The Obstetric Reason for Lordosis
and the Implications for
Lifting and Low Back Pain
(technical paper submitted to SPINE and to the Journal of Bio-Medical Engineering) UK
J D Gorman
MA (Cantab.) (Mechanical Sciences)
C.Eng. (Chartered Engineer)
MIEE (Member of the Institution of Electrical Engineers)
MI.Mech.E (Member of Institution of Mechanical Engineers)
MC (McTimoney Chiropractor)
MMCA (Member of the McTimoney Chiropractic Association)
The author thanks the Oxford Orthopaedic Engineering Centre (Oxford UK) and the European Chiropractic College (Bournemouth UK) for permission to examine X-ray records, and the Natural History Museum (London UK) for permission to examine chimpanzee skeletons and the fossil casts of Lucy.
The author manufactures chairs for back sufferers and claims associated patent rights.
A review of the reasons proposed for lumbar lordosis suggests that the obstetric reason is primary. Evaluation by scale drawing of the moment arms of erector spinae muscles shows how evolution has taken advantage of the lordosis and angle of the sacrum to maximise lifting strength. However, muscular moment arms are reduced by flexion and can be reduced sufficiently to provide the conditions of flexion and compression which may cause disc prolapse. It is proposed that prolapse is not common because a warning pain inhibits activity when either the L4-5 or the L5-S disc is flexed (or can flex) beyond parallel sided. It is further proposed that civilised sitting will tend to increase the mobility of these joints and thus cause low back pain.
(Keywords: lordosis, obstetric, weight lifting, sitting)
Note: This paper was prepared and submitted in 1987 before the author became a chiropractor and has not been modified since.
The Reason for Lordosis
In the bipedal human being the primary function of the lumbar spine is to act as a column to support the upper body, head and arms above the pelvis and legs. One would expect such a column to be straight.
In fact the lumbar spine is sharply curved at the base. The sacral endplate is frequently more than 50° from the horizontal and the lowest two discs are sharply wedge shaped. Figure 1 is a typical shape redrawn from an X-ray tracing in a report on a population chosen because of "the rarity of the disc syndrome" (25) (50° endplate angle as shown in that paper).
There have been many attempts to explain this lordosis in the human lumbar spine.
One suggestion is that it is simply a result of bringing the upper body vertical for bipedalism while the pelvis retains its orientation in the ape. "It seems probable that the lordosis is an adaptation to upright stance with straight knees" (8). In fact the relation of pelvis to thorax in the human is approximately the same as in the chimpanzee or the orang-utan. (The chimpanzee line and the human line only separated recently, maybe as recently as 5 million years ago (27). It is generally assumed that our common ancestor was similar to the chimpanzee. This is supported by the similarity of the chimp and orang pelves.)
An alternative suggestion is that the S-shape of the spine (including the neck) acts as a spring to vertical shocks or displacements. "The normal curves of the spine help it to act as a shock absorber by reducing its longitudinal stiffness (by a similar mechanism a metal rod can absorb shock if it is curved to form a helical spring)" (1). This assumes that the lordosis in the lumbar and cervical spine will increase to absorb vertical shock, i.e. that the cervical and lumbar spines will be extended by shock or displacement. In fact both are flexed by a vertical shock or force as for instance when landing from a jump. "When a load is placed on the shoulders of an erect individual we observe a reduction of the lumbar lordosis" (18). This is because the vertical line from the centre of gravity of the head is in front of the cervical spine and a vertical line up from the acetabulum is in front of most of the lumbar spine.
Gracovetsky et al suggest that the lordosis is a means by which the spine can flex to tighten the ligaments. "The passive part of the lumbodorsal fascia requires the reduction of lordosis in order to become tightened" (18). This could explain an even lordosis throughout the lumbar spine. It does not seem to explain the sharp lordosis at the lowest lumbar joints.
It is natural to assume that all lumbar discs should be similar and only slightly wedge shaped in the erect position. Many diagrams, including those in technical journals, are drawn to show this (1, 8, 9, 3). In fact the lowest two discs in non-back sufferers are sharply wedge shaped. Figure 2 (of a child) shows how extreme this can be in the population chosen "because of the rarity of the disc syndrome" (25). In a group chosen because "none had suffered any back pain within 12 months or a history of back pain" (33), the mean wedging of the L5-S disc was 18° and of L4-5, 16° , in the erect standing position". The wedging in extension would be increased further to about 23° at L5-S (This angle information was not included in the initial report (33). Analysis was done recently by this author from the original X-ray films. For subjects Nos. 2, 3, 4, 5, 6, 7, 8, 9 10, 11 and 13, the wedging at L4-5 was 14° , 12° , 22° , 17° , 15° , 22° , 13° , 18° , 17° , 13° and 16° , and at L5-S it was 10° , 15° , 28° , 22° , 17° , 20° , 17° , 20° , 18° , 8° and 21° respectively).
The angles from non-back sufferers in a sedentary population and the similar angles shown in the example from the more naturally living group confirm the sharpness of the lordosis and the very pronounced wedge shape of the lowest two lumbar discs.
Any hypothesis proposed as a reason for lordosis must explain this shape in the lower lumbar and sacral area of the spine. Mechanical arguments to do with columns and cantilevers will probably not do so. They will tend to suggest that the spine should be straight or slightly and gently curved.
The obstetric explanation
The shape of the lower lumbar spine is defined by the angle of the sacrum in the pelvis. Figure 3 shows this. The ventral surface of the sacrum is almost exactly the arc of a circle centred on the pubic symphysis.
