Back muscle response to transient whole-body vibration

International Journal of Industrial Ergonomics, 12 (1993) 49-59

49

Elsevier

Back muscle response to transient whole-body vibration Ralph Bliithner a, Barbara Hinz a, G e r h a r d Menzel b, H e l m u t Seidel a " Federal Institute for Occupational Health, Group 5.3, N6ldnerstr. 40-42, D-10317 Berlin, Germany b Federal Institute for Occupational Health, Group 3.3, N61dnerstr. 40-42, D-10317 Berlin, Germany Received March 31, 1992; accepted in revised form December 1, 1992

Abstract This study was performed in order to clarify, (1) if trunk muscles react immediately to a transient whole-body vibration (WBV), (2) to which extent the timing of EMG depends on the direction of transient WBV and/or on the muscle group, and (3) to which degree after-effects of transient WBVs have to be considered. Five healthy males were exposed to transient displacements (nearly sinusoidal or half-sinusoidal waveforms with peak accelerations of about ± 2.7 ms 2). Four EMGs (m. erector spinae at 3 levels and m. trapezius) were averaged and normalized. The alternating activation-inactivation of the EMG-responses nearly exhibited a mirror symmetry when the direction of displacements changed. Responses occurred earlier at the shoulder than at the lumbar level. An increased health risk was predicted for (1) the initial phase of a sudden upward displacement without motion in the history preceding the transient WBV and (2) a downward displacement with a dominating frequency near 6-8 Hz. The immediate muscular reactions suggest the necessity to include muscle forces in calculations of the spinal load under transient WBV, except for the first 50 to 100 ms of an event without motion in its preceding time history. The direction and preceding history of a transient WBV should be considered in future evaluation procedures as a characteristic of WBV-exposure.

Relevance to industry The paper contributes to the evaluation of health risks caused by transient WBV. The avoidance of an unfavourably timed back muscle activity by an appropriate control of the vibration input might enable a significant contribution to the prevention of work-related low back pain.

Keywords Timing; EMG; Lumbar spine; Load; Shock

Introduction Back muscles show a systematic and reproducible myoelectric activity (EMG) during sinusoidal WBV with a frequency range between 0.315 and 10 Hz (Seidel et al., 1986; Seidel, 1988). The timing of EMG during an exposure to transient whole-body vibrations (transient WBVs) has re-

Correspondence to: R. Bliithner, Federal Institute for Occupational Health, Group 5.3, Postfach 5, D-10266 Berlin, Germany.

mained an open question. It has significance for (i) the human biodynamics, (ii) the stabilization/ destabilization of the spine during dynamic load, and (iii) the evaluation of a transient WBV with regard to its after-effect that might constitute at the same time the history preceding a subsequent event. The muscle force is a very significant component of the compressive force acting on the lumbar spine during WBV (Sandover, 1981; Blfithner et al., 1986; Seidel et al., 1986). Muscles control the time-variant coupling of partial masses of the human body. A decoupling of the shoulder during a transient increase of acceleration, e.g.,

0169-8141/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

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R. Bliithner et al. / EMG response to vibration

attenuates the shock-induced internal load. The muscular stabilization of the spine reduces relative motions - mainly shear - between neighboured vertebrae, i.e. it diminishes the mechanical strain of the vertebral segment. This E M G study was performed in order to clarify, (a) if trunk muscles react immediately to a transient WBV, (b) to which extent the timing of the E M G depends on the initial direction of a transient displacement a n d / o r on the muscle group, (c) if the E M G suggests a muscular stabilization or destabilization of the spine during transient WBV-loads, and (d) to which degree after-effects of transient WBVs have to be considered. The data were to be applied to derive at a conclusion, whether a vibration analysis in the time domain could provide an instrument for the evaluation of the health risk caused by transient WBVs.

