analysis and dystrophic man · foundbypalpation andthe needle inserted at this point. three disa...

J. Neurol. Neurosurg. Psychiat., 1964, 27, 386

Analysis of electrical activity in healthy anddystrophic muscle in man

R. G. WILLISONFrom The Institute of Neurology, Queen Square, London

In clinical electromyography it is often difficult toanalyse the differences between the electrical activityof healthy and of dystrophic muscle. The measure-ment of the duration and amplitude of single motorunit potentials is perhaps the most reliable methodof quantitative assessment in current use (Kugelberg,1949; Pinelli and Buchthal, 1953). For this purposeit is necessary to record the waveforms of individualmotor units at levels of voluntary effort so low thatthey may be seen in isolation. Unfortunately thesemeasurements do not characterize one of the moststriking features ofthe activity of dystrophic muscles,which is the appearance of large numbers ofpotentials at relatively low tensions.Another approach involves the analysis of the

pattern of activity during strong contraction when itis no longer possible to identify single motor unitpotentials on the trace. The only technique in currentuse for this purpose is frequency analysis (Richard-son, 1951 ; Walton, 1952) but even this has not gainedwide acceptance, and subjective assessment of theelectromyograph (E.M.G.) by eye and ear is stillcurrent practice in the diagnosis of muscle disease.Subjective methods are not only undesirable in thediagnosis of established cases (Gilliatt, 1962) but areeven more unsuitable for the detection of potentialcarriers of inherited muscular dystrophy in whomthe abnormalities may be expected to be slight(Barwick, 1963; van den Bosch, 1963). The first stepaway from this situation is to describe the patternseen during voluntary effort in simple numericalterms. In order to do this a method has been de-veloped which involves the analysis of voluntaryactivity recorded with a concentric needle electrodeat standard tensions; photographic records aremeasured by a counting device in order to obtain thenumber of potential changes per second present inthe trace and the mean amplitude of these potentials.The details of the counting method have beenpreviously described (Willison, 1963) but a summaryis given below.

MATERIAL AND METHODS

Observations were made on 24 control subjects. They

were healthy subjects associated with the laboratory,their ages ranging from 18 to 40 years. Twenty patientswith muscular dystrophy and polymyositis were examined.Their ages ranged from 12 to 61 years.

Subjects lay supine on a couch in a warm room. Thearm under examination was supported by two slingsapproximately 90 cm. in length suspended from a gantry(Fig. 1). One sling was placed around the elbow and thesecond around the wrist so that the forearm could bemoved freely in the horizontal plane without touchingthe chest. The position of the head and shoulders wascarefully adjusted with pillows so that the subject wascomfortable and able to relax. A webbing cuff was alsoplaced round the wrist. A nylon cord attached to this ledover a pulley mounted on a pillar behind the head,weights being placed on a hook on this line. The relativepositions of the pulley and the subject were adjusted sothat the cord was as nearly as possible parallel with theupper arm in both vertical and horizontal planes.Maximal tension was measured by means of a spring

balance attached to the cord, the subject being requestedto extend the arm as steadily and forcefully as possible,the balance being used to keep the elbow joint at 90°. Therange of tensions found in the healthy subjects extendedfrom 6-5 to 16 kg. Next, trial weights were applied so thatthe subject became accustomed to maintaining the elbowat a right angle while resisting the load of the weight. Inhealthy subjects and in most patients with dystrophy thecontribution of the long head of the triceps to extensionof the elbow at the tensions used appeared to be small.The E.M.G. was therefore recorded from the lateral headof the triceps. The bulkiest portion of the lateral head wasfound by palpation and the needle inserted at this point.