This shape and angle of the sacrum is absolutely necessary to the passage of the baby’s head through the mother’s pelvis in childbirth. The dimensions x and y in Figure 3 are the pelvic inlet and outlet a-p diameters respectively and are used by the gynaecologist and midwife in predicting the difficulty of childbirth.
It is proposed here that this is the reason for the lordosis in the human spine.
This is confirmed in an analysis of the changes that took place in the evolution of Australopithecus afarensis (e.g. Lucy) from the previous chimpanzee-like ape (36).
"Among primates, hominid (i.e. Australopithecus or Homo) morphology is unique. The non-hominid primate (e.g. chimpanzee) ilium is cranio-caudally elongated separating the sacroiliac and hip joints … As an adaptation to bipedality among hominids the lower iliac height shortened, thereby narrowing the distance between the sacroiliac and hip joints. However the approximation of these joints is obstetrically disadvantageous as the anteroposterior diameter of the pelvic inlet becomes constricted … The difference in sacral position between non-hominids and hominids also is obstetrically significant. Among non-hominids the sacral apex does not extend down to the level of the ischial spines; the dorsal walls of the pelvis midplane and outlet are muscular. Among hominids, the reduced distance between the sacroiliac and hip joints lowers the sacrum into the pelvic cavity; the ventral surface of the sacrum forms a prominent bony wall of the true pelvis. Obstetrically, the sacrum reduces the sagittal dimensions of the hominid birth canal, particularly at the midplane and outlet. In contrast with other primates, the position of the hominid sacrum may contribute to parturitional difficulty." (36).
Figure 4(a) shows the chimpanzee pelvis positioned as in a vertical body. The height of the pelvis ensures that the sacrum does not obstruct the birth canal. The sacrum is approximately vertical resulting in a straight lumbar spine.
Figure 4b shows the pelvis of Lucy. The overall height of the pelvis is greatly reduced. The ilia have been modified such that the iliac crest is closer to and above the hip joint. This allows the muscles from the iliac crest to control the leg laterally when walking. "As an adaptation to a bipedal gait in which the thigh passes underneath and behind the hip joint, hominid iliac rotated to a more sagittal plane … The gluteal muscles (medius, minimus and upper part of maximus) became positioned to function as abductors. These muscles are principal in controlling pelvic balance in the transverse plane during the single support phase of stride." (36).
With the shape of the ilia thus defined by the needs of bipedalism, there is no possible configuration of a vertical sacrum and straight spine, which would not place the spine too far behind the acetabulum. Figure 4(b) shows the configuration. The sacral angulation in Lucy is less than the mean of human females but within the range of human male or female angles (36). This almost certainly resulted in a lumbar spine very similar to the modern human with a marked lumbar lordosis. The sacrum probably did not have the characteristic curvature of the human sacrum and the limiting dimension for childbirth was between "… the pubic symphysis and the centre of the sacrum" (d in Figure 4 b), (36).
In comparison with the chimpanzee, childbirth was almost certainly difficult for Lucy ("adaptations for locomotion and visceral accommodation and support functions at the expense of parturitional ease"), (36). The human adult brain is almost four times as large as that of Lucy (similar to chimpanzee (36); chimp 300-400 cc (24), human 1350 cc). As brain size grew in steps from Lucy to Homo habilis to Homo erectus to the Neandertal Homo sapiens and to ourselves, childbirth must have been a major problem and a limit on our evolution.
There are four main parts to the solution that has evolved. Firstly we are born early with a small and incompletely developed brain. Comparing our lifespan with other primates, we should have a gestation period of about 18 months instead of 9 (16). "Among extant hominoids (includes the chimps, etc), humans have the lowest newborn-to-adult endocranial ratio" (36).
The second part of the solution is to allow the human cranium to be "collapsible" at birth with the four main cranial bones overlapping one another and greatly reducing the transverse and a-p diameters of the foetal cranium. This mechanism does not exist in the chimpanzee or any other ape.
Thirdly, during human parturition, both the pubic symphysis and sacroiliac joints can separate slightly to increase pelvic diameters.
The fourth part of the solution must be to maximise the size of the birth canal in the pelvis where this is at all possible and maybe at the expense of perfect adaptation for other purposes.
Conclusion to Section 1
Throughout our evolution as a bipedal animal childbirth has been difficult. The need to get the increasingly large foetal head through the pelvis has defined the angle and shape of the sacrum in the pelvis and the concurrent evolution of the spine for bipedalism had to proceed with this as an unalterable constraint.
Thus, the lordotic shape of the lower lumbar spine including the sharp wedging of the lowest two spinal discs is a direct result of the needs of childbirth.
Disadvantages of Lordosis
Advantages of Lordosis
Investigations of more naturally living peoples speak of "the rarity of the disc syndrome" (25) among one group, and in another "primitive population" the incidence of disc narrowing was very much lower than among "North Americans and North Europeans" (11).
It seems likely that low back problems are a result of some aspect of civilised life rather than an inherent fault in the spine. The human spine has evolved for at least four million years with this constraint and seems to be relatively free of problems for more naturally living peoples.
Implications of Lordosis for Bipedalism
Walking on two legs requires balance and control but does not put great mechanical stress on the spine. In bending and lifting, however, the forces are multiplied by ten or more (32). Many articles in technical journals confirm that lifting can stress the spine close to its limits (2, 18, 19, 22, 31, 32).
In evaluating the forces on the spine in lifting the body must be seen as a cantilever like the jib of a crane (Figure 5). There is a compression component and a tension component and the offset between them is critical to the strength.
