz. then motor units are too many for the highest firing rate (ie. the reciprocal). individual MUAPs can be identified and the baseline is partly visible. the reciprocal of the recruitment interval. a third motor unit is recruited. corresponding to a recruitment frequency of about 10-11 Hz. a single motor unit is seen firing at 6 Hz. When the first unit to fire (MUAP A) reaches 10 Hz. decreased recruitment). If the recruitment ratio approaches 10. The recruitment ratio is 5 (15/3). This example show s a discrete interference pattern in a patient w ith amyotrophic lateral sclerosis. Recruitment frequency is defined as the firing frequency of the first motor unit w hen a second motor unit is recruited. it is 12 Hz. This rapid firing unit indicates a neurogenic pathology. The orderly recruitment of successive motor units may be described as a rough approximation by the "rule of fives. In the example just discussed. If the first recruited motor unit is seen 3 times or more before the second unit is activated. If it is reduced to less than 4 or 5. Note the low amplitude and short duration of individual units. the EMG becomes continuously denser and the maximal peaks in the signal have a higher amplitude. The number of units required to maintain a given force increases in proportion to the inefficiency of the individual motor unit discharge." The recruitment frequency is decreased. As a result of such motor unit loss. With increasing force. This recruitment pattern in myopathic conditions is called "early recruitment" or "increased recruitment. defined above as the firing rate of the first motor unit at the point when the second motor unit is activated. this second unit is missing and an increase in force can be achieved only by increasing the firing rate of the first unit." This pattern of decreased recruitment may occur whenever a lesion results in a reduced number of functionally intact motor neurons and axons.5/26/13 Motor Unit Recruitment in EMG Damage may occur to the neural portion of a motor unit. therefore. and all the muscle fibers previously innervated by this axon will be denervated. In a healthy subject providing maximal voluntary effort of the muscle under investigation. Successful activation of a second motor unit occurs only at a higher level of muscular effort than in the normal condition. the firing frequency of individual motor units increases and progressively more and larger units are activated. Even with minimal muscular effort. given the first motor unit firing rate. Recruitment With Maximal Volitional Effort: Interference Pattern Analysis With increasing effort. or corresponding axon. Early Recruitment in Myogenic Conditions In muscle diseases such as polymyositis or muscular dystrophies. It is an early finding after acute nerve injury (eg. The recruitment pattern with maximal voluntary contraction is called "interference pattern" because of the increasing degree of superimposition of action potentials from different units. In this electromyographic (EMG) study of a patient w ith inclusion body myositis. The firing rate is calculated from the presence of 3 MUAPs on a screen of 200 milliseconds. this pattern is called "decreased recruitment" or "reduced recruitment. Decreased recruitment in neurogenic conditions. American Association of Electrodiagnostic Medicine defines the interference pattern as "electric activity recorded from the muscle with a needle electrode during maximal voluntary effort. anterior horn cell. Early recruitment in myopathic conditions. many motor units are activated simultaneously at a low level of muscle contraction. whether it is the result of actual motor unit loss or temporary conduction block as in neurapraxia. Because in such cases fewer MUAPs are active than expected. the underlying condition in this patient is amyotrophic lateral sclerosis. This rapid firing unit indicates a neurogenic pathology. fewer motor units are available for muscle activation. typically 2 or more units may be activated. a "full" or "complete" interference pattern is emedicine. Such injury may result in wallerian degeneration of the motor axon. This single motor unit is firing at 15 Hz. A number of motor units are unaffected but the muscle fiber content of each motor unit is reduced. See the image below." During a maximal voluntary muscle contraction of a healthy individual. is therefore increased in a neurogenic lesion. Normally. a second unit should begin firing with increasing muscular effort. radiculopathy from disk herniation or nerve trauma) and may precede other evidence of denervation in the EMG study. when the first recruited motor unit reaches a firing frequency of 10 Hz. In a myopathy.medscape. Such an abnormally fast firing motor unit is called "rapid firing unit" (RFU). the action potentials of individual motor units no longer can be separated from each other but are mixed with the signals of other units. the force output of each unit is diminished. Compensation occurs by having multiple motor units begin firing simultaneously. In a neurogenic condition. isolating a single firing motor unit often is impossible. muscle fibers are damaged. The recruitment frequency.com/article/1141359-overview#showall 5/8 . An incomplete interference pattern typically signifies a decreased number of MUAPs being activated with maximal effort. In general. intermediate interference pattern): Some of the individual MUAPs may be identified. individual MUAPs can be identified and the baseline is partly visible. resulting in relatively large high-frequency content. This example show s a discrete interference pattern in a patient w ith amyotrophic lateral sclerosis. Both the amplitudes/second and the turns/second increase with strength of muscle contraction. The position of the needle should be such that it is recording relatively sharp action potentials. A turn was defined as a change in the direction of the signal of at least 100 microvolts (µV).com/article/1141359-overview#showall 6/8 . possibly as a result of poor cooperation or pain. See the image below. the turns/amplitude analysis resulted in 4 