New methods of estimating the number ofmotor units in a muscle

Journal of Neurology, Neurosuirgery, and Psychiatry, 1976, 39, 258-265

New methods of estimating the number of motorunits in a muscle

H. S. MILNER-BROWN AND W. F. BROWN

From the Department of Clinical Neurological Sciences, University Hospital, London, Ontario, Canada

SYNOPSIS Two electrophysiological methods have been described for estimating the number ofmotor units (MU) in a muscle. The methods are based on the original methods of McComas andothers, which involve dividing the maximum compound potential (MCP) of muscle evoked bysupramaximal electrical stimulation of nerve by a mean motor unit potential (MMUP). The im-portant modifications in the methods are the incorporation of the fluctuations in the response

of MU to electrical excitation and a possible correction for the overlap in the firing levels of motoraxons. The methods have been used in the estimation of the number of motor units in the firstdorsal interosseous, thenar, hypothenar, and extensor digitorum brevis muscles of normal subjectsand patients with various neuromuscular disorders. The results indicate that previous motor unitestimates were in general erroneously high.

In the preceding paper (Brown and Milner-Brown, 1976) it was shown that two of the mostcritical assumptions made in the electro-physiological method of estimating the numberof motor units in a muscle introduced byMcComas et al. (1971) were not correct. Modifi-cations to the original method have recentlybeen introduced by Ballantyne and Hansen(1974) and Panayiotopoulos et al. (1974), butboth these two methods are influenced by thesame two critical assumptions.The first important factor in the estimation

of the number of motor units in a muscle, whichhave so far been disregarded by the aboveauthors, are inherent properties of motor axons:the fluctuations in their electrical excitability andtheir overlap in firing levels. The second im-portant factor is that motor units whose sizesare much larger than the incremental stepsevoked by nerve stimulation can be isolated bythe isometric voluntary contraction method(Milner-Brown et al., 1973a), the F-recurrentdischarge method (Brown and Feasby, 1974),and by stimulation of multiple points along the

1 Address for correspondence: Dr W. F. Brown, Department ofClinical Neurological Sciences, University Hospital, 339 WindermereRd., London, Ontario N6G 2K3, Canada.(Accepted 10 October 1975.)

258

length of the nerve (Brown and Milner-Brown,1976). Hence, any motor unit estimate whichexcludes these properties of motor units cangenerally lead to erroneous results. This paperdescribes two methods of estimating the numberof motor units in a muscle based on the originalmethod of McComas et al. (1971), but with theimportant modification of incorporating thefluctuations in the response of motor units toelectrical excitation and a possible correction forthe overlap in motor axon firing levels.

METHODS

The basic method is unchanged and consists firstin obtaining a mean motor unit action potentialrepresentative of the motor units in the muscle. Thenumber of motor units is then estimated by dividingthe maximum compound potential evoked in muscleby supramaximal electrical stimulation to the nerveby the mean motor unit potential (MMUP). Thetwo methods to be described were developed atdifferent times. The first method, which was morerecently developed, has been used in the motor unitestimation (MUE) of mainly the first dorsal inter-osseous (IDI) muscle, but the second method hasbeen applied extensively to the extensor digitorumbrevis (EDB) and thenar and hypothenar muscles.The methods will be presented separately, followedby the basic biophysical principles justifying them.

New methods of estimating the number of motor units in a muscle

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FIG. 1 Schematic diagrams to illustrate the fluctua-tions in excitability and overlap in firing levels ofmotor axons. A. At a constant voltage, the evokedpotential can vary between a minimum (Amn) and a

maximum (Amx), but with the predominant potential(Apd). B. Incremental steps which can be attributedto the following factors: (1) the recruitment ofadditional motor units-ifcertain criteria are satisfied,(2) the fluctuations in the excitability of motor units,or (3) the overlap in firing levels of motor units. C.The increase in firing index from 0 to 100% withincreasing voltage of six MU and the overlap in firinglevels of motor units MU1, MU2, MU3 and MU4,MU5, MU6.