The main compression component is the column of vertebral bodies and discs. There are several different tension components and there is considerable disagreement in the technical literature as to which are the most important. Referring to the posterior ligamentous system Gravovetsky et al write: "This we suggest is the primary force transmission mechanism linking the pelvis to the upper extremities" (18). Conversely, in the model used by McGill & Norman (30) "The passive ligament system created no moment because the subjects elected to maintain a flatback posture."
The defining question here is whether lifting is or should be done with the spine fully flexed. The function of the ligaments is to limit the flexion of the spine and for these to be tight the spine must be allowed to flex fully. This may be possible in the very specilised activity of competitive weight lifting which was the subject investigated by Gravovetsky et al (18) but would be impractical in most normal every day applications. To flex the spine fully means an inclination of some 75° of the thorax with regard to the pelvis (52° in lumbar spine (33) plus L1/2 flexion of 8° at L1/T12, T12/11, T11/10). This would be impractical for the average builder, forestry worker or farmer lifting loads of variable height. To fully flex the spine for lifting would also be the exact opposite of the advice given to such workers by back schools and lifting training. It would also be the opposite for the advice of such postural techniques as yoga and the Alexander Technique and the general human experience that bending to lift is dangerous.
Another reason for rejecting full flexion for normal lifting is that such lifts must be done quickly. "A collagen fibre creeps when it is stretched and then kept in that state for a prolonged period. When the PLS (posterior ligamentous system) is tightened as a result of the reduction of spinal lordosis, the creeping fibres reduce the ability of the PLS to transmit loads, thus explaining why lifts must be performed at speed", (18). This form of fast lifting would not be suitable for most workers who need to lift and manoeuver objects frequently and continuously during a full day.
In their evaluation of the pain and discomfort caused by various lifting postures Hart et al (22) found that "the kyphotic posture … caused more discomfort than any other posture". McGill & Norman also quote the studies of White & Panjabi, Berkson et al, Cappozzo et al and Andersson et al in which "ligaments (were not) recruited to check flexion during strenuous lifts", (30).
It will be assumed in this paper that normal subjects will use the muscles in lifting and will allow the body to flex to a convenient degree for the task in hand if adequate strength can be maintained.
Evaluation of Offsets
In order to estimate the offset, or moment arm length of muscles used in lifting, some categorisation of muscle groups and strengths must be made. Here again, there is considerable variation in the literature. Gracovetsky et al write "in the best case, the maximum moment contribution can be calculated to be about 250 Nm" (18) but McGill & Norman calculate a best value of 449 Nm (30). This is a difference of 80% between the values calculated in two detailed analyses by computer model. It is difficult for the reader to work through and check such analyses because the quantity of data and detail is not included in the published papers.
The analysis in this paper will concentrate on the offset between the compression and tension components at their connection to the pelvis. As with the crane jib this is critical to the lifting strength.
Method of Evaluating Offsets (moment arm length)
Offsets are taken from a scale drawing of the pelvis and spine in the saggital plane. Figure 6 is a full-scale reproduction of the central part of the drawing. The pelvis is shown in the normally defined standing position with the anterior superior iliac spines in the same vertical plane as the pubic tubercles. The spine is shown erect (shape 1) and with three different flexions. Shape 2 shows a flexion of about 5° per joint. Shape 3 corresponds to full flexion in the two papers discussed in Section 1 (25, 33). (The important characteristics of shape 3 are described in Section 3.) Shape 4 shows each spinal joint flexed to an extent that looks reasonable for the joint (10° - 15° per joint wedged with wider end dorsally).
In trying to select a "typical" pelvic shape from X-rays great variation was found, particularly in the size, shape, position and angle of the sacrum. A larger than average male subject was therefore chosen (182 cms – 80kg) who was know to be strong in lifting and free of LBP till the age 43 (but recently suffered an attack with no visible cause in X-rays). The X-rays were scaled down in ratio 5:4 approximately (film focal distance 100 cm), this ratio being confirmed by in vivo measurement of pelvic height. This subject is near the top of the height/weight range used by McGill & Norman (30) but lighter than competitive weight lifters (19). The sacral angle is 50° which is the angle shown by Jonck & Van Niekerk (25) in their Figure 1. The mean of the sample used by Pearcy et al (33) was 48° (estimated by this author from the X-rays of the subjects mentioned in Section 1. "The sacral angle" as used here is the angle of the sacral endplate from the horizontal with the pelvis aligned as defined above).
. Figure 6 shows the lower right part of the drawing. The full sized(A0) drawing was used to evaluate offsets of muscle and ligament lines of action from the centre of the L5-S disc (X). This picture is reduced so that the whole drawing can be seen on the screen at the same time.
Shapes 1,2,3and 4 are of erect (1), slight flexion (2), full flexion (3) and hyperflexion (4).
Points A,B,C,D,E and F define origins on the pelvis of muscle or ligament groups defined in the text with the same letter
Point A1 defines the point of insertion of the muscle group A in the spine in shape 1 and similarly for muscle groups B,C etc and for shapes 2,3 and 4.
The lines of action are not shown for clarity and where a point (e.g. A1) is outside the range of Figure 6, the line of action is shown near the edge of the drawing.
Table1 lists the offsets(or moment arm lengths)
Lines of Action of Muscles and Ligaments
In separating the spinal muscles into groups the anatomical definitions of Bogdug’s "reappraisal of the anatomy of the human lumbar erector spinae" (7) will be used. However it is useful mechanically to group these according to their connection points. The following groupings will be used.
