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Some effects on the spine from driving
Clinical Biomechanics 1988; 3:236-240
Some effects on the spine from driving Rosemary Bonney, BSc Department of Production Engineering and Production Management, University of Nottingham, UK.
Summary Changes in stature have been shown to relate to loads imposed on the spine which can arise from work activities or postural demands, including the seating requirement. It was deduced, therefore, that the shrinkage method could be used to identify the effects of different factors in the driving situation by measuring changes in spinal column length. The results showed that when the spine was vibrated changes in stature did not show a simple relationship to estimated load. A non-invasive technique was thus developed to assess the in vivo displacements of the upper, central and lower regions of the spinal column, in order to investigate why these anomalies were occurring.
Introduction It is recognized that a large proportion of people exposed to truck and automobile driving suffer from low back pain. This could be the result of many factors, such as poor posture, length of exposure to the driving task or manual handling as part of the task m An important contribution to these high levels of injury is believed to be a high load transmitted via the lumbar spine. Biomechanical analysis 3 has led to recommendations that the load on the spine at LJS~' should not exceed certain levels, calculated using a static biomechanical model incorporating certain conventions 4. The effects of dynamic loads, frequency, amplitude and duration are taken into account by the use of recommended factors derived from experiments. To be able to measure more directly the effects on the spine of work activities, postures and working equipment, Eklund ,and Corlett 5 developed a precision stadiometer to measure changes in stature, using these as a measure of disc compression, and demonstrated how the rate of shrinkage was a function of load on the spine. Shrinkage when sitting in different chairs was compared and the results were in agreement with disc pressure measurements reported in the literature. From these initial findings, it may be deduced that the shrinkage method could be used to identify the effects of a range of factors affecting vehicle driving, and believed to have an influence on spinal load and subsequent low back problems. A number of experiments were therefore proposed to explore the effects of each factor, using as a
Correspondence and reprint requests to: R. Bonney, Department of Production Engineering and Production Management, University of Nottingham, University Park, Nottingham NG9 2RD, UK. © 1988 Butterworth & Co (Publishers) Ltd 0268-0033/88/040236-05503.00
major dependent variable the change in stature of the subject during exposure. The normal body height decreases during the day 5 by around 15 mm. In order to make the measure useful for ergonomic evaluations in these studies, an experimental period of at least 30 minutes and a precision of 1 mm or less was needed. A version of the apparatus as used in studies 1 and 2 described below was modified technically and the procedure changed before study 3 was performed. This improved the precision of measurement and provided computerized recording. The measuring equipment has been described elsewhere 6, and is shown in Figure 1.
Experiments Study 1: Effects of head posture on stature This study investigated the effects of viewing a television screen at three different head positions, whilst in a driving simulator: i) 0O (eyes straight ahead); ii) 20 ° (forward inclination); and iii) 40 ° (forward inclination). Eight young male subjects participated in the study. After 1 hour watching a TV screen at each of the three head angles, statistically significant differences in stature reduction were measured. At 0 ° no change was found, at 20 ° about 0.9 mm was recorded, whilst at 40 ° almost 1.5 mm was measured. These results were supported by the biomechanical calculations undertaken by Columbini et al.7, and are reported in detail elsewhere 6.
Study 2: The effects of different factors in the driving situation on stature A small pilot study was conducted by Corlett and Rose 8 to investigate the effects of seat shape, driving activity
Bonney: and whole body vibration on the back load of motor car drivers. The results suggested that the driving activities of steering and pedal operation were the major contributions to spinal load. It appeared to show that neither the seat nor the vibration contributed to spinal load, a result which was not accepted due to certain features of the values chosen for the seat and vibration variables.
