 The Downside to Upright |
11 Mar 09 |
The Downside to Upright
Today, roughly 80 per cent of adults experience back problems sometime in their lives - a consequence of our upright posture. Humans have likely suffered this way since our ancestors first stood up, transforming our backbone from a bridge or arch to a column that must bear the full weight of the upper body. Especially vulnerable is the lower, or lumbar, region of the column, where pressure on the disks that separate our vertebrae can cause them to bulge or herniate.
Yesterday’s Model
Humans come from a long line of ancestors, from reptile to mammal to ape, whose skeletons were built to carry weight on all fours. Our ape ancestors probably evolved around 20 million years ago from small primates that carried themselves horizontally. Then, six or seven million years ago, our ancestors stood up and began to move about on their hind legs. By the time the famous Lucy appeared in East Africa 3.2 million years ago, they had adopted walking as their chief mode of getting around.
It was a radical shift. “Bipedalism is a unique and bizarre form of locomotion. Of more than 250 species of primates, only one goes around on two legs”Craig Stanford, anthropologist at the University of Southern California.
Stanford and many other scientists consider bipedalism the key defining feature of being human. “Some may think it’s our big brain, but the rapid expansion of the human brain didn’t begin until less than two million years ago, millions of years after we got upright and began using tools.” Evolutionary biologists agree that standing upright launched a cascade of anatomical alterations that paved the way, through changes of behavior, that lead to other evolutionary changes, such as the expansion of the human brain.
Just How Do We Do It?
The biomechanics of upright walking is so drastically different from quadrupedal locomotion that anatomical adaptation became a necessity. The skull and spine were realigned, bringing the head and torso into a vertical line over the hips and feet. To support the body’s weight and absorb the forces of upright locomotion, joints in limbs and the spine enlarged and the foot evolved an arch. As for the pelvis: It morphed from the ape’s long, thin paddle into a wide, flat saddle shape, which thrust the weight of the trunk down through the legs and accommodated the attachment of large muscles, namely the hamstrings, gluteals, and quadriceps. These adaptations improved the stability of upright walking and gave tremendous efficiency over long journeys but severely constricted the birth canal making child birth a considerably trickier proposition than for other mammals.
The Advantages
Why would we give up the stability and speed of quadrupedalism for the fragile joints, vulnerable spines, and difficult, risky births that led to the deaths of countless babies and mothers for bipedalism?
Two theriories dominate this debate. First is the significant advantage of freeing the arms from locomotion for gathering food, holding babies, and the development and utilization of tools. Second and the more popular is the huge energy savings in bipedal locomotion.The efficiency of the human walking action is far superior to our closest relative the chimp by as much as 65 percent. The key lies in our human features:
1. the ability to fully extend our knees,
2. the way our lower back curves forward and our thighbone slopes inward from hip to knee so that our feet straddle our center of gravity,
3. the action of the gluteal abductors, the muscles attached to the pelvis that contract to prevent us from toppling over sideways mid-stride when our weight is on a single foot.
These adaptations allow the human gait to operate like an alternating pendulum that swings the body up and over a stiff leg, so that the energy required to shift the body in its rise roughly equals the energy conserved in its decent … what? Take a two metre plank lying flat on the ground. Lift one end up to 90 degrees, this takes some energy, then let the plank fall away from you, this requires zero energy, and you have moved the end of the plank you originally picked up four metres. The significant advantage lies in the reduced energy required to move from point A to point B. If you can save energy while gathering your food supply, that energy can go into growth and reproduction. If food became scarce individuals who moved across the ground more economically gained a significant advantage.
The Cost
Back pain is one of the most common health complaints, accounting for more than 15 million doctor visits annularly in the United States.
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