METHOD I The subjects are comfortably seated andtheir hands and forearms are strapped to a paddedboard. Silver disc electrodes filled with conductivepaste are placed on the belly of the IDI muscle withthe indifferent electrode 3.0 cm away on the proximaljoint of the second finger. The ulnar nerve is stimu-lated (Devices 3290, 3072) at the wrist by means ofbipolar electrical pulses 0.1 ms in duration at 1/s.The evoked potentials from 1DI are amplified byGrass P15 preamplifiers at a frequency bandwidth30 Hz-10 kHz, displayed on an oscilloscope andstored on magnetic tape. The usual procedure is toincrease the stimulus voltage gradually until the firstmotor unit is excited. The voltage is then fixed at

this threshold voltage (V1) and 50 stimulus pulses

applied to the ulnar nerve. The motor unit potentialamplitude recorded from the muscle might varybetween a minimum (Amn(l)) and a maximum(Amx(l)). Between Amx(l) and Amn(l) there maybe other amplitudes; however, there is always apredominant amplitude (Apd(l)). Amx(l), Amn(l)and Apd(l) are noted (Fig. IA).The stimulus voltage is then gradually increased

until an incremental step greater than Amx(l) isobtained. Fifty stimulus pulses are applied to thenerve at this voltage (V2) and Amx(2), Amn(2), andApd(2) noted. Ten or more different voltages (Vn),with corresponding amplitudes Amx(n), Amn(n),Apd(n) are obtained. The procedure is repeated anumber of times, and the maximum compoundpotential obtained, at the end.

In estimating the mean motor unit potential(MMUP), the main criteria for identifying therecruitment of an additional unit are that Apd(n+ 1)> Apd(n) and Amn(n + 1) > Amx(n)-that is, thepredominant amplitude or area of the compoundpotential (CP) evoked by stimulating n +1 motorunits is greater than the compound potential evokedby stimulating n motor units and also the minimumCP evoked by stimulating n+ 1 MU should equalor exceed the maximum CP evoked by stimulating nMU with long train pulses and under the sameexperimental conditions. These criteria are based onthe general assumption that if a stimulus voltage V1is required to recruit the first n MU and a highervoltage V2 is needed to recruit the (n + l)th MU,then V2 should excite the same n MU recruited atvoltage V1, plus the additional (n+ I)th MU, underthe same experimental conditions. Thus it is evidentfrom the above criteria that 10 incremental steps donot necessarily represent the recruitment of 10 motorunits, but usually represent a fewer number of motorunits. A justification of the criteria will be consideredin the next section.

METHOD II The experimental arrangement used forMUE in EDB and thenar and hypothenar muscleshas been previously described in detail (Brown, 1973).As the stimulus voltage is increased, 0-2 motor unitswith distinct thresholds may be recruited. After thisnumber (n), any increase in stimulus voltage (V),generally results in a fluctuation in the incrementalsteps ('alternation') as shown in the preceding paper.V is then kept constant and 50 stimulus pulses appliedto nerve to produce Nmx incremental steps. TheMMUP is estimated as follows: if the number ofincremental steps with distinct thresholds-that is,firing points-is n (varies between 0 and 2 in normalsubjects), then Nmx-n steps were involved in thealternation. The maximum possible combinations ofN motor units, with overlapping firing levels needed

259

H. S. Milner-Brown and W. F. Brown

to produce Nmx-n steps is given by 2N-1. Fromthe relation 2N- > Nmx-n, N is found and theMMUP is computed by dividing the negative com-pound potential by the corrected number of motorunits recruited, which is equal to N+ n. The proce-dure is repeated a number of times, the negativeMCP obtained, and the MUE calculated. The experi-mental justification of this method is presented byBrown and Milner-Brown (1976). A simple theoreti-cal basis will be outlined in the next section.

