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Analysis and measurement of the loads on the lumbar spine during work at a table
J. Biomechmics Vol. 13, pp. 513-520. 0 Pergamon Press Ltd. 1980. Printed
@X2-9290/80/060-0513 in Grea
SO?.oOD
Britain.
ANALYSIS AND MEASUREMENT OF THE LOADS ON THE LUMBAR SPINE DURING WORK AT A TABLE* G. B. J. ANDERSSON?,R. C)RTENGWN$ and A. SCHULTZ$ l
Department of Orthopaedic Surgery I and tDepartment of Clinical Neurophysiology, Sahlgren Hospital, S-413 45 Giiteborg and 4 Department of Materials Engineer& University of Illinois, Chicago Circle, Chicago, IL 60680, U.S.A.
Abstract-A study has been made in which the loads on the lumbar spine and trunk muscles were calculated in static submaximal armlifts, and the method validated in part by measurements ofmyoelectric back muscle activity. Ten subjects performed thirty work tasks while seated at a tabk. The lumbar spine loads and lumbar trunk muscle contraction forces required for task execution were predicted using mechanical analysis. The myoelectric activity was recorded at eight sites in the low back region. A good correlation was obtained between the contraction force predictions and the myoelectric signal levels.
INTRODUCnON One of the most common hazards industrial workers contend with is low back pain. The higher rates of back injuries in physically heavy work has led to the assumption that work load is related to low back symptoms. So one consideration in the design of safe and efficient work places and work-execution schemes is to keep the loads on the lumbar spine low. Manual industrial work is often performed at a table. Objects are moved about on the table surface and between boxes and shelves and the tabk. It can be assumed that many factors affect the loads on the lumbar spine under these circumstances: the extent to which the upper trunk is bent over ; the positions of the head, neck, arms and hands; whether the arms and hands are suspended or resting on the table or on armrests; the weight of each of these body segments; the weights and locations of any objects handled ; and the nature of any pushes or pulls exerted on those objects. The loads imposed on the lumbar spine by different physical activities can be measured. Intradiscal pressure mcaaurcmcnts have been used as semi-direct indicators of these loads (Nachemson and Morris, 1964; Nachemson and Elfstriim, 1970; Andersson et al., 1976; Andersson, &tengren and Nachemson, 1977), and measurements of myoelectric activity in the back muscles (brtengren and Andersson, 1977) have been used as indirect indicators of the loads. Load measurement by such mean is not always practical; measurement procedures can be invasive, the costs of the needed equipment and personnel are high, and the data analysis is often cumbersone. Moreover, because so many factors affect the loads, the number of measurements necessary to describe even the more common situations and maneuvers would be un-
* Received 27 August 1919. 513
wieldy. A more practical approach to the problem would be to know how to estimate the loads imposed by an activity, without the need for sophisticated measurements. The purposes of this paper are to present a method to calculate the loads imposed by table work on the lumbar spine and on the trunk muscles, and to validate the method in part by expezimental measurements of myoelectric activity in the muscles of the back. EXPERIMENTALMEABUREMENTOFBACKMUBCLE ACTIVITY Material The study was performed on ten male university students. Ages ranged from 23 to 40 years with a mean age of 29 years. Heights were between 171 and 191 cm (mean 182 cm), and weights ranged from 56 to 85 kg (mean 74 kg). There was no history of back pain or previous back trauma among the subjects. Physical examination revealed no pathological findings. Activities
investigated
Subjects were seated at a table on an ordinary office chair. The height of the chair was adjusted to 3 cm lower than the knee joint height when the subject was standing, and the height of the table to 3 cm higher than the tip of the elbow when the subject was sitting with the arms hanging down close to the body. A backrest was placed at the L3 level of the spine. The subject was placed in a comfortable position. He was asked to look straight ahead and not to support his left arm. The position of the head was kept unchanged throughout the experiments. This was done with an adjustable pad, which served only as an indicator; support was not permitted. Measurements were made with the right hand in ten different positions (Fig. 1); one anterior and to the left of the trunk, three directly anterior to the trunk, three anterior and to the right of
G. B. J. ANDERSSON, R. C)RTENCREN and A. SCHULTZ
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Fig. 1. Top view to scale of the work surface. Weights were held 5 cm above the table in all positions except in Position E, where the height was 60 Cal.
the trunk, and two directly to the right of the trunk. In one of the anterior positions, the hand was held 60 cm above the table. In the other nine the hand was held five centimltres above the table. In each position, the subjects held no weight at first, then a 20 N weight, and finally a 40 N weight. The positions were maintained isometricalIy for 15 s while measurements were made. A one minute rest period was allowed after each measurement. This rest period was found adequate with the comparatively low loads applied. Measurements were also ma& when the subjects were in resting positions. Recording procedure The myoelectric signals were picked up by means of bipolar recessed surface electrodes. The electrodes were placed three centimetres lateral to the midline on both sides of the spine at the C4, T8, Ll, L3 and L5 levels. At the level of L3 an additional pair ofelectrodes was placed six centimetres lateral to the midline. The electrodes were glued to the skin using an alphacyanoacrylate adhesive, Cyanolita. The gap under the electrode discs was filled with electrode jelly, injected through a hole in the electrode. The maximal electrode impedance was less than 50 kR The electrode signals were fed to preamplifiers built into a small box (weight 0.4 kg), which was strapped to the subject’s chest. The signals were further amplified in main amplifiers and recorded on magnetic tape. During recording the signal quality was continuously checked on an oscilloscope screen. Signal processing and evaluation procedure The recorded signals were played back and fed to multi-channel r.m.s. detectors, and further to the analog-to-digital converter of a computer. The r.m.s. detectors included low-pass filters with a 3 dB frequency limit of 0.8 Hz. The analog-to-digital conversion rate was 6 Hz. All 12 channels were digitized simultaneously for each subject. The data were processed first by dividing the signals into sequences corresponding to the 15s periods of
recording. During thii process the data were reviewed and any artifacts excluded. The gain for each channel was then determined by means ofsine wave calibration signals that had been recorded at the end of each testing session. Signal amplitudes were expressed in microvolts @V). The mean signal amplitude was calculated for each recording period, channel and subject, and the standard deviation of the mean determined. The only use of this standard deviation was for further check of the signal quality; abnormal values lead to a visual check of the signal as recorded on the magnetic tape. This check included searching for any artifacts and a study of the symmetry of the signal around the zero line. Further statistical analysis consisted of calculations of mean amplitudes and standard deviations of the means over subjects, for each electrode position in each task. The myoelectric data were plotted together with the corresponding estimated muscle force for each position, and a regression analysis was used and correlation coefficients established. -MATtON
OF THE SPINE LOADS AND MU!3CLE FORCES
There are two main steps involved in the estimation of the load on the spine and the contraction forces in the trunk muscles. First it is necessary to estimate the net lower trunk support reaction needed. Then the trunk muscle forces needed to produce that reaction can be determined. These in turn determine the load on the spine.