Muscle Group A
Included in Group A are the muscles which connect the thoracic area to the sacrum. Muscular connection to the thorax is widely spread between all the ribs and the thoracic vertebrae from T12 to T6. Connection to the sacrum is via the erector spinae aponeurosis which is "ultimately attached to the lumbar and sacral spinous processes " (7).
The muscles are "the thoracic fibres of the medial division of the lumbar erector spinae" which "arise from the deep surface of the erector spinae aponeurosis" (7) together with "the thoracic fibres of the lateral division of the erector spinae" which "arise from the erector spinae aponeurosis (esa)" (9).
The esa connects onto the dorsal surface of the sacrum and the sacral spinous processes and also onto the posterior iliac spine. However "those (muscles) to successively more rostral (thoracic) levels arise progressively more medially across the erector spinae aponeurosis" (7). The point of origin of the esa in Figure 6 has therefore been placed on a line from the most prominent sacral spinal process to the posterior iliac spine but nearer the sacral process in a ratio 2:1.
The point of origin on the thorax has been chosen to be on T9 (central between T6 and T12) and half way between the transverse processes and the most dorsal point on the rib. This point is A1 for spinal shape 1 and A2, A3 and A4 for spinal shapes 2, 3 and 4 respectively. Because these points are outside the size of Figure 6 they are shown on the line of action near the edge of the drawing.
Muscle Group B
This muscle group includes those muscles that arise from the ilium via the lumbar intermuscular aponeurosis (lia) near the posterior iliac spine. They connect to the accessory and transverse processes of L1, L2 and L3.
A common line of action to L2 is shown in Figure 6 as B1, B2, B3 or B4 for spinal shapes 1, 2, 3 and 4 respectively. B2, B3 and B4 are outside the range of Figure 6 and are shown on the line of action near the edge of the figure.
The muscles are: The lumbar fibres from L1, L2 and L3 of the medial division of the lumbar erector spinae together with the lumbar fibres from L1, L2 and L3 of the lateral division.
Muscle Group C
These muscles arise directly from the ilium near the posterior iliac spine and connect onto the accessory or transverse processes of L4 and L5.
The origin is shown as point C on the medial surface of the iliac crest near the posterior iliac spine and the insertion is shown on L4 as point C1, C2, C3 and C4 for spinal shapes 1, 2, 3 and 4 respectively.
The muscles are those lumbar fibres of the medial and lateral divisions of the lumbar erector spinae which connect to L4 and L5.
Muscle Group D
This is the multifidus. "In the sacral region, the fasciculi arise from the back of the sacrum as low as the fourth sacral foramen from the aponeurosis of origin of the sacrospinalis, from the medial surface of the posterior superior iliac spine and from the posterior sacro iliac ligaments" (20). This would suggest an origin similar to point A but with more connection to the posterior iliac spine. Point D is shown above point A.
It is assumed that L2, L3, L4 and L5 will have direct connection and D1 is shown half way between the midpoints of the spinous processes of L3 and L4. Connection points are D1, D2, D3 and D4 for spinal shapes 1, 2, 3 and 4 respectively.
Ligament Group E
This is the L4 to iliac connection of the deep lamina of the posterior layer of the thoracolumbar fascia as described by Bogdug & Macintosh (6). The point E has been estimated roughly by transferring the ratio of vertical heights in the photograph Figure 4 in their report to the Figure 6 here.
This is the only direct connection from the supraspinous ligament to the pelvis. The supraspinous ligament is continuous and strong from the cranium (as the nuchal ligament) down to L4. However, surprisingly, "the ligament end(s) between L4 and L5" (23). This is confirmed by Risannan (34).
The supraspinous ligament does not reach L5 or the sacrum and for this reason the corresponding lamina running from L5 to the iliac crest will not be considered in this analysis.
Ligament group E connects to the tip of the L4 spinous process at E1, E2, E3 and E4 for spinal shapes 1, 2, 3 and 4 respectively.
Ligament Group F
These are the individual connections via the lateral raphe from the spinous processes of L3 and L2 to the iliac crest (6).
The position of the connection point F on the iliac crest was estimated as described for E above.
The vertebral connection points F1, F2, F3 and F4 for spinal shapes 1, 2, 3 and 4 respectively are placed at a point half way between the tips of the spinous processes of L2 and L3.
It should be noted that this ligament connection can only be effective at all if there is a tension in the abdominal muscles to hold the lateral raphe in place. This may significantly affect the line of action but this has not been considered.
Figure 6 shows these points for all muscle and ligament groups. For clarity lines of action are not drawn.
The compression component, the column of vertebral bodies and discs, connects to the pelvis at L5/S" and point X marks the centre of this disc. The offset from X of the line of action of each group is listed in Table 1. (Lines of action are taken as straight except where these will pass through bone.)
(normal full flexion)
Muscle Group A
Ligament Group E
Mean of Muscle Groups
(A, B, C and D)
Discussion of Results
It is clear that the maximum offset can be achieved by muscle group A. This is the muscle group that arises from the back of the sacrum via the esa and connects over the whole of the back of the thorax to ribs and vertebrae.
From the "jib of a crane" analogy one would expect this group to provide a substantial part of the whole strength of the spine in lifting. In the crane jib there will always be a tension member which runs from near the support (with maximum offset from the compression member) to meet the compression member near the point of attachment of the load. (Note that connection on the vertebrae is not to the spinous processes but to the transverse processes. This is what one would expect in order to bring the line of tension close to the line of compression, the vertebral body, near the support point of the load.)
