Of all the reasons people go see their doctors worldwide, back pain comes in right at number 6. Back pain is coming for us all. You, your friends, your parents, everyone you’ve ever known. If you live long enough, you’re gonna get back pain. Oh, yes, you’re thinking, “Well, that’s just aging, isn’t it? Doesn’t everything decay and crumble as we hurtle towards oblivion.” Well, yes, but also, humans are the only mammals to suffer many of these musculoskeletal maladies. You don’t hear chimps and gorillas complaining about their herniated discs. I bring up chimps because chimpanzees share a common evolutionary lineage with us. So understanding how our anatomy differs from our chimp cousins, can help us understand some of the mysterious aches, and pains, and quirks about us. Why pregnant women waddle like this? Or why your grandma gets shorter and stooped over like this. Essentially, it can help us understand why the human spine is an engineering nightmare. [MUSIC] Let’s run through some of the greatest hits from the musculoskeletal problems that affect only humans and not other mammals album: Fractured hips, bunions, hernias, fallen arches, torn meniscuses, shin splints, herniated discs, fractured vertebrae, spondylolysis, scoliosis, kyphosis, osteoporosis, just to name a few. All of this pain can be traced to, arguably, the most important adaptation humans had made, standing upright. This one very human feature causes two big issues. Let’s break them down. Taking a short, stiff horizontal spine that was adapted for climbing and trying to balance our big heads over our 26 vertebrae and our feet is a tall order. Evolutionary anthropologist, Bruce Latimer, studies the evolution of human locomotion. He compares this task to trying to stack 26 tea cups and saucers and then balance a human head on top of that. To make this configuration possible, we evolved some anatomical quirks. Latimer uses the fossil record as well as data about modern apes and modern people’s spines to trace the evolution of these mechanical changes. Chimpanzees have a straight stiff spine, which supports climbing. Our spine is flexible and curvy, which supports our weird upright gait. You probably know this spot in the lower back as the iconic resting place for your hands when you’re complaining about how getting old is the worst and your back just isn’t what it used to be. It’s anatomical name is the lumbar lordosis. Our spine had to curve inwards here to avoid the birth canal while also keeping the torso balanced over the feet. This is why women who are very pregnant sometimes lean back this way. It’s to correct for the way the weight of the fetus throws off the center of balance. While these bends in our spine help us balance and walk upright, from an engineering standpoint, they’re weak spots just begging for failure. Meet T8, the most commonly fractured vertebra in the spine. It’s had a mechanically weak location, plus as we age, and our discs and joints get worn down, we get more and more likely to suffer a break. Once T8 goes, you’ve changed the load bearing situation of the spine, and it’s not long until T12 and L1 break. When that happens, you’re putting pressure on the large joints at the bottom of the spine until they break. This is the reason your grandma gets shorter and stooped over and gets a [inaudible] Okay. That’s the first big problem. The curves are mechanical weak spots. Here’s another big factor, our gait. Whether you’re a human, a flea, an ostrich, or a tiger, you’ve got to do two things to get anywhere in this world. You need to create enough force and friction with the ground to propel yourself forward, and you also need to protect your body from the force that’s generated each time your feet strike the ground. Don’t underestimate the importance of this aspect. It’s critical. Latimer uses a Siberian tiger to illustrate his point here. When an 800 pound tiger is galloping at 35 miles an hour, the force that’s being generated each time its paws hit the ground is enormous, enough to blow the cat to pieces. In quadrupeds, like the tiger, their tricep and shoulder muscles contract in a long gait in a special way that absorbs the mechanical energy and disperses it as harmless heat. But because we traded in four limbs for arms, we don’t have our shoulder muscles available as shock absorbers. In humans all of that force is entering the body through a single point, our heel and then traveling up our leg, so we rely on the muscles of our lower legs to absorb that energy and protect our joints. When your heel strikes the ground, you flex your knee around 5-6 degrees allowing the quad to absorb the energy. In a push off stage, when the ball of your foot pushes off the ground, the quad muscle contracts which creates the force against the ground that propels you forward. That’s a lot of force coming through one contact point. So we’ve adapted our gait to mitigate all that pressure. With each step, you throw your right arm and left leg forward at the same time. Basically, we’re constantly twisting. That’s our adaptation for further dispersing the torsional load from our weirdo bipedal gait. If we didn’t make this twist, the reactive force from the ground would be strong enough to spin you around. This twisting is another major source of our back problems. In between each bone in your spine, there rests a cushion of collagen with a mucoprotein gel filling. This cushion is the intervertebral disc. Its structure is often compared to that of a jelly doughnut. The discs allow our spine to twist and flex and absorb some of the ground force reaction. They also help evenly distribute the load that our spine is supporting. But twisting all day, every day, millions of times over our lifetime, really does a number on the discs. The tough outer layer wears out. Sometimes the jelly escapes the doughnut. When that happens, you’ve got a herniated disc. The jelly can’t push into nerves causing pain or numbness. By the time we hit 50 or 60, we are all looking at that pain. Some of us will see it earlier or worse than others too. If you’re a gymnast or you swim the butterfly for instance, you’re putting your lumbar lordosis through some intense pressure and wear and tear. If you carry extra weight, you’re putting more load on the spine and its discs. Of course, if you do a lot of repetitive twisting, lifting, or pulling at your job, that’s adding more twist and load cycles to the spine and the discs as well. We can try to stave off the worst of it by strengthening our core muscles for added spinal support and stability, and we can also remember that this is a relatively small price to pay. Standing upright is what makes us human, and the number one most successful primates. [MUSIC] Thanks for watching. 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