Fastballs get power from the pitcher’s waist

"The goal of throwing is to transmit that power, from the waist, out to your hand," says Madhusudhan Venkadesan. "We do that by storing and releasing elastic energy in the shoulder. That is what we think is happening." (Credit: Chris Swann/Flickr)

How do people, not just professional baseball players, throw stuff a such high speeds?

Humans in general are the throwing aces of the animal kingdom. Chimpanzees, on the other hand, can throw but no better than a small child.

When we throw an object, our arm undergoes angular velocities of more than 9,000 degrees per second—that’s the fastest movement in the human body.

Madhusudhan Venkadesan, professor of mechanical engineering and materials science at Yale University, has studied images of baseball players captured with 3D cameras and built devices in his lab to replicate throwing motions.

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Calculations show it can’t be the upper-body muscles alone that produce that kind of power.

“What happens is that the power is generated down by your hip and waist, where we have huge muscles,” he says.

That the waist can rotate is crucial to throwing, since it allows the body to store elastic energy, which occurs when objects are stretched or compressed. “The goal of throwing is to transmit that power, from the waist, out to your hand. We do that by storing and releasing elastic energy in the shoulder. That is what we think is happening.”

Another big part of our throwing ability comes from what’s known as the glenoid joint, the collarbone-and-socket combination in the shoulder. This joint points upward in chimps, which makes it look like they’re always shrugging their shoulders. In humans, it’s lateral, which allows us to turn the arm into a huge lever powered by the elastic energy stored in the tendons and ligaments across the chest and shoulder muscles—like giant rubber bands.

The last component of this system is the twisted geometry of the humerus, the bone that runs from shoulder to elbow. The twist in the bone allows throwers to rotate the arm far back to increase the stored elastic energy.

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These three factors—the mobile waist, the sideways glenoid joint, and the twist in the humerus—first appear together in the fossil record about 1.8 million years ago in homo erectus. That’s just around the time when there’s strong evidence of hunting. The throwing-hunting connection makes sense. Humans, who specialize in long-distance running could chase down large animals until they were exhausted. But then what?

“You really don’t want to be within an arm’s reach of the animal,” Venkadesan says, adding that just one kick from the animal would bring an abrupt end to the hunt. “What you want to do is fling a rock or something hard really fast to kill the animal.”

While the research answers a lot of questions about throwing, there are still more that need to be addressed, Venkadesan says. For instance, how do we throw so accurately at high speeds? And why do we throw overhanded?

“Of all the possible ways someone could throw, why is this the best way?”

Source: Yale University