The probable importance of this muscle group is also suggested by the very good spread of muscle connections to the thorax. Eleven ribs have muscle connections and seven vertebrae (7).
In the structure of the pelvis there are also reasons for expecting muscle group A to be a major contributor to lifting strength. These are as follows:
Muscle group A is thus of considerable importance both in the strength of the spine and of the pelvis. It is not however the only muscle of importance. Muscle groups A, B, C and D can all have offsets in excess of 54 mm and will together provide the tension components of the spine required for lifting.
Together these muscles connect to the whole of the area of the back of the sacrum and to the medial surface of the posterior iliac spine area of the ilium. The possible surface for muscular connection is also greatly increased by the presence of the aponeuroses, esa and lia, the former having direct ligamentous connection to hip joint extensor tendons.
Thus, the muscle groups considered here make maximum use of the whole of the back of the pelvis, both sacrum and iliac spines, to achieve the most substantial muscular attachment with maximum offset.
There is, however, a problem with this arrangement. The offset is seriously reduced by flexion of the spine. If the mean of the geometrical offsets of muscle groups A, B, C and D is taken (giving equal important to each muscle group) then the strength in shape 4 is reduced to only 49% of that in shape 1. This must make serious damage to the spine possible.
If, in attempting to lift a heavy weight, the offset becomes seriously reduced, the muscles may not be able to limit the flexion of the lowest lumbar joints. The supraspinous ligament will limit the flexion at all joints above L4 but this does not exist at L4-5 and L5-S (23, 34). A constant weight and tiring muscles may allow further flexion. Alternatively extension of the strong hip joint may flex L4-5 and L5-S instead of raising the weight. Whatever the reason, further flexion at these joints will reduce the offset even more and reduce the bending force that they can withstand. The resulting further flexion will again result in a reduced offset and reduced resistance to flexion.
The disc at the relevant joint will be subjected to very high compression while its flexion increases. This combination of flexion and compression is exactly the combination used "in vitro" to produce the "slipped" or prolapsed disc (2). Ligaments such as the capsular ligaments will have too small an offset to stop further flexion and may be damaged. The interspinous ligament has a slightly greater offset but it runs obliquely posterocranially (23, 34) which reduces its strength considerably and it will also be liable to rupture in such a case.
The reduction in offset with flexion explains many of the serious injuries of the lower spinal joints, particularly the prolapsed disc. This simple mechanical problem seems to confirm the common preconception that lifting with a bent back is dangerous and liable to result in a back injury.
The multifidus muscle (group D) has the second largest offset and is probably of great importance in protecting L4/5 and L5/S. This may be significant in view of the statement of Mattila et al (29) that "we are willing to consider that the small size of the type 2 fibres (in the multifidus) indicates atrophy both in the patients and in the controls … A possibility is that sedentary people in modern society do not use their multifidus muscles in such a way that the type 2 fibres would retain their normal size".
The total offset in shape 2 is reduced by 9% only and may therefore be perfectly adequate for heavy lifting. It is not suggested that the spine must be kept fully extended for maximum weight lifting, particularly if the compressive strength of vertebrae can be increased by some degree of flexion as suggested by Adams & Hutton (1). The greatest loss of offset occurs as shape 3 is approached.
The role of the ligament groups E and F will be discussed under the heading Contra-Arguments below.
Conclusion from Results
It seems therefore that the muscular groups considered provide a very substantial tension component for the structure of the spine as a cantilever. Muscle connections spread over the whole of the back of the thorax and the spine itself and make connection to all parts of the back of the pelvis with additional strength from aponeuroses which are continuous with ligamentous connections to hip extensor tendons. The attachment area on the back of the pelvis also provides the maximum offset for these muscle connections and this is maximised by concentrating some connections on the sacrum which is prominent for obstetric rather than mechanical reasons.
The disadvantage of this arrangement is that the high offset is only achieved if the spine is not flexed fully. An angle of flexion which would be perfectly reasonable for the spinal joints (10° - 15° wedging in flexion per joint in shape 4) will reduce the offset of all these muscle groups by more than half.
If a person lifts a weight with a spine approaching this degree of flexion there is a serious danger of tearing ligaments near the joint (capsular or interspinous) and of prolapse of the intervertebral disc itself.
Evolution has taken advantage of the prominence of the sacrum because this maximises lifting strength but has had to accept the consequent reduction in strength on flexion. No evolutionary reshaping of the pelvis was possible because the shape of the whole pelvis is mainly defined by obstetric considerations.
If, as suggested by Gracovetsky et al (18) the posterior ligamentous system (PLS) is able to provide the required tension component for heavy lifting (even for competitive weight lifting) then no joint damage should ever occur. As the muscular offset reduced and the flexion increased the PLS would come tight and would stop further flexion and protect the disc and capsular ligaments etc. Since such damage and problems do occur it is worth examining the adequacy of the posterior ligamentous system for this purpose.
Down as far as L4 the supraspinous ligament is strong, well placed on the tips of the spinous processes and directly parallel to the axis of the spine. It is not surprising that injury is almost always confined to the L4/5 and L5/S joints below L4. The supraspinous ligament does not extend below L4 (23, 34).
The connections from the supraspinous ligament to the pelvis are the groups E and F defined earlier. "This, we suggest, is the primary force transmission mechanism linking the pelvis to the upper extremities" (18).
