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The most commonly used industrial robots are typically six-axis, but recently, I came across a collaborative robotic arm that has seven degrees of freedom and I couldn’t help but wonder why. It wasn’t until I saw an online discussion that finally clarified things. The question was: How many degrees of freedom does the human arm (from wrist to shoulder) have? I learned that it’s actually seven—this is a true mimicry of the human arm's natural movement.
Most people never think about this, let alone understand the specific degrees of freedom involved. Even engineers might struggle to explain them clearly. I was surprised to find someone who answered with such depth and insight. Their explanation was not only technical but also philosophical, which made me want to share it.
The answer came from a netizen named Yang Shuo, and here’s what he had to say:
"Frankly, I’m quite familiar with robot manipulation, but at first glance, the answers above didn’t make much sense. It wasn’t black and white, and for non-experts, it was hard to grasp. I’ll try to explain it in simple terms. First, the answer to the question is: Just count them!"
The answer is 7 degrees of freedom. Someone asked if 5 or 6 makes a difference. In a 5-degree system, only one joint rotates when turning a key, and the motion comes from twisting the forearm. In a 6-degree system, only the wrist rotates when placing a mouse on the table.
There are several images illustrating these concepts, showing different configurations and movements.
So why does the human arm have 7 degