IL NUOVO CIMENTO VOL. 2 D, N. 2 Marzo-Aprile 1983

Magnetic Signals from Human Skeletal Musdes ( ').

S. KoGA. and A. ~AI~A~I_mA (**)

Electrotechnieal Laboratory - 1-1-4, Umezono, Sakuramura, Niiharigun, Ibaraki, Japa~

(rieevuto il 12 Novembre 1982)

S u m m a r y . - - The pulse wave form for the magnetomyogram (~I~G) of the human right leg was examined. The typical pulse durations are about (10--20) ms. The spatial variation of the MMG suggests a source in the anterior muscles.

PACS. 87.40. - Biomagnetism (including magnetocardiography). PACS. 85.25. - Superconducting devices; superconducting magnets.

1 . - I n t r o d u c t i o n .

The record of the magne t i c field p roduced b y skeletal muscles is called t h e m a g n e t o m y o g r a m (MMG) (1-3) and its character is t ics have been discussed b y

Com~N and GIVL~I~ (1), b u t the direct observa t ion of the wave fo rm of indiv idual

MMG pulses has no t been repor t . I n this s t udy we examine the spat ial

d i s t r ibu t ion of the leg MMG and also succeeded iI~ observing t he individual

pulse wave form.

(*) Paper presented at the (, IV International Workshop on Biomagnetism ~, held in Rome, September 14-16, 1982. .(**) Present address: Electronics Materials Laboratory, Sumitomo Metal Mining Co. Ltd. (1) D. C o I ~ and E. GIVL~R: Appl. Phys. Lett., 21, 114 (1972). (2) M. ]~]~IT]~, J. E. ZIMMERM/kN, J. El)RICH and ft. ZI~ER~AI~: Electroencepalogr. Clin. Neurophysiol., 40, 59 (1976). (a) S. B&RBANERA, P. CARELLI, R. LEONI, G. L. ROMANI, F. BORDNI, I. MODENA, R. F~NICI and P. Zm'PILLI: in Biomagnetism, edited by S. N. ER~. , H.-D. HAKLBOHM and tt. Li~BBIG (Berlin and ~ew York, N. Y., 1981), p. 139.

~42

MAGNETIC SIGNALS FROM HUMAN SKELETAL MUSCLES 6~'~

2 . - E x p e r i m e n t a l .

A b l o c k d i a g r a m o f t h e e x p e r i m e n t a l a p p a r a t u s is s h o w n in fig. l a ) . T h e

e x p e r i m e n t s were done w i t h a f i r s t - d e r i v a t i v e g r a d i o m e t e r in t h e E T L m a g -

:Fig. 1. - a) Block diagram of the SQUID system: the l iquid-helium dewar which con- ta ins the detect ing coil is supported by a wooden mount. Al l electronics including ~he 30 MHz preamplif ier are outisde of the shielded room. b) ETL magnetical ly shielded room: the wall of the shielded room has three metal layers insulated from each other by a wooden frame. These layers are made of 2 mm permal loy plates, 5 mm allumi- num plates and 0.035 mm copper foils, but the innermost layer has no permal loy plate. The shielding factor at 50 Hz is 80 dB.

6 4 4 s. KOGA a n d A. NA.KAMUR/h

Fig. 2. - a) The right leg and the dewar tail: the leg is ready to contract, though i t is relaxed in this photograph, b) Typical MMG: on contracting the muscle of the leg, the 1KiKG pulse train is obtained. The baseline shift is small.

~AGNETIC SIGNALS FRO~ HUM'AI~ SKELETAI, mUSCLES 6 4 ~

net ical ly shielded room (4), shown in fig. lb). The diameter of the gradiometer coils is 4.4 cm and the baseline is 15 cm. All the SQUID electronics, including t he 30 MHz preamplifier, are located outside the room. The band width of the measurement system is d.c. to 4 kHz and the background white-noise level is about 1 .10 -la T/~/H-z. The 50 Hz power line noise is smaller t han 1.10 -18 Tp.p:

Figure 2a) shows the dewar tai l and the r ight leg. The detect ion coil was placed *angential to the skin for all our measurements.

3 . - R e s u l t s .

A typical MMG is shown in fig. 2b). This was observed at the position shown in fig. 2a), i.e. just above the tibialis anterior muscles. The baseline drift of the M1VIG is small compared with t ha t observed at the elbow (1) and is the reason why we chose the leg to observe the wave form of the MMG pulse. The spee-

Fig. 3. - a) Distribution of the MMG pulse amplitude (H) : H is maximum at B, i.e. just above the tibialis anterior muscle, b) Position for the experiment: 4 points, A, B, G and D, are shown.

(4) S. KOGA and A. N~KAMURA: Proceedings o/the _~irst Sensor Symposium, Japan, 1981 (Tokyo, 1982), p. 305.