data points above the normal area. Turns/amplitude Analysis The interference pattern is dependent on the shape of the individual motor unit potentials (eg. myopathies have low amplitude and short duration components. the interference pattern may be incomplete because of marked loss of muscle fibers. An amplitude count is produced for a fixed voltage change. Stalberg introduced a new version of the method. occur with incomplete effort of muscle contraction. Attempts have been made to quantify the characteristics of the interference pattern.5/26/13 Motor Unit Recruitment in EMG present. even though low-amplitude MUAPs may be noted on the recording of the interference pattern. usually 100 µV. an inherited neuropathy. The baseline is obscured completely by motor unit activity. amplitude. by plotting the turns/second against the mean amplitude change/turn in an XY-diagram. In contrast. In this patient w ith Charcot-Marie-Tooth disease type 1 (also called hereditary motor and sensory neuropathy type 1). the so-called "cloud. This is a major limitation of this method. the so-called turns/amplitude (T/A) analysis (see images below). however. No individual MUAPs can be identified clearly (this is normal). Despite maximal voluntary effort. Thus. Incomplete interference pattern may be divided as follows: Reduced interference pattern (ie. the turns count is normal or low and the amplitude high.medscape. duration. Single unit pattern: A single motor unit fires at rapid rate during maximum voluntary effort. the turns count is high and the amplitude low. Discrete activity: Each of several different MUAPs can be identified." emedicine. and the number of active units.[7] The recordings are made during an epoch of steady contraction (typically 1 s. 20 recordings are collected. while other individual MUAPs cannot be identified because of overlap. Turns/amplitude analysis. and in myopathic lesions. Incomplete interference pattern. the firing rates. Willison in 1964 described a technique to analyze "turns" and "amplitudes" within the EMG trace. between successive turns.[6] The EMG is converted into 2 trains of pulses that are counted to characterize the signal in terms of amplitude and turns. This may be suggestive of a neurogenic lesion resulting in a decreased number of functional motor units. polyphasia). In myopathic conditions. the force exerted by the muscle is critical for the analysis and needs to be kept constant at a defined level. In 1983. in very advanced stages of muscle disorders. It may. In neurogenic conditions. making it less dependent on force. however. neurogenic conditions have low-frequency components due to the large amplitude and long-duration motor unit potentials. Typically. the interference pattern is typically complete. but 250-300 ms may be sufficient) at various levels of muscle contraction. MD Chief. MD is a member of the following medical societies: American Academy of Neurology and American Association of Neuromuscular and Electrodiagnostic Medicine Disclosure: Nothing to disclose. PhD. a normal area called "cloud" exists that was defined by plotting recordings from a large number of healthy subjects at the 95% confidence level. George Washington University School of Medicine and Health Sciences. University of Nebraska Medical Center College of Pharmacy. and for a healthy individual no more than 1 data point of 20 recordings should be outside the cloud. A recording indicates a neurogenic condition if 2 or more data points are above the cloud of normal values and a myopathic condition if 2 or more data points are below the cloud. PhD. Head.medscape. MD.com/article/1141359-overview#showall 7/8 . MD Assistant Professor of Neurology. Division of Neurology. Georgetown University School of Medicine. American Academy of Neurology. Specialty Editor Board Stephen A Berman. MD is a member of the following medical societies: American Academy of Neurology and American Association of Neuromuscular and Electrodiagnostic Medicine Disclosure: Nothing to disclose. emedicine. MBA is a member of the following medical societies: Alpha Omega Alpha. Francisco Talavera. Clinical Neurophysiology Laboratory. Department of Neurology. Normal values fall into this cloud. Contributor Information and Disclosures Author Friedhelm Sandbrink. and Phi Beta Kappa Disclosure: Nothing to disclose. University of Central Florida College of Medicine Stephen A Berman. MBA Professor of Neurology. Department of Medicine. Coauthor(s) Eliad Culcea. Chronic Pain Clinic. PhD Adjunct Assistant Professor. MD. Director. University of Pittsburgh Medical Center-Shadyside Neil A Busis. EMG Laboratory and Chief. thus supporting the impression of a myopathic condition. Medscape Drug Reference Disclosure: Medscape Salary Employment Neil A Busis. Benefis Medical Group Eliad Culcea. the turns/amplitude analysis resulted in 14 of 20 data points below the cloud of normal values. For each muscle. MD Consulting Staff. PharmD. Washington Veterans Affairs Medical Center Friedhelm Sandbrink. Assistant Clinical Professor of Neurology. MD is a member of the following medical societies: American Academy of Neurology and American Academy of Pain Medicine Disclosure: Nothing to disclose.5/26/13 Motor Unit Recruitment in EMG In this patient w ith muscular dystrophy. Department of Neurology. Editor-in-Chief. 179(1):57-66. Nandedkar SD. Analysis of electrical activity in healthy and dystrophic muscle in man. Dumitru D. Petajan JH. Neurology Specialists and Consultants Nicholas Lorenzo. 1995:221-9. [Medline]. A simulation model of the surface EMG signal for analysis of muscle activity during the gait cycle. Willison RG. 8. Focal peripheral neuropathies. Motor unit firing rates and firing rate variability in the detection of neuromuscular disorders. Aug 11 2005. Munson JB. Jul 16 2005.56(6):672-81. Ross BH. Can the size principle be detected in conventional EMG recordings?. [Medline]. Hodson-Tole EF. Muscle Nerve. Carpenter DO.18(4):435-9. 5. J Clin Neurophysiol. May 1965. Stefano AD. Electroencephalogr Clin Neurophysiol. 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