Estimation of the net reaction .To carry out the first step, we assume that there is a single resting position in which neither hand holds a weight and both arms hang freely at the sides. The weights of the head, neck, arms and upper trunk and the moments of these weights are supported by the lower half of the body, but we will not consider the forces and moments required to support this resting position. When an activity is carried out, changes from this resting situation occur. We will consider only the increases in the loads on the spine, and the increases in the contraction forces of the trunk muscles brought about by these changes. We assume in this study that no changes occur in the positions of the head, neck, left arm, or upper trunk. An imaginary transection of the trunk is made by a transverse plane passing through the lumbar level in question (Fig. 2). A free-body diagram of all body parts superior to that cutting plane is then considered. It shows the forces and moments provided by the lower half of the body to support the upper. Under the influence of these net reaction forces and moments and the external loads that act on it, the frabody must remain in equilibrium. In the situations examined here, external loads arise only from the weight of the body segments superior to
515
Analysis and measurement of the loads on the lumbar spine
A coordinate system is established at the center of the vertebral body where the load increase is to be calculated, with the x, y, and z axes positive to the right, anteriorly, and superiorly. The line of action of W, intersects the transecting plane at (x1, yt), and the line of action of W, intersects that plane at (x, y,). Equilibrium of the free-body requires : V = W, + W, (equilibrium of vertical forces)
(1)
M,, = x,W, + x1 W, (equilibrium moments)
of frontal
M,, = y.W, + y, W, (equilibrium moments).
of sagittal plane (3)
plane (2)
Specification of the two weights and the four coordinates determines these three components of the net support reaction. Estimation of the muscle contraction forces Fig. 2. Geometry for analysis of net reaction. The line of action of the upper extremity weight W, intersects the horizontal plane at (x, y,); that of the weight held, W,, at (x,,y,). These two weights are equilibrated by the vertical reaction V at the origin, and by the extension and left-lateral moments, M,, and M,,. the lumbar
spine and any weight
held by the hand.
Since our concern is the load increase caused by changes from the rest position, we will consider only the weight loads relevant to those changes (Fig. 2). These are the weight of any load lifted, W,, and the weight of the right upper limb, W,, which acts at the limb’s center of mass. Because these weight forces act vertically, they can be equilibrated only by a vertical support reaction, V ; a support moment tending to extend the trunk, M,,; and a support moment tending to bend the trunk to the left, M,, (Fig. 2).
Once the two moment components of the support reaction, M, 1 and M,,, are known, the contraction forces in the muscles crossing the lumbar levels of the trunk needed to equilibrate’these moments can be estimated. Figure 3 is a schematic diagram of the trunk cross-section at the lumbar level through which the imaginary transection plane passes. To simplify calculations, we will represent the lower trunk muscles by few single-equivalent muscles. For the activities studied here, three of these are sufficient to develop the required values of M,, and M 11. They are the equivalents of the left and right side posterior trunk muscles (erectors), which contract with tensions R and 15, and an equivalent of the left side lateral abdominal wall muscles (obliques), which contracts with tension A (Fig. 3). A and R are antagonists in generating either frontal or sagittal plane moments. R
Fig. 3. Geometry for analysis of contraction forces in the single-equivalent muscles. A is the tension in the equivalent of the left-lateral abdominal wall (oblique) muscles, L and R the tensions in the equivalents of the left and right side posterior back (erector) muscles. Either A or R is assumed to have zero value. C is the compression on the lumbar spine motion segment.
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AND#B.WN, R. C)RTENQREN
We will assume that antagonistic activity is minimal, so that either A or R has zero value. Suppose the centroids of the L and R muscle equivalents are placed at a distance (c) lateral to, and a distance (a) posterior to the center of the vertebral body, while the ccntroid of A lies at a distance (d) lateral to and a distance (b) anterior to the center of the vertebral body (Fig. 3). Development of the required sagittal plane moment requires that: = (L + R)a - Ab,
4, while development quires that:
(3)
of the frontal plane moment re-
M,, = (L - R)c + Ad.
(4)
If A = 0, the two equations can be solved for L and R : L=
CM,,+ aMl 1 2ac
, R-
CM.,- aMl 1 2ac
.
(9
IfR=O,thetwoequationscanbesolvedforLandA: L
_
(ud+bc)
’
A=
*
(ad + bc)
@)
These formulae become even simpler if the involved dimensions are multiples of one another. A convenient set, reasonably representative of the dimensions of real trunk cross-sections, is: a=b==c
and d-2a.
In that situation, the formulae become: L=
Me,- Mu
4, + MII ,R=
2a
2a
;
(7)
ifA=O,and L
= MI, + 2M.x 3a
’
MI, - M,x A=
3a
’
Estimation of the increase in the spine compression The net reaction support force, V, from Equation (I), is v=
w,+
w,.
According to the model, that support force is the resultant of the increases in compression on the spine and the tensions in the muscles. So, the required equilibrium of vertical forces can be expressed as: V=W,+W,=C-A-R-L, and this can be solved for the compression increase C=A+R+L+
W,+
W,.
(9)
One of the first two terms will be zero. We previously noted that the values of W, W,, x, y, xl and y, were needed to find the net reactions. Our assumptions have been such that the only other value needed to solve Equations (7) or (8), and (9), and thereby determine the muscle contraction forces and the increase in the load on the spine, is the cross-section dimension, (a). Implementation of the calculation scheme
CM.,- aM1I
bMu + Wx
and A. SCHULTZ
(8)
if R = 0. Since M,, is positive in ail the cases examined here, it can be seen that under the assumptions made, A willbezerowhenM,,>M,,andRwillbetnowhen M,, < MX1, since physiologically neither A nor R can be negative. The anterior and the right lateral’ abdominal wall muscles are not considered in the model. In all the activities analyzed here, their contractions would be antagonistic to those of the three equivalents included. In other situations this will not be true, but it is an easy matter to determine what muscle equivalents need to be included in order to develop the required moments. With the above assumptions, the problem of predicting what lower trunk muscle activity is needed to equilibrate the moments developed in the thirty tasks studied here can be solved directly. Each task must be evaluated from Equations (2) and (3) for the two moments components it produces, M,, and M,,. Then either Equations (7) or (8) are used to find the two equilibrating equivalent muscle forces.
