|
|
 |
* Please Note: The following article is provided by Body Mechanics for the sole purpose of
educating and informing our current and future patients.
Quadrupedalism, bipedalism, and human pregnancy
M. M. Abitbol
INTRODUCTION
The transformation from quadrupedalism to bipedalism necessitated a complete reorganization of the orthostatics and orthodynamics of the human body. Actually, any non-human mammal, specifically any non-human primate, can stand on its hind limbs. The fundamental difference, and in fact the definition of human erect posture, is that it is effortless. Any non-human mammal standing on its hind limbs wastes a tremendous amount of energy, while humans who stand erect tire much less (Abitbol 1988). How humans manage erect posture effortlessly is achieved through complete reorganization of the anatomy, specifically the bony anatomy. It is in the vertebral spine and pelvis that reorganization is most noticeable, although it is also noticeable in the lower limbs (hip joint rotation) and other parts.
THE VERTEBRAL SPINE CURVATURES
An erect vertebral spine does not mean a straight vertebral spine - far from it. The vertebral spine may appear to be straight when seen from an anterior-posterior (AP) view, but it presents numerous vertebral curvatures when viewed laterally.
Essentially, for the body to stand without effort, the center of gravity (CG) of each segment of the human body has to fall within the base of support formed by the segment immediately underneath. This requires displacement (moving back and forth) of various segments of the vertebral column in order for the CG in each segment of the trunk to align vertically. For the body to be erect without effort, the vertebral. In a frontal view, the vertebral column is perfectly vertical, but on lateral view the vertebral curvatures become evident column has to undergo four AP curvatures: cervical, thoracic, lumbar, and sacral. The direction' of each curvature depends on the part of the body involved. The second, third, and fourth curvatures are not genetically (ontogenetically) determined and are acquired as the human infant and young child is progressively acquainted with proper erect posture (Abitbol 1993b). The reason the human infant is, slow in acquiring its final erect posture is because spinal curvatures have not yet taken their final forms (Abitbol 1987). It is only progressively that the curvatures are formed and posture becomes more or less effortless (Abitbol 1988).
The first curvature starts to form at the lumbosacral region because proper (effortless) erect posture requires that the biauricular and biacetabular lines be in the same vertical plane for alignment of the CG of the upper and lower body (Fig. 31.2). If the biauricular line is located ventrally in relation to the biacetabular line (Fig. 31.3), as happens in pongids when they use erect posture, the entire body has to bend forward to re-establish an orthostatic equilibrium. If the biauricular line is located ventrally in relation to the biacetabular line (Fig. 31.4A), as happens in young children first attempting an erect posture, a marked lordotic curvature is necessary to reestablish the orthostatic equilibrium. Under these two conditions (pongids and human infants attempting erect posture), tremendous energy is necessary to maintain erect posture. It is no wonder that the pongid or the human infant cannotstay erect for a long time (Abitbol 1988). It is only in human adults that erect posture becomes effortless, since the biacetabular and biauricular lines are almost in the same vertical plane. Indeed, on a lateral X-ray film of an erect human, the center of the sacroiliac joint (SIJ) and the acetabulum more or less form a vertical line. This vertical alignment is an important factor contributing to effortless erect posture. Of course, this is also helped by the biauricular and biacetabular lines getting close to each other.
The anatomical consequence of the biauricularbiacetabular vertical alignment is the anterior displacement and rotation of the sacrum in the SIJ, necessary for the biauricular line to align vertically with the biacetabular line. The SIJ rotation results in compensatory curvatures at other levels of the vertebral spine.
Formation of the sacral curvature is followed by multiple anatomic changes. The upper sacrum becomes almost horizontal, while the lower sacrum, which is retained in its original position by various ligaments, remains almost vertical. The upper sacrum widens and the opposite auricular surfaces of the two SIJs increase in size better to support the load from above. The auricular surface becomes L-shaped instead of elongated. Therefore, the base of the sacrum (superior surface of the first sacral vertebra) is no longer horizontal as it is in pongids (or any other nonhuman mammal) who use an erect posture. The angle of the sacral promontory becomes acute (less than 90°) instead of forming a right angle as it does in non-human primates.