From Table 3, these will have an offset of 63 mm in normal full flexion (shape 3). If the "required moment of about 1200 Nm for maximal lifts" (18) is taken as suggested by Gracovetsky et al then the axial component of the ligament tension is 19,000 N or approximately 2 tonnes. This is divided between the two sides to give 1 tonne each but the ligaments do not run axially.
Even when flexed the fascial connection from L4 to the iliac crest will be at an angle of about 45° from the spinal axis which will increase the tension to about 1.4 tonnes.
This is the force that must be applied by the two ligament groups E and F on each iliac crest in order to provide the tension component required.
In marked contrast to the muscle connections of group A, B, C and D, which are spread widely over the whole of the back of the pelvis and sacrum with direct connection to ligaments and hip extensors, these ligament groups connect very close to one another on the iliac crest (points E and F in Figure 6). Furthermore, the force of 1.4 tonnes is applied in a direction approximately perpendicular to the plane of the ilium at that point. The connection on the ilium, extending about 30 mm along the iliac crest, seems small for this force when compared with the wide spread of the erector spinae aponeurosis and lumbar intermuscular aponeurosis which together attach along a line some 220 mm long (across the medial side of the posterior iliac spine down and across the sacrum and the opposite side).
No experimental results could be found which defined the strength of these ligamentous connections and therefore it is not possible to confirm that they are inadequate.
However, for the compression component, the column of vertebral bodies and discs, there is extensive experimental data. The tension force of 19,000 N calculated earlier must also apply as a compression force on the vertebral bodies. Granhead et al (19) evaluated the compression forces on vertebrae in similar lifting situations to those referred to by Gracovetsky et al (18). Although 19 kN is at the lower edge of the range calculated by Granhead et al (19 kN-36 kN) many of the conclusions must still apply. Granhead et al found these figures "astonishingly high. They by far exceeded not only the results of similar earlier analyses, but also the load values, which were two or three times higher than what has ever been found to be the maximum ultimate strength of experimentally tested vertebral bodies. … (In vitro static loading test(s) (have) repeatedly shown that most lumbar vertebrae … fail at compressive loads not higher than 10-13 kN)" (19). It does not seem likely that this sort of discrepancy can be explained by inaccuracies in the estimation of offsets or by proposing that training can increase the bone mineral content (BMC) sufficiently to allow "the vertebral body (to) approach the behaviour of an almost solid block of bone" (19), particularly as previous studies have suggested "a positive linear correlation between the BMC and the ultimate compressive strength in these vertebrae" (19). (The mean BMC of the age and weight group was 5.18. The mean for the lifters was 7.06: An increase of 36%.)
It seems far more likely that the extra strength found in competitive weight lifting is due to intra abdominal pressure (IAP). The role of IAP in slow or continuous lifting is limited because "it is unreasonable to claim that this pressure can rise above the arterial blood pressure" (17), but this may not be a limitation for the very quick action in competitive weight lifting.
This would be a far better explanation of why such lifts have to take place very quickly. The IAP could be raised to a very high level at the instant of starting the lift but would be released as soon as the body approached the upright position where the spinal strength would be sufficient. The problem of arterial blood flow would only be very short.
It has been suggested previously that the IAP rise may be very short. "In Bartelink’s experiments (4) the equipment to measure the sudden high elevations of pressure was not available, which was unfortunate because he believed that the reflex contraction of the abdominal wall muscles lasted only a very short time" (10). IAP was not measured either by McGill & Norman (30) or by Granhead et al (19).
Gracovetsky et al (18) explains the need for speed as follows. "When the PLS is tightened as a result of the reduction of spinal lordosis, the creeping fibres reduce the ability of the PLS to transmit loads thus explaining why lifts must be performed at speed" (180). If the posterior ligamentous system were really strong enough to withstand the loads required one would expect it to be able to support such loads for at least a minute or so. Plastic deformation or creep of a ligament which is only loaded within its elastic limit should not be instant but relatively slow.
It is suggested instead that the thoracolumbar fascia part of the PLS (ligament groups E and F) is not adequate for strenuous weight lifting and functions as the limit in flexion of the lower lumbar spine in less stressful activities such as sitting, crouching and bending.
The angled orientation of these ligament fibres severely reduces their strength for weight bearing but is very well suited to providing the flexion limit in sitting. The orientation makes their action very non-linear and very progressive. Thus the plastic deformation or creep caused by variable periods of sitting will not result in greatly increased ligament length and joint mobilities.
This is exactly the same function which is provided by the postero-cranial orientation of the interspinous ligament (23) where the purpose is to ensure equal or corresponding joint mobilities at different joints in the spine and to avoid localisation of mobility (15).
That there is no adequate load carrying protective ligament for the lowest two lumbar joints is surprising but seems to fit the known facts of back pain. One possible alternative route for such a ligament would be direct connection between the supraspinous ligament and the erector spinae aponeurosis. There seems however to be no evidence of such a connection in a form that could provide an adequate ligamentous connection to the pelvis.
Summary of Section 2
In normal lifting the maximum strength and the maximum offset is provided by the muscles of the back.
The erector spinae and multifidus muscles connect to the pelvis over all of the back of the sacrum and the posterior iliac spines with maximum attachment strength and direct connection to sacrotuberous and sacrospinous ligaments and to tendons of origin of hip extensors such as the gluteus maximus.
The areas of attachment on the sacrum and iliac spines also give the greatest offset from the L5/S disc which can be achieved with this configuration of pelvis.
The configuration of the pelvis however is defined by the needs of childbirth. Thus evolution has taken advantage of the prominence of the human sacrum to achieve a maximum offset for muscle attachment and therefore maximum lifting strength.