Effects on spine from driving
237
DC motor driving a variable eccentric, which could be set at different levels according to the amplitude and vibration required. Vibration waveforms and levels were simultaneously monitored by means of an Endeveco 2265/20 piezo resistive accelerometer (UK), mounted beneath the car seat. The acceleration was set to 1 ms -2 to keep the vibration experienced within the reduced proficiency boundary limits as defined by ISO 26319. The position of the pedals and steering wheel could be set to a variety of representative driving positions from sports car to heavy truck. The wheel and pedals controlled a driving task displayed on a television monitor. Subjects were required to perform the task whilst the trials were being run so as to simulate driving activity. The whole apparatus was connected to a pen recorder so the driving activities of the subjects could be monitored. Ten male subjects participated in the trials. Each subject was randomly assigned to one of the two driving positions described as 'car or lorry' (low or high seat and with the steering wheel more horizontally orientated for the lorry position) so there were five subjects at each level. They were then required to carry out six trials, varying the seat back angle and oscillating frequency. Their statures were measured before and after the trials. The times of day at which the experiments were conducted were the same for all subjects. The results showed that when the three levels of vibration were compared, 8 Hz vibration caused 0.14 mm growth, 0 Hz caused 0-13 mm growth and 4 Hz caused 0.55 m m shrinkage. The results were not significant. Similarly, when the two driving configurations were compared, the car driving position caused 0.44 m m shrinkage and the lorry position caused 0-13 mm growth. These results were not significant either. When the two sitting postures were compared it was shown that an inclined posture, (back rest-seat angle set to 110%) caused 0.81mm growth and an upright posture, (back rest angle set to 90 °) caused 1.27 mm shrinkage. The difference between the levels was significant at the 0.01 level (Figure 2).
A more comprehensive study was devised to investigate firstly, whether seat back angles of 90 °, and 110° affected stature; secondly, whether the relationship between seat and steering wheel configuration (high and low) affected stature; and thirdly, whether changes in vibration at frequencies of 0 Hz, 4 Hz and 8 Hz affected stature.
Modification of apparatus The results obtained from these preliminary studies created concern about the accuracy of the measured results, due to the large differences in individual readings for some of the measurements. For this reason a number of aspects of the measurement technique were investigated, with the aim of identifying the causes of any anomalies. It was first considered that the differences might be related to height and weight of the individual subjects. However, when the raw data were re-examined in relation to height and weight this did not appear to be the case. For this reason, it was decided to take a closer look at the apparatus used. The major changes made were:
Apparatus An experimental rig was built which carded an adjustable car seat with seat pan and back rest instrumented to record loads. The rig was vibrated by a variable speed
I. Replacement of the dial gauge on the stadiometer with a linear transducer connected to a digital printout, so that it was no longer necessary to rely on the experimenter for the readings taken.
Figure 1. The stadiometer used to record changes in stature (height) Further development
Clin. Biomech. 1988; 3: No 4
238 2.0
1.5
110 ° S e a t - b a c k
angle
1.0 E E 0.5
r: x:
90 ° S e a t - b a c k
angle
0
t-
~ -0.5
-1.0
which the subject was exposed affected the amount of change in stature which occurred (Figure 3). The results also showed that at 4 Hz, close to the natural frequency of the trunk, stature increased, in agreement with Amin et al.~0 but at 8 Hz there was no change for the conditions presented. However, it must be noted that, when vibration was absent subjects' stature decreased. It was hypothesized that the differences in stature changes with different frequencies were due to changes in phase between different sectors of the spine, which gave rise to relative movements between these sectors causing extension or shrinkage. The effect would be related to the forcing (imposed) frequency and the natural frequency of the various sectors. Hence it was important to consider the oscillators of different sectors of the spine and assess whether this hypothesis was valid or not.
-1.5
-2.0
-2.5
Figure 2. Changes in height during simulated driving with two different sitting postures. The means and standard deviations are shown. 2. Introduction of a nose pointer to control head position in the sagittal plane. Hence the subject did not need to control head position by viewing a reflection in a mirror. Modifications were also made to the measurement procedure; subjects were not required to take off their shoes each time a reading was taken, as had been the case previously. Trials were carded out on the modified apparatus and individual scores for five sets of readings were recorded to within a range of 0-33 mm. The period of exposure to vibration originally chosen was 30 minutes. This was as a result of pilot experiments 8, where it appeared that this was sufficient to show significant differences between exposure conditions. The period was extended to 1 hour for the following experiments.
