A good deal of what we know about the comparative strengths of the hand and wrist comes from a series of improbable experiments undertaken by a French physician, Pierre Barbet, in the 1930s. Barbet was a surgeon at the Paris Saint-Joseph Hospital who became obsessed with the physical challenges and limitations of human crucifixion. To test how well humans would remain in place on a cross, he nailed real human corpses to wooden crosses using different types of nails driven through different parts of the hands and wrists. He discovered that nails driven through the palm of the hand—the method traditionally depicted in paintings—would not support the weight of a body. The hands would literally tear apart. But if the nails were driven through the wrists, the body would stay in position indefinitely, thus proving that the wrists are much more robust than the hands. And by such means does human knowledge creep forward.
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Our other disproportionately bony outposts, the feet, receive a lot less praise and attention when it comes to discussing the things that make us special, but in fact the feet are pretty marvelous, too. The foot has to be three different things: shock absorber, platform, and pushing organ. With every step you take—and in the course of a lifetime you will take probably something in the region of 200 million of them—you execute those three functions in that order. The foot’s curved shape, like that of the Roman arch, is immensely strong, but it’s also pliant, lending a springy rebound to every step. The combination of arch and springiness gives the foot a recoil mechanism that helps to make our walking rhythmic and bouncy and efficient in comparison with the more lumbering movements of other apes. The average human walks at a pace of about 4.25 feet per second, or 120 steps per minute, though obviously this varies a great deal depending on age, height, urgency, and much else.
Our feet were designed to grasp, which is why you have a lot of bones in them. They were not designed to support a lot of weight, which is one reason they ache at the end of a long day of standing or walking. As Jeremy Taylor points out in Body by Darwin, ostriches have eliminated this problem by fusing the bones of their feet and ankle, but then ostriches have had 250 million years to adjust to upright walking, roughly forty times as long as we have had.
All bodies are compromises between strength and mobility. The bulkier an animal is, the more massive its bones must be. So an elephant is 13 percent bone, whereas a tiny shrew needs to devote just 4 percent of itself to skeleton. Humans fall in between at 8.5 percent. If we had stronger scaffolding, we couldn’t be as nimble. The price we pay for being able to scamper and sprint is, for many, backache and knee pain in later life—or indeed not so late in life. Such is the pressure on the spine from our upright posture that pathological changes can be detected “as early as the eighteenth year,” as Peter Medawar noted.
The problem, of course, is that we come from a long line of beings whose skeletons were designed to take our weight on four legs. We will look at the benefits and consequences of this massive change to our anatomy more closely in the next chapter, but for the moment it’s enough to bear in mind that becoming upright meant a wholesale redistribution of our weight load, and with that has come a lot of pain that we would not otherwise have suffered. Nowhere is this more uncomfortably evident in modern humans than in the back. Becoming upright put extra pressure on the cartilage disks that support and cushion the spine, in consequence of which they sometimes become displaced or herniated in what is popularly known as a slipped disk. Between 1 and 3 percent of adults have slipped disks. Back pain is the most common of chronic complaints as we age. An estimated 60 percent of adults have taken at least a week off work at some time with back pain.
Our lower limb joints are also highly vulnerable. Every year in the United States, surgeons perform over 800,000 joint replacements, principally of hips and knees, mostly from wear and tear on the cartilage lining the joints. It is pretty impressive that cartilage lasts as well as it does, especially when you consider that it cannot repair or replenish itself. Think of how many pairs of shoes you have worn out in your life, and you begin to appreciate just how durable your cartilage is.
Because cartilage isn’t nourished by blood, the best thing you can do to maintain it is to move around a lot, to keep the cartilage bathed in its own synovial fluid. The worst thing you can do is to pack on a lot of extra body weight. Try walking around all day with a couple of bowling balls tied to your belt and see if you don’t feel it in your hips and knees at the end of the day. Well, that’s essentially what you are doing already all day every day if you are twenty-five or thirty pounds overweight. It’s little wonder that so many of us end up undergoing corrective surgery as the years catch up with us.
For many people, the most problematic part of their infrastructure is their hips. Hips wear out because they have to do two incompatible things: they must provide mobility for the lower limbs, and they must support the weight of the body. This exerts a lot of frictional pressure on the cartilage on both the head of the femur and the hip socket into which it fits. So instead of swiveling smoothly, the two can start to grind painfully, like a pestle in a mortar. Well into the 1950s, there wasn’t much medical science could do to relieve the problem. Complications from hip surgery were so great that the usual procedure was to “fuse” the hip, an operation that relieved the pain but left the person with a permanently stiffened leg.
Surgical relief was always short-lived because every synthetic material tried would soon wear down until the bones were grinding painfully again. In some cases, the plastics used in hip replacements squeaked so loudly when people walked that they were embarrassed to go out. Then a dogged orthopedic surgeon in Manchester, England, named John Charnley devoted himself heroically to finding materials and devising methods that would solve all the problems. Essentially, he realized that wear was greatly reduced if the femur was replaced with a stainless steel head and the socket—acetabulum, to use the formal name—was lined with plastic. Almost no one has heard of Charnley outside orthopedic circles (where he is venerated), but few people have brought relief to greater numbers of sufferers than he did.
Our bones lose mass at a rate of about 1 percent a year from late middle age onward, which is of course why elderly people and broken bones are so unhappily synonymous. Broken hips are especially challenging for the elderly. About 40 percent of people over seventy-five who break their hips are no longer able to care for themselves. For many, it is a kind of last straw. Ten percent die within thirty days, and nearly 30 percent die within twelve months. As the British surgeon and anatomist Sir Astley Cooper liked to quip, “We enter the world through the pelvis and leave it through the hip.”
Happily, Cooper was exaggerating. Three-quarters of men and half of women don’t break any bones at all in old age, and three-quarters of all people go through the whole of life without any serious problems with their knees, so it is not all bad news. Anyway, as we are about to see, when you consider how many millions of years of risk and hardship our forebears went through to get us comfortably upright, we really don’t have much to complain about at all.