However, due again to the configuration of the pelvis and particularly the shape and angle of the sacrum, the large offset is seriously reduced by large angles of flexion in the spine. Flexion of L4/5 and L5/S to a shape which is perfectly acceptable for L1/2 or L2/3 (10° wedge with wide end dorsally) will reduce the offset by over 50% and therefore the lifting strength also.
If strenuous lifts were performed with this flexion serious damage would be very likely to occur at the lowest two spinal joints and in particular the conditions of flexion and compression would be applied to those discs which would produce a prolapsed disc in "in vitro" experiments.
The Shape Warning Pain
The loss of offset with flexion explains why the spine can suffer serious injury. Damage such as the prolapsed disc will occur if heavy lifting is performed with an excessively flexed lower spine.
However prolapsed intervertebral discs are rare although severe back pain is common. There must be some protective mechanism which ensures that such serious damage does not occur.
Such protection does not seem to be provided by adequate ligaments. To provide full protection the strength of such ligaments would need to equal or exceed the strength of the muscles. They would have to have the same or greater offset and the same or equally substantial connections. This does not seem to be the case for ligamentous connections to the pelvis.
In considering lifting strength in Section 2, little mention was made of ligaments near the joint such as the longitudinal, capsular or interspinous ligaments. An example will show why these are of little use to lifting strength. If, in a fairly heavy lift, the muscle moment at L5 is 500 Nm with an offset of 50 mm, then the compression on the disc is 10,000 N or about 1 tonne. This is within the compressive strength of the vertebrae. If, while still lifting, the spine flexes such that the capsular ligaments come tight with offset of 25 mm and take a tension equal to the muscle tension then muscle and ligament tension will be reduced to two-thirds or a tonne. However, the vertebral compression will be increased to 1.3 tonnes which would probably exceed its capacity. Since the compressive strength of vertebrae is one of the known limitations on lifting strength the capacity has actually been reduced by involving such ligaments in lifting. For maximum lifting strength the offset is the most important factor.
The longitudinal, capsular and interspinous ligaments will of course protect the disc from excessive flexion in most activities but will themselves be liable to damage in flexed weight lifting. In his sample, Rissanen found ruptures in the interspinous ligament "in 21% of persons over 20 years of age". "All were in the lowest 3 lumbar interspinous spaces" (34) (93% of these in the lowest 2 spaces).
It seems that neither the supraspinous ligament with its fascial connection to the pelvis nor the smaller ligaments between adjacent vertebrae can provide an adequate ligament system to protect the lumbar spine in weight lifting. It is probably not possible to envisage such a system as it would have to duplicate all muscle connections to thorax, pelvis and elsewhere.
The alternative is a warning pain of sufficient intensity to force the person to stop lifting or otherwise overstraining the spine.
There will always be warning signals from muscles and ligaments and the spinal muscles appear to be well endowed with such warnings. However, this does not cover the case of reduced offset due to flexion.
When the lowest joints are flexed beyond shape 3 the reduced offset will not only make a prolapsed disc possible as explained earlier: It will also mean that the extra flexion can occur without sufficient muscular stretching to provide a warning from the normal tissue damage pains. A warning pain is required which is specifically associated with angle of flexion.
IT IS SUGGESTED THAT THE WARNING PAIN WILL BE TRIGGERED WHENEVER EITHER OF THE LOWEST JOINTS OF THE SPINE (L4-5 AND L5-S) FLEXES SIGNIFICANTLY BEYOND THE PARALLEL SIDED SHAPE.
As explained earlier, these two joints are very wedge shaped and flexion only "half way" to parallel sided still gives a perfectly adequate mobility.
This limit is suggested by the observation of Pearcy et al that "this (i.e. flexion resulting in the anterior edges of the endplates coming closer together than the posterior edges) was not seen to occur at L5-S" (33). In their sample of non back sufferers there were cases of the L4-5 joint flexing beyond parallel sided but in the subset of these checked by this author (listed in Section 1) there was no case.
In the similar evaluation of mobility in a more naturally living group the example of a flexed spine in Figure 1 of that report (25) shows approximately the same. L5/S is flexed less than parallel sided by about 4° while L4/5 is just beyond parallel sided by 1° or 2° .
The rule therefore seems to apply slightly more strictly to L5/S than to L4/5 but both these very dissimilar groups of non back sufferers seem to be similar in flexed shape (and also, as mentioned earlier, in erect standing shape). The word "significantly" in the rule proposed will probably mean 1° or 2° but will require further investigation.
The presence of a warning pain triggered only by the shape of the lowest lumbar joints would fit many of the known facts of back pain such as:
The actual source of the pain could be almost anywhere in the spine from any nerves which give the required geometrical information. There does not need to be any damage to the nerves. "Signals which sometimes announce an innocent event will at other times evoke pain" (37). It seems, however, that "the intervertebral disc most likely causes the pain … pain resembling that which the patient had experienced previously could be registered only from the outer part of the annulus or nerve root" (31). There certainly are nerves in the annulus of the disc which could be the source of the pain (38). Ways could be suggested in which such nerves could "detect" the transition through the parallel-sided shape.
