European Journal of

Eur J Appl Physiol (1983) 51:61-66

Applied Physiology and Occupational Physiology 9 Springer-Verlag 1983

Changes in EEG Power Density Spectrum During Static Muscle Work G. Krause, P. Ullsperger, L. Beyer, and H.-G. Gille Central Institute for Occupational Medicine of the GDR, Department of Work Physiology, DDR-1134 Berlin

Summary. Changes in parameters of the EEG power density spectrum were demonstrated in four healthy male subjects during fatiguing static muscle contractions (M. biceps brachii dexter). For the following EEG frequency bands (delta 1 - 4 Hz, theta 4 - 8 Hz, alpha 8-13 Hz, beta 13-20 Hz), the relative power and the peak frequencies were determined. A significant increase in the alpha peak frequency was observed during muscle work. This trend was evident in both the central leads (C3-A1, C4-A1) and the occipital (O1-A1), particularly when the subjects had their eyes closed. These changes in peak frequency might be connected with an increase in central nervous activation.

Key words: EEG power spectrum - Static muscle work - Central nervous activation

Introduction Changes in bioelectrical muscle activity and in the responses of the cardiovascular system to fatiguing static work have been well investigated (e.g., Monod and Scherrer 1957; Rohmert 1960; Rhein et al. 1974; Khemici et al. 1980; Kt~chler et al. 1980; Kramer et al. 1981). Relatively little, however, is known about the interactions between peripheral and central reactions to muscular fatigue. Wood (1979) and Morris (1977) found that the visual reaction time changed after muscular activity, but they did not analyze the EEG parameters. Roitbak et al. (1960) observed changes of the alpha- and my-activities in the EEG recorded on paper for different forms of muscle work. Beyer and Schumann (1981) found parameters of the EEG interval histogram influenced after 400-m Offprint requests to: Dr. rer. nat. Gabriele Krause, Zentralinstitut f,fr Arbeitsmedizin der DDR, Fachbereich Arbeitsphysiologie, DDR-1134 Berlin, N61dnerstr. 40/42

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runs on a treadmill, and interpreted their findings as resulting f r o m changes in n e r v o u s activation level. S o l o g u b (1976), w h o investigated the changes of the E E G t h r o u g h d y n a m i c muscle w o r k , u n d e r l i n e d the necessity o f E E G s r e c o r d e d especially during muscle activity. T o s u m up, it can be said that there are facts indicating changes in s p o n t a n e o u s bioelectrical brain activity while muscle w o r k is being p e r f o r m e d . T h e aim of this pilot study is to find informative p a r a m e t e r s of the E E G p o w e r density spectra which c h a n g e in the course of static muscle work.

Material and Methods Four healthy male subjects (age: 25, 33, 37, 40 years) participated in this pilot study. The subjects were asked to perform sustained isometric contractions of M. biceps brachii dexter with 30% of maximum voluntary contraction (MVC) for as long as possible. The maximum flexor force was picked up at the right wrist and measured mechanoelectrically by means of a strain gauge force transducer. The EEG activity was recorded from central and occipital leads (C3-A1, C4-A1, O1-A1 according to the International 10-20 System; Jasper 1958). The electrooculogram (EOG) was used for artifact detection. The bioelectrical signals were amplified (RFT Bioscript BST 1, GDR) and tape recorded (Lyrec TR 86, Denmark). EEG and EOG were recorded during the following experimental phases: rest period (3 rain), the period of contraction, and during the first 3 min of recovery. The duration of the muscle contraction amounted on average to 4.5 min, and matched the range of related data in the literature (cf. Monod 1957; Kramer et al. 1981; but see also Rohmert 1960). As the length of the muscle contraction periods varied from subject to subject, the further calculation was made on the basis of percentage values in relation to the whole period of contraction. At first, the experiment was carried out with the eyes open, and then, after a break of 5 min, with the eyes closed. On another day, the experiment was repeated by every subject, but this time with closed eyes at the beginning and opened eyes at the end of the test. Figure 1 shows the experimental arrangements. EEG power density spectra were calculated off-line by means of a laboratory computer (Nicolet, Med-80, USA, cf. Fig. 2). The relative power density and the peak frequencies were computed in four frequency bands (delta 1-4Hz, theta 4-8Hz, alpha 8-13 Hz, and beta 13-20 Hz) for every 20-s interval of the experimental phases (excluding intervals with artifacts). These spectral parameters were examined by analyses of variance and multiple comparisons of mean values (Newman-Keuls-Test) to show the effects of muscle contractions. The level of significance was set at 5%.