JUSTIFICATION OF METHODS Method I Suppose theminimum voltage required to excite a motor unitis V. If the voltage is kept fixed at V, and 50 pulsesgiven to nerve, the motor unit will fire only about10% of the time-that is, response probability orfiring index of 10%. As the voltage is increasedgradually, the firing index will increase until atvoltage V+ AV, the firing index reaches 100%. Thisfluctuation in excitability is an inherent property ofanimal and human axons (Blair and Erlanger, 1933;Pecher, 1939; Verveen, 1962; Ten Hoopen andVerveen, 1963; Bergmans, 1970; Brown and Milner-Brown, 1976). Figure 1C shows a schematic diagramof the firing index plots of six motor units. At avoltage V,, motor unit MU1 has a firing index of80%o and is represented by the potential A1 (Fig. IB).Motor unit MU2 has a firing index of 20%, indicatedby an infrequent potential A2; A2 may representMU2 alone or MU1+MU2. When the voltage isincreased slightly (by AV) to V2, both MU1 and MU2will have a firing index of 100% and their sum mayproduce a predominant potential A3 and a lessfrequent potential A4. If this happens, then A2 can-not be counted as an incremental step, because itrepresents MU2 alone and not the sum MU1 + MU2.In this situation, the incremental steps A1, A2, A3would represent the recruitment of two motor unitsand not three. In previous methods, MU2 couldhave been counted twice, which would decrease theMMUP, and hence overestimate the number ofmotor units.

Method II Figure IC will again be used, but withthe modification that MU1, MU2, MU3 are actuallythe first three units recruited and whose firing levelsoverlap less than 5000. (In the Justification ofMethods section (I) above, MU1, MU2, MU3 werenot necessarily the first three motor units recruited,and their firing levels greatly overlapped). In thiscase, suppose there were no alternation until thevoltage was V4 and motor unit MU4 and MU5 hadFIs equal to 20% and 3500 respectively. When 50electrical pulses (at V4) are given to nerve, MU4 andMU5 may fire separately to produce the incrementalsteps A4, A5 (Fig. 1B) or fire simultaneously to give

a larger incremental step A6. This indicates that twomotor units MU4, MU5 with overlapping firinglevels can produce three incremental steps. Whenthe voltage is increased to V5, an additional motorunit MU6 (FT = 20%) will be recruited and the threemotor units can evoke a maximum of seven incre-mental steps from the possible combinations MU4,MU5, MU6, MU4+MU5, MU4+MU6, MU5+MU6, MU4+ MU5 + MU6. From these two examplesit can be deduced that N motor units with over-lapping firing levels can generate 2N_1 incrementalsteps. This is the theoretical basis of the method ofcorrection for the overlap in motor axon firinglevels, when calculating the MMUP. Direct experi-mental evidence and a more detailed quantitativetreatment are given by Brown and Milner-Brown(1976).

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FIG. 2 Histogram of motor unit(s) responses toelectrical excitation. At each stimulus voltage, 50 ormore electrical pulses are applied to nerve. Eachpoint represents the compound potential of a numberof motor units evoked by a single electrical pulse.The 'groups', numbers 1 to 8, represent the recruit-ment of single MU. These groups must satisfy thecriterion Amn(n+ 1) > Amx(n). When there areuncertain groups (?) the MMUP will be calculatedwith and without the uncertain groups, and theresulting MUE will then lie within a range, usuallyless than 1000 apart. The EMG amplitudes of theMU recruited can be obtained from the peak orpredominant amplitudes (Apd) in each group. TheEMG amplitudes are (p-p) voltages.

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New methods of estimating the number ofmotor units in a muscle

RESULTS

Method I was developed while electrophysio-logical studies were being carried out on IDImuscle. Silver disc surface electrodes (9 mm dia-meter) filled with electrode paste were used, andsignals amplified by Grass P15 preamplifiers(frequency bandwidth 30 Hz-10 kHz). In addi-tion, the EMG parameters measured were p-pamplitudes and areas. Method 1I, however, wasapplied to EDB and thenar and hypothenarmuscles, using clip surface electrodes, and theEMG parameter measured was the negativepeak amplitude. Non-commercial amplifiers atfrequency bandwidth 1.0 Hz-l.0 kHz were used.Because of these differences in measuring tech-niques, the results obtained using both methodswill be presented separately.

