The Contribution Of Dynamic Electromyography To Gait Analysis
SECTION TWOChapter OneThe Contribution of Dynamic Electromyography toGait Analysisby Jacquelin Perry, M.D.Dr. Perry is Director of the Pathokinesiology Lab at the Rancho Los Amigos Medical Center in Downey, California.INTRODUCTIONThe purpose of dynamic electromyography is toaccurately define the muscle action that controls jointmotion . While gross function of muscle groups can beinferred from motion and moment calculations, specificity of muscle function requires a more The Functional ChallengeWalking relies on selective timing and intensity ofappropriate muscles at each joint to provide weightbearing stability, shock absorption, and progression overthe supporting foot during stance and to advance thelimb in swing . Energy is conserved by activating onlythe muscles optimally aligned for each task and bysubstituting momentum and passive tissue tension fordirect muscle activity wherever possible.Throughout this sequence of functions, the musclesperform in groups, as shown in Figure 1 (1) . While thedominant motions of the lower limb occur in the sagittalplane (i .e., flexion and extension for the demands ofprogression), there also are significant actions in theother two planes (coronal and transverse) to enhancesingle limb balance and body rotations . Each muscle hasa unique three-dimensional (3D) effect determined byits alignment across the joint or joints it crosses . Inaddition, most muscles are members of two or moreANKLE / FOOT(plantar flexion)ICTOFigure I.Normal sequence of synergistic activity of the major extensormuscle groups during stance . Linear display of the EMG amplitudes(vertical scale) of the individual muscles identify their relativeintensity and timing . Hamstrings (biceps femoris, semimembranosus,semitendinosus) ; Vasti (intermedius, lateralis, medialis longus,medialis oblique) ; Plantar flexors (soleus, gastrocnemius, tibialisposterior [biphasicl) . IC (initial contact) indicates onset of stance.Note extensor muscles begin in late swing.functional groups . This redundancy assures 3D balanceand serves to simplify the integration of adjacent jointaction. Relative intensity of action of a particular33
34RRDS Gait Analysis in the Science of Rehabilitationmuscle is determined by which of its functions ismomentarily dominant. Hence, just understanding normal function requires a detailed study of individualmuscle action . Such information alsocan identify theeffects of orthoses, muscle training regimens, etc.Dynamic electromyography offers the means of precisely relating muscle action to the specific function.The Influence of PathologyThe normal, complex walking pattern can bedisrupted in many ways . Muscles may be weakened bydisuse, pain, or direct injury . Fibrous tissue contracturemay limit passive mobility . Orthoses incidentally restrict adjacent motion while purposefully protecting thearea of concern. Brain and spinal cord injury maydisrupt the primary motor control and feedback pathways . Persons with spastic paralysis, stroke, or headtrauma, present the greatest diagnostic challenge asmuscle function is disrupted at many levels and theoverlay of spasticity often causes the clinical tests todiffer significantly from the muscle pattern used duringwalking . Even lower motor neuron lesions can presentunpredictable situations . Individuals preserve their ability to walk by substituting, to the extent their selectivecontrol allows . Alternate motions and muscle actionsare used to overcome the limitations imposed bypathology . Such substitution capability varies markedlyamong individuals . Consequently, the person's walkingpattern is a mixture of primary functional loss andsubstitutive actions . The results are mixtures of inadequate, excessive, inappropriately timed, or out-of-phasemuscle action . To best design retraining protocols,optimize orthotic assistance, or to plan an appropriatereconstructive surgical procedure, it is essential to knowmuscle function as it is occurring rather than assumed.This requires dynamic electromyography.METHODSMyoelectric Signal AnatomyElectromyography (EMG) is a system that recordsthe electrical signals activating the muscle fibers . Fromsuch information, one can determine the timing andrelative intensity during both normal and abnormalfunction . Under specific circumstances, muscle forcealso can be calculated .a.MUSCLE FIBER/NEURONiMUSELECTRICAL FIELDFigure 2.a) Muscle Fiber Structure : Each muscle fiber is a bundle ofmyofibrils (chains of contractile units called sarcomeres) . Interplayof the thin and