Study 3: The effects of pure sinusoidal vibration on changes in stature The effects of pure sinusoidal vibration at frequencies of 0 Hz, 4 Hz, 6 Hz and 8 Hz on stature were investigated with eight subjects. The seat and pedals were arranged to give minimum stature change during driving, based on the earlier study. The arrangement was as for the car driving position with a seat-back angle of 90 °. Each subject was exposed to each frequency for 1 hour's duration using the driving rig and measurement procedures previously described, with measurements taken before and after each exposure. The stature changes were subjected to analysis of variance, which showed no difference between subjects but a significant difference between vibration and no vibration, and furthermore a significant difference between 4 Hz, and the 6 Hz and 8 H_z frequencies. The results demonstrated that the frequencies to
3.01
2.0 E E Ix .c o~
1.0
. u
¢¢-
I 0
ol to cO
I q
6
i
-1.0
-2.0 j
Frequency (Hz)
Figure 3. Changes in height during vibration at different frequencies. The means and standard deviations are shown.
Bonney: Effects on spine from driving
239
marked using well defined body landmarks and palpation. The subject was required to sit in an erect position with the vertical centre line of his back in line with the centre line of the back rest but not leaning on a back rest. One of the spinal probes was positioned level with the L 3 vertebra and the other level with the T 4 or C 5 vertebra. The subject leaned on these probes, exerting a 9.81 N force on their spring-loaded end. Vibration recordings were made from the accelerometers fitted to the probes and to the seat, and the procedure was repeated for each of the trial conditions. All the recorded data were transferred to a digital oscilloscope via a Kemo Low Pas Filter and processed using a Masscomp Computer system operating under UNIX. Results
Figure 4. Spinal probe positioned over La of a vibrat-
ing subject Study 4: Segmental analysis of the oscillatory behaviour of the spinal column The aim here was to develop a non-invasive technique to assess the in vivo displacements of upper, central and lower regions of the spinal column. It was required to eliminate the problem of the motion of the superficial soft tissues and identify the movement of the bony elements themselves. Using such equipment, the objective of the subsequent experiments would be to measure the amplitude and phase relationship between different groups of vertebrae in order to investigate the phenomena seen to be occurring in the previous experiments. Two flexible spinal probes with low friction characteristics were manufactured to sit onto the skin over the projections of the spinous processes of individual vertebra. An Endeveco 2265/20 piezo resistive accelerometer (UK) was attached to each of the probes so that motion in the vertical mode could be detected and recorded. The probes were supported by a non-vibrating stand, so that any movement detected by the accelerometer would be due to the movement of the person on the rig (Figure 4). A set of trials was conducted using one of the subjects who had also participated in the experimental conditions for study 3. This was done so that the individual results from the study could be compared to the changes in stature measured. A repeated measures design was used so that the subject was exposed to each frequency level twice. The order of presentation of the conditions was as follows: 6 Hz, 4 Hz, 8 Hz, 6 Hz, 8 Hz, 4 Hz. To compare the oscillatory behaviour along the spine three vertebrae were selected; C 5, T 4 and L 3. Vibration along the spine could then be compared to the vibration waveforms occurring at the seat pan. However, since there were only two probes it meant that two trials had to be run for each vertebral level so that the oscillatory behaviour of all three vertebrae could be compared. The correct positions of the vertebrae were identified and
The results showed that the relative amplitude measured along the spine changed in a complex way with increasing distance from the seat pan. It demonstrated, therefore, that the response of the spine to vibration is not uniform, but that some parts of the spine are more affected than others. From the results it appeared that the region of the L 3 vertebra was more stressed than those of the C 5 and T 4 vertebrae. Relative amplitude was also affected by frequency; the spine was more stressed at 6 Hz than at 4 Hz and 8 Hz, where the effects of the vibration were very much reduced. A major resonant frequency of the spine obviously occurred around 4 - 6 Hz, which is in agreement with the results reported on in the literature", but the experiment did not enable us to identify its frequency precisely. The results also showed that the oscillatory behaviour of the spinal column at the L 3 level was affected when the position o f the probes was changed on the spine. Thus, the measured