METHOD I Motor unit estimates (MUE) wereobtained from the IDI muscles of 12 normalsubjects and five patients with neuromusculardisorders between 20 to 40 years of age. Usingthe criteria for identifying the recruitment of anadditional motor unit (Apd(n +1) > Ap(n) andAmn(n+ 1) > Amx(n)), the compound potentialsof 8-10 motor units were read directly from thestorage oscilloscope: the MMUP, MCP, and

hence MUE were computed. The results arepresented in Table 1. The method was made moreaccurate by using a computer to calculate thepeak-to-peak amplitude and areas of individualresponses. A histogram of number of responsesagainst peak-to-peak amplitudes and areas ofmotor unit potentials was plotted (Fig. 2). Eachpoint represents the motor unit potential evokedby a single electrical pulse. A number of distinctpeaks are evident, as well as a scatter of pointsat the base of each 'group'. The scatter of pointsaround the base of each 'group' (the grouprefers to the vertical columns noticeable in thehistogram) can be attributed to a number offactors. (1) At a stimulus voltage Vn, n motorunits recruited may be firing 100% of the time,giving rise to the predominant response in thegroup. However, if the (n+ l)th and (n+ 2)thunits have thresholds overlapping Vn, theycould be firing very infrequently, to produce thescatter in the motor unit potentials. (2) Therecan be a slight variation in the potentials ofindividual motor units evoked by a stimulus atconstant voltage. (3) Mechanical factors such asmovement of the stimulating electrode by even afraction of a millimetre. In view of all thesefactors which contribute to the random fluctua-tion of motor unit potentials, the number of

BLE 1

COMPARISON OF MEAN MOTOR UNIT POTENTIAL AND MOTOR UNIT ESTIMATE CALCULATED BY DIRECT MEASUREMENT(OSCILLOSCOPE) WITH CALCULATION BY COMPUTER

Subject M7WMUP (l V) MCP MUE Comments(m V)

Oscilloscope Computer Oscilloscope Computer

HM 240 246 29 120 127 NSB 220 25.6 116 NHM 250 266 29 115 109 NDM 233 27 116 NJS 210 230-254 31.9 152 125-145 NAH 250 180-204 23.6 95 115-130 NSW 180 190 25 139 130 NSW 200 200 25 125 125 NCT 183 18.4 100 NMJ 200 155-175 20 100 128-140 NSY 200 170 24 120 140 NSY 220 190 24 110 125 NDZ 127-140 16 114-125 PPB 440-508 63.5 128-144 MDKH 220 16.4 75 MDDM 480-533 55.3 104-115 MDRH 140 14.7 105 MD

MMUP: Mean motor unit potential. MCP: maximum compound potential. MUE: motor unit estimate.N: normal. P: polymyositis. MD: muscular dystrophy.

261


calculated by dividing Apd (8), which is equalto 1250 ,±V, by 8 and 9, to give 156 ,uV and139 ,uV respectively. The MUE calculated froma maximum compound potential of 14.7 mVwill be in the range 94-105 ,uV. By reading thepredominant (Apd) amplitude in each group theEMG amplitudes of the MU recruited can bedirectly obtained. (The EMG amplitude of the(n +I )th MU recruited is equal to Apd(n +1)-Apd(n)). As an example, the EMG amplitudesof the MU recruited in Fig. 2 are 75, 125, 100,100, 290, 115, 195, and 230 ,uV. The EMGamplitudes of about 100 MU obtained fromsimilar histograms of 11 subjects are shown inFig. 3. The predominance of smaller motor unitsis similar to what was observed during isometricvoluntary contraction (Milner-Brown et al.,1973a, b, c; Brown and Milner-Brown, 1976).Also, the presence of larger MU indicates thatthe samples of MU used in the calculation ofthe mean MU potential are reasonably repre-sentative of the total population of motor units.

EMG AMPLITUDE (mV)

FIG. 3 EMG amplitudes of about one hundred MUobtained from the histograms (see Fig. 2) of 11 sub-jects. The amplitude of the (n+J)th MU recruitedwas obtained by subtracting the predominant ampli-tude of the previously recruited n MU (Apd(n)) fromthe predominant amplitude of the (n+ J)th groupApd(n+ 1). The EMG amplitudes are (p-p) voltages.