One aim of the study was to develop a method that can be used to estimate the loads on the spine during work, so that these loads can be considered in workplace designs. Simplifications were accepted because in real work situations complete control of all the variables that afIect the spine loads would be impractical. This explains for example, our approximations of cross-sectional dimension ratios, and our use of estimates of mass center locations. We did not make calculations for each individual subject, but as all our subjects were of medium height and build we dealt only with one representative subject. The data of Clauser et al. (1969) were used for the weights of the hand, forearm, and upper arm. In our subjects the upper limb weighs approximately 3 1 N. and this was the value used for W,. The location of the mass center of these segments and the position of the load relative to the vertebral body centers in each of the ten positions investigated was estimated visually to within the nearest few cm with the aid of a tape measure. The cross-sectional dimension (a) was assumed to be 5 cm. The weight lifted was 0.20, or 40 N. From these data, we calculated M,, and MI 1 for each exercise, and then the tensions in the muscle equivalents and the load increases on the lumbar spine. Because the external loads all have vertical lines of action and the dimensions used are only approximate, these compression increase calculations would apply to any level of the lumbar spine. RJLWLTR
Myoelectric activity The myoelectric activity on either the left or the right side of the back was similar at all lumbar levels. Table 1 shows the activity at the L3 level. Comparing the left and right sides, the activity was asymmetric. This
517
Analysis and measurement of the loads on the lumbar spine Table 1. Representative EMG signal levels, PV L3 left
A B C D E F G H I J
Hand positions
ON
Antero-contralateral Anterior Anterior Anterior Anterior Antero-ipsilateral Antero-ipsilateral Antero-ipsilateral Lateral Lateral
lO(4) 13 (4) 170) 18(5) 17(6) 15(5) 14(5) 21(6) lf(6) 14(7)
L3 light 40N 17(6) 28 (7) 36 (8) 43 (9) 44 (9) 39 (10) 42(11) 54(11) 28 (12) 37(11)
ON 24 (4) 20 (5) 23 (6) 26 (6) 19 (7) 16 (7) 15 (6) 15(7) 14 (6) 15 (7)
40N 39 (7) 30(8) 39 (9) 45 (10) 34(10) 22 (12) 24(13) 25 (14) 20(13) 22(13)
Myoelectric signal amplitudes at L3 level adjacent to the midline. Mean values in FV and, within parenthesis, standard deviation of the mean. The letter identifications of hand positions correspond to those in Fig. 1. has two sources : only the right hand was used for holding weights, so body segment weight was distributed asymmetrically; and all but positions B through E were located to the left or the right side of the mid-sagittal plane. When the weight was held on the left side (position A) the right side back muscles showed more activity relative to those on the left. When the weight was held on the right side (positions F through J), the right side muscles showed less activity relative to those on the left. Moreover, the myoelectric activity increased when the net reaction moments increased. For example, comparing among positions B, C, and D, the lateral bending moment does not differ very much, but the flexion moment asymmetry
increases as the arm is extended from B through C to D. The myoelectric activity increased correspondingly, and the increase was substantial when the moment increase was emphasized by the holding of the 40N weight. Myoelectric activity at the C4 and T8 levels was also measured, and showed load-activity trends similar to those found in the lumbar region. Complete data tabulations are available from the authors. Force estimates Over the ten configurations of the right arm that were studied, the mass center of the arm complex had anterior offsets from the the vertebral body centers
Table 2. Approximate locations of the mass centers and the net reaction moments Hand positions
Anterior offset of mass centers (cm)
ArmCv,) A B C D E F G H I J
Antero-contralateral Anterior Anterior Anterior Anterior Antero-ipsilaterai Antero-ipsilateral Antero-ipsilateral Lateral Lateral
22 15 24 28 22 10 10 22 -5 0
W&ht 01,) 30 30 45 60 45 30 30 45 0 0
Lateral offset (cm)
Arm (x,) A Antero-contralateral B C D E F G H I J
Anterior Anterior Anterior Anterior Antero-ipsilateral Antero-ipsilateral Antero-ipsilateral Lateral Lateral
I
20 15 10 17 24 28 26 33 42
Weight (x1) -20 0 0 0 0 20 30 45 55 75
Total extension moment, K,, (Nm) ON
7 5 7 9 7 3 3 7 -2 0
20N 13 11 16 21 16 9 9 16 -2 0
40N 19 17 25 33 25 15 15 25 -2 0
Total left lateral moment, MI r, (Nm) ON
2 6 5 3 5 7 9 8 10 13
20N -2 6 5 3 5 11 15 17 21 28
The letter identifications of the right hand positions correspond to those shown in Fig. 1.
40N
-6 6 5 3 5 15 21 26 32 43
G. B. J.
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ANDERSON, R. ~IZNGREN
and A.
!SCHULTZ
500.
Loft Erector
S. Musck Tension
0
Spin* Compression lncrwr G
a i5
-Antorior
l
2-b $83
I
H
-Anhrior-
‘Z
J
clokml4
lpsilataal Pmitiam
of RiRht Hand
Fig. 4. Muscle tensions and compression increase on the spine predii from the analysis. The positions rue those shown in Fig. 1. The three levels on each bar, from letI to right, correspond to the weights held (0,40, and 80 N).
ranging from - 5 to 28 cm, and right lateral offsets ranging from 7 to 42 cm. The extensor moments, M,, ranged from -2 to 33 Nm, while the left lateral moments, Mir, tanged from -6 to 43 Nm (Table 2). The contraction forces in the muscle equivalents were predicted using the Table 2 data and equations (7) or (8). The predicted contraction forces in the left oblique muscle equivalent ranged from 0 to 290 N; in the left lateral erector equivalent from 50 to 500 N ; and in the right lateral erector equivalent from 0 to 300 N (Fig 4). Substitution of the appropriate data into equation (9) led to predicted increases of the compressive load on the spine tanging from 140 to 720 N (Fig 4). Of the activities studied here, it can be seen that the ones which increase the load on the spine the most are the ones involving large teaches, either laterally or anteriorly.