phase angles at the L 3 level were smaller when L 3 and T 4 were compared to one another than when L 3 and C 5 were compared. However, the relative amplitude was greater at the L 3 level when the L 3 and T 4 vertebrae were compared than when the relative magnitude was measured at the L 3 level when the L 3 and C 5 vertebrae were compared. These results do not lend themselves to simple conclusions, but suggest that the oscillatory behaviour of the L 3 vertebra is affected by the position of the second probe higher up the spine. It could be surmised that very slight changes in muscle action, head posture and spinal shape could explain the differences in amplitude and phase angle which are occurring. It was earlier hypothesized that a difference in phase shift at different sections of the spinal column could be used to explain the increase in stature which occurred at different frequencies. Since changes in the position of the probes relative to one another obviously introduced changes in the phase angles measured, the results could not clearly support the argument. However, the results did show that phase shift is smallest at 4 Hz, congruent with the views of Wilder et al.t2 Pope et al.13, and a personal communication from Griffin (1988); 4 Hz is also
240
Clin. Biomech. 1988; 3: No 4
approximately the resonant frequency of the internal organs which are oscillating with the spine. It could be argued that the reaction of the internal organs acting on the spine causes an upward thrust which more than compensates for the downward force causing an unloading effect. Since it is known that the spine expands more rapidly when load is removed than it shrinks when loaded m4, this may be the reason for the differential behaviour, but there is no clear evidence from these studies. Conclusions
The probe method has obvious advantages in that it reduces the effects of the movement of soft tissue over the underlying bone and so can be used to measure spinal column behaviour non-invasively. It would seem useful to take the probe system a stage further and implement certain design modifications in order that it can be used as a practical tool. Firstly, more detailed measures of any changes in posture could be achieved by using a CODA system. Secondly, the use of three or more probes would enable the response of different sections of the spinal column to be monitored both simultaneously and individually. The apparatus could then be used to achieve a better understanding of what happens to the spine under vibration, and so explain the apparently dramatic changes in height which occur with only very minor changes in the level of vibration exposure at low acceleration levels.
References
1 Frymoyer JW, Pope MH, Clements JH, Wilder D, MacPearson BR, Ashikagt B. Risk factors in low back pain. J Bone Jt Surg 1985; 65A, 213-8
2 Kelsey J, Hardy R. Driving of motor vehicles as a risk factor for acute herniated lumbar disc. Am J Epidemiol 1975; 102:63-7 3 Schultz BA, Andersson B. Analysis of loads on the lumbar spine. Spine 1981; 6:76-81 4 National Institute for Occupational Safety and Health. A work practice guide for manual lifting, Technical Report No. 81-122, 1981 US Dept of Health and Human Sciences flfflOSH), Cincinnati, Ohio 5 Eklund J, Corlett EN. Shrinkage as a measure of the effect of load on the spine, Spine 1984; 9, 189-94 6 Bonney RA. Effects of driving on the spine. Thesis to be submitted, Department of Production Engineering and
Production Management, University of Nottingham, UK 7 Columbini D, Occipinti E, Frigo C, Pedotti A, Grieco A. Biomechanical, electromyographical and radiological study of seated postures. In: Corlett EN, Wilson JR, Manenica I, Ergonomics of working postures. London; Taylor and Francis, 1986:331-44 8 Corlett EN, Rose T. The effect of driving factors on spinal load. In: Proceedings of the Human Factors Society, 29th Annual Meeting, 1985:264-5 9 ISO 2631. Guide for the evaluation of human exposure to whole body vibration. International Organisation for Standardisation, Case Postule 56, CH-1211, Geneva 20, Switzerland, 1985 10 Amin AN, Corlett EN, Bonney RA. Does wearing a seat belt alter the load on the back whilst driving. In: Proceedings of the Ergonomics Society's 1988 Conference, 1988:538--43 11 Coermann RR. The mechanical impedance of the human body in sitting and standing position at low frequencies. Hum Fact 1962; 1:227-53 12 Wilder D, Frymoyer JW, Pope M. The effects of vibration on the spine. Automedica 1985; 6:3-35 13 Pope MH, Svensson M, Broman H, Andersson GBJ. Mounting of the transducers in measurement of segmental motion of the spine. J Bone Jt Surg 1986; 68A: 895-702 14 Tyrell AA, Reilly J, Troup JDG. Circadian variation in stature and the effects of spinal loading. Spine 1985; 10: 161---4
Some effects on the spine from driving Rosemary Bonney, BSc Department of Production Engineering and Production Management, University of Nottingham, UK.