Materials and methods Subjects

Five normal healthy males without disorders of the spine and nervous system (21 to 28 years old, mean 23.8 years; body mass from 61 to 82 kg, mean 73.0 kg; height from 168 to 186 cm, mean 178.8 cm) volunteered for the experiment and gave their informed consent. Vibration exposure

Transient displacements with nearly sinusoidal or half-sinusoidal waveforms and peak accelerations of about _+2.7 ms -z were produced by a computer-controlled electro-hydraulic vibrator. Two durations (D) were tested for each waveform: sinusoidal waveform - D1 = 500 ms, D2 = 250 ms; half-sinusoidal - D1 = 250 ms, D2 = 125 ms. Each combination of D and waveform was repeated 60 times with a random variation of the initial direction of the displacement - upwards (up) or downwards (down). The presentation of durations and waveforms was randomized across the subjects as well as the vibration-free intervals ranging from 1,000-1,400 ms.

Experimental procedure and design

The subjects were instructed to control a constant moderately erect sitting posture by video technique (Seidel et al., 1980). Music was presented via headphones in order to distract the attention of the subjects who were asked not to contract muscles voluntarily. The subjects were informed that the direction of transient WBVs and the length of the intervals in between would vary at random. Data acqu&ition and processing

Four bipolar electromyograms (EMGs) were derived with surface electrodes: three of them were lateral-symmetric derivations of the m. erector spinae at the levels L3 (EMG1), T l l / 1 2 (EMG2), T 1 / 2 (EMG3) with one electrode on each side (Chapman and Troup, 1969); one unilateral derivation was obtained from the left m. trapezius, pars descendens (EMG4). The E M G s were preamplified near the electrodes, amplified and stored by FM magnetic tape recording. They were off-line band-pass filtered (Butterworth, from 16 to 400 Hz) and averaged in relation to the displacement signal from the seat after analogue-to-digital conversion (sampling rate 1 kHz) and linear rectification. The individual average E M G s were normalized with respect to the mean myoelectric activity during the last 200 ms of the vibration-free intervals in order to reduce the between-subject differences. After averaging of normalized rectified E M G s across all subjects (Seidel, 1988), peaks were determined and integrals of EMG-segments below or above the activity during the vibration-free intervals were computed by programmes written in PASCAL. For a better comparison, an average activity was calculated for each interval dividing the integrated E M G by the duration of the segment. Accelerations of the seat and of parts of the body (skin above the spinous processes of the third and fourth lumbar vertebrae, shoulder and head) were measured simultaneously with the resulting dynamic, i.e. with the body weight set to zero, force FZ acting on the interface between the vibrator and the hard seat without backrest (for details cf. Hinz and Seidel, 1987; Hinz et al., 1988b).

R. Bliithner et al. / EMG response to vibration

Results

Fig. 1 illustrates the waveforms of the halfsinusoidal transient WBV at D2. The sinusoidal transient WBV at D1 exhibited similar differences between the waveforms of displacements and accelerations, the latter characterized by distinct small additional peaks with a higher frequency content at the start and stop of the vibrator. The spectral analysis of the acceleration input depicted in Fig. 1 indicated a broad-band spectrum between 1 and 15 Hz with a dominance near 9 Hz. Preliminary results on the body's dynamic response have been published by Seidel et al. (1992). The body does not move synchronously with the displacement of the seat, due to its visco-elastic properties. Therefore, the comparison with the maximum FZ, and not with the displacement, was considered as more relevant for the assessment of a possible stabilizing effect of the muscular activity during the dynamic load. Fig. 2 provides the mean waveforms of the EMGs 1 and 3 during sinusoidal transient WBV and halfsinusoidal transient WBV at D2, averaged across all subjects. They clearly indicate that the myoelectric response does not depend on a continuous sinusoidal exposure, but occurs right at the beginning of a transient vibration. The pattern of the myoelectric response was determined by the initial sign of the displacement. Whether the my-

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Fig. 1. Example of the transient vibration input: half-sinusoidal displacement beginning with an upward motion. Continuous line = displacement of the seat with the extreme value indicated in mm; point = acceleration of the seat with the extreme value indicated in ms -2.

