BECAUSE WE NEVER SHOULD HAVE STOOD UPRIGHT (Thanks Lucy*)
Basically, we can thank our ancestors for our back pain. It was their
switch from four-legged locomotion to two-legged movement that changed our basic structure, but not quite enough.
We're relatively new to this type of locomotion - our quadrupedal ancestry can be traced by hundreds of millions of years, while our bipedal ancestry is at most three million years old.
Technically speaking, this 'new' transformation from quadrupedalism to bipedalism necessitated
a complete reorganization of the orthostatics and orthodynamics of the human body, however it did save energy, free up our hands and
allow us to search the grassy savannahs for both food and prey.
According to a recent study, the human walking on two legs consumes only a quarter of the energy that a chimpanzee uses while “knuckle-walking” on all fours.
The finding, detailed in the July 17, 2007 issue of the journal for the Proceedings of the National Academy of Sciences, supports the idea that early humans became bipedal as a way to reduce energy costs associated with moving about.
Of course, now we expend much of that extra energy buying heat and ice packs, going to doctors, masseuses, acupuncturists and chiropractors to get that upright back fixed.
So which design was more efficient in the long-run?
SPINE CURVATURES AND BALANCING
The erect vertebral spine is not a straight vertebral spine.
The spine may appear to be straight when seen from a rear or frontal view, but its vertebral curvatures are readily seen when viewed from the side.
For a human to stand without effort, the center of gravity of each spinal section must fall within the base of support formed by the segment immediately below it.
This requires flexibility in various sections of the vertebral column as they move back and forth in order to maintain the center of gravity within each portion of the trunk as it aligns vertically.
From the side, it is evident that the spinal column has to undergo four anterior-posterior or front to back 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 determined and are acquired as a human infant and young child develops.
The spinal curvatures are interdependent: any kind of normal movement (bending) or misaligned movement (side flexion and rotation) in one curvature initiates a chain reaction altering all other curvatures.
The double sinusoid curve formed by the vertebral column is dynamic rather than static.
All the vertebrae have joints between them. The only exception is the sacral vertebrae, which are fused together in all mammals.
Due to the relative recency of our bipedalism, the basic structure and mechanics of the most of the human body remains essentially quadrupedal.
The transition was progressive. It started about three million years ago with the australopithecines and, even with the Neanderthals 40 000-100 000 years ago, the transition was not entirely complete. It is only with modern Homo sapiens that erect posture and bipedal stride became what they are today.
Our bipedal achievements are easily subjected 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. Evolution did not prepare humans for old age.
Our ancestors of 10,000 years ago died in relatively young, by our current measures. They experienced fewer orthopedic defects and their relatively 'delicate' bipedal posture and locomotion were never stressed by age.
As humans age, vertebral curvatures tend to fail: the cervical curvature decreases as the head has a tendency to fall forward, and the thoracic curvature increases in a compensatory manner.
The lumbar curvature also tends to decrease, hip joints are strained, and the lumbar spine slides over the sacrum and chronic backache begins.
Backache occurs because the center of erect posture is at the lumbosacral level, and this level is the first to suffer. Backache is more frequent with advanced age and is the cost we pay for a combination of erect posture and advanced age, both of which are very recent in homo sapiens evolution.
Erect posture is not only an unstable equilibrium, but also a succession of unstable equilibria that work to balance and correct each other. The main reason for instability of erect posture is that, in quadrupedal posture, the center of gravity is low and the base of support is wide.
Balancing on two legs, the opposite occurs. The center of gravity is high and there's a narrow base of support. The fragility and instability of erect posture can be demonstrated in different ways.
Homo sapiens takes the longest time to acquire and perfect posture during development.
The process starts at between one year and 18 months of age, but is not settled until 6-7 years of age. During the last few months of the first year of life, children practice both quadrupedal and bipedal stances.
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.
Coming soon -- ways in which to maintain back and spine health.
* Lucy - a hominid and representative of the Australopithecus afarensis, the last ancestor common to humans and chimpanzees living from 3.9 to 3 million years ago.
Only 3 feet 8 inches (1.1 m) tall, Lucy weighed 29 kilograms (65 lb) and looked somewhat like a Common Chimpanzee.
Observations of her pelvis proved that she walked upright and more in the manner of humans.