Summary Changes in stature have been shown to relate to loads imposed on the spine which can arise from work activities or postural demands, including the seating requirement. It was deduced, therefore, that the shrinkage method could be used to identify the effects of different factors in the driving situation by measuring changes in spinal column length. The results showed that when the spine was vibrated changes in stature did not show a simple relationship to estimated load. A non-invasive technique was thus developed to assess the in vivo displacements of the upper, central and lower regions of the spinal column, in order to investigate why these anomalies were occurring.
Introduction It is recognized that a large proportion of people exposed to truck and automobile driving suffer from low back pain. This could be the result of many factors, such as poor posture, length of exposure to the driving task or manual handling as part of the task m An important contribution to these high levels of injury is believed to be a high load transmitted via the lumbar spine. Biomechanical analysis 3 has led to recommendations that the load on the spine at LJS~' should not exceed certain levels, calculated using a static biomechanical model incorporating certain conventions 4. The effects of dynamic loads, frequency, amplitude and duration are taken into account by the use of recommended factors derived from experiments. To be able to measure more directly the effects on the spine of work activities, postures and working equipment, Eklund ,and Corlett 5 developed a precision stadiometer to measure changes in stature, using these as a measure of disc compression, and demonstrated how the rate of shrinkage was a function of load on the spine. Shrinkage when sitting in different chairs was compared and the results were in agreement with disc pressure measurements reported in the literature. From these initial findings, it may be deduced that the shrinkage method could be used to identify the effects of a range of factors affecting vehicle driving, and believed to have an influence on spinal load and subsequent low back problems. A number of experiments were therefore proposed to explore the effects of each factor, using as a
Correspondence and reprint requests to: R. Bonney, Department of Production Engineering and Production Management, University of Nottingham, University Park, Nottingham NG9 2RD, UK. © 1988 Butterworth & Co (Publishers) Ltd 0268-0033/88/040236-05503.00
major dependent variable the change in stature of the subject during exposure. The normal body height decreases during the day 5 by around 15 mm. In order to make the measure useful for ergonomic evaluations in these studies, an experimental period of at least 30 minutes and a precision of 1 mm or less was needed. A version of the apparatus as used in studies 1 and 2 described below was modified technically and the procedure changed before study 3 was performed. This improved the precision of measurement and provided computerized recording. The measuring equipment has been described elsewhere 6, and is shown in Figure 1.
Experiments Study 1: Effects of head posture on stature This study investigated the effects of viewing a television screen at three different head positions, whilst in a driving simulator: i) 0O (eyes straight ahead); ii) 20 ° (forward inclination); and iii) 40 ° (forward inclination). Eight young male subjects participated in the study. After 1 hour watching a TV screen at each of the three head angles, statistically significant differences in stature reduction were measured. At 0 ° no change was found, at 20 ° about 0.9 mm was recorded, whilst at 40 ° almost 1.5 mm was measured. These results were supported by the biomechanical calculations undertaken by Columbini et al.7, and are reported in detail elsewhere 6.