Three Disa needles (type 13 K 51) were used in thestudy. Their length was 30 mm., outside diameter0-45 mm., core area 0-07 mm.2, and bevel angle 15°.Before use each needle was prepared in 0 9y% saline byapplying 3V A.C. at 50 c/s across core and shaft with asuitable meter in series. A minimal impedance of 2 5 to7 kilohms was usually reached within 15 sec., after whichno further fall occurred. If the impedance did not fallrapidly on application of current the needle was wipedfirmly with a sterile swab and the process repeated. Theneedle led through its standard cable and socket to a pairof cathode-follower input stages mounted on the couchclose to the subject's arm. The screen of the cable wasearthed and an earth lead was taken to a metal plate3-5 cm. x 6 cm. strapped to the forearm. The core andshaft of the needle were each connected via 0-15 uF

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Analysis of electrical activity in healthy and dystrophic muscle in man

pulse counter is operated once and the extent of the move-ment of the rack is recorded on a pair of rotary ratchetcounters by means of a series of gears and levers. Theratchet counters are arranged so that the displacement ineach direction is totalled separately.A portion of trace from a healthy subject is shown in

Fig. 2, the direction of analysis being from left to right.The interrupted lines indicate the movements of the cursorwhich occur without movements of the rack and showthe backlash of 100 uV. At each transition from an inter-rupted line to a solid line a count is registered on the pulsecounter. At the same point the measurement of amplitudeis started and it ceases at the end of the movement of thecursor in that direction. In the epoch shown here thepulse counter would register nine counts and the ratchetcounters would register the amplitude of five excursionsin an upward direction and four excursions downwards.In the calculation of mean potential amplitude the totalamplitude is divided by the number of counts in the sameportion of the record and it is clear from Fig. 2 that100 HtV must be added to this figure as a correction forthe backlash.

FIG. 1. Position of subject, slings, and cathode follower.

capacitors to the grids of the double triode used in thecathode-follower stages. The input impedance of eachhalf to earth was approximately equivalent to 75 megohmswith 7 pF in parallel. The output impedance of thecathode-follower was 2-5 kilohms and long leads weretaken to a Tektronix FM 122 pre-amplifier and 502oscilloscope. The frequency response of the pre-amplifierwas 3 db down at 40 Kc/s and 8 c/s and the amplificationwas approximately 1,000 times. A calibration signal of1 mV derived from the oscilloscope was always appliedto the input of the cathode-follower at the end of eachrecording. A time scale of pulses at 10 msec. intervals wasderived from a crystal-controlled oscillator and frequencydivider and this was displayed on the lower beam of theoscilloscope, except in a few earlier experiments when a50 c/s signal was used. Continuous records were taken onmoving film at approximately 30 cm./sec. with a stationaryspot. A separate oscilloscope and loud speaker were usedto monitor the activity of the muscle.The needle was inserted into the muscle and records for

measurement of the activity-tension curve were takenwith four standard weights of 0-5, 1-0, 1-5, and 2-0 kg.Care was taken not to allow the forearm or the needle tomove so that records were taken at the same musclelength from the same site. Following this, recordings weremade with the needle in other positions.For analysis the film was projected on to the baseboard

of a 35 mm. enlarger and examined with the specialcounting device. Using this, the wave form is tracedvisually with a transparent straight edge which is free tomove backwards and forwards on the enlarger baseboard(i.e., in the Y axis). A piece of fine-toothed rack is con-nected to the cursor through a sliding joint which pro-vides backlash equivalent to an amplitude of 100 uV onthe trace. This rack does not therefore follow movementsof the cursor which are less than the equivalent of 100 4tV.When the cursor is moved a distance greater than this a

socsFIG. 2. Tracingfrom triceps in a healthy subject. Directionof analysis indicated by arrows. Interrupted lines indicatebacklash of counter, solid lines show portions included inamplitude count (by permission of the Editors, Journal ofPhysiology).

In both healthy subjects and patients single sections ofrecord one second in duration and free from artefactwere examined, and the count rate and mean amplitude,expressed in mV, were calculated. Patients with musculardystrophy who were too weak to maintain steady tensionswere excluded from the study as the analysis of electricalactivity recorded during fluctuating effort was unsatis-factory; furthermore very weak patients often showedtrick movements. For example, the contribution of thelong head of the triceps to elbow extension might besubstantially greater than is normally the case. From thepractical point of view severely affected patients do notrequire this form of analysis as gross and easily recog-nizable changes in motor unit potential wave form areapparent on inspection of the trace.