10
20
30
Myoelectric
40
50
Activity
(PV)
40
50
60
[Fig. 5.1
Corre~~~o~ between myoe&ctric signal levels and estimated forces The measured myoelectric signal levels correlate extremely well with the prediction of the contraction forces in both the ri@ and left side erector muscle equivalents. In 14 of the 30 activities studied, the right-side erector tension was predicted to be greater than zero. A linear regression analysis was made over these 14 activities, correlating the predicted value of the tension in the right erector equivalent with the sum of the subject-mean right-side myoelectric signal levels over the Li, the two L3, and the LS electrodes, divided by four. The correlation coefficient for this regression was
OJ
0
10
20
30
Myo&ctric
60
Actkity fftV)
Figs. 5 and 6. ReIationships between estimated tension increases and myoektric signal levels. Each point shows the relationship in one of the thirty activities studied. The signal amplitude showu is the mean over the four lumbar ekctrodr?s of the means over the ten subjects for each electrode. The least-squares regression line through the points and its correlation co&cient (r) is shown.
Analysis and measurement of the loads on the lumbar spine
0.988 (Fig. 5). A similar linear regression analysis was made on the left side, where non-zero contraction forces were predicted for all 30 activities. The correlation coefhcient for this regression was 0.984 (Fig. 6). Linear regression analyses that included the myoelectric signal levels corresponding to predictions of zero contraction forces were also made, and showed equally good correlations. We did not measure myoelectric activity in the oblique muscles, and so did not attempt to validate the prediction of contraction forces in these muscles. DISCUSSION
The results show linear correlation between the predicted muscle tension increases and the increases in myoelectric activity. These findings imply that, in situations similar to those studied here, myoelectric activity can be used as a direct indication of muscle contraction force. In order to do this on one individual, the signal from one electrode pair would first be calibrated, The calibration would need to be made in a situation where the muscle tension can be predicted with confidence; that is, in a well-controlled and fullyquantified configuration. Once this is done, so long as the electrode pair is not moved, the signal level should remain a measure of the muscle tension in that individual. Then muscle tension in less well-controlled configurations or configurations difficult to quantify mechanically, where tension prediction would be inpractical, could be determined dire&y by measure ment of the myoelectric signal levels. It is not necessary that the myoelectric activity-contraction force relationship be linear for the idea to work; only that the relationship be monotonic. We believe that in the present experiments, because the largest contraction forces used were small compared to the maximum voluntary contraction forces, a linear activity-force relationship adequately described our situations. Inter-individual differences in myoelectric activity, because of such factors as electrode placement and soft tissue cover of muscles, make it desirable to record activity in groups of subjects and use mean values of activity over the subject group. The findings further imply that the assumptions used to construct our mathematical model and make the force predictions did capture the essentials of the work situations, and that our estimates of input data were reasonable. So, good predictions of muscle contraction forces and loads on the spine can be made using relatively simple mathematical models, and relatively imprecise input data. This means that the loads on the spine in a wide variety of industrial work situations can be determined easily. The model makes clear what needs to be done to keep the loads on the spine low : keep the magnitude of the external loads low, and keep the loads and the upper body segments as close horixontahy to the lumbar spine as possible. These precautions will keep the moments M,, and M,, low, and they are R.M13,6---E
519
the major determinants of the load on the spine. Among other things, this means that the work space should be designed so that it is not necessary to lean the head or trunk forward to accomplish the required task. Similarly, it can be reasoned that neither the tableor the chair heights have, in themselves, any direct effect on the spine load. But, if the table is too high, or the chair too low, it will be necessary to spread the arms laterally to perform work on the table surface. This will cause the mass centers to move out laterally, increasing the moments and so the spine loads. Similarly, if the table is too low, or the chair too high, it will be necessary to bend the head and trunk forward to accomplish the work, again increasing the moments and so the loads on the spine. None of the thirty tasks studied here was very stressful to the spine. Figure 4 shows that the largest compression increase on the spine was approximately 720N. In contrast, the tests of trunk strengths in extension reported by McNeil1 et al. (1979) showed that in the mean, healthy males developed spine compression loads of approximately 4OOON, more than five times as large. It was clear from observation of the subjects that some of the exercises significantly stressed the muscles of the shoulder girdle. SUMMARYAND CONCLUSJONS
1. A scheme for the calculation of the loads on the lumbar spine produced by industrial work tasks was described, along with a number of assumptions to minimize complexity in the use of the scheme. 2. Myoelectric activity was measured on ten subjects at eight places over the posterior aspect of the lumbar spine, while carrying out thirty work tasks at a table. 3. The agreement between the muscle tensions predicted by the calculation scheme and changes in myoelectric activity levels was very good, tending strongly to validate the calcmation scheme and the assumptions used. 4. The thirty tasks produced spine compression increases up to 720 N, and trunk muscle contraction force increases up to SOON compared to the resting position. 5. To keep the loads on the lumbar spine small during table-work, keep the external loads small and design the work-space to allow ali work to be done close to the chest. Acknowledgements- The support of the Swedish Work Environment Fund, the Swe&sh Medical Research Council, and US Public Health Service Grant OH 00514 and Develot+ ment Award AM 00029 for this work is gratefully acknowledged. Roland Bjiirk, Margareta Nordin, Ingrid Melchersson and Piir Arkelsjii all provided significant assistance in the research. REFERENCES Andersson, G. B. J., &tengren, R., Nachemson, A. and Elfstrom, G. (1979) Lumbar disc pressure and myoelectric
520
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R.
ORTENGREN
back muscle activity during sitting. Stand. J. Reha6. Med. 3, 104-114. Andcrsson, G. B. J., &tcngrcn, R. and Nachcmson, A. (1977) Intradiscal pressure, intra-abdominal pressure and myo&cttic back muscle activity related to posture and loading. Clin. Orthop. 129, 156-164. Clauscr, C, McCoaville. J. and Young, J. (1969) Weight volume and center of mass of segments of the human body. AMRL-TR-49 70, Wright-Patterson Air Force Base, Ohio. McNcill, T., Warwick, D., Andcrsson, G. and Schulk, A. (1970) Trunk strengths in attempted flexion, extension, and
and A.
SCHULTZ
lateral bending in healthy subjects and patients with low back disorders. Spine. Nachcmson, A. and ElfstrGm, G. (1970) Intravital dynamic pressure measurements in lumbar discs. Stand. J. Rehab. Med. Suppl. 1. Nachemson, A. and Morris, J. M. (1964) In uivo measure mcnts of intradiscal pressure. J. Bone Jr Surg. 464 1077-1092. &tengrcn, R. and Andersson, 0. B. J. (1977) Electromyographic Studies ofTrunk Muscles, with spatial reference to the anatomy of the lumbar spine. Spine 2,44-52.