Study 2: The effects of different factors in the driving situation on stature A small pilot study was conducted by Corlett and Rose 8 to investigate the effects of seat shape, driving activity
Bonney: and whole body vibration on the back load of motor car drivers. The results suggested that the driving activities of steering and pedal operation were the major contributions to spinal load. It appeared to show that neither the seat nor the vibration contributed to spinal load, a result which was not accepted due to certain features of the values chosen for the seat and vibration variables.
Effects on spine from driving
237
DC motor driving a variable eccentric, which could be set at different levels according to the amplitude and vibration required. Vibration waveforms and levels were simultaneously monitored by means of an Endeveco 2265/20 piezo resistive accelerometer (UK), mounted beneath the car seat. The acceleration was set to 1 ms -2 to keep the vibration experienced within the reduced proficiency boundary limits as defined by ISO 26319. The position of the pedals and steering wheel could be set to a variety of representative driving positions from sports car to heavy truck. The wheel and pedals controlled a driving task displayed on a television monitor. Subjects were required to perform the task whilst the trials were being run so as to simulate driving activity. The whole apparatus was connected to a pen recorder so the driving activities of the subjects could be monitored. Ten male subjects participated in the trials. Each subject was randomly assigned to one of the two driving positions described as 'car or lorry' (low or high seat and with the steering wheel more horizontally orientated for the lorry position) so there were five subjects at each level. They were then required to carry out six trials, varying the seat back angle and oscillating frequency. Their statures were measured before and after the trials. The times of day at which the experiments were conducted were the same for all subjects. The results showed that when the three levels of vibration were compared, 8 Hz vibration caused 0.14 mm growth, 0 Hz caused 0-13 mm growth and 4 Hz caused 0.55 m m shrinkage. The results were not significant. Similarly, when the two driving configurations were compared, the car driving position caused 0.44 m m shrinkage and the lorry position caused 0-13 mm growth. These results were not significant either. When the two sitting postures were compared it was shown that an inclined posture, (back rest-seat angle set to 110%) caused 0.81mm growth and an upright posture, (back rest angle set to 90 °) caused 1.27 mm shrinkage. The difference between the levels was significant at the 0.01 level (Figure 2).
A more comprehensive study was devised to investigate firstly, whether seat back angles of 90 °, and 110° affected stature; secondly, whether the relationship between seat and steering wheel configuration (high and low) affected stature; and thirdly, whether changes in vibration at frequencies of 0 Hz, 4 Hz and 8 Hz affected stature.
Modification of apparatus The results obtained from these preliminary studies created concern about the accuracy of the measured results, due to the large differences in individual readings for some of the measurements. For this reason a number of aspects of the measurement technique were investigated, with the aim of identifying the causes of any anomalies. It was first considered that the differences might be related to height and weight of the individual subjects. However, when the raw data were re-examined in relation to height and weight this did not appear to be the case. For this reason, it was decided to take a closer look at the apparatus used. The major changes made were:
Apparatus An experimental rig was built which carded an adjustable car seat with seat pan and back rest instrumented to record loads. The rig was vibrated by a variable speed
I. Replacement of the dial gauge on the stadiometer with a linear transducer connected to a digital printout, so that it was no longer necessary to rely on the experimenter for the readings taken.
Figure 1. The stadiometer used to record changes in stature (height) Further development
Clin. Biomech. 1988; 3: No 4
238 2.0
1.5
110 ° S e a t - b a c k
angle
1.0 E E 0.5
r: x:
90 ° S e a t - b a c k
angle
0
t-
~ -0.5
-1.0
which the subject was exposed affected the amount of change in stature which occurred (Figure 3). The results also showed that at 4 Hz, close to the natural frequency of the trunk, stature increased, in agreement with Amin et al.~0 but at 8 Hz there was no change for the conditions presented. However, it must be noted that, when vibration was absent subjects' stature decreased. It was hypothesized that the differences in stature changes with different frequencies were due to changes in phase between different sectors of the spine, which gave rise to relative movements between these sectors causing extension or shrinkage. The effect would be related to the forcing (imposed) frequency and the natural frequency of the various sectors. Hence it was important to consider the oscillators of different sectors of the spine and assess whether this hypothesis was valid or not.