The second curvature is the formation of a lumbar lordosis to make up for the sacral rotation. This lordosis is barely present in non-human mammals who use an erect posture.
The third curvature is at the cervical spine. It takes a lordotic shape because the human head has to be kept in equilibrium over the atlas-axis complex. Also, the Frankfurt horizontal (that joining the external acoustic meatus to the inferior border of the orbital cavity) has to be kept horizontal for the eyes to look straight ahead. In terms of energy, another difference between humans and animals who use an erect posture is that the human head is in more or less effortless equilibrium over the cervical spine.
The fourth curvature is at the thoracic spine. It is 'caught' between the lumbar and cervical lordosis, resulting in the formation of the thoracic `kyphosis' to keep the body in equilibrium.
Essentially, the curvatures are interdependent: any kind of physiologic (e.g. bending) or pathologic (e.g. hyperlordosis) alteration in one curvature initiates a chain reaction altering all other curvatures. The double sinusoid curve formed by the vertebral column is really dynamic rather than static.
All the vertebrae have joints between each other. The only exception is the sacral vertebrae, which have fused together in all mammalian species to present a solid body to the iliac side of the SIJ (Abitbol 1987). The immobilization of the sacral vertebrae was 'replaced' by limited mobility of the SIJ, as described by Dorman (1992, 1993). He has demonstrated that .synarthrosis joints accumulate energy during compression and, in view of their elasticity, release this energy duringthe relaxation period. Dorman applied this principle to the SIJ and it can be extended to the intervertebral joints, as previously demonstrated (Abitbol 1995).
THE ORTHOPEDIC INSTABILITY OF THE HUMAN BODY
The orthopedic problems associated with pregnancy are simple to follow if one keeps in mind that they are the consequence of our orthopedic evolution. The basic facts are:
1. Phylogenetically speaking, our animal ancestors were quadrupedal, and it is only secondarily that we became bipedal. Our quadrupedal ancestry has been well established for hundreds of millions of years, while our bipedal ancestry is at most three million years old (Stern Susman 1983).
2. The orthostatics and orthodynamics of the human body are, therefore, originally and essentially quadrupedal. The transition was progressive. It started about three million years ago with the australopithecines (Stern & Susman 1983) and, even with the Neanderthals 40 000-100 000 years ago, the transition was not entirely complete (Rak & Arensberg 1987). It is only with modern Homo sapiens that erect posture and bipedal stride became what they are today.
3. In view of the above, one can conclude that our bipedal achievement is only 'skin deep' and is easily subject to imperfections, misadaptations, and regression. These problems can occur at any age but become more and more frequent past maturity and are predominant in old age. It is essential to stress that old age exists only in humans and in some animals living in contact with humans. Evolution has never 'perfected' or prepared humans for old age (Abitbol 1996a). There' is a strong likelihood that our ancestors of 10 000 years ago died in full maturity (Aries 1992) and therefore with fewer orthopedic defects. Of' the systems or organs of the human body to fall apart when aging, bipedal posture and locomotion are among the first. Animals are spared most orthopedic defects because they have no old age. Incidentally, another problem of very old age in humans is a diminution of mental faculties because these also are a recent acquisition from the evolutionary point of view.
With old age, vertebral curvatures have a tendency to fail: the cervical curvature diminishes as the head has a tendency to fall forward, and the thoracic curvature increases in a compensatory manner; the lumbar curvature has a tendency to decrease, the SIJs and hip joints are strained, the lumbar spine slides over the sacrum (the latter can occur at any age), and backache (a universal human complaint) appears (Burton 1995). Backache appears because the center of erect posture is at the lumbosacral level, and this level is the first to suffer. The result - backache - can occur at any age but is more frequent with advanced age. Backache is the price humans pay for the combination of erect posture and advanced age, both very recent in human evolution.
Another disturbance to the stability of human erect posture is during pregnancy.
