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Monkeying Around

On the playground, they call them monkey bars.

A child can swing from rung to rung—or skip a rung as reach increases and strength allows.

A child likely doesn’t know it, but they are engaging the highly complex muscles of the shoulder including supraspinatus, infraspinatus, teres minor, subscapularis, pectoralis major, pectoralis minor, the deltoids, trapezius, and the serratus anterior.

No other joint in the human body moves with more variety than the shoulder. That same flexibility, however, makes our shoulders vulnerable to instability and injury. The glenohumeral joint is held in place by a complex series of muscles, tendons, and ligaments.

But let’s think again about those “monkey” bars. They only allow swinging with arms extended overhead, the rest of their torso and body hanging as weight. However, if you have ever watched monkeys scrambling around a tree canopy, you know that they climb up and down with remarkable ease. How are they so well adapted to climb down as well as up?

Well, thanks again to science and scientists for stepping in with an answer.

Some ancestral primates, after all, were the first to climb a tree. Maybe after food? A view? (We’re joking.)

But how to get down?

Now, in a study published recently in the journal Royal Society Open Science, scientists have found that our ancestors evolved flexible shoulder and elbow joints, a “braking system,” as the New York Times describes it, “to finely control their descent from trees.”

The study was led by Nathaniel Dominy, an evolutionary biologist at Dartmouth University. The analysis suggests that the improvement in the shoulder’s capabilities persisted as early humans left forests for grassland habits. In fact, their renewed and upgraded shoulder joints may have led to their abilities to forage, hunt, and defend, too.

One key researcher was Mary Joy, a co-author of the study. According to the New York Times, she studied videos to compare how chimpanzees and sooty mangabeys move up and down in trees. (Sooty managabeys are an Old World monkey native to West and Central Africa.) The chimps, it turned out, showed much more shoulder extension both when climbing up (14 degrees more) and down (34 degrees more) than the sooty mangabeys.

Joy called the chimps’ downward climb like a “controlled fall.” Scientists are comparing shoulder and elbow joint shapes and will soon look into larger monkeys (mandrills and baboons) and their ability to climb up—and down.

So the next time you find yourself out in the forest and are tempted to climb a tree, think to yourself, “how am I going to get down?”







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