thick filament within the sarcomere creates themuscle force . b) Muscle Fiber Myoelectric Signal : An electrical fieldis created by stimulation from the neuron activating the musclefiber's chemical receptors (shaded circles), which in turn, send anelectrical charge up and down each myofibril to activate the chain ofsarcomeres . Adapted from reference (2) . Used with permission.Each muscle fiber consists of multiple long chains(myofibrils) of contractile units (2) called sarcomeres,which create the force of muscle action (Figure 2a) . Asthe local neuron chemically activates the muscle fiber atits myoneural junction, an electrical charge is sent upand down each myofibril (Figure 2b), stimulating thesarcomeres to contract (3) . This event creates anelectromagnetic field, which can be used to trackmuscle activity (4) . By volume conduction, the localsignal spreads through the tissues making it technicallypossible to record the signal at the skin surface as wellas internally.Neural control is simplified by having large groupsof muscle fibers controlled by a single motor cell bodylocated in the anterior horn of the spinal cord . This
35Chapter One : EMG Dynamicscomposite of cell body, connecting neuron, and themuscle fiber cluster is called a motor unit . Thegastrocnemius, for example, is composed of approximately one million muscle fibers clustered in 600 motorunits (5) . Animal experimentation has shown that themuscle fibers of each motor unit are widely dispersedthrough the muscle . Only a few units are needed tocreate a weak effort throughout the whole muscle . In themultipennate soleus, for example, one motor unit isspread across 60 percent of the muscle's volume, asshown in Figure 3 (6) . Theoretically, just two motorunits would be sufficient to traverse the whole muscle.In contrast, a motor unit in the unipennate tibialisanterior covers only 16 percent of the volume (7) . Now6 motor units would be needed . The practical interpretation of this anatomical fact is that during walking andother physiological functions, muscle action can berecorded regardless of the location of the electrode overor within the muscle.Interspersion of tendonous tissues, however, reduces the concentration of muscle fibers ; thus, themiddle of the muscle belly is the site where the largestsignals are obtained . To be even more precise, maximum signal occurs at the muscle's motor point (8) . Using the gastrocnemius as an example, 6 motor unitswould represent only 0 .1 percent muscular effort, whilea clinical strength grade of 2 (poor), which represents amuscle too weak to accept even the resistance ofgravity, averages 5 percent . Theoretically, this represents 30 motor units, a minimum contraction situation.As more motor units are activated, the intensity of themuscular response increases and the EMG signalbecomes larger . Clinically, this is reflected as a greaterfunctional force.Myoelectric Signal QualitiesThe signal recorded during functional EMG isdescribed as random because it does not have aconsistent waveform . Instead, the individual spikes varyin amplitude and duration without an identifiablesequence . This inconsistency reflects the fact that everymuscular effort is a composite of multiple motor units,each activating multiple muscle fibers . In addition, eachfiber's response to stimulation is a brief twitch and,thus, repeated stimulation is required to generate auseful force . Hence, the EMG signal of muscle action isFigure 3.Motor Unit Territory : The vertical shaded areas in the anterior andlateral projections show the distribution of the muscle fibers of onemotor unit within the rat soleus . The cross section identifies theindividual muscle fiber distribution (dots) for that motor unit.Adapted from reference (6) . Used with permission.a train of randomly shaped action potentials. Inaddition, the raw recorded electronic signal is contaminated by noise (i .e., unwanted signals arising fromtissue motion and the environment, such as lights,neighboring motors, and so forth) . The unwantedelectronic noise is excluded by filtering and the use ofdifferential amplifiers, which reject common modesignals.Waveforms are classified by their content ofdifferent sine wave frequencies Fourier analysis (4) . Insimplistic terms, sharply peaked waves have a highfrequency while broad waves have a low frequency . Thecomplex nature of myoelectric signals includes a very