Mechanical offset and Warning Pain
The spinal shape defined by the above criterion, that the lowest two joints should not flex beyond parallel sided , seems to correspond approximately to a reasonable minimum offset for the muscle groups. The mean offset of muscle groups in shape 3 is still 46 mm (Table 1) which is probably acceptable, whilst the further reduction produced by shape 4 to 34 mm seems excessive. Figure 7a, b and c show more clearly (though less accurately) the loss of offset that occurs beyond the defined limit. A straight line between the spinous processes of L3 and S2 represents approximately the line of the erector spinae aponeurosis. ("The ligamentous sheet … tends to straighten out between L3 and S2 …" (12).
Figure 7b represents the flexion limit with L4/5 and L5/S parallel sided. This corresponds to shape 3. The line is still clear of the L4 an L5 spinous processes. Further flexion to the shape of Figure 7c (corresponding to shape 4) brings the line very close to the L5/S disc. This represents a serious reduction in offset and looks unacceptable.
This correspondence between the parallel-sided shape of L4/5 and L5/S and the minimum offset for safety is a coincidental effect of the shape of the sacrum. The sacrum was shaped primarily in order to maximise the pelvis inlet and outlet diameters for childbirth, but this shape seems to have been particularly suitable for the generation of a warning pain in the disc.
It is suggested that much persistent and acute back pain is a warning pain operated in the L4/5 and L5/S discs when these discs flex or are able to flex beyond parallel sided. This needs to be distinguished from warning pains that may come from stretched muscles or ligaments, from pains that may come from damaged tissue, from pain caused by actual disc protrusions and from similar pain that seems also to be triggered by hyperextension. Diagnosis based on flexed shape may be difficult because of the success of the pain and muscular spasms in "freezing" the back and limiting mobility.
Civilised Sitting as a Cause
The previous sections have suggested that the L4/5 and L5/S discs should not flex beyond parallel sided. This is equivalent to saying that a minimum level of lordosis should be maintained at these joints in all circumstances (shape 3 in figure 6).
The limit to the flexion will be provided mainly by the ligaments of the spine. These will only come tight to limit the flexion occasionally in most activities but will be tight continuously in some sitting postures. Ligaments are collagen and "a collagen fibre creeps when it is stretched … for a prolonged period" (18). The ligaments can therefore be stretched by some sitting postures and the limit in flexion of some spinal joints can be increased. Sitting can therefore eliminate the lumbar lordosis completely and allow the L4/5 and L5/S joints to flex beyond the defined limits.
Many authors have concentrated on the extent and the importance of the effect of sitting on the lordosis (28, 13, 35, 3). "The mechanics and statics of sitting have been carefully studied by Schoberth. He showed roentgenographically that, on a change of posture from standing to sitting, the pelvis is rotated on the average 40° , and that this rotation is accompanied by a simultaneous compensatory kyphotic movement of the lumbar spine" (35).
In civilised sitting we frequently wish to have the spine upright and fairly straight despite the pelvis being tipped backwards by 40° or so. This will tend to concentrate the 40° into the lowest few joints of the spine, typically the critical ones, L4/5 and L5/S. This is particularly true in the working posture where the upper body may even be beyond the vertical and face down to the work, and in the vehicle driving posture where the upper body needs to be upright and active to control the vehicle. The truck driver needs the upper body to be even more vertical than the car driver.
Epidemiological studies confirm that these postures are associated with back pain. "Truck drivers are about five times more likely to develop an acute herniated lumbar disc than males who are not truck drivers" (26), and (referring to the same research) "it is suggested that prolonged sitting, regardless of whether it is in a motor vehicle, is detrimental, since length of time spent sitting on the job was in fact found to be associated with acute herniated lumbar disc. The relative risk for sitting while driving, however, was nearly twice as high as that for sitting in a chair regardless of the type of chair" (35).
These results were published in 1975 by recent work "confirmed previous findings that motor vehicle transportation was an indicator for first time experience of LBT" (5). (LBT or Low Back Trouble includes all pain instead of the single factor of herniated lumbar disc.) It is suggested therefore that civilised forms of sitting, particularly the desk working posture, and the vehicle driving posture, are likely to increase the flexion of the L4/5 and L5/S joints beyond the defined limit and thus cause one category of back pain associated with the shape warning pain. A person whose spine is thus weakened by being able to flex beyond the defined limit is also in danger of suffering actual damage to the spine if he or she then lifts with too flexed a lower spine. Such damage may include the prolapsed disc.
Sacral Angle Variation
In the examination of X-rays as a part of this study one variation was observed which may help to explain the variation in the incidence of back pain among those with identical occupations (e.g. truck drivers). The variation in sacral angle was seen to be very large, both among the sample of non back sufferers used by Pearcy et al. (33) and among back pain sufferers in patient records.
Among the sample used by Pearcy et al (33) the extremes were 22° and 70° . (This is the angle between the sacral endplate and the horizontal with the pelvic position adjusted such that the anterior superior iliac spines are in the same vertical plane as the pubic tubercles.) These figures are necessarily only approximate because the whole pelvis was not included in the X-ray film.
A sacral angle of 70° will tend to make the lower lumbar joints very wedge shaped when standing. Much more flexion will then be required for these joints to reach the parallel-sided shape than if the sacral angle is only 22° .
It would seem likely therefore that a larger sacral angle will make a person less susceptible to this form of serious back pain.
If, however, the large sacral angle is advantageous, the large variation is then surprising. It would be interesting to know whether a similar variation was found among the sample from a population chosen because of "the rarity of the disc syndrome" (25). This information was not included in that report.
The challenge of pain
The Listener, 26 July 1984
Yoshizawa H, O’Brien JP, Smith WT, Trumper M
The neuropathology of intervertebral discs removed for low back pain
J Pathol 132: 95-104, 1980