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ANALYSIS AND MEASUREMENT OF THE LOADS ON THE LUMBAR SPINE DURING WORK AT A TABLE* G. B. J. ANDERSSON?,R. C)RTENGWN$ and A. SCHULTZ$ l
Department of Orthopaedic Surgery I and tDepartment of Clinical Neurophysiology, Sahlgren Hospital, S-413 45 Giiteborg and 4 Department of Materials Engineer& University of Illinois, Chicago Circle, Chicago, IL 60680, U.S.A.
Abstract-A study has been made in which the loads on the lumbar spine and trunk muscles were calculated in static submaximal armlifts, and the method validated in part by measurements ofmyoelectric back muscle activity. Ten subjects performed thirty work tasks while seated at a tabk. The lumbar spine loads and lumbar trunk muscle contraction forces required for task execution were predicted using mechanical analysis. The myoelectric activity was recorded at eight sites in the low back region. A good correlation was obtained between the contraction force predictions and the myoelectric signal levels.
INTRODUCnON One of the most common hazards industrial workers contend with is low back pain. The higher rates of back injuries in physically heavy work has led to the assumption that work load is related to low back symptoms. So one consideration in the design of safe and efficient work places and work-execution schemes is to keep the loads on the lumbar spine low. Manual industrial work is often performed at a table. Objects are moved about on the table surface and between boxes and shelves and the tabk. It can be assumed that many factors affect the loads on the lumbar spine under these circumstances: the extent to which the upper trunk is bent over ; the positions of the head, neck, arms and hands; whether the arms and hands are suspended or resting on the table or on armrests; the weight of each of these body segments; the weights and locations of any objects handled ; and the nature of any pushes or pulls exerted on those objects. The loads imposed on the lumbar spine by different physical activities can be measured. Intradiscal pressure mcaaurcmcnts have been used as semi-direct indicators of these loads (Nachemson and Morris, 1964; Nachemson and Elfstriim, 1970; Andersson et al., 1976; Andersson, &tengren and Nachemson, 1977), and measurements of myoelectric activity in the back muscles (brtengren and Andersson, 1977) have been used as indirect indicators of the loads. Load measurement by such mean is not always practical; measurement procedures can be invasive, the costs of the needed equipment and personnel are high, and the data analysis is often cumbersone. Moreover, because so many factors affect the loads, the number of measurements necessary to describe even the more common situations and maneuvers would be un-
* Received 27 August 1919. 513
wieldy. A more practical approach to the problem would be to know how to estimate the loads imposed by an activity, without the need for sophisticated measurements. The purposes of this paper are to present a method to calculate the loads imposed by table work on the lumbar spine and on the trunk muscles, and to validate the method in part by expezimental measurements of myoelectric activity in the muscles of the back. EXPERIMENTALMEABUREMENTOFBACKMUBCLE ACTIVITY Material The study was performed on ten male university students. Ages ranged from 23 to 40 years with a mean age of 29 years. Heights were between 171 and 191 cm (mean 182 cm), and weights ranged from 56 to 85 kg (mean 74 kg). There was no history of back pain or previous back trauma among the subjects. Physical examination revealed no pathological findings. Activities
investigated
Subjects were seated at a table on an ordinary office chair. The height of the chair was adjusted to 3 cm lower than the knee joint height when the subject was standing, and the height of the table to 3 cm higher than the tip of the elbow when the subject was sitting with the arms hanging down close to the body. A backrest was placed at the L3 level of the spine. The subject was placed in a comfortable position. He was asked to look straight ahead and not to support his left arm. The position of the head was kept unchanged throughout the experiments. This was done with an adjustable pad, which served only as an indicator; support was not permitted. Measurements were made with the right hand in ten different positions (Fig. 1); one anterior and to the left of the trunk, three directly anterior to the trunk, three anterior and to the right of
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Do C6E
Fig. 1. Top view to scale of the work surface. Weights were held 5 cm above the table in all positions except in Position E, where the height was 60 Cal.
the trunk, and two directly to the right of the trunk. In one of the anterior positions, the hand was held 60 cm above the table. In the other nine the hand was held five centimltres above the table. In each position, the subjects held no weight at first, then a 20 N weight, and finally a 40 N weight. The positions were maintained isometricalIy for 15 s while measurements were made. A one minute rest period was allowed after each measurement. This rest period was found adequate with the comparatively low loads applied. Measurements were also ma& when the subjects were in resting positions. Recording procedure The myoelectric signals were picked up by means of bipolar recessed surface electrodes. The electrodes were placed three centimetres lateral to the midline on both sides of the spine at the C4, T8, Ll, L3 and L5 levels. At the level of L3 an additional pair ofelectrodes was placed six centimetres lateral to the midline. The electrodes were glued to the skin using an alphacyanoacrylate adhesive, Cyanolita. The gap under the electrode discs was filled with electrode jelly, injected through a hole in the electrode. The maximal electrode impedance was less than 50 kR The electrode signals were fed to preamplifiers built into a small box (weight 0.4 kg), which was strapped to the subject’s chest. The signals were further amplified in main amplifiers and recorded on magnetic tape. During recording the signal quality was continuously checked on an oscilloscope screen. Signal processing and evaluation procedure The recorded signals were played back and fed to multi-channel r.m.s. detectors, and further to the analog-to-digital converter of a computer. The r.m.s. detectors included low-pass filters with a 3 dB frequency limit of 0.8 Hz. The analog-to-digital conversion rate was 6 Hz. All 12 channels were digitized simultaneously for each subject. The data were processed first by dividing the signals into sequences corresponding to the 15s periods of
recording. During thii process the data were reviewed and any artifacts excluded. The gain for each channel was then determined by means ofsine wave calibration signals that had been recorded at the end of each testing session. Signal amplitudes were expressed in microvolts @V). The mean signal amplitude was calculated for each recording period, channel and subject, and the standard deviation of the mean determined. The only use of this standard deviation was for further check of the signal quality; abnormal values lead to a visual check of the signal as recorded on the magnetic tape. This check included searching for any artifacts and a study of the symmetry of the signal around the zero line. Further statistical analysis consisted of calculations of mean amplitudes and standard deviations of the means over subjects, for each electrode position in each task. The myoelectric data were plotted together with the corresponding estimated muscle force for each position, and a regression analysis was used and correlation coefficients established. -MATtON
OF THE SPINE LOADS AND MU!3CLE FORCES
There are two main steps involved in the estimation of the load on the spine and the contraction forces in the trunk muscles. First it is necessary to estimate the net lower trunk support reaction needed. Then the trunk muscle forces needed to produce that reaction can be determined. These in turn determine the load on the spine.