51

oelectric response started with a distinct increase or decrease of activity, was determined by the direction of the initial displacement - either downwards (Figs. 2a and 2c) or upwards (Figs. 2b and 2d) -, respectively. At the lumbar level, the EMGs exhibited an initial small opposite response, e.g. a slightly decreased activity with an initial downwards displacement (Figs. 2a and 2c). The myoelectric responses of alternating contractions and relaxations nearly exhibited a mirror symmetry, when the initial sign of displacement changed (cf. Figs. 2a and 2c vs. Figs. 2b and 2d). The timing of the EMGs also depended on the muscle group examined (Table 1). In Table 1, the first extreme values of the displacements were set to 150 (D1) or 100 ms (D2) for a better comparison. Predominantly, EMGs 3 and 4 exhibited an earlier response than EMGs 1 and 2, exemplified by EMGs 1 and 3 in Figs. 2a-d. In EMG1, the first maximum activations during half-sinusoidal transient W B V / d o w n / D 2 and sinusoidal transient W B V / d o w n / D 2 occurred after the maximum displacement, in EMG3 before (Figs. 2a and 2c). Similar time relations between the activation of these muscle groups can be seen at the first minimal activities during half-sinusoidal transient W B V / u p / D 2 and sinusoidal transient W B V / u p / D 2 (Fig. 2b and 2d). The myoelectric responses clearly outlasted the transient WBVs (Fig. 2). Fig. 2 also hints at a dependence of timing on the history of the WBV-input preceding an event. The maximum downward displacement of the transient WBV starting without motion in the preceding history goes along with an increasing activity of EMG1 (Fig. 2a), whereas the maximum downward displacement of the transient WBV with a preceding history of an upward motion is accompanied by a decreasing activity of EMG1 (Fig. 2d). The comparison of the responses during an upward displacement (Fig. 2b vs. Fig. 2c) suggests similar slight differences indicating a smaller time delay of the EMG-response after a preceding history with motion. Another evidence for the influence of the history is provided by the after-effect of the myoelectric bursts, which caused a continued alternating increase and decrease of EMG-activity after the stop of the vibrator. On average, the EMG-amplitudes at D2 exhibited a tendency to be more pronounced

52

R. Bliithner et al.

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R. Bliithner et aL / EMG response to vibration

after sinusoidal transient WBV than after halfsinusoidal transient WBV (Table 2) and to occur earlier (Fig. 2). The quality of the after-effect

depended on the direction of the motion before the stop of the vibrator - the stop after an upward motion was followed by an increased

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R. Bliithner et al. / E M G response to L,ibration

EMG-activity (Figs. 2a, d), that after a downward motion by a decreased E M G (Figs. 2b, c). The longer duration of the transient WBVs with a half-sinusoidal waveform went along with larger EMG-amplitudes, whereas the duration of sinusoidal displacements had no distinct effect on the average amplitude of the responses (Table 2).

Discussion

The first distinct increased activity, i.e. more than 120% of the no-vibration activity, took place very early after the beginning of the downward displacement of the seat. Mechanisms which might be responsible for triggering a WBV-synchronous EMG-response were discussed earlier in detail (Berthoz and Wisner, 1968; Seidel et al., 1986; Seidel, 1988). At half-sinusoidal transient W B V / d o w n / D 2 and sinusoidal transient W B V /

d o w n / D 2 , e.g., it occurred after about 30 ms (EMG1) and 65 ms (EMG3), and reached maxima after about 40 ms (EMG1) and 83 ms (EMG3) (cf. Figs. 2a, c). These latencies are well within the range of stretch reflexes. They agree with the latency of 22.5 ms observed in animal experiments after an unexpected sudden decrease in gravity (Lacour et al., 1981). Since the E M G increased, however, a few milliseconds after the beginning of the accelerations measured at the skin above L3 and before those measured at the shoulder, the latency between a mechanical stretch and the response seems too short for eliciting a reflex. The very early EMG-response could be explained by a stretch reflex only, if the phase lead of bone acceleration vs. skin acceleration would be of an order of more than 10 ms - a phase lead that was supposed earlier on the basis of model calculations (Hinz et al., 1988a,b). Model calculations for the present experiment suggested a phase lead of the bone at the spinous process of

Table 1 Mean values (5 subjects) of time relations in (ms) between the extreme values of the myoelectric responses (maxima are marked by *) and of the different displacements Waveform/initial direction/duration

