2: Shoulders

I knew almost nothing about how shoulders actually work before mine got injured, and I only understand a little more now. After reading a few popular articles and skimming some papers, this is my impression: Shoulders are not perfectly designed for climbing. They are probably not perfectly designed for any one thing that humans use them for.

If asked to optimize the design of the joint for a specific task (say, climbing), most engineers would probably toss it out and start from scratch.

Evolution rarely works that way, and instead we’ve inherited a joint that was repurposed many times as the life histories of our ancestors have changed. What were once fins, used for swimming in our common ancestors with fish, became weight-bearing limbs in tetrapods that walked with their wrists and elbows sprawled out to their sides. In early mammals, those weight-bearing limbs moved underneath the body, made possible by changes in the structure of the shoulder blade, which allowed for faster and more efficient locomotion.

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Eventually, our quadruped ancestors took to the trees, developing specialized arms and shoulders for climbing, such as those seen in lemurs or orangutans. Perhaps unfortunately for the modern climber, we then went through a ground-dwelling period where we looked a lot like chimpanzees and bonobos, which climb a lot, but more often use their arms for walking. In the subsequent 613 million years, the human shoulder kept evolving in ways that make it easier to use tools, throw weapons, and carry objects while upright. Interestingly, the result of all this evolution is that our shoulders now superficially resemble those of our highly arboreal early ancestors more than those of our closely-related ground-dwelling apes, and some of our intermediate ancestors may have lived in trees.

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Rock climbing requires shoulder mobility, strength, and tool use. New River Gorge (photo Chris Gillis)

How this impacts our ability to climb, and whether we would be better off with the shoulders of chimpanzees, orangutans, or lemurs, I don’t know. I’m pretty happy that I can use tools and walk upright (though apes of course do both of these things, just not as well as we do). Regardless, evolutionary history leaves its mark, and our shoulders would probably look different if we hadn’t spent so many millions of years using them to support legs and fins, or if we had a few more millions of years to use them as arms. Moreover, the amazing variety of tasks for which we use our arms means that human shoulder anatomy is a compromise between those tasks. When we push or pull too hard on one function, we often have trouble.

 

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Pulling hard in Little Rock City, TN

I think it’s no accident that I have the same injury on both sides of my body, and that so many other climbers suffer from that injury. It’s becoming clear that human climbers are prone to SLAP tears. The biggest evolutionary “mistake” here might be the insertion of the long head of the biceps into the labrum. When you fall onto an outstretched arm overhead, a lot of the weight is carried by this tendon, and its pull on the labrum is a common cause of SLAP tears. Perhaps humans are well adapted for climbing, but not for repeated falling.

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The biceps. The long head of the biceps tendon, in red, runs over the top of the humerus and enters the shoulder capsule, where it attaches to the labrum. (image Wikipedia)

The human biceps has a very strange anatomy. The biceps has two heads (hence the ‘bi’), the bottoms of which attach to the forearm. The top of one of these heads passes over the ball of the humerus, makes a 90 degree turn, enters the shoulder capsule, and attaches to the labrum. It’s an oddity, probably an evolutionary spandrel, that the biceps spans two joints (the elbow, which it helps to bend, and the shoulder, which it mostly doesn’t).

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Squamish. (photo Nina Varsava)

Biomechanical studies haven’t found a very good reason why the biceps needs to attach to the labrum, or even enter the shoulder joint, and several doctors think that climbers around the world would be better off if the biceps just attached directly to the arm bone.

Which brings me to the surgery that does just that. Stay tuned for the next post.

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Until next time. Squamish. (photo Moos van Caspel)

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