-1.5
-2.0
-2.5
Figure 2. Changes in height during simulated driving with two different sitting postures. The means and standard deviations are shown. 2. Introduction of a nose pointer to control head position in the sagittal plane. Hence the subject did not need to control head position by viewing a reflection in a mirror. Modifications were also made to the measurement procedure; subjects were not required to take off their shoes each time a reading was taken, as had been the case previously. Trials were carded out on the modified apparatus and individual scores for five sets of readings were recorded to within a range of 0-33 mm. The period of exposure to vibration originally chosen was 30 minutes. This was as a result of pilot experiments 8, where it appeared that this was sufficient to show significant differences between exposure conditions. The period was extended to 1 hour for the following experiments.
Study 3: The effects of pure sinusoidal vibration on changes in stature The effects of pure sinusoidal vibration at frequencies of 0 Hz, 4 Hz, 6 Hz and 8 Hz on stature were investigated with eight subjects. The seat and pedals were arranged to give minimum stature change during driving, based on the earlier study. The arrangement was as for the car driving position with a seat-back angle of 90 °. Each subject was exposed to each frequency for 1 hour's duration using the driving rig and measurement procedures previously described, with measurements taken before and after each exposure. The stature changes were subjected to analysis of variance, which showed no difference between subjects but a significant difference between vibration and no vibration, and furthermore a significant difference between 4 Hz, and the 6 Hz and 8 H_z frequencies. The results demonstrated that the frequencies to
3.01
2.0 E E Ix .c o~
1.0
. u
¢¢-
I 0
ol to cO
I q
6
i
-1.0
-2.0 j
Frequency (Hz)
Figure 3. Changes in height during vibration at different frequencies. The means and standard deviations are shown.
Bonney: Effects on spine from driving
239
marked using well defined body landmarks and palpation. The subject was required to sit in an erect position with the vertical centre line of his back in line with the centre line of the back rest but not leaning on a back rest. One of the spinal probes was positioned level with the L 3 vertebra and the other level with the T 4 or C 5 vertebra. The subject leaned on these probes, exerting a 9.81 N force on their spring-loaded end. Vibration recordings were made from the accelerometers fitted to the probes and to the seat, and the procedure was repeated for each of the trial conditions. All the recorded data were transferred to a digital oscilloscope via a Kemo Low Pas Filter and processed using a Masscomp Computer system operating under UNIX. Results
Figure 4. Spinal probe positioned over La of a vibrat-
ing subject Study 4: Segmental analysis of the oscillatory behaviour of the spinal column The aim here was to develop a non-invasive technique to assess the in vivo displacements of upper, central and lower regions of the spinal column. It was required to eliminate the problem of the motion of the superficial soft tissues and identify the movement of the bony elements themselves. Using such equipment, the objective of the subsequent experiments would be to measure the amplitude and phase relationship between different groups of vertebrae in order to investigate the phenomena seen to be occurring in the previous experiments. Two flexible spinal probes with low friction characteristics were manufactured to sit onto the skin over the projections of the spinous processes of individual vertebra. An Endeveco 2265/20 piezo resistive accelerometer (UK) was attached to each of the probes so that motion in the vertical mode could be detected and recorded. The probes were supported by a non-vibrating stand, so that any movement detected by the accelerometer would be due to the movement of the person on the rig (Figure 4). A set of trials was conducted using one of the subjects who had also participated in the experimental conditions for study 3. This was done so that the individual results from the study could be compared to the changes in stature measured. A repeated measures design was used so that the subject was exposed to each frequency level twice. The order of presentation of the conditions was as follows: 6 Hz, 4 Hz, 8 Hz, 6 Hz, 8 Hz, 4 Hz. To compare the oscillatory behaviour along the spine three vertebrae were selected; C 5, T 4 and L 3. Vibration along the spine could then be compared to the vibration waveforms occurring at the seat pan. However, since there were only two probes it meant that two trials had to be run for each vertebral level so that the oscillatory behaviour of all three vertebrae could be compared. The correct positions of the vertebrae were identified and