Kind of signal

1st extreme value

2nd extreme value

3rd extreme value

ST/do/D1 ST/do/D1 ST/do/D1 HT/do/D1 HT/do/D1 HT/do/D1 ST/up/D 1 ST/up/D1 ST/up/D1 HT/up/D1 HT/up/D1 HT/up/DI ST/do/D2 ST/do/D2 ST/do/D2 HT/do/D2 HT/do/D2 HT/do/D2 ST/up/D2 ST/up/D2 ST/up/D2 HT/up/D2 HT/up/D2 HT/up/D2

displacement EMG1/2 EMG3/4 displacement EMG1/2 EMG3/4 displacement EMG1/2 EMG3/4 displacement EMG1/2 EMG3/4 displacement EMG1/2 EMG3/4 displacement EMG1/2 EMG3/4 displacement EMG1/2 EMG3/4 displacement EMG1/2 EMG3/4

150 98 * / 1 1 7 * 91 * / 9 7 * 150 104 * / 1 0 8 * 104 *,/96 * 150 85/121 75/107 150 137/142 102/107 100 106 * / 1 0 7 * 67 *,/61 * 100 110 * / 1 0 6 * 67 *,/74 * 100 100/108 65/69 100 93/92 66/64

390 209/197 184/212 179/226 190/208 388 269 * / 1 8 0 208 * / 2 0 4 223 */241 239 * / 2 1 7 218 158/200 151/125 171/179 126/151 215 164 * / 1 5 8 143 * / 172 172 * / 1 6 9 156 * / 1 3 2

486 * / 4 7 7 * 459 * / 4 6 0 *

* * * *

* * * *

479/486 437/462 292 * / 2 7 9 * 267 * / 2 7 6 * 272/302 259/259 -

Note: S - sinusoidal, H = half-sinusoidal; T = transient WBV, do = downward, up - upward; D = duration of the transient WBVs: D1 = 250 ms for ST or 125 ms for HT, D2 = 500 ms for ST or 250 ms for HT.

R. Bliithner et al. / EMG response to vibration

L3 between 15 and 20 ms. Hence, a mechanical stretch with a sufficiently long latency before the response can be assumed. The early onset of the muscular response suggests a quickly timed dynamic activity of trunk muscles and contradicts the widespread negligence of their extraordinarily fast dynamic function. To a certain degree, it contradicts also the stretch reflex times of 5 4 - 8 7 ms for neck muscle extensors (semispinalis and splenius eapitis) after a "controlled jerk" reported earlier (Foust et al., 1974). The shorter latency of EMG3 is a surprising fact. An additional experiment with 4 successive sinusoidal displacements similar to sinusoidal transient WBV demonstrated the constancy of a different timing of EMG1 and EMG3 (Fig. 3). A vestibulo-spinal reflex could explain it, since this pathway to T1 is shorter than to L3. However, the head acceleration exhibited a distinct phase lag behind the seat (e.g., 52 ms for the maximum head acceleration behind that of the seat in sinusoidal transient W B V / d o w n / D 2 ) and could not, therefore, trigger the response. It might, however, be explained by the larger stretch at this level caused by the considerable time lag between the motion of the shoulder and the trunk which can

55

be seen already right at the beginning of the transient WBV. The consequence is a distinct and sudden relative motion between the spine and the shoulder which means a more pronounced stretch at the T1 level. Thus, the earlier response at a greater distance from the vibration input becomes understandable. Another possible hypothetical explanation would be that the different timing of EMG1 and EMG3 follows a centrally generated pattern that aims (as an outcome of phylogenetic development) at two different effects: (i) the control of decoupling/coupling the shoulder o f / t o the trunk as a shock-attenuating mechanism and (ii) the stabilization of the lumbar spine at high internal loads as a mechanism reducing relative motions, i.e. internal strain. The present regulations for the measurement and evaluation of whole-body vibration do not consider the sign of an acceleration input. The direction of the motion had a distinct effect on the muscular response. The increase of the EMG with a downward movement could be a result of the stretch caused by the delay of the upper part of the body with respect to the lower part - an assumption confirmed by our measurements of

Table 2 Average activity of normalized EMGs in subsequent intervals below or above the activity during the vibration-free intervals set to 100. The average activity in response to different displacements was computed as the integrated EMG in each interval divided by the duration of the interval Waveform/initial direction/duration