Estimation of the net reaction .To carry out the first step, we assume that there is a single resting position in which neither hand holds a weight and both arms hang freely at the sides. The weights of the head, neck, arms and upper trunk and the moments of these weights are supported by the lower half of the body, but we will not consider the forces and moments required to support this resting position. When an activity is carried out, changes from this resting situation occur. We will consider only the increases in the loads on the spine, and the increases in the contraction forces of the trunk muscles brought about by these changes. We assume in this study that no changes occur in the positions of the head, neck, left arm, or upper trunk. An imaginary transection of the trunk is made by a transverse plane passing through the lumbar level in question (Fig. 2). A free-body diagram of all body parts superior to that cutting plane is then considered. It shows the forces and moments provided by the lower half of the body to support the upper. Under the influence of these net reaction forces and moments and the external loads that act on it, the frabody must remain in equilibrium. In the situations examined here, external loads arise only from the weight of the body segments superior to
515
Analysis and measurement of the loads on the lumbar spine
A coordinate system is established at the center of the vertebral body where the load increase is to be calculated, with the x, y, and z axes positive to the right, anteriorly, and superiorly. The line of action of W, intersects the transecting plane at (x1, yt), and the line of action of W, intersects that plane at (x, y,). Equilibrium of the free-body requires : V = W, + W, (equilibrium of vertical forces)
(1)
M,, = x,W, + x1 W, (equilibrium moments)
of frontal
M,, = y.W, + y, W, (equilibrium moments).
of sagittal plane (3)
plane (2)
Specification of the two weights and the four coordinates determines these three components of the net support reaction. Estimation of the muscle contraction forces Fig. 2. Geometry for analysis of net reaction. The line of action of the upper extremity weight W, intersects the horizontal plane at (x, y,); that of the weight held, W,, at (x,,y,). These two weights are equilibrated by the vertical reaction V at the origin, and by the extension and left-lateral moments, M,, and M,,. the lumbar
spine and any weight
held by the hand.
Since our concern is the load increase caused by changes from the rest position, we will consider only the weight loads relevant to those changes (Fig. 2). These are the weight of any load lifted, W,, and the weight of the right upper limb, W,, which acts at the limb’s center of mass. Because these weight forces act vertically, they can be equilibrated only by a vertical support reaction, V ; a support moment tending to extend the trunk, M,,; and a support moment tending to bend the trunk to the left, M,, (Fig. 2).
Once the two moment components of the support reaction, M, 1 and M,,, are known, the contraction forces in the muscles crossing the lumbar levels of the trunk needed to equilibrate’these moments can be estimated. Figure 3 is a schematic diagram of the trunk cross-section at the lumbar level through which the imaginary transection plane passes. To simplify calculations, we will represent the lower trunk muscles by few single-equivalent muscles. For the activities studied here, three of these are sufficient to develop the required values of M,, and M 11. They are the equivalents of the left and right side posterior trunk muscles (erectors), which contract with tensions R and 15, and an equivalent of the left side lateral abdominal wall muscles (obliques), which contracts with tension A (Fig. 3). A and R are antagonists in generating either frontal or sagittal plane moments. R
Fig. 3. Geometry for analysis of contraction forces in the single-equivalent muscles. A is the tension in the equivalent of the left-lateral abdominal wall (oblique) muscles, L and R the tensions in the equivalents of the left and right side posterior back (erector) muscles. Either A or R is assumed to have zero value. C is the compression on the lumbar spine motion segment.
516
G. B. J.
AND#B.WN, R. C)RTENQREN
We will assume that antagonistic activity is minimal, so that either A or R has zero value. Suppose the centroids of the L and R muscle equivalents are placed at a distance (c) lateral to, and a distance (a) posterior to the center of the vertebral body, while the ccntroid of A lies at a distance (d) lateral to and a distance (b) anterior to the center of the vertebral body (Fig. 3). Development of the required sagittal plane moment requires that: = (L + R)a - Ab,
4, while development quires that:
(3)
of the frontal plane moment re-
M,, = (L - R)c + Ad.
(4)
If A = 0, the two equations can be solved for L and R : L=
CM,,+ aMl 1 2ac
, R-
CM.,- aMl 1 2ac
.
(9
IfR=O,thetwoequationscanbesolvedforLandA: L
_
(ud+bc)
’
A=
*
(ad + bc)
@)
These formulae become even simpler if the involved dimensions are multiples of one another. A convenient set, reasonably representative of the dimensions of real trunk cross-sections, is: a=b==c
and d-2a.
In that situation, the formulae become: L=
Me,- Mu
4, + MII ,R=
2a
2a
;
(7)
ifA=O,and L
= MI, + 2M.x 3a
’
MI, - M,x A=
3a
’
Estimation of the increase in the spine compression The net reaction support force, V, from Equation (I), is v=
w,+
w,.
According to the model, that support force is the resultant of the increases in compression on the spine and the tensions in the muscles. So, the required equilibrium of vertical forces can be expressed as: V=W,+W,=C-A-R-L, and this can be solved for the compression increase C=A+R+L+
W,+
W,.
(9)
One of the first two terms will be zero. We previously noted that the values of W, W,, x, y, xl and y, were needed to find the net reactions. Our assumptions have been such that the only other value needed to solve Equations (7) or (8), and (9), and thereby determine the muscle contraction forces and the increase in the load on the spine, is the cross-section dimension, (a). Implementation of the calculation scheme
CM.,- aM1I
bMu + Wx
and A. SCHULTZ
(8)
if R = 0. Since M,, is positive in ail the cases examined here, it can be seen that under the assumptions made, A willbezerowhenM,,>M,,andRwillbetnowhen M,, < MX1, since physiologically neither A nor R can be negative. The anterior and the right lateral’ abdominal wall muscles are not considered in the model. In all the activities analyzed here, their contractions would be antagonistic to those of the three equivalents included. In other situations this will not be true, but it is an easy matter to determine what muscle equivalents need to be included in order to develop the required moments. With the above assumptions, the problem of predicting what lower trunk muscle activity is needed to equilibrate the moments developed in the thirty tasks studied here can be solved directly. Each task must be evaluated from Equations (2) and (3) for the two moments components it produces, M,, and M,,. Then either Equations (7) or (8) are used to find the two equilibrating equivalent muscle forces.