646 s. KOGA a n d A. NAKA~URA

t r a l densi ty of the signal has a m a x i m u m at 40 Hz and extends up to 400 Hz, similar to t h a t of the elbow (1).

The m a x i m u m peak- to-peak ampl i tude (H) of the signal was measured as

a funct ion of posit ion along the leg and the results are shown in fig. 3a). H is m a x i m u m at the point B in fig. 3b), i.e. jus t above the tibialis anterior muscle" The dependence of H at B on the distance (d) be tween the surface of the skin and the dewar tai l was then obtained and is shown in fig. 4. H -1 was found to

/ /

2

T

C~o / / i

1 / I , . 0 5

d.lstanee ~ (r

Fig. 4. - Distance dependence of the MMG pulse amplitude (H): the distance d be- tween the dewar tail and the skin is obtained from the thickness of the spacer a s

indicated in the insert. The MMG pulse amplitude / / follows the empirical formula H oc (d - - do) -1, where do = 2.3 ore.

va ry approximate ly l inearly with d, viz.

where

H r (d - - do) -1 ,

do - - 2.3 e m .

Figure 5 shows a typica l MMG pulse t r a in which was obtained at the posit ion shown in fig. ~a). To obtain an es t imate of the wave forms for an individual pulse, an averaging technique was used for which the tr igger source was the signal itself. Signals t ha t exceeded the tr iggering level are averaged and smaller signals ignored. The baseline drif t of the leg MMG is sufficiently small to permi t this self-triggering. A typica l result, obta ined b y averaging 39 samples, is

MAGNETIC SIGNALS FROM HUMAN SKELETAL MUSCLES 647

(n C:

i

I t

5x i0 "11 T

40 ms

I I I I I I I ,

t /me

Fig. 5. - Typical MMG pulse t ra in ; i t was observed b y expanding the t ime scale. The spacing of the pulses depends on the contraction strength of the muscles.

~ u)

16ms

I I I I I ~ ~ I I .

t /me

Fig. 6. - Averaged MMG pulse (1): this typical wave form was obtained at the posi- t ion shown in fig, 2a) b y averaging 32 samples. The t r igger source for the t ime averaging is the ~IMG signal itself and the tr iggering level is shown b y an arrow.

6 4 8 s. KOGA a n d A. N A K A ~ A

shown in fig. 6. The arrow in the figure shows the triggering level. The sharp peak at the tr iggered point in the figure is a noise art ifact resulting from this triggering procedure and is not related to the signal f rom the muscles. The wave form shown in fig. 6 is almost symmetr ic and the pulse width at half am- pli tude is 4 ms. An asymmetr ic wave form was also observed as shown in fig. 7 when the leg was ro ta ted about 45 ~

~ , . . 5 XlO"-ITT

h

c:

E

I I 16 m s

t I I I I I I I I

time

Fig. 7. - Averaged I~I~G pulse (2): this was obtained by the same method as that of fig. 6 but at a slightly different position, i.e. the leg was rotated roughly 45 ~ about its axis.

4 . - D i s c u s s i o n .

An interesting empirical feature of our results is the linear dependence of H -1 as noted in fig. 4. Although we have not ye t a t t empted any detailed modelling. I t is interesting, however, to note tha t the value of 2.3 cm th a t we obtained for do is comparable with the depth of the tibialis anterior muscle below the detector coil (the detector-skin minimum distance is 1.3 cm). ~ Previous studies on action potentials, using inserte4 electrodes, have shown tha t the durat ion of the single-motor-unit action potentials of the r an- terior muscle are (2 - -19 )ms (6). The magnitude of the MMG is large just

(s) B. FEINST~IN, B. LINDmG~.P~D, :E. NY~AN and G. WO~'ART: Acta Psychiatr. ~eurol. Scan&, 29, 189 (1954).

MAGNETIC SIGNALS FR0~ HUMAN SKET~ETAL MUSCLES 6~9

above the t ibialis anter ior muscle and the overal l dura t ion of the MMG pulse

ob ta ined just above the muscle is (10--20) ms. Thus the source of the MMG

pulse m a y be the single moto r uni t of the tibia]is anter ior muscles. We note t h a t our averaging procedure tends to b roaden the observed pulse

width. Ind iv idua l pulses (fig. 5) are significantly sharper t h a n the average (fig. 6)~ hav ing a half -width of about 4 ms. This is less t h a n the width sug- gested b y C o m ~ and GIVLEI~. However , the difference could arise f rom dif-

ferences in detector coil geome t ry and location.

�9 R I A S S U N T O (*)

stata esamina~a la forma d'onda d'impulso per il magnetomiogramma (~vIMG) dell'ar$o inferiore destro umano. Le durate tipiche degl'impulsi sono di circa (10--20) ms. La variazione spaziale del MIVIG suggerisce un'origine nei muscoli anteriori delia tibia.

(*) T r a d u z i o n e a c u r a de l la R e d a z i o ~ e .

42 - I[ Nuovo C, imento D.