Kind of the EMG-signal

Interval 1

Interval 2

Interval 3

Interval 4

Interval 5

ST/do/D1 ST/do/D1 HT/do/DI HT/do/D1 ST/up/D1 ST/up/D1 HT/up/D1 HT/up/D1 ST/do/D2 ST/do/D2 HT/do/D2 HT/do/D2 ST/up/D2 ST/up/D2 HT/up/D2 HT/up/D2

EMG1 EMG3 EMG1 EMG3 EMG1 EMG3 EMG1 EMG3 EMG1 EMG3 EMG1 EMG3 EMG1 EMG3 EMG1 EMG3

153/128 144/150 171/131 149/156 84/94 88/85 83/93 86/83 155/134 138/129 156/133 130/131 88/95 88/93 92/97 93/94

97 /98 96 /93 88 /92 84 /84 154 /120 140 /148 159 /129 180 /178 78 /92 88 /95 78 /94 91 /92 138 /117 139 /134 146 /117 141 /136

126 128 147 159 92 97 79 83 145 153 135 124 82 92 92 92

76/91 88/87 86/93 86/86 131/115 142/144 123/113 127/127 82/93 88/93 98/99 96/98 140/118 136/130 116/108 111/113

131/118 116/123

/2 /4 /2 /4 /2 /4 /2 /4 /2 /4 /2 /4 /2 /4 /2 /4

/112 /133 /122 /171 /96 /94 /92 /87 / 124 /152 / 117 /128 /95 /95 /98 /90

86/96 95/96 124/119 115/111 94/100 96,/99 -

Note: S = sinusoidaI, H = half-sinusoidal; T = transient WBV, do = downward, up = upward; D = duration of the transient WBVs:

DI = 250 ms for ST or 125 ms for HT, D2 = 500 ms for ST or 250 ms for HT.

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R. Bliithner et al. / EMG response to vibration

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Fig. 3. Averaged (across 5 subjects) rectified normalized EMG-responses (lower curves, in percent) of erector spinae muscles (L3 = thick line; T 1 / T 2 = thin line) to 4 successive sinusoidal displacements with a period of 500 ms (a) or 250 ms (b), beginning with a downward motion. U p p e r curves: average displacement of the seat (thick line) with the first extreme value indicated in m m and resultant dynamic force FZ at the interface between the seat and the subject (thin line) with the second extreme value indicated in N.

R. Bliithner et al. / E M G response to tqbration

accelerations at the different parts of the body. In contrast to the stretch, this delay means a shortening of back muscles with an upward transient WBV, followed by an inactivation of muscles. Unexpected shortening typically silences E M G activity, presumably by unloading muscle spindles (Sanes and Evarts, 1983). The different degrees of the EMG-responses (cf. Table 2) may be explained by the frequency content of the transient accelerations which cannot be derived simply from the duration. With regard to the resonance of the whole body, the transient WBVs starting with a downward motion, e.g., may be arranged in the following sequence with both, the dominating frequency of the input spectrum and the quotient "maximum acceleration of L3 by maximum acceleration of the seat", given in brackets: half-sinusoidal transient W B V / D 1 (4 Hz; 1.81); sinusoidal transient W B V / D 1 (3 and 6 Hz; 1.66); sinusoidal transient W B V / D 2 (5-6 Hz; 1.58); half-sinusoidal transient W B V / D 2 (8 Hz; 1.41). Obviously, the halfsinusoidal displacement with a duration of 250 ms is closest to the resonance of the whole body. When sinusoidal displacements are repeated four times, the response is larger at D2 than at D1 (cf. Fig. 3). The spectral analysis for this type of input reveals narrow-band spectra with dominating frequencies of 2 and 4 Hz corresponding to the durations of one sinusoid at D1 and D2, respectively. The fact that signals with " D 2 " are closer to the resonance of the whole body explains the larger EMG-response in this case. It remains an open question, to which extent the reflex-like muscular activity during transient WBVs can stabilize and protect the spine. The E M G can be used to predict the mechanical activity of muscles. The prediction of the muscular mechanical activity has to consider the electro-mechanical delay, i.e. the time between the electrical and mechanical activity of the muscle. According to Seidel et al. (1986), a delay of about 50 ms can be assumed for back muscles. During the first part of upward movements of sinusoidal transient W B V / u p and half-sinusoidal transient W B V / u p at D1 and D2, the averaged F Z exhibited local maxima with about 210 N at 89 ms (D1) and 120 N (D2) at 43 ms before the maximum upward displacements. At these moments, the muscles did not yet react mechanically and the

