Monthly Archives: September 2014

MYSTERIES OF THE BRAIN Part 26: Why is a smaller chimp much stronger than a larger human?


This is episode no 26 of the VERITAS series on the mysteries of the brain. Older parts can be found here:

Chimpanzees are the closest living relatives of humans. The genetic difference between humans and chimpanzees is less than 2%. But the strange thing is that chimpanzees are much stronger than humans. Some scientists have done experiments that show that an average chimp is 8 times stronger than a human. Others estimate that the difference is between 2 to 5 times. These experiments were done using pulling and lifting weights. So chimps can pull or lift many times more weight than humans. And in a human-chimp fight, the human would be massacred. There have been reports of a small chimp nearly killing a much bigger human. So an average chimp is many times stronger than an average human. This is remarkable especially given the fact that an average chimp is much smaller in size when compared with an average man. A male chimp is about 5’3 inch tall and weighs between 40 and 60 kg. So how is it that a much smaller animal is so much stronger than us humans? And why are we discussing this in a VERITAS series on the mysteries of the human brain? Well, you will find the answer to both these questions below.

The difference between a human and a chimp’s strength does not lie in the muscles. The difference lies in the way the nervous system controls the muscles of these two creatures.  To understand this first we need to know about grey matter and motor neurons. Grey matter is a major component of our central nervous system. The grey matter includes the regions of the brain and spinal cord responsible for muscle control, speech, hearing, emotions and decision making. The motor neurons originate from the gray matter located in the spinal cord and connect to the muscle fibers in our limbs. The signals are sent by the brain/spinal cord through the motor neurons and they move the muscles which are connected to them. Now, there are two kinds of motor neurons- some motor neurons are connected to many muscles forming what is called a large motor unit. Other motor neurons are connected to a few muscles and these form the small motor unit.

So if I need to jump, climb a tree or lift a heavy weight, the large motor unit comes into play and many muscles are moved by a single “command” passed through a motor neuron. But if I need to hold a pen, draw a picture or use chopsticks to eat noodles, it is the small motor unit that becomes active- only a few muscles are contracted.

Another way to look at this is that large motor unit lets us do tasks that involve power and strength. On the other hand, small motor unit allows us to do delicate, precise tasks.

Now, lets come back to chimps and humans. Chimps have much less grey matter and this means that they have less motor neurons. Therefore each motor neuron is connected to many muscles creating large motor units. So most muscles in a chimp are parts of large motor units. This enables them to run, climb and lift heavy weights. On the other hand, humans have a lot more small motor units. Most of our neurons control smaller sets of muscles allowing us to do specific tasks- we can write, we can play musical instruments, we can type!

In summary, the reason why chimps have greater “raw” strength is because their nervous system has less fine motor control. What this means is that the neurons of a chimp cannot pick and choose a few muscles at one time.

So we see that strength does not just depend on how many muscles a creature has. A chimp may have less muscles but has more muscles employed for “power” tasks like climbing, running and lifting and very few employed for delicate/specific tasks. In the case of humans it is the opposite.

Raw strength is not what humans are good at. Our thousands of years of evolution from our ape ancestors has not been in a direction that enhances our ability to employ power. Instead, we have become more mental creatures and our muscles let us do more complex tasks to bring our compex ideas into a physical form. Our strength is not in things like running or lifting. Our “strength” is in activities like calligraphy and using sophisticated tools.


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Go wondrous creature, mount where science guides

go measure earth, weigh air, state the tides,

instruct the planets in what orbs to run

correct old time, regulate the sun

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