motor units recruited are selected from thedistinct peaks in the histogram (Fig. 2), whichalso satisfy the criterion Amn(n+ 1) > Amx(n)-that is, the minimum evoked response of n+ 1

units should equal or exceed the maximumresponse of the same previously recruited n unitsunder the same experimental conditions. Theresults of this computation are shown in Table 1,using only the amplitudes of motor unit poten-tials. At times when one or two groups are notdistinct, the MMUP was calculated with andwithout the uncertain groups. In such cases thetwo estimated values of MMUP and MUE areboth given in Table 1. In the example given inFig. 2, eight groups (numbered 1-8) are evident,in addition to one or possibly two indistinctgroups. In this example, the MMUP will be

METHOD II This method has been applied to34 normal and 41 abnormal extensor digitorumbrevis, hypothenar, and thenar muscles. Themethod is based on the experimental evidencethat generally the firing levels of the first 3-10motor units excited electrically overlap. Themethod then assumes that if N motor units withoverlapping firing levels are excited by 50-100electrical pulses at a constant voltage above theircritical thresholds for firing, then it is highlyprobable that most or all ofthe possible combina-tions of N, given by 2N- 1 will occur. Figure 4is an illustration of how this method is used toestimate the mean MU potential. When thestimulus current was at 13.8 mA, only a smallMU (-ve peak 0.01 mV) was excited. Thecurrent was increased to 20 mA and a secondMU was excited as shown in A. At 20.8 mAalternation occurred resulting in five incrementalsteps, indicated by the points on the graph andthe evoked potential increments in the lowerfigure (B). From the relation 2N-1 > 5, N=3motor units produced the additional incrementalsteps. These three MU plus the first two MUwith distinct firing points, would add up tofive MU. Hence MMUP is obtained by dividingthe compound potential in B by 5 not 7.

In Fig. 5 a comparison is shown between the

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FIG. 4 Illustration of method II for measuring theMMUP. The upper graph is the negative evokedpotentials recorded from the hypothenar muscles of anormal subject as the stimulus current was increasedfrom 10-20.8 mA. The lower Figures A, B, are theactual evoked potential traces on the storage oscillo-scope: A. shows the first two potentials at stimuluscurrents 13.8 and 20 mA (0.05 mV/DIV) and B thecombined potentials at 20.8 mA (0.1 mV/DIV).Using the formula 2N'-, the probable number ofMUinvolved in the five alternating steps would be three,and hence the total number of MU which producedthe compound potential B would be three plus thefirst two MU recruited.

conventional method of McComas et al. (1971)and the present 'alternation correction' methodin the estimation of the MMUP. In the upperfigure (A), six incremental steps were producedby graded electrical stimulation, and the MMUPcalculated from the negative compound potentialwas 66 ,uV. In the lower figure (B), 100 stimuluspulses at motor threshold produced 15 incre-mental steps. By using the alternation correctionformula 2N-1, the probable number of MU toproduce the observed 15 steps would be four,and the MMUP calculated equal to 100 ,uV.This comparison was done frequently in theinitial stages, and it was found that the MUEusing the alternation correction was, on average,20% lower than the conventional MUE. In later

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FIG. 5 A. Thenar MU potentials evoked by gradedelectrical stimulation to the median nerve. B. Evokedpotential steps produced by 100 stimulus pulses atmotor threshold. A close comparison betwen A and B,shows that some of the steps in A are the same as thesteps produced in B as a result of alternation. Thenegative MMUP from A was 66 1 V, while, using thecorrection formula 2N-1, the MMUP from B was100 (LV.

stages of the investigation, the increasing aware-ness of the possibility of alternation led to thetermination of the use of and the comparisonwith the conventional MUE. The alternationcorrection method has been used to estimate thenumber of motor units in extensor digitorumbrevis, thenar, and hypothenar muscles of 34normal subjects and 41 patients with peripheralneuropathies, predominantly pressure or entrap-ment neuropathies. The results are summarizedin Table 2.

DISCUSSION

In the estimation of the number of motor unitsin a muscle, one is faced with a number of in-evitable biophysical and experimental problems.However, the accuracy of the estimates dependssignificantly on how well the following importantcriteria are satisfied.