OUR QUADRUPEDAL ANCESTRY
Mammalians and, in fact, most terrestrial vertebrates except humans, have a vertebral spine that is more or less horizontal. This is how terrestrial vertebrates functioned for a billion years, from the most primitive vertebrates to the higher primates. The higher primates have flirted with erect posture, but they are essentially still quadrupeds. All mammals keep this posture when pregnant and when giving birth. Schultz (1969) has doubted that human erect posture has reached its final stage and has suggested that there is more evolution to come.
Erect posture is not only an unstable equilibrium, but also a succession of unstable equilibria that manage to balance and correct each other. The main reason for instability of erect posture is that, in quadrupedal posture, the CG is low and the base of support is wide, whereas in bipedal posture the opposite occurs, with a high CG and a narrow base of support. How fragile and unstable erect posture is can be demonstrated in different ways.
Homosapiens takes the longest time to acquire and perfect posture. The process starts at between 1 year and 18 months of age, but is not perfect until 6-7 years of age (Abitbol 1988). During the last few months of the first year of life, the child practices both quadrupedalism and bipedalism.
Since erect posture is acquired and practiced before complete ossification, one can conclude that erect posture tremendously influences (in fact, determines) the anatomy of the human body, and specifically the anatomy of the pelvis and spinal column, during the time erect posture is being acquired (Abitbol 1987).
The whole orthostatics and orthodynamics of the mammalian body became altered when the trunk rotated 903 around the hip joint (Fig. 31.4) and the pelvis had to support the whole trunk. The repercussions on parturition are tremendous.
In conclusion, erect posture is not only a phylogenetic (genetic) inheritance, but also onto-genetic (Abitbol 1993b). The whole anatomy and physiology has had to readapt to erect behavior; this readaptation is not always perfect, and even less so during parturition. The typical example like to quote is the position of the heart within the chest. In a quadrupedal posture, this organ rests peacefully on the sternum for an entire lifetime, while in erect posture it 'floats' in the middle of the chest with practically no strong anatomic support. The impression one gets from a review of the forms that preceded Homo sapiens is that present erect behavior is unstable and easily subjected to defects. There is hardly any organ or function in the human body that has not readapted, or rather that has not poorly readapted, to erect posture and locomotion. The two organs that underwent the most readaptation, or sometimes misadaptation, to erect behavior are the pelvis and the vertebral spine. The readaptation-misadaptation of the pelvis to erect behavior has been the subject of much research. The readaptation-misadaptation of the spinal column to erect behavior, mainly as far as the spinal curvatures are concerned, has previously been discussed (Abitbol 1995).
The main manifestation of imperfect erect locomotion is wobbling. Lateral wobbling is observed easily in frontal view. It occurs in nonhuman mammals attempting erect locomotion, in human infants starting bipedal locomotion, in women with advanced pregnancy, in a human walking while very tired, and in old age. In these cases, the gluteal muscles do not function, or function less, as abductors. The graceful appearance of the human stride is lost.
ENCEPHALIZATION OR ENLARGEMENT OF THE HUMAN BRAIN
Complete reorganization of the pelvis by erect posture is only half of the problem facing human parturition. The other half is the tremendous enlargement of the fetal head at term. The fetal head of the australopithecines was not much different in size from that of a chimpanzee (Leutenegger 1972), but the pelvis presented an accentuated platypelloidy associated with erect posture (Tague St Lovejoy 1986). With Thnno erectus, the fetal head was most likely midway in size between that of the chimpanzee and the present human condition (Walker and Leakey 1993), and it is only with Neanderthal man that the fetal head most probably reached the present human size and became (as it still is) a serious obstetric problem. As a practicing obstetrician, I can definitely state that the relationship between the narrow human pelvis and the enlarged fetal head is disproportional or at least a tight fit.
Humans have adapted so much to this disproportion that it is now considered natural.
I will discuss the readaptation-misadaptation of the human female body to pregnancy, birth, delivery, and the postpartum period, in relation to erect posture and locomotion.