36RRDS Gait Analysis in the Science of Rehabilitationbroad spectrum of frequencies, with the range from 10Hz to 1,000 Hz being considered significant to identifymuscle function related to joint motion (Figures 4a and4b) . Tissue displacement accompanying a muscle contraction can generate 10-Hz signals and floor impactduring walking gives rise to signals of 25—30 Hz.Hence, 40 Hz has become a customary lower value forgait EMG . In addition, a notch filter is used to excludethe common 60-Hz signals from electrical equipment.Signals above 1,000 Hz do exist but they represent lessthan 1 percent of the signal power and add nothing toour knowledge of muscle function, so instrumentationwith this capability is unnecessary . Hence, a bandwidthof 40-1,000 is appropriate.Muscle Specificity : Surface versus Wire ElectrodesFor functional EMG, the sensing electrode may beeither surface contacts (Figure 5a) or penetrating wires(Figure 5b) . The criteria for selecting an appropriateelectrode include the purpose of the EMG recording,a.1000 15005002000 2500Frequency (Hz)b.CUMULATIVE POWER (%)MUSCLEVastus Lateralis (wire)Vastus Lateralis surface10FREQUENCY (Hz)50901302036050580100Sartorius surface204080Figure 4.The typical normalized power spectral density (PSD) for wire andsurface electrodes : a) Percent total signal power per frequencyinterval . Determined by direct Fourier transform of data digitized at5000 samples/sec, total spectrum 5-2500 Hz with a 5 Hz resolution;b) Thresholds of power spectrum distribution. Frequency belowwhich 10, 50, 90% of the power spectrum occurs . From reference(9) . Used with permission .muscle anatomy, signal dispersion through the tissues,and tolerance of skin penetration with a fine needle.Surface ElectrodesThese EMG sensors have the advantage of convenience and comfort . An active electrode system merelyneeds to be taped over the center of the target muscle.Passive disc electrodes require a gel and skin cleansingto improve signal transmission . Of the 28 major musclescontrolling each lower limb that can be delineated byEMG, the majority are superficial . The dominant periodof activity of these subcutaneous muscles can be readilyidentified by surface electrodes.The major disadvantages to surface electrodes arecross talk and low signal reception . Their adverseeffects complicate the definition of muscle timing andthe relative intensity of the activity.Cross TalkDuring periods of low muscle activity, there is thepossibility that the EMG record may include signalsfrom musculature other than the muscle of interest.Surface electrodes sense all the signals that reach itsreception area. Volume conduction allows wide dispersion of the myoelectric signals through the tissues (10).The thin films of fascia between adjacent musclespresent no significant barrier to the myoelectric signalsfrom nearby muscles . Also, muscles function in groupsrather than in isolation . As a result, the recording from asurface electrode, by picking up the signals of asynergist may indicate activity in the designated musclewhen actually it is quiet.Several investigators have documented the presence of cross talk by comparing the output of wire andsurface electrodes . Perry et al . (11) confirmed groupmuscle action by demonstrating simultaneous activity ofthe soleus, gastrocnemius, and tibialis posterior duringtraditional manual strength tests purported to isolate thetargeted muscle . Peak muscular effort, however, corresponded to the designated muscle . The finding that thesurface electrodes included EMG from the adjacentmuscles implied greater activity than was confirmed bythe wire data . De Luca and Merletti (12) studied thesignal spread that accompanied electrical stimulation ofthe tibialis anterior . They found signals in the peroneusbrevis and soleus that approximated 17 percent of themaximum tibialis anterior EMG . Koh and Grabner (13),using both stimulation and voluntary quadriceps activa-
37Chapter One: EMG DynamicsFigure 5a.Electrodes for DynamicEMG . Surface : (left) Apassive electrode paircontaining 2-mm diameter silver silver-chloridedisc centers . (Center andright) Examples of activeelectrodes with signalpreamplification circuitryimbedded in the electrodehousing . The elements ofthe center electrodes are1-cm by 0 .1-cm barsspaced 1 cm apart. Theright electrode elementsare 1-cm discs with aninterelectrode spacing of3 .5 cm.Figure 5b.Intramuscular wire electrodes are a pair of 50micron, nylon insulatednickel-chromium alloywires ' with the distal2-mm bare tips, placed ina 3 .81 cm 25- or 30gauge needle for intramuscular insertion . Inset:Note, to allow singleneedle insertion, the external barbs must differ inlength to avoid contactbetween the bared tips.' California Fine-WireCompany, Grover Beach,CA 93433 .