57

predicted muscular force was nearly equal to that of the no-vibration condition. It is of interest that the maximum accelerations measured on the skin above L3 occurred at D1 even another 9 ms earlier than the maximum FZ. Hence, during the increased load at the very beginning of a sudden upward movement, back muscles did not stabilize the spine. Higher average FZ (302 N at sinusoidal transient W B V / D 1 and 374 N at half-sinusoidal transient W B V / D 1 , 195 N at sinusoidal transient W B V / D 2 and 204 N at half-sinusoidal transient W B V / D 2 ) were measured 22 and 25 ms (at D1) and 31 and 25 ms (at D2) after the lowermost displacements during transient WBVs beginning with a downward motion. At D1, these maximum FZ coincided with a predicted high contraction of muscles shortly after their maximum activity. At D2, however, the predicted mechanical force of the lumbar erector spinae muscles was only somewhat higher than the control level, and at the moments of maximum average accelerations at the skin above L3 which preceded the maximum FZ by about 20 ms, lumbar muscles did not show an increased mechanical activity at all. The accelerations predicted for the bone L3 (Hinz et al., 1988a) occurred another 10 to 20 ms earlier. An assessment of the functional significance of the E M G with regard to a stabilization of the spine is rendered difficult by the uncertainty of the transfer function between the E M G and muscle force under dynamic conditions (Seidel et al., 1986). Considering the electro-mechanical delay, our results indicate that the early peak of increased gravity caused by a sudden transient WBV from a motionless state to an upward displacement cannot be stabilized by back muscles. The transient WBV from a motionless state to a downward displacement displays an early peak of decreased gravity followed by a large peak of increased gravity. Back muscles most probably stabilize the lumbar spine during the latter case, if the frequency content of the motion does not exceed the resonance of the whole body, i.e. at D1 in our study. The data suggest that the muscular response is too slow to protect the spine from this peak force during similar transient WBVs with a shorter duration, i.e. with a maximal power spectral density between 6 and 8 Hz. At the stop of all transient WBVs tested, a generally high

58

R. Bliithner et al. / E M G response to uibration

stabilization was predicted. However, this assumption is restricted to that kind of examined "regularly" alternating motion. Considerable peaks of F Z indicating a damped oscillation were observed when the vibrator stopped (Figs. 2 and 3). At these moments, generally either a distinctly increased or maximum mechanical activity was predicted for back muscles. Distinct relaxations were observed during the outlasting responses after transient WBV (Fig. 2). The onset of these intervals with a decreased muscular activity depended on the direction of the last displacement (Fig. 2). An enhanced vulnerability of the spine can be predicted, if an "irregular" force peak would hit the spine during these intervals without "muscular protection" lasting up to about 100 ms. These considerations illustrate again the significance of the history preceding a transient WBV.

Conclusions Although a time constant of 125 ms was recently applied for the description of biodynamic effects of transient vibrations (Dupuis et al., 1991), the rapid fluctuations of the muscular responses in this study suggest in conformity with Griffin (1990) that it is inappropriate for shocks with short duration. An increased health risk can be assumed, when an enhanced need for stabilization of the spine does not coincide with an equivalent mechanical activity of back muscles. It can be predicted for the following cases: (a) the initial phase of a sudden upward displacement without motion in the preceding history and (b) a downward displacement with a dominating frequency near 6 - 8 Hz. A very unfavourable condition can be predicted, if a load occurs during the interval with relaxation of back muscles following a sudden stop of the seat, with a time delay that depends on the direction of the motion before the stop. The immediate muscular reactions suggest the necessity to include muscle forces in calculations of the internal spinal load under similar exposure conditions, except for the first 50 to 100 ms of an event without motion in its preceding history. The marked effect of the direction of a transient WBV on the kind of response

hints at the sign of acceleration as a characteristic of vibration exposure which should probably be considered in future procedures evaluating transient WBV. As the biological effect depends also on the history preceding an event, the due consideration of the latter by means of a suited procedure might be another promising future approach. The evaluation of transient WBVs in the time domain requires further research. It may be suited to amend the present unsatisfactory evaluation and design mainly using the frequency domain. The avoidance of an unfavourably timed back muscle activity by an appropriate control of the vibration input might enable a significant contribution to the prevention of work-related low back pain.