1. Do the incremental responses evoked by gradedelectrical stimulation correspond to the activationof single motor units? Because of the fluctua-tions in the excitability and overlap in firing levelsof motor units, an incremental response does notnecessarily correspond to the recruitment of an

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TABLE 2MOTOR UNIT ESTIMATES USING METHOD TI

Muscle

EDB Hypothenar Thenar

Normal Neuropathies Normal Neuropathies Normal Neuropathies

Mean ±SD 163 ± 84 72± 33 300± 125 145 ± 145 261±116 127 ± 57Range 61-351 33-131 127-470 20-545 101-541 6-206Number 10 12 10 11 14 18

additional motor unit. If two motor units MU1and MU2 with overlapping firing levels are

stimulated a number of times with abovethreshold stimuli, both MU1 and MU2 may firesimultaneously, or separately. The evokedresponses may show three incremental steps, dueto MU1, MU2, and MU1+ MU2. This 'alterna-tion' was recognized, but ignored, by the originalauthors (McComas et al., 1971 ; Sica et al., 1974).The two modified methods that have been de-cribed are attempts to solve the very importanterror of overestimating the number of motorunits recruited in the first 10 or more incrementalsteps, using the conventional method ofMcComas et al. (1971).The two methods evolved from experimental

evidence on two inherent properties of motorunits. Firstly, if the minimum voltage requiredto excite a motor unit is V, then, over 50 trials,the firing index will increase from 0 to 100%as the stimulus voltage is increased from V toV+ AV (AV 0.05V). Secondly, with the excep-tion of certain neuromuscular disorders such as

amyotrophic lateral sclerosis, in which thenumber of motor units is greatly reduced, thefiring levels of even the first few motor unitsexcited generally overlap. Considering thepossible differences in the biophysical state ofdifferent motor units at any given time andmechanical effects such as electrode and limbposition, it is very unlikely that two motor unitswill have coincident firing index versus voltagecurves (excitability curves). This makes it possibleto isolate distinct peaks as shown in the histo-gram in Fig. 2, representing the recruitment ofsingle motor units, and helps to eliminate thepossibility of counting a motor unit twice. Thesecond method, however, depends on the theor-etical and experimental data that, if N motor

units with overlapping firing levels are stimu-lated with 100 continuous electrical pulses abovethreshold, there is a high probability that themaximum number of possible combinations,given by 2N- 1 will occur.

Ballantyne and Hansen (1974) have introduceda computerized method in which motor unitsare identified by five parameters: latency, dura-tion, amplitude, area, and number of phases.They stated that 'A given motor unit is likelyto differ from every other unit in at least one ofthese variables'. The results from stimulating amotor unit several times at a constant voltageindicate that any of the five parameters canchange for a given motor unit. Thus, if a slightfluctuation in any of the above parameters isregistered by the computer as the recruitmentof an additional unit, then obviously the meanmotor unit potential will be low, and the motorunit estimate high. In an attempt to improveupon the original method of McComas et al.(1971), Panayiotopoulos et al. (1974) used photo-graphy to discriminate increments within thenoise level, which they counted as single motorunits. Some of the small variations could havebeen the result of fluctuations in the amplifiernoise and even changes in stimulus artefact. Theclassification of these fluctuations as smallamplitude motor unit action potentials, wouldgive very low MMUP and erroneously highmotor unit estimates such as 555 and 599obtained by Panayiotopoulos et al. (1974) forEDB muscles.

2. Are the evoked motor unit action potentialsused in the calculation of the mean motor unitpotential representative of those generated by thetotal population of motor units? Single MUisolated during voluntary isometric contractions

264


and the F recurrent discharge method havesurface potentials which range from less than100 NV to over 1 mV. However, the amplitudesof the incremental responses read off directlyfrom a storage oscilloscope rarely exceed 200 sV.This obviously indicates that the sample of MUused in the estimation of the MMUP is notrepresentative. The modified method I has awide range of MU amplitudes comparable withthe voluntary MU, and is evident by the muchlarger MMUP (215 + 24 ,V) compared withprevious methods (McComas et al., 1971;Ballantyne and Hansen, 1974; Panayiotopouloset al., 1974; Sica et al., 1974). In the precedingpaper, a method was described in which singlemotor units were obtained by stimulation atmultiple locations along the length of the nerve.The amplitudes of the MU isolated by thismethod ranged from less than 50 ,uV to over2 mV, with mean + SD of 332 + 625 ,uV. Un-fortunately, because there is no built-in correc-tion for the differences in the latencies to peak,the mean amplitude cannot be used directly inthe calculation of the MUE. A computerizedmethod is now being devised in this laboratory,which will hopefully solve this problem, andmake this method potentially useful for esti-mating motor unit numbers in the future. In acontinuation of this investigation, the two modi-fied methods, and the potentially third method,will be used simultaneously in the estimation ofthe number of motor units in different musclesin patients with muscular dystrophy.