Results N o significant effects o f muscle contractions were f o u n d in the relative p o w e r densities of the four E E G f r e q u e n c y bands, neither did the p e a k frequencies of delta, t h e t a and beta waves indicate any influence that could be confirmed statistically. O n the o t h e r h a n d , the alpha p e a k f r e q u e n c y evidently increased by nearly 0.6 H z f r o m the beginning of the rest p e r i o d up to the e n d of the contraction period. T h e m e a n alpha p e a k frequencies f r o m spectra o b t a i n e d with the subjects' eyes closed are d e m o n s t r a t e d in Fig. 3 for the three experimental phases.

EEG and Static Muscle Work

63

( ", /

amplifiers and registration unit

~t

r

lob. computer (spectratanalysis )

off line

tope recorder

1

oscilloscope Fig. 1. Arrangement of the experiment. EEG - Electroencephalogram (Derivations: C3-A1, C4-A1, O1-A1). EOG - Electrooculogram. EMG - Electromyogram

RA2

KA RA1

b

'

lb

'

2b

f" r e q u e n c y ~Nz~

Fig. 2. Relative EEG power density spectra in the course of the experiment (example of one subject, eyes closed, derivation O1-A1). RA1 -- Start of rest period. Ka - Start of static muscle contraction. Ra2 - Start of recovery

64

G. Krause et al. 10,6,

.2:.10,4 c

_L

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I0,0

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< 9,6

Ii1 RA1

0-20 20-l,0 z,0-60 60-B0 80-100 re[. duration of contraction[~

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Fig. 3. Mean values and standard errors of alpha peak frequency (Derivation: O1-A1, with closed eyes) for the three experimental phases rest, contraction and recovery. RA1 - Start of rest period. RE2 -- Third minute of recovery

Table 1. Mean values of EEG alpha peak frequencies (Hz) calculated for 40 s intervals at the beginning and the end of three experimental phases ("closed-eyes" condition) EEG derivation

C3-A1 C4-A1 O1-A1

Experimental phases

Significant influence of factors

RA1

RE1

KA

KE

RA2

RE2

A

B

C

9.7 9.6 9.8

10.0 10.1 10.1

1 0 . 3 1 0 , 4 1 0 . 4 10.0 1 0 . 2 1 0 . 3 1 0 . 4 10.1 1 0 . 3 1 0 . 5 1 0 . 4 10.1

+ + +

+

+ + +

RA1 Start of rest period RE1 End of rest period KA Start of static muscle contraction (M. biceps brachii, 30% of maximal voluntary contraction) KE End of static muscle contraction (the discomfort of contraction could not longer be tolerated) RA2 Start of recovery Rm The 3rd rain of recovery Factor A: Subject effect Factor B: Treatment effect (rest, contraction, recovery) Factor C: Time effect (start and end of experimental phases)

D u r i n g the first 3 m i n of recovery, the p e a k f r e q u e n c y d e c r e a s e d again, b u t did n o t r e a c h the v a l u e o b t a i n e d at the b e g i n n i n g of the rest p e r i o d b e f o r e c o n t r a c t i o n started. W i t h the eyes closed, a n a l o g o u s t r e n d s of the alpha p e a k f r e q u e n c y w e r e o b s e r v e d also for leads C3-A1 a n d C4-A1 (cf. T a b l e 1). N o significant effects of m u s c l e c o n t r a c t i o n s o n the alpha p e a k f r e q u e n c y were f o u n d w h e n the s u b j e c t s h a d their eyes o p e n , a l t h o u g h a similar t e n d e n c y was o b s e r v e d .