38RRDS Gait Analysis in the Science of Rehabilitationpragmatic approach might be to eliminate the lowintensity signals representing 17 percent of maximum orfrom 7 to 10 percent of a typical submaximal peakintensity . This could clarify some of the phasinginterpretations.50a.b.Cocontraction6 .50ICFigure 6.Cross talk : a) Surface electrode recording of antagonistic flexor ( )and extensor ( ) muscles during walking to identify cocontraction.Shaded area identifies occurrence of simultaneous EMG by bothelectrodes . Adapted from reference (14) . b) Wire electrode recordingof similar hamstrings ( ) and quadriceps () action . Note thecontinuous baseline of EMG in the surface recording that is notpresent in the wire electrode data . This is a display of signal crosstalk from adjacent muscles . The taller shaded areas in bothrecordings represent true cocontraction of antagonist muscles.Adapted from reference (I) . Used with permission.tion to study cross talk, found EMG signals in themedial and lateral hamstrings averaging 11 percent and17 percent of a maximum effort, respectively . Theyattributed the difference in these means to the greaterdistance of the medial electrode from the quadricepsmuscle mass . Hence, a significant level of cross talkfrom adjacent and even moderately remote muscles hasbeen confirmed for both the thigh and lower leg . Thiscomplicates the determination of onset and cessationtimes of muscles' action ; thereby confusing the preciseidentification of muscle phasing, which is a commonclinical objective.At present, there is no established method forcircumventing these data complications . Research studies have demonstrated that double differentiation canreduce the cross talk to half or less (12,14) . Thenecessary instrumentation, however, is just becomingavailable for use in the multiple muscle studies conducted clinically . Faced with this limitation, a possibleThe interpretation of simultaneous EMG in anagonist and antagonist may be confounded by thepresence of cross talk . As Koh and Grabiner concluded,low-to-moderate signals recorded with surface electrodes may be a cross talk artifact rather than cocontraction (13,14) . This was demonstrated in a recent study ofcocontraction of antagonists in children (15) . Theauthors showed continuous EMG throughout the gaitcycle. Superimposed on an average 6.5 percent maximum intensity baseline were regularly interspersedpeaks of 20 percent maximum (Figure 6) . Wireelectrode recording from the literature shows that thehamstrings and quadriceps normally overlap in theirfunctions only during limb loading (1) ; hence, truecocontraction was phasic not continuous.Wire ElectrodesIntramuscular placement of the EMG sensorscircumvents the specificity limitations of surface electrodes . By having the electrode located within the targetmuscle, a much stronger signal is obtained and itsfrequency content is higher (Figure 7) . Both qualitiesserve to virtually eliminate the problem of cross talk.While myoelectric signals from neighboring musclesmay still spread through the tissues, their intensity isinsignificant due to their distance from the electrodes.A second advantage of wire electrodes is theopportunity to use the same signal gain for all muscles.A gain of 1,000 with wire electrodes provides a strongsignal for all muscles . This allows the clinician tovisually estimate the relative intensity of one muscle'saction compared with the others . In contrast, the lowreception of surface electrodes (Figure 7) commonlyrequires increasing the gain many fold to obtain areadable signal and the cross talk signals would besimilarly magnified . Variability in soft tissue resistancealso often necessitates adjusting the gain for individualmuscles in order to obtain a readable signal . Thus, wireelectrodes allow precise differentiation in the activity ofadjacent muscles, making this technique preferable forsurgical decisions.
39Chapter One : EMG Dynamics2505075100Muscle Effort (% Max)b . 100-75 -050C.12wwire25 surface0III255075100Muscle Effort (% Max)Figure 7.a) Total signal power of wire and surface with different spacing between the paired electrodes . Wire 2 .5-cm spacing inserted with separateneedles . Wire 0 .1-cm spacing represents single needle insertion . Surface 2 .5- and 5 .0-cm spacing indicates distance between the centers of two1-cm diameter discs . b) The Effect of Normalizing : For each electrode (wire and surface), the EMG recorded at each effort level (%Max) wasexpressed as a percent of the EMG obtained during the isometric maximum muscle test (MMT) . From reference (16) . Used with permission.
40RRDS Gait Analysis in the Science of RehabilitationThe disadvantage of wire electrodes is the need forskin penetration as the wire electrodes are inserted intothe muscle with a fine needle (gauge 25-30) . Unless thesubject has a bleeding tendency (which wouldcontraindicate wires), the only penalty is momentarydiscomfort . This is minimized by tensing the skin,knowing the desired location and making a rapidinsertion . Children as young as 4 years of age can besuccessfully tested with wire electrodes . Basmajian andStecko's technique of inserting both wires with a singleneedle has simplified electrode location (17) . A criticalfactor, however, is electrode fabrication . The end of thebarbs must be of different lengths so that their baredtips will not contact each other and short-out the signal(Figure 5b, inset).For both electrode systems, it is essential that thelocation relative to the target muscle be accuratelydetermined . Following electrode application, activity ofthe target muscle is determined by palpable contractionand/or tension of its tendon during a low effort muscletest . Wire electrodes also allow precise localization bylight electrical stimulation through the electrodes . Electrodes must be moved until the desired muscle actioncoincides with the EMG.EMG Signal TimingAs each muscle provides a specific function, thebasic information to be gained by dynamic electromyography is phasing within the gait cycle . The fundamental question is the time of onset and cessation ofeach muscle's activity relative to the limb mot
Gait Analysis by Jacquelin Perry, M.D. Dr. Perry is Director of the Pathokinesiology Lab at the Rancho Los Amigos Medical Center in Downey, California. INTRODUCTION The purpose of dynamic electromyography is to accurately define the muscle action that controls joint motion. While gross function of muscle groups can be
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