ORTHOPEDIC PROBLEMS IN PREGNANCY
Over the last three million years, hominids (including humans) have managed to find a modus vivendi, or workable arrangement, for efficient bipedal behavior. For the largest segment of their lives, humans are adapted to erect behavior. During pregnancy, however, this arrangement, which is already unstable to begin with, falls apart. The pelvis and the vertebral spine do not adapt well to human parturition because postural and obstetric requirements make demands in opposite directions on the pelvis (Abitbol 1993a, 1993b). Contrary to parturition, erect posture would benefit from lumbar lordosis, triangular and narrow pelvis, and protruding epiphyses (sacral promontory, ischial spines, and sacral apex).
Erect posture took precedence over pelvic obstetric requirements from a phylogenetic as well as an ontogenetic point of view (Abitbol 1993a, 1993b).
Human intervention can either 'ease' obstetric demands on the pelvis or completely bypass them, and this can make obstetric demands less imposing from an evolutionary point of view (Abitbol 1993a).
The final impression is that the human female body (specifically the vertebral spine and pelvis) is not entirely .and perfectly adapted to human parturition. The human female practices erect posture all her life, while advanced pregnancy concerns, only a short period. In that respect, erect posture is more important than parturition. The demands of erect posture and locomotion on the lumbar spine and pelvis have created the following obstetric problems.
Mechanical prseclems during pregnancy
First, the abdominal cavity is already, relatively speaking, smaller in a superior-inferior direction in view of the sacralization of the last two lumbar vertebra (L6 and L7) and their integration into the sacrum (Abitbol 1987), and smaller in an anterior-posterior direction in view of the formation of the lumbar lordosis. Human pregnancy barely has adequate space in the abdominal cavity, and this results in compression on all surrounding organs, such as the major abdominal vessels (Abitbol 1976), anterior abdominal wall, vertebral and pelvic joints, muscles, ligaments, etc. (Fig. 31.6). This is contrary to the quadrupedal condition in which the pregnancy rests comfortably over the anterior abdominal wall with the necessary space available and with no undue pressure on any organ.
Second, the human fetus grows in a ventral direction and, at or near term, the CG of the human body no longer falls over the base' of ; support formed by the feet. To regain her equilibrium, the pregnant woman at term has to lean backwards, and this results in important orthostatic and orthodynamic consequences. Spinal curvatures are completely disorganized. The lumbar curvature moves dorsally and may or may not be accentuated (Snijders 1995), the thoracic curvature has to compensate accordingly, and the equilibrium of the body has to be maintained in an unusual (painful) position. Because of the dorsal displacement of the trunk, the rectus abdominis muscles have to increase their tension and the intra-abdominal pressure is increased, producing elevation of the diaphragm. The cardiopulmonary consequences are numerous.
Third, from the orthodynamic point of view, the situation is even worse. Because of the backward position of the trunk in relation to the pelvic girdle, the glutei lose some of their abductor function (Warwick & Williams 1973) and the pregnant patient at term now wobbles, as previously described. The elegant striding walk, the pride of human erect behavior, is all but gone in advanced pregnancy.
Obstetric problems during labor
During labor, the fetal head and body have to find passage at the level of the three pelvic planes through a bony pelvis that has been adapted to and distorted by erect posture and locomotion.
1. Pelvic inlet. The sacral promontory has moved forward, thus restricting the AP diameter of the pelvic inlet. When the fetal head presents at the pelvic inlet, it pushes the sacral promontory dorsally, thus producing a backward rotation of the upper sacrum around the SIJ.
2. Midpelvis. The ischial spines have moved inward, thus restricting the transverse (TR) diameter of the midpelvis. When the fetal head presents at the midpelvis, it attempts to widen the TR diameter of this pelvic plane, and thus stretches the SIJ wide open.
3. Pelvic outlet. The bi-ischial diameters and the subpubic angle are narrower when the pelvis is android (30% of cases; Berman 1955, Warwick & Williams 1973), thus restricting the diameters of the outlet. When the fetal head presents at the outlet, it pushes the sacral apex dorsally and therefore produces a backward rotation of the lower sacrum around the SIJ.
In conclusion, the obstetric complications resulting from fetal body-maternal pelvic distortion produced by erect posture are numerous and far-reaching.