One aim of the study was to develop a method that can be used to estimate the loads on the spine during work, so that these loads can be considered in workplace designs. Simplifications were accepted because in real work situations complete control of all the variables that afIect the spine loads would be impractical. This explains for example, our approximations of cross-sectional dimension ratios, and our use of estimates of mass center locations. We did not make calculations for each individual subject, but as all our subjects were of medium height and build we dealt only with one representative subject. The data of Clauser et al. (1969) were used for the weights of the hand, forearm, and upper arm. In our subjects the upper limb weighs approximately 3 1 N. and this was the value used for W,. The location of the mass center of these segments and the position of the load relative to the vertebral body centers in each of the ten positions investigated was estimated visually to within the nearest few cm with the aid of a tape measure. The cross-sectional dimension (a) was assumed to be 5 cm. The weight lifted was 0.20, or 40 N. From these data, we calculated M,, and MI 1 for each exercise, and then the tensions in the muscle equivalents and the load increases on the lumbar spine. Because the external loads all have vertical lines of action and the dimensions used are only approximate, these compression increase calculations would apply to any level of the lumbar spine. RJLWLTR
Myoelectric activity The myoelectric activity on either the left or the right side of the back was similar at all lumbar levels. Table 1 shows the activity at the L3 level. Comparing the left and right sides, the activity was asymmetric. This
517
Analysis and measurement of the loads on the lumbar spine Table 1. Representative EMG signal levels, PV L3 left
A B C D E F G H I J
Hand positions
ON
Antero-contralateral Anterior Anterior Anterior Anterior Antero-ipsilateral Antero-ipsilateral Antero-ipsilateral Lateral Lateral
lO(4) 13 (4) 170) 18(5) 17(6) 15(5) 14(5) 21(6) lf(6) 14(7)
L3 light 40N 17(6) 28 (7) 36 (8) 43 (9) 44 (9) 39 (10) 42(11) 54(11) 28 (12) 37(11)
ON 24 (4) 20 (5) 23 (6) 26 (6) 19 (7) 16 (7) 15 (6) 15(7) 14 (6) 15 (7)
40N 39 (7) 30(8) 39 (9) 45 (10) 34(10) 22 (12) 24(13) 25 (14) 20(13) 22(13)
Myoelectric signal amplitudes at L3 level adjacent to the midline. Mean values in FV and, within parenthesis, standard deviation of the mean. The letter identifications of hand positions correspond to those in Fig. 1. has two sources : only the right hand was used for holding weights, so body segment weight was distributed asymmetrically; and all but positions B through E were located to the left or the right side of the mid-sagittal plane. When the weight was held on the left side (position A) the right side back muscles showed more activity relative to those on the left. When the weight was held on the right side (positions F through J), the right side muscles showed less activity relative to those on the left. Moreover, the myoelectric activity increased when the net reaction moments increased. For example, comparing among positions B, C, and D, the lateral bending moment does not differ very much, but the flexion moment asymmetry
increases as the arm is extended from B through C to D. The myoelectric activity increased correspondingly, and the increase was substantial when the moment increase was emphasized by the holding of the 40N weight. Myoelectric activity at the C4 and T8 levels was also measured, and showed load-activity trends similar to those found in the lumbar region. Complete data tabulations are available from the authors. Force estimates Over the ten configurations of the right arm that were studied, the mass center of the arm complex had anterior offsets from the the vertebral body centers
Table 2. Approximate locations of the mass centers and the net reaction moments Hand positions
Anterior offset of mass centers (cm)
ArmCv,) A B C D E F G H I J
Antero-contralateral Anterior Anterior Anterior Anterior Antero-ipsilaterai Antero-ipsilateral Antero-ipsilateral Lateral Lateral
22 15 24 28 22 10 10 22 -5 0
W&ht 01,) 30 30 45 60 45 30 30 45 0 0
Lateral offset (cm)
Arm (x,) A Antero-contralateral B C D E F G H I J
Anterior Anterior Anterior Anterior Antero-ipsilateral Antero-ipsilateral Antero-ipsilateral Lateral Lateral
I
20 15 10 17 24 28 26 33 42
Weight (x1) -20 0 0 0 0 20 30 45 55 75
Total extension moment, K,, (Nm) ON
7 5 7 9 7 3 3 7 -2 0
20N 13 11 16 21 16 9 9 16 -2 0
40N 19 17 25 33 25 15 15 25 -2 0
Total left lateral moment, MI r, (Nm) ON
2 6 5 3 5 7 9 8 10 13
20N -2 6 5 3 5 11 15 17 21 28
The letter identifications of the right hand positions correspond to those shown in Fig. 1.
40N
-6 6 5 3 5 15 21 26 32 43
G. B. J.
518
ANDERSON, R. ~IZNGREN
and A.
!SCHULTZ
500.
Loft Erector
S. Musck Tension
0
Spin* Compression lncrwr G
a i5
-Antorior
l
2-b $83
I
H
-Anhrior-
‘Z
J
clokml4
lpsilataal Pmitiam
of RiRht Hand
Fig. 4. Muscle tensions and compression increase on the spine predii from the analysis. The positions rue those shown in Fig. 1. The three levels on each bar, from letI to right, correspond to the weights held (0,40, and 80 N).
ranging from - 5 to 28 cm, and right lateral offsets ranging from 7 to 42 cm. The extensor moments, M,, ranged from -2 to 33 Nm, while the left lateral moments, Mir, tanged from -6 to 43 Nm (Table 2). The contraction forces in the muscle equivalents were predicted using the Table 2 data and equations (7) or (8). The predicted contraction forces in the left oblique muscle equivalent ranged from 0 to 290 N; in the left lateral erector equivalent from 50 to 500 N ; and in the right lateral erector equivalent from 0 to 300 N (Fig 4). Substitution of the appropriate data into equation (9) led to predicted increases of the compressive load on the spine tanging from 140 to 720 N (Fig 4). Of the activities studied here, it can be seen that the ones which increase the load on the spine the most are the ones involving large teaches, either laterally or anteriorly.