Acknowledgement The authors gratefully acknowledge the contribution of Dipl.-Ing. Udo Erdmann, who developed hard- and software used in this study. We thank R. Vizcaino and M. G o d a u for their help and assistance. The research project was performed with financial support from the G e r m a n Federal Ministry for Research and Technology, Grant 01HK0613.

References Berthoz, A. and Wisner, A., 1968. Striated muscle activity and biomechanical effects in m a n submitted to low frequency vibrations. Electromyography 8 (Suppl. 1): 101-109. Blfithner, R., Hinz, B. and Seidel, H. 1986. Zur M6glichkeit einer Absch~itzung der Wirbelsaulenbeanspruchung durch Ganzk6rpervibration unter experimentellen Bedingungen. Z. ges. Hyg.,32(2): 111 113. Chapman, A.E. and Troup, J.D.G., 1969. The effect of increased maximal strength on the integrated electrical activity of lumbar erectores spinae. Electromyography, 9(3): 263 28(/. Dupuis, H., Hartung, E. and Haverkamp, M., 1991. Acute effects of transient vertical whole-body vibration. Int. Arch. Occup. Environ. Health, 63(4): 261-265. Foust, D.R., Chaffin, D.B., Snyder, R.G. and Baum, J.K., 1973. Cervical range of motion and dynamic response and strength of cervical muscles. SAE Transactions 82(4): 3222-3234. Griffin, M.J., 1990. Handbook of h u m a n vibration. Academic Press, London, pp. 86-88.

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Hinz, B. and Seidel, H., 1987. The nonlinearity of the human body's dynamic response during sinusoidal whole-body vibration. Industrial Health, 25(4): 169-181. Hinz, B., Seidel, H., Br~iuer, D., Menzel, G., Blfithner, R. and Erdmann, U., 1988a. Examination of spinal column vibrations - A non-invasive approach. Eur. J. Appl. Physiol., 57(6): 707-713. Hinz, B., Seidel, H., Brfiuer, D., Menzel, G., Bliithner, R. and Erdmann, U., 1988b. Bidimensional accelerations of lumbar vertebrae and estimation of internal spinal load during sinusoidal vertical whole-body vibration: A pilot study. Clinical Biomechanics, 3(4): 241-248. Lacour, M., Vidal, P.P. and Xerri, C,, 1981. Early directional influence of visual motion cues on postural control in the falling monkey. Ann. NY Acad. Sci., 374: 403-411. Sandover, J., 1981. Vibration, posture and low-back disorders of professional drivers. Report No DHS 402. Dept. of Human Sciences, University of Technology, Loughborough, 141 pp. Sanes, J. and Evarts, E.V., 1983. Regulatory role of proprioceptive input in motor control of phasic or maintained voluntary contractions in man. In: J.E. Desmedt (Ed.),

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Motor control mechanisms in health and disease; Advances in Neurology. Raven Press, New York, 39: pp. 47-59. Seidel, H., 1988. Myoelectric reactions to ultra-low frequency and low-frequency whole body vibration. Eur. J. Appl. Physiol., 57(5): 558-562. Seidel, H., Bliithner, R. and Hinz, B., 1986. Effects of sinusoidal whole-body vibration on the lumbar spine: The stress-strain relationship. Int. Arch. Occup. Environ. Health, 57(3): 207-223. Seidel, H., Hinz, B., Menzel, G. and Bliithner, R., 1992. Estimation of internal spinal load during transient wholebody vibration: An analysis in the time domain. Arbete och H~ilsa, 17 (Book of Abstracts "International Scientific Conference on Prevention of Work-related Musculoskeletal Disorders PREMUS" Sweden May 12-14, 1992): 257259. Seidel, H., Bastek, R., Br~iuer, D., Buchholz, Ch., Meister, A., Metz, A.-M. and Rothe, R., 1980. On human response to prolonged repeated whole-body vibration. Ergonomics, 23(3): 191-211.