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RESULTS

ACTIVITY-TENSION RELATIONSHIP IN HEALTHY SUBJECTSThe relationship between electrical activity and ten-sion was determined in 22 control subjects. In eachcase the needle was placed in the triceps at a pointapproximately 2 cm. anterior to the junction of thelower third and upper two-thirds of a line joining theacromion to the lateral epicondyle of the humerus.At a depth of 2 to 2-5 cm. a site could usually befound which showed clearly defined activity on themonitor CRT at 0-5 kg. tension.The four weights were applied in turn, recordings

being made for a period of 2 to 3 sec. as soon as theactivity had become uniform after the subject hadtaken up the load. The position of the forearm wascarefully watched to ensure that the elbow remainedat 900.The records were measured with the counter and

the results are shown in Fig. 3 in which the countrate is plotted against tension. In several subjectsactivity-tension curves were determined on bothsides and on different occasions. The curve for thesesubjects included in Fig. 3 is that which showed thehighest count rate obtained at a tension of 2-0 kg.The graph shows considerable variation from

subject to subject; from the mean figures for thegroup as a whole (interrupted line) it is clear thatthere was a linear rise in count rate with increasing

500

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FIG. 3. Count rates ofpotential changes in triceps plottedagainst tension in 22 control subjects. Mean valuesindicated by interrupted line.

tension. It will be noted that the projection of thisline would not pass through the origin but wouldreach the baseline at a negative value of approxi-mately -0 5 kg. This may be the reflection of thetension required to take up the slack in the muscle,and in the cuff and sling. These activity-tensioncurves show that the upper figures for the group asa whole are 275, 400, 480, and 543 counts/sec. fortensions of0 5, 10, 1-5, and 2-0 kg. respectively.READINGS AT 2-0 KG. IN HEALTHY SUBJECTS Thevariation between subjects was more fully examinedin 23 healthy subjects. At 2-0 kg. tension the needlewas placed in different positions in the muscles, thedepth varying from 2-75 to 1-5 cm. During contrac-tion the trace was observed in order to find the areasshowing the greatest activity. Because the countinghad to be carried out after the observations had beenmade it was necessary to make numerous photo-graphic records in order to be sure that the highestfigure was measured. Even with practice it was notpossible to estimate the count rate from the oscil-loscope and loudspeaker to within 50 counts/sec., theerror sometimes being considerably higher than this.The activity during a period of one second wasexamined using the counter. The count rates ob-tained are shown in Fig. 4 where they are plottedvertically above the index number of the subject. Inorder to determine whether the readings in individualsubjects are representative of the group as a wholean analysis of variance has been made of these data.The estimate of variance between samples is 17507,within samples 2572. The ratio of variance is 6-8 andusing Snedecor's variance ratio test, this indicatesthat the samples from the individual subjects showsignificantly less variation than the population as awhole (F = 2 0 at 1 % level, df(vl) = 115, df(v2) =22).The count rates at 2-0 kg. tension may be used to

define the upper portion of the normal range. Thusthe highest figure is 543/sec., the only count over500/sec., whereas five subjects had counts between450 and 500/sec. The highest count at 2-0 kg. tensionwas found in 11 out of 21 instances at the first needleinsertion when the activity at four tensions wasexamined. In 10 other subjects a higher figure wasfound during the subsequent search for areas show-ing greatest activity at 2-0 kg.

RESULTS IN PATIENTS WITH MUSCLE DISEASE Eighteenpatients suffering from muscle disease were studied.They included eight suffering from progressivemuscular dystrophy, five with dystrophia myotonica,and five with polymyositis. The techniques weresimilar to those used in the control series, the tricepsmuscle and the same standard weights being used for

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I 0

* * FIG. 4. Count rates of potential. changes at 2-0 kg. tension in

triceps with different needle posi-* 1 tions in 23 control subjects.

8 10 24 7 3 2 6 12 20 9 14 21 4 18 16 13

SUBJECT

23 15 22 1 11 19 17

the activity-tension curves. Triceps was by no meansthe weakest muscle in many of the patients but, sincepreliminary observations had shown that count ratesvaried considerably in different muscles, it was clearthat valid comparisons between healthy subjects andpatients could only be made by using the samemuscle in both groups.