EEG and Static Muscle Work

65

Discussion

A significant increase in alpha peak frequency during static muscle activity occurred when the subjects had their eyes closed. None of the other parameters of the EEG power density spectra were influenced significantly by isometric contractions of the biceps brachii muscle. Roitbak et al. (1960) found an increase in alpha- and my-activities in the EEG during static muscle work. The authors also observed a depression in alpha activity at the beginning of work. The results of Roitbak et al. can hardly be compared with those of the present study, as no quantitative data have been specified. The results concur in so far as the essential effect of muscle activity has been found in the alpha band of the EEG. The reason for these effects on EEG activity is not yet clear; one might envisage a change of central nervous activation due to a permanent efferent and afferent flow of information between CNS and the contracting muscle. For the general activation level, importance must also be attributed to the flow of information from intero- and proprio-receptors, e.g. muscle spindles (Haschke 1980). In a similar way, Ullsperger et al. (1977) have explained that the drop in amplitude of auditory evoked responses during isometric muscle contractions is connected with changes in activation level. Contamination of the alpha frequency band through bioelectrical activity of neck muscles is not very likely. Such an influence should be more pronounced in the beta band (O'Donnel et al. 1974), which actually was not the case. The fact that the alpha peak frequency remained higher even in the relaxed condition is another argument against such an assumption. The present preliminary results indicate that the function of the CNS changes during muscle activity, which is reflected in the EEG. Thus, future investigations should give more consideration to EEG parameters, in which the alpha peak frequency could be an important indicator.

References Beyer L, Schumann H (1981) M6glichkeiten neurophysiologischer Untersuchungen in der Sportmedizin anhand von zwei ausgewfihlten Beispielen. Med Sport 3 : 65-70 Haschke W (1980) Grundziige der Neurophysiologie, 2. Aull. Fischer, Jena, S 65 Jasper H (1958) The ten twenty electrode system of the International Federation. Electroencephalogr Clin Neurophysiol 10:371-375 Khemici R, Kramer H, Mucke R (1980) Zum EinfluB der Pausenlfinge auf die lokale muskulfire Ausdauer bei wiederholten isometrischen Handschlug-Kontraktionen. Z Gesamte Hyg 26 : 573-576 Kramer H, Rehfeldt H, Mucke R (1981) Einige arbeitsphysiologische Aspekte der Herz-Kreislaufreaktionen bei statischer Muskelarbeit. Z Gesamte Hyg 27 : 34-38 Kfichler G, Frauendorf H, Kramer H (1980) Muskelaktivitfit und Anpassung des Transportsystems Herz-Kreislauf. Wiss Z Karl-Marx-Uuiv Leipzig, Math-Naturwiss R 29:103-110 Monod H, Scherrer J (1957) Capacitd de travail statique d'un groupe musculaire synerguique chez l'homme. C R Soc Biol (Paris) 151:1358-1362 Morris AF (1977) Effects of fatiguing isometric and isotonic exercise on resisted and unresisted reaction time components. Eur J Appl Physiol 37:1-11

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O'Donnel RD, Berkhout J, Adey WR (1974) Contamination of scalp EEG spectrum during contraction of cranio-facial muscles. Electroencephalogr Clin Neurophysiol 37:145-151 Rhein A, Buchholz Ch, Kramer H, Ktichler G (1974) Ver~nderungen der bioelektrischen Muskelaktivit/it w~hrend langdauernder Willkfirkontraktionen bei vorgegebener Haltekraft. Acta Biol Med Ger 33:231-239 Rohmert W (1960) Ermittlung von Erholpausen ffir statische Arbeit des Menschen. Int Z Angew Physiol 18:123-164 Roitbak AJ, Dedabrischwili ZM, Gotziridse IK (1960) Izmenenija elektroencefalogrammy pri staticeskich usilijach. Problemy Fiziologii Sporta 2:100-107 Sologub JB (1976) Elektroenzephalografie im Sport. Barth, Leipzig Ullsperger P, Reimer W, Mucke R, Bastek R, Rehfeldt H, Ktichler G (1977) Einflug statischer Muskelanspannung auf akustisch evozierte Hirnpotentiale, Hautwiderstandsfinderungen und bioelektrische Muskelaktivit~t. Acta Biol Med Get 36:213-219 Wood GA (1979) Electrophysiological correlates of local muscular fatigue effects upon human visual reaction time. Eur J Appl Physiol 41:247-257 Accepted January 14, 1983