The results showed that the relative amplitude measured along the spine changed in a complex way with increasing distance from the seat pan. It demonstrated, therefore, that the response of the spine to vibration is not uniform, but that some parts of the spine are more affected than others. From the results it appeared that the region of the L 3 vertebra was more stressed than those of the C 5 and T 4 vertebrae. Relative amplitude was also affected by frequency; the spine was more stressed at 6 Hz than at 4 Hz and 8 Hz, where the effects of the vibration were very much reduced. A major resonant frequency of the spine obviously occurred around 4 - 6 Hz, which is in agreement with the results reported on in the literature", but the experiment did not enable us to identify its frequency precisely. The results also showed that the oscillatory behaviour of the spinal column at the L 3 level was affected when the position o f the probes was changed on the spine. Thus, the measured phase angles at the L 3 level were smaller when L 3 and T 4 were compared to one another than when L 3 and C 5 were compared. However, the relative amplitude was greater at the L 3 level when the L 3 and T 4 vertebrae were compared than when the relative magnitude was measured at the L 3 level when the L 3 and C 5 vertebrae were compared. These results do not lend themselves to simple conclusions, but suggest that the oscillatory behaviour of the L 3 vertebra is affected by the position of the second probe higher up the spine. It could be surmised that very slight changes in muscle action, head posture and spinal shape could explain the differences in amplitude and phase angle which are occurring. It was earlier hypothesized that a difference in phase shift at different sections of the spinal column could be used to explain the increase in stature which occurred at different frequencies. Since changes in the position of the probes relative to one another obviously introduced changes in the phase angles measured, the results could not clearly support the argument. However, the results did show that phase shift is smallest at 4 Hz, congruent with the views of Wilder et al.t2 Pope et al.13, and a personal communication from Griffin (1988); 4 Hz is also
240
Clin. Biomech. 1988; 3: No 4
approximately the resonant frequency of the internal organs which are oscillating with the spine. It could be argued that the reaction of the internal organs acting on the spine causes an upward thrust which more than compensates for the downward force causing an unloading effect. Since it is known that the spine expands more rapidly when load is removed than it shrinks when loaded m4, this may be the reason for the differential behaviour, but there is no clear evidence from these studies. Conclusions
The probe method has obvious advantages in that it reduces the effects of the movement of soft tissue over the underlying bone and so can be used to measure spinal column behaviour non-invasively. It would seem useful to take the probe system a stage further and implement certain design modifications in order that it can be used as a practical tool. Firstly, more detailed measures of any changes in posture could be achieved by using a CODA system. Secondly, the use of three or more probes would enable the response of different sections of the spinal column to be monitored both simultaneously and individually. The apparatus could then be used to achieve a better understanding of what happens to the spine under vibration, and so explain the apparently dramatic changes in height which occur with only very minor changes in the level of vibration exposure at low acceleration levels.
References
1 Frymoyer JW, Pope MH, Clements JH, Wilder D, MacPearson BR, Ashikagt B. Risk factors in low back pain. J Bone Jt Surg 1985; 65A, 213-8
2 Kelsey J, Hardy R. Driving of motor vehicles as a risk factor for acute herniated lumbar disc. Am J Epidemiol 1975; 102:63-7 3 Schultz BA, Andersson B. Analysis of loads on the lumbar spine. Spine 1981; 6:76-81 4 National Institute for Occupational Safety and Health. A work practice guide for manual lifting, Technical Report No. 81-122, 1981 US Dept of Health and Human Sciences flfflOSH), Cincinnati, Ohio 5 Eklund J, Corlett EN. Shrinkage as a measure of the effect of load on the spine, Spine 1984; 9, 189-94 6 Bonney RA. Effects of driving on the spine. Thesis to be submitted, Department of Production Engineering and
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