Humans are distinct from chimpanzees when it comes to throwing objects at high speed and with great accuracy. This ability, which first evolved in humans some two million years ago, helped them while hunting animals.

Humans do not have any of the characteristics so essential for hunting — speed, strength, agility, claws or sharp canines to hunt down and kill a prey. Hence the ability to throw a spear or stone at high speed and great precision to kill/injure an animal and to do so from a distance so as not to put himself at close range and run the risk of being attacked was crucial.

Based on experimental studies, a paper published on Thursday in Nature has found that throwing capabilities have resulted from several anatomical features that allow the storage of elastic energy at the shoulder.

“The human shoulder acts much like a slingshot during a throw, storing and releasing large amounts of energy,” notes a George Washington University release.

Before throwing an object we tend to rotate our arm backwards away from the target. This rotation stretches the tendons and ligaments crossing the shoulder. Elastic energy is stored in the stretched ligaments and tendons.

The elastic energy so stored in the shoulder springs like a coil and causes the hand to move forward “generating the fastest motion the human body produces” and the fastest throw speed possible.

The observation

“Our focus on the shoulder was motivated in part by the observation that in very high speed throws, the angular velocity of the humerus [upper arm bone] can be as high as 9,000 degrees per second [3,000 rotations per minute (rpm)],” said Dr. Madhusudhan Venkadesan, Assistant Professor at the Bangalore-based National Centre for Biological Science (NCBS) in an email to this Correspondent. He is one of the authors of the paper.

“By estimating the power output associated with that joint, we show that the muscles around that joint are incapable of producing that much energy in that short a time. But, if there was substantial elastic energy storage and release at the shoulder, it could explain the kind of extreme power outputs that we (and others) measure at the shoulder.”

But can the shoulder by itself generate all the power and hence the speed of the throw? “No, our experiments show that the shoulder cannot by itself account for the throwing speed that we measure.

“Rather, the work done at the waist, transmitted by elastic energy storage at the shoulder are together responsible for the high speed,” he explained.

Chimpanzees, on the other hand, do not have “much mobility at the waist, and also lack higher humeral tension and do not have a laterally oriented glenohumeral joint. All these together do not allow them to throw objects at a great speed.”

That throwing athletes tend to suffer from shoulder injuries and tear to ligaments are proof that elastic energy is stored in the shoulder. While humans have evolved to throw objects at great speed, overusing this ability tend result in injury.

More In: Science | Sci-Tech