10
20
30
Myoelectric
40
50
Activity
(PV)
40
50
60
[Fig. 5.1
Corre~~~o~ between myoe&ctric signal levels and estimated forces The measured myoelectric signal levels correlate extremely well with the prediction of the contraction forces in both the ri@ and left side erector muscle equivalents. In 14 of the 30 activities studied, the right-side erector tension was predicted to be greater than zero. A linear regression analysis was made over these 14 activities, correlating the predicted value of the tension in the right erector equivalent with the sum of the subject-mean right-side myoelectric signal levels over the Li, the two L3, and the LS electrodes, divided by four. The correlation coefficient for this regression was
OJ
0
10
20
30
Myo&ctric
60
Actkity fftV)
Figs. 5 and 6. ReIationships between estimated tension increases and myoektric signal levels. Each point shows the relationship in one of the thirty activities studied. The signal amplitude showu is the mean over the four lumbar ekctrodr?s of the means over the ten subjects for each electrode. The least-squares regression line through the points and its correlation co&cient (r) is shown.
Analysis and measurement of the loads on the lumbar spine
0.988 (Fig. 5). A similar linear regression analysis was made on the left side, where non-zero contraction forces were predicted for all 30 activities. The correlation coefhcient for this regression was 0.984 (Fig. 6). Linear regression analyses that included the myoelectric signal levels corresponding to predictions of zero contraction forces were also made, and showed equally good correlations. We did not measure myoelectric activity in the oblique muscles, and so did not attempt to validate the prediction of contraction forces in these muscles. DISCUSSION
The results show linear correlation between the predicted muscle tension increases and the increases in myoelectric activity. These findings imply that, in situations similar to those studied here, myoelectric activity can be used as a direct indication of muscle contraction force. In order to do this on one individual, the signal from one electrode pair would first be calibrated, The calibration would need to be made in a situation where the muscle tension can be predicted with confidence; that is, in a well-controlled and fullyquantified configuration. Once this is done, so long as the electrode pair is not moved, the signal level should remain a measure of the muscle tension in that individual. Then muscle tension in less well-controlled configurations or configurations difficult to quantify mechanically, where tension prediction would be inpractical, could be determined dire&y by measure ment of the myoelectric signal levels. It is not necessary that the myoelectric activity-contraction force relationship be linear for the idea to work; only that the relationship be monotonic. We believe that in the present experiments, because the largest contraction forces used were small compared to the maximum voluntary contraction forces, a linear activity-force relationship adequately described our situations. Inter-individual differences in myoelectric activity, because of such factors as electrode placement and soft tissue cover of muscles, make it desirable to record activity in groups of subjects and use mean values of activity over the subject group. The findings further imply that the assumptions used to construct our mathematical model and make the force predictions did capture the essentials of the work situations, and that our estimates of input data were reasonable. So, good predictions of muscle contraction forces and loads on the spine can be made using relatively simple mathematical models, and relatively imprecise input data. This means that the loads on the spine in a wide variety of industrial work situations can be determined easily. The model makes clear what needs to be done to keep the loads on the spine low : keep the magnitude of the external loads low, and keep the loads and the upper body segments as close horixontahy to the lumbar spine as possible. These precautions will keep the moments M,, and M,, low, and they are R.M13,6---E
519
the major determinants of the load on the spine. Among other things, this means that the work space should be designed so that it is not necessary to lean the head or trunk forward to accomplish the required task. Similarly, it can be reasoned that neither the tableor the chair heights have, in themselves, any direct effect on the spine load. But, if the table is too high, or the chair too low, it will be necessary to spread the arms laterally to perform work on the table surface. This will cause the mass centers to move out laterally, increasing the moments and so the spine loads. Similarly, if the table is too low, or the chair too high, it will be necessary to bend the head and trunk forward to accomplish the work, again increasing the moments and so the loads on the spine. None of the thirty tasks studied here was very stressful to the spine. Figure 4 shows that the largest compression increase on the spine was approximately 720N. In contrast, the tests of trunk strengths in extension reported by McNeil1 et al. (1979) showed that in the mean, healthy males developed spine compression loads of approximately 4OOON, more than five times as large. It was clear from observation of the subjects that some of the exercises significantly stressed the muscles of the shoulder girdle. SUMMARYAND CONCLUSJONS
1. A scheme for the calculation of the loads on the lumbar spine produced by industrial work tasks was described, along with a number of assumptions to minimize complexity in the use of the scheme. 2. Myoelectric activity was measured on ten subjects at eight places over the posterior aspect of the lumbar spine, while carrying out thirty work tasks at a table. 3. The agreement between the muscle tensions predicted by the calculation scheme and changes in myoelectric activity levels was very good, tending strongly to validate the calcmation scheme and the assumptions used. 4. The thirty tasks produced spine compression increases up to 720 N, and trunk muscle contraction force increases up to SOON compared to the resting position. 5. To keep the loads on the lumbar spine small during table-work, keep the external loads small and design the work-space to allow ali work to be done close to the chest. Acknowledgements- The support of the Swedish Work Environment Fund, the Swe&sh Medical Research Council, and US Public Health Service Grant OH 00514 and Develot+ ment Award AM 00029 for this work is gratefully acknowledged. Roland Bjiirk, Margareta Nordin, Ingrid Melchersson and Piir Arkelsjii all provided significant assistance in the research. REFERENCES Andersson, G. B. J., &tengren, R., Nachemson, A. and Elfstrom, G. (1979) Lumbar disc pressure and myoelectric
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R.
ORTENGREN
back muscle activity during sitting. Stand. J. Reha6. Med. 3, 104-114. Andcrsson, G. B. J., &tcngrcn, R. and Nachcmson, A. (1977) Intradiscal pressure, intra-abdominal pressure and myo&cttic back muscle activity related to posture and loading. Clin. Orthop. 129, 156-164. Clauscr, C, McCoaville. J. and Young, J. (1969) Weight volume and center of mass of segments of the human body. AMRL-TR-49 70, Wright-Patterson Air Force Base, Ohio. McNcill, T., Warwick, D., Andcrsson, G. and Schulk, A. (1970) Trunk strengths in attempted flexion, extension, and
and A.
SCHULTZ
lateral bending in healthy subjects and patients with low back disorders. Spine. Nachcmson, A. and ElfstrGm, G. (1970) Intravital dynamic pressure measurements in lumbar discs. Stand. J. Rehab. Med. Suppl. 1. Nachemson, A. and Morris, J. M. (1964) In uivo measure mcnts of intradiscal pressure. J. Bone Jr Surg. 464 1077-1092. &tengrcn, R. and Andersson, 0. B. J. (1977) Electromyographic Studies ofTrunk Muscles, with spatial reference to the anatomy of the lumbar spine. Spine 2,44-52.