Soft tissue problems in human pregnancy
Bony readaptations-misadaptations are not the only anatomic consequences of erect posture. Soft tissue structures are also altered and adaptedmisadapted. In quadrupedal posture, all the abdominal viscera, including the pregnant uterus, are supported by the anterior abdominal wall, but in erect posture all these organs pound against the pelvis. Adequate and strict control of the rectal and bladder sphincters becomes imperative (Mostwin 1991). To carry our these new functions, the pelvic floor and the perineal body have become stronger and firmer. In addition, the sphincter of the uterine cervix has become hypertrophied in order to close the cervical canal tightly. The result is a build-up of the soft tissues. All these anatomic changes are directly or indirectly connected with the demands of erect posture and locomotion (Abitbol 1996b).
All these soft tissue structures that have been built to perform a support function in erect posture are markedly stretched during labor and delivery and may even become obstructive. In this case, they are literally torn apart to allow the passage and birth of the fetus. The stretching may be temporary but, more often than not, it takes on tearing proportion. The permanent scars of these tears may be serious unless they are properly handled (Droegemueller et al 1987). Incompetent cervical is a congenital or acquired weakness of the cervical sphincter in which the cervix opens in midpregnancy and the fetus is expelled. Postpartum relaxation of the pelvic floor results in urinary and even fecalincontinence and prolapse of the bladder, rectum, and uterus. All these defects require surgical correction.
The hormones of pregnancy, specifically relaxin, are produced during gestation and contribute to relaxation of all the body joints, including inter-vertebral and pelvic joints (Birnberg & Abitbol 1959).
There is a need to resume erect posture and locomotion immediately or as soon as possible after giving birth, to avoid complications of excessive bedrest such as phlebitis and thromboembolism. This early resumption of erect posture and locomotion may impose some stretching of the' pelvic joints before they return to their normal prepregnancy condition because they have been softened by hormones of pregnancy (progesterone and relaxin, among others). The damage may be permanent (leading to backaches, etc.).
My personal impression is that, as previously mentioned, the human body has more or less adapted to erect posture and locomotion, and has modified its anatomy accordingly (Fig. 31.9). 'However, the body of the female human has not entirely adapted to parturition. Parturition, rather, might have become extinct for various reasons, among which are difficulties of giving birth.
CONCLUSIONS
Fig. 31.9 (A) Neandertal man: the head is not in equilibrium over the cervical spine and therefore requires more energy to be held properly. (B) Modern man: the head is in equilibrium over the cervical spine suffers somehow from the anatomic modification resulting from erect posture. Pregnancy, labor, and delivery are meant to be carried more by a quadrupedal than by a bipedal body. If it were not for progressive interference of Homo sapiens with its own birth process (labor and delivery attendance, primitive then modern midwifery, assisted and directed labor, oxytocin stimulation, cesarean section, for example), the survival of the human species would have been difficult, and this is what might have happened to the other hominid species (now extinct) that preceded modern Homo sapiens (Abitbol 1993a) and who:
1. Erect posture is a recent (three millionyear-old) acquisition in human evolution. The equilibrium in erect posture is not as stable because the CG is high and the base of support is narrow, in contrast to quadrupedal posture, in which the CG is low and the base of support is wide.
2. On lateral view, the vertical alignment of the biauricular and biacetabular lines is necessary for an almost effortless erect posture, and this is associated with spinal curvatures.
3. To carry her pregnancy to or near term, the human female has had to lean backward in order to keep her equilibrium. This results in disequilibrium of the vertical alignment of the body, with a reorganization of the spinal curvatures. This may be associated with a partial loss of abductor function of the gluteal muscles and wobbling.
4. The hormone relaxin stretches all the lumbar and pelvic joints, and makes erect locomotion and even erect posture painful and tiring.
5. The distortion and narrowing of the three planes make labor and delivery painful, prolonged, and traumatic.
6. The soft tissues of the pelvis are temporarily stretched or permanently damaged by the passage of the fetus.
7. Erect posture and encephalization, therefore, make human parturition particularly difficult (Abitbol 1996b).
|