Activity-tension curves were estimated in 18patients. Illustrative records from a healthy subjectand a patient with muscular dystrophy are shown inFigure 5. In this case there is an obvious differencein the number of potential changes, which is con-firmed by the count rates shown below each trace.The figures relating to mean amplitude will be dis-cussed in a later section.

Results from the 18 patients are shown graphicallyin Fig. 6 in which the normal range illustrated inFig. 3 is shown as a shaded area. Count rates up tonearly three times the highest normal figure wereobtained in some patients. In all but three patientsthe curves lie outside the normal range in whole orin part. The increase in count rate is most con-spicuous at 2-0 kg. tension so that this appears toprovide the most useful level of activity fordifferentiating between healthy subjects and patients.The highest count rate at this tension was 1,452/sec.

obtained from a patient, Mrs. A.Y. (NH no. Al 5511),who was 56 years of age and had suffered from slowlyprogressive muscular dystrophy of facio-scapulo-humeral type for over 30 years. The maximal tensionwhich she was able to sustain was 4 kg. (normalrange 6-5 to 16 kg.). Thus her effort at 2-0 kg. tensionwas half-maximal compared with the healthy subjectsin whom 2-0 kg. tension is between one-third andone-eighth of maximum. The four patients who hadcounts over 1,000/sec. at 2-0 kg. tension showedmaximal tensions of 4 kg. or less; at the other end ofthe range there were six patients with count rates ofover 600/sec. whose maximal effort was within thenormal range of 6-5 to 16 kg., so that the relation

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FIG. 5. Portions of primary trace from triceps in healthysubject (A) andpatient with progressive muscular dystrophy(B) with count rates and mean amplitudes at four standardtensions. The count rates are higher in the patient than inthe healthy subject and confirm the visible difference.

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FIG. 6. Count rates ofpotential changes in triceps plottedagainst tension in 18 patients with muscle disease. Shadedarea indicates normal range.

between low maximal tension and high count ratesis approximate rather than precise.

MEAN AMPLITUDE OF POTENTIAL CHANGES The meanamplitude of potential changes is calculated bydividing the total amplitude of all excursions ex-pressed in mV by the total count for the same portionof the record. Figure 7 shows 22 activity-tensioncurves in the healthy subjects in which mean ampli-tude, expressed in mV, is plotted against the tensionin kilograms. These figures represent the mean ampli-tudes corresponding to the count rates shown inFigure 3. The scatter is wide but the mean values forthe group are 0 35, 0 40, 0 47, and 0-51 mV at ten-sions of 0 5, 1-0, 1P5, and 2-0 kg. respectively. It isclear that there is a slight rise in mean amplitude forthe group with increasing tension but that the per-centage increase in the amplitude is less than thepercentage increase in mean count rate (45% asopposed to 138% over the tension range examined).On examination of individual curves in Fig. 7 there

TENSION (kg.)

FIG. 7. Mean amplitudes of potential changes in tricepsplotted against tension in 22 control subjects. Thesecorrespond to the curves shown in Figure 3.

are obvious irregularities, the mean amplitude fallingmarkedly with increasing tension in several instances.Such exceptions to the general behaviour may be dueto slight movement of the recording electrode or toalterations in the pattern of activity at the needle tipcaused by the change in tension. In one subject inwhom the mean amplitudes were 0 53 mV and0-36 mV at 0 5 kg. and 1-0 kg. tension respectivelythe count rates were 101/sec. and 254/sec. Examina-tion of the records reveals the presence of a numberof small potentials in the record at 1I0 kg. These werenot seen at the lower tension which contained feweroscillations of greater amplitude, a single unit closeto the needle making a major contribution to thetrace; thus the mean amplitude was higher at thelower count rate. In only five instances were thereinconsistent falls of mean amplitude between twotensions greater than 10% of the figure at the lowertension. It is interesting that these accompaniedcount rates of 360/sec. or less. If the combination ofsmall needle movements and sampling error is

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Analysis of electrical activity in healthy and dystrophic muscle in man 391

FIG. 9. Portionof primary tracein patient with

iSmV dermatomyositisto show largeamplitude, briefrise time, andhigh count rateof the potentialchanges.