In conclusion, the authors would like to statethat the modified methods described in this paperdo not solve all the problems involved in theelectrophysiological methods of estimating thenumber of motor units in a muscle, but shouldhelp to make future estimates more accurate.For the present, then, it is necessary to continueto question the validity of the motor unit esti-mates in health and disease in order to makeuse more properly of the information suchestimates provide.

We gratefully acknowledge financial support from theMuscular Dystrophy Association of Canada; Dr H. S.Milner-Brown is a MDAC Post-doctoral Fellow. We

also wish to thank all the hospital and laboratory staffwho served as controls, Mrs Liza Morchat for typing themanuscript, and Mr Stephen Yates for writing thecomputer programs. Illustrations were by Mr GeorgeMoogk and photography by Mr Michael Donnelly.

REFERENCES

Ballantyne, J. P., and Hansen, S. (1974). A new method forthe estimation of the number of motor units in a muscle.1. Control subjects and patients with myasthenia gravis.Journal of Neurology, Neurosurgery, and Psychiatry, 37,907-915.

Bergmans, J. (1970). The Physiology of Single Human NerveFibers. Vander: Louvain.

Blair, E. A., and Erlanger, J. (1933). A comparison of thecharacteristic of axons through their individual electricresponses. American Journal ofPhysiology, 106, 524-564.

Brown, W. F. (1973). Thenar motor unit count estimates inthe carpel tunnel syndrome. Journal of Neurology, Neuro-surgery, and Psychiatry, 36, 194-198.

Brown, W. F., and Feasby, T. (1974). Estimates of motoraxon loss in diabetics. Journal of the Neurological Sciences,23, 275-293.

Brown, W. F., and Milner-Brown, H. S. (1976). Some elec-trical properties of motor units and their effects on themethods of estimating motor unit numbers. Journal ofNeurology, Neurosurgery, and Psychiatry, 39, 249-257.

McComas, A. J., Fawcett, P. R. W., Campbell, M. T., andSica, R. E. P. (1971). Electrophysiological estimation ofthe number of motor units within a human muscle. Journalof Neurology, Neurosurgery, and Psychiatry, 34, 121-131.

Milner-Brown, H. S., Stein, R. B., and Yemm, R. (1973a).The contractile properties of human motor units duringvoluntary isometric contractions. Journal of Physiology,228, 285-306.

Milner-Brown, H. S., Stein, R. B., and Yemm, R. (1973b).The orderly recruitment of human motor units duringvoluntary isometric contractions. Journal of Physiology,230, 359-370.

Milner-Brown, H. S., Stein, R. B., and Yemm, R. (1973c).Changes in the firing rate of human motor units duringlinearly changing voluntary contractions. Journal ofPhysiology, 230, 371-390.

Panayiotopoulos, C. P., Scarpalezos, S., and Papapetro-poulos, Th. (1974). Electrophysiological estimation ofmotor units in Duchenne muscular dystrophy. Journal ofNeurological Sciences, 23, 89-98.

Pecher, C. (1939). La fluctuation d'excitabilite de la fibrenerveuse. Archives Internationales de Physiologie et deBiochemie, 49, 129-152.

Sica, R. E. P., McComas, A. J., Upton, A. R. M., andLongmire, D. (1974). Motor unit estimation in smallmuscles of the hand. Journal of Neurology, Neurosurgery,and Psychiatry, 37, 55-67.

Ten Hoopen, A., and Verveen, A. A. (1963). Nerve-modelexperiments on fluctuation in excitability. Progress inBrain Research, 2, 8-21.

Verveen, A. A. (1962). Fiber diameter and fluctuation inexcitability. Acta Morphologica Neerlands-Scandinavica, 5,79-85.

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New methods of estimating the number ofmotor units in a muscle