Just how do we tall, two-legged creatures manage to stay upright while in motion, or even standing still? How can we possibly keep our balance on those two small platforms we call feet? When running in particular, which experts liken to a controlled forward fall, why aren't face-plants the norm rather than the exception?
"Would you ever make a chair out of two legs?"
The answer lies in a remarkable web of coordinated systems—voluntary and reflexive, neural, muscular, and skeletal—that collectively allow us to walk along a sidewalk or run along a forest path, largely without a second thought. We take these systems for granted, but they're a breathtaking testament to the engineering powers of natural selection.
We are jerry-built
Our distant ancestors were quadrupeds; they walked on four legs. Somewhere along the line—scholars debate exactly when or why—transformations began in our forebears' quadrupedal body plan that, over millions of years, resulted in our bipedal body plan. Resulted, that is, in you and me and our ability, unique among all 200-some primate species on Earth, to walk as a matter of course on two feet.
These changes were not planned—evolution doesn't proceed toward anything—and they were kind of cobbled together. "Would you ever make a chair out of two legs?" asks Jeremy DeSilva, an anthropologist at Boston University who specializes in the evolution of the human foot. "It's just stupid. You'd have to balance it perfectly, or you'd have to give it gigantic clown feet."
In fact, when we consider forward motion in us compared to that in quadrupeds like chimps or gorillas, "we've got the worst of both worlds," says Daniel Lieberman, an expert on early human locomotion at Harvard University. Not only do we have just two legs, but our center of gravity is significantly higher off the ground—just above our waist—which makes us naturally tippy. Imagine trying to tip over a chimp.
"And, if you think about it, when you walk, you only have one leg on the ground, and when you run, sometimes you have none," Lieberman told me. "So it's actually worse than that."
Don't I know. Yet experts agree that bipedalism was key in making us human, with all the advantages conferred by our bigger brains. And most of the time, we stand and walk and even run flawlessly using this jerry-rigged system. How?
Stacked in our favor
For starters, our ancient ancestors, the hominins—creatures more closely related to us than to other apes—became columns. In us humans, just as in a weight-bearing column, everything is in alignment, from our skull down through our spine and pelvis to our legs and feet. "We can stand and walk very efficiently, because gravity is essentially pulling straight down the column," says Carol Ward, a paleoanatomist at the University of Missouri.
"What really makes us human is the pelvis."
This makes all the difference for upright posture and travel. Just think how you'd feel if you tried to walk bent over forward like a chimp? "You could probably walk around all day bent forward like that," Ward says. "But I bet your back muscles will be so sore that you wouldn't be able to walk for another two days."
To make this verticality happen, our ancestors' quadruped skeletons went through an overhaul. The most significant change in terms of bipedalism, most experts agree, occurred to the pelvis. "I study the foot, I love the foot, but the pelvis is where it all starts," DeSilva says. Craig Stanford, a biological anthropologist at UCLA, concurs. "I have colleagues who write books about the evolution of the brain, which is fascinating," Stanford says. "But what really makes us human, in the larger sense of being in the human family, is the pelvis."
The pelvis of a typical quadruped like a gorilla is long and tilts forward (see diagram above). Ours is short, squat, and, perhaps most important, vertical—"a whole different kind of organism," as Stanford put it to me. All other changes to our bones that benefited upright posture, Stanford says, were really about keeping everything in the same plane—that is, columnar.
For example, the foramen magnum, the hole through which the brain connects to the spinal cord, migrated from the back of the skull to the bottom. The spinal column went from essentially horizontal to vertical. (The curves that make our spine S-shaped were needed to keep our forward-weighted head and trunk balanced over our pelvis and legs.) The femurs, the big bones of our upper legs, angled inward toward our knees, while our shin bones became perpendicular relative to our feet. Finally, our toe bones became short and flattened, and our big toe aligned with its four mates—all ideal for bipedal walking on the ground.
"Keeping things in the same plane is key," DeSilva agrees. "Once out of alignment with each other"—such as when doing a controlled forward fall along a trail—"we need to use muscles."
Muscling our way
Our muscles went through an overhaul of their own. In fact, bipedalism brought about a reversal in the roles played by the three major upper-leg muscles that all primates share—the large, medium, and small gluteal muscles.
In a chimp, the medium and small gluteals work to extend the trunk; they help chimps run and climb. In us, the large gluteal muscle, the gluteus maximus, serves this purpose. Whenever we climb stairs or run a forest path, we make use of our butt muscle, our body's largest.
Our medium and small gluteals, meanwhile, are abductors—muscles that move our legs away from the central axis of our body. (Think of feet together versus feet apart.) "The abductors' purpose is to keep the body from falling over to one side when standing on one leg," DeSilva says. Chimps and other primates don't have this stabilizing ability, and if they tried to stand on one leg, they would tip over.
Of course, our muscles do nothing without instructions. These come via our neural networks, the impossibly intricate web of nerves trailing up our spinal cord and into our brain.
Amazingly enough, some instructions happen entirely via the spinal cord. "A lot of basic movements never make it to your brain," Lieberman says. "A runner doesn't have to tell her legs what to do each time she takes a step, because there are basic reflexes that tell the legs what to do each time." Such movements are not just without a second thought but without a first.
Most of the time when we're in motion, however, we rely on our brain to voluntarily override or modify those reflexes to avoid obstacles—such as the exposed root I didn't see. Altogether, at any given moment during a walk or run, our brain is coordinating the movements of literally hundreds of muscles throughout our body. It's not just the muscles of the legs and feet but those of the torso, shoulders, arms, and neck, all of which help stabilize us during forward movement.
"A lot of basic movements never make it to your brain."
Information on where our body is in space, what hazards lie ahead, and so on streams into our brain from three principal sources. One is our eyes. Another is specialized nerve cells (known as proprioceptors) that sense positional changes in our muscles and joints. And lastly, there is our vestibular system, the extraordinary balancing apparatus in our inner ear.
Our vestibular system is absolutely vital to controlling our forward falls, or even to simply standing upright. One function of the system is voluntary: to continuously let our brain know how our head is moving. The other involves three involuntary reflexes. Different than the ones mediated by the spine, the vestibular reflexes are akin to the multiple gyroscopes that inform a pilot about the pitch, roll, and yaw of his plane.
During my unexpected plunge, all three vestibular reflexes came into play. The first maintains stability of our visual field. That enabled me, even as I nosedived, to effortlessly keep my eye trained on a fellow jogger who, wouldn't you know, had just appeared ahead and whom I was looking at the very instant I tripped. The second reflex stabilizes the head relative to the trunk, so that as my upper body went suddenly horizontal, my head didn't remain vertical—which might have given me whiplash—but stayed in line with my body. The third tries to maintain the body's center of gravity so we don't fall down. Obviously, that reflex was of no help at that humbling moment.
Not time enough
Why? Well, even lightning-quick reflexes take time. In his book The Evolution of the Human Head, Lieberman notes that it takes 5 to 10 milliseconds for our system to sense a stimulus—such as the warning that my right foot had collided with something—then another 30 milliseconds or more for muscle stimulation to initiate force—such as moving my arms and hands up to protect my face. Forty-one-thousandths of a second might sound virtually instantaneous, but it proved too long for me to successfully break my fall. The result? Catastrophe.
Fortunately, it was minor, and I simply regained the vertical, mumbled something self-abasing to the jogger then passing me, and got on with my controlled forward fall.