800 900 1000>

FIG. 8. Mean amplitudes (log scale) at 2-0 kg. tension intriceps plotted against count rates over 350/sec. found in23 healthy subjects (closed circles) and 18 patients withmuscle disease (open circles).

responsible for these irregularities it is reassuring tofind that except in certain cases with low count ratesthese factors will not produce inconsistent results.The relationship between mean amplitude and

count rate is shown in Fig. 8 for all counts over

350/sec. at 2-0 kg. tension. Each figure is calculatedfrom measurements of a one-second period ofactivity; 23 control subjects (closed circles) and 18patients with muscle disease (open circles) are

shown. It is clear that the affected patients show a

much wider range of amplitude (0-21 to 1 0 mV) thanthe healthy subjects in whom the figures range from0-40 to 0-87 mV.The traces seen in some patients with muscle

disease which show high count rates and high meanamplitudes are of great interest because the largemean amplitude implies that active subunits are

close to the tip of the recording needle, probablywithin 0 5 mm. or less (Buchthal, Guld, and Rosen-falck, 1957). The high count rate also indicates thatmany units near to the tip of the needle are activebecause only potential changes over 100 ,tV are

counted and distant activity would be attenuatedbelow this level. A specimen of record in a patientwith polymyositis (Mrs. J.L. MH no. K21810) isillustrated in Fig. 9 to demonstrate the large ampli-tude of individual potentials. This, in associationwith the brief rise time, suggests that there was pro-

fuse activity near the needle. The mean amplitude inthis muscle was 0-79 mV with a count rate of

1,286/sec. This record also shows that the brevityof the potential changes makes it possible for highcounts to be reached in spite of their large ampli-tude, as suggested by Buchthal and Rosenfalck(1963).At the lower end of the range of mean amplitude

there are 24 readings in eight patients which arebelow those encountered in healthy subjects. Half ofthese have count rates within the normal range. Thiscombination of findings is frequently encountered inseverely affected muscle. The explanation of this isclarified by consideration of a patient whose weak-ness was so extreme that he was not included inFigure 8. This was a 61-year-old man (Mr. A. F.NH no. A9450) with progressive muscular dystrophyof 35 years' duration. The muscle was so weak thatmovement of the joint was only just possible. Aportion of the record is shown in Fig. 10 from whichit can be seen that individual potential changes areof low amplitude and many are of brief duration.Table I shows the figures obtained. Many potentialswere below 100 /xV in amplitude and so were onlycounted when the significance level was reduced, thecount rate being highest and the mean amplitudelowest when potentials of 25 ,uV and above werecounted. It can be envisaged that in an advancedcase not only would the amplitude of individual unitsbe low but even the number would be reduced as it isgenerally acknowledged that this occurs in the latestages of the disease. Thus low count rates and lowmean amplitudes may be expected in the mostseverely affected patients. In such patients countingat levels lower than 100 ,uV is necessary if a realisticpicture of electrical activity is to be obtained.From the data presented in Fig. 8 it is clear that

patients with myopathy may be distinguished fromhealthy subjects by the occurrence of high countrates or abnormal mean amplitudes which may be

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3A92.. G.WAiisAon

A.F. 11-163 RT TRICEPS-NO LOAD 2Omsec.

FIG. 10. Portion of primary trace in patient with severe progressive musculardystrophy. Triceps was severely affected (M.R.C. scale 2) and this record wastaken at maximal effort. Count rates and mean amplitudes are shown in Table L

TABLE ICOUNT RATES AND MEAN AMPLITUDE AT THREE LEVELS INA PATIENT WITH SEVERE WEAKNESS (PRIMARY TRACE SHOWN

IN FIG. 10)Count Counts/Second MeanLevel Amplitude([AV) (mV)

1005025

72157335

above or below the normal range. There remaineight readings encountered in three patients in whichboth count rate and mean amplitude are within thenormal range. However, in each of these patientsthere were other areas in the muscle which were

abnormal by the criteria already described so itproved possible to delineate every patient from thecontrol group. This illustrates the advantage ofrecording from more than one site.

DISCUSSION

In clinical electrodiagnosis previous attempts toanalyse the pattern of activity on voluntary efforthave been directed towards automatic methods suchas frequency analysis (Walton, 1952; Richardson,1951). In physiological studies Bergstr6m (1959)compared spike counts with the integrated activity,and Close, Nickel, and Todd (1960) used a pulsecounter to count spikes during voluntary effortagainst tension, but such methods do not yet appearto have been applied to the study of diseased muscle.The method used in the present study is designed

to circumvent some of the unknown factors whichmay influence automatic frequency analysis or pulsecounting using a conventional counter. Thus thepresent technique depends upon following the trace,using the point of change of direction of potentialchanges as the index mark for assessing the amplitudeof the following potential; this overcomes the diffi-culty inherent in using a fixed baseline. In practiceboth pulse counters and integrating devices are in-fluenced by the voltage level which depends on thecoupling capacitances in the system. A conventionalRC coupled pulse counter will fail to count smalloscillations superimposed on slower and larger

waves unless the coupling time constant is reducedto a value which may interfere with the transmissionof slow changes.The relationship between potential changes

counted by this method and the action potentials ofindividual motor units requires consideration becauseall the records described in the paper have beenmade at levels of voluntary activity at which it is notpossible to identify the action potentials of individualunits. It might be thought that each potential changerepresented one phase of an action potential but ifthis was so the amplitude should remain constantwith tension. Figure 7 shows that mean amplitudeincreases with rising tension, as does the count rate.This suggests that summation of the activity ofdifferent units may occur by coincidence in time ofpotential changes in the same direction. No doubtsome cancellation also occurs when potentials ofopposite sign are simultaneous but, if cancellationwere dominant, mean amplitude might be expectedto decrease with increasing tension. The precise con-figuration of the waveforms of individual units maybe expected to influence this complex interaction. Itfollows that we may consider potential changes asrepresenting motor unit potentials only in a generalway, summation and cancellation obscuring a clearrelationship. Thus it is incorrect to refer to thecounts as action potential counts, and the count ratesdescribed in this study should not be regarded ashaving a definite relation to the discharge rate ofmotor units.

In order to measure the relation between countrate and tension some of the sources of variation,such as the particular muscle examined, limb postureand hence muscle length, the type of needle used, andthe high input impedance, were kept constant.Despite this it is clear that there was a considerablevariation from subject to subject (Fig. 3) and thatneedle position (Fig. 4) affected the count rate. Largeelectrodes might sample a greater number of motorunits but the use of a small recording electrode isnecessary if patchy involvement of muscle is to bedetected in polymyositis. It is of interest in this con-nexion that the highest potential counts recorded inthe soleus by Close, Nickel, and Todd (1960) were ofthe order of 200/sec. using wire electrodes bared for

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i in. at the tip. These were taken at maximal tension,and, assuming that the pulses recorded by them weremonophasic, this figure becomes 400/sec. in terms ofpotential changes measured by the present method.It may be that small needle electrodes give highercount rates for a given tension than large electrodesand this point requires further examination, buttheir method of counting differed considerably fromthe technique described here and only approximatecomparisons may be made.

If there is significant summation and cancellationof action potentials it might be expected that thebriefer the individual components, the more likely itis that they will appear separately and be counted.Thus at maximal effort we might expect highercounts in patients with muscle disease than in healthysubjects because the action potential duration isreduced in muscle disease and polyphasic potentialsare more frequent. It is also probable that individualmotor units develop less tension than in healthysubjects (Lenman, 1959). Therefore the examinationof activity at low tensions would be expected tobring out a difference between the density of activeunits in the region of the needle in healthy andaffected muscles. The use of a standard tension of2-0 kg. suitable to the triceps allowed comparisonsto be made between subjects whereas maximaltension varied so widely that it was not a usefulstandard.Although it has been said that increase in mean

amplitude is a result of summation of coincidentaction potential phases in the same direction, it ispossible that motor units recruited at high tensionsare of greater amplitude than those recruited atminimal effort (Norris and Gasteiger, 1955). Thepotentials of 10 to 15 mV amplitude in Fig. 9 wereonly occasionally seen in records from the sameregion of the muscle taken at tensions less than 2-0 kg.This question requires further investigation.Some patients with muscle disease showed mean

amplitudes less than those found in healthy subjectsat 2'0 kg. tension. The count rates in these patientswere often within or close to the normal range. It isnot clear whether motor unit potentials were smallerin this group than in those showing high mean ampli-tudes or whether potentials were attenuated by thepresence of inactive tissue such as fat or connectivetissue. The findings shown in Fig. 10 and Table Isuggest that at least in some patients it seems likelythat action potential amplitudes were smaller.Possibly the use of more selective electrodes wouldhelp to clarify the issue. If areas of very low activitywere found in severely involved muscle it would seemlikely that such were occupied by inactive tissue.With the electrodes used in the present study, areasshowing little activity were found frequently even in

patients showing high count rat'es in other areas. Thusit was always necessary to make a deliberate searchfor portions of muscle showing high count rates.A disadvantage inherent in examining photo-

graphic records is the long delay before the countrate is known. It is not always possible to be certainwhen the areas showing high counts have been found,judging by the visual display and loudspeaker, andit may be necessary to carry out counts on the recordsfrom several areas in order to find the highest value.Even with experience it is not possible to make morethan rough estimates of the count rate by judgmentalone and this study has made it clear that subjectiveassessment is unreliable except in instances of grossalterations in the pattern.Examination of the muscle could be made more

thorough and more rapid with automatic methods ofcounting potential changes and measuring amplitude.Automatic methods would also help to extendstudies to other muscles in healthy subjects and inpatients with muscle disease. Certainly such studiesare required, for it is not possible at the present timeto investigate the weakest muscles in patients or themost suitable muscles in carriers of the sex-linkedrecessive forms of dystrophy.The development of an electronic method of

counting analogous to that used in the mechanicalcounter is in progress but there are several problemsto be overcome in designing a system which willanalyse both the most rapid and the slowest potentialchanges encountered in this study.

SUMMARY

A new method of analysing the rate and amplitudeof potential changes in the triceps muscle has beenused in 24 healthy subjects and in 20 patients withmuscle disease. The method involved the examina-tion of photographic records taken under standardconditions and at known tensions. The analysis isperformed by counting the rate and mean amplitudeof potential changes using a mechanical countingsystem. The reliability of the results is independentof the frequency and the amplitude of the recordedpotentials. Counts up to three times the highest en-countered in a healthy subject were seen in patientswith muscle disease. Abnormalities in the meanamplitude of potential changes were also found. Thesignificance of these changes is discussed.

I am most grateful to Professor R. W. Gilliatt andMr. H. B. Morton for advice and encouragement and tothe Medical Research Council for help with apparatus.A special grant from the Muscular Dystrophy Group fortechnical assistance and counting apparatus is alsogratefully acknowledged.

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REFERENCESBarwick, D. D. (1963). In Research in Muscular Dystrophy; Proc.

2nd Symposium, Muscular Dystrophy Group, pp. 10-19. PitmanMedical Publ. Co., London.

Bergstrom, R. M. (1959). The relation between the number ofimpulsesand the integrated electric activity in electromyogram. Actaphysiol. scand., 45, 97-101.

Buchthal, F., Guld, C., and Rosenfalck, P. (1957). Volume conductionof the spike of the motor unit potential investigated with a new

type of multielectrode. Ibid., 38, 331-354., and Rosenfalck, P. (1963). In Muscular Dystrophy in Man and

Animals, edited by G. H. Bourne and M. N., Golarz, p. 197.Karger, Basel.

Close, J. R., Nickel, E. D., and Todd, F. N. (1960). Motor-unitaction-potential counts. Their significance in isometric andisotonic contractions. J. Bone Jt Surg., 42A, 1207-1222.

Gilliatt, R. W. (1962). Electrodiagnosis and electromyography inclinical practice. Brit. med. J., 2, 1073-1079.

Kugelberg, E. (1949). Electromyography in muscular dystrophies.Differentiation between dystrophies and chronic lower motorneurone lesions. J. Neurol. Neurosurg. Psychiat., 12, 129-136.

Lenman, J. A. R. (1959). Quantitative electromyographicchanges associated with muscular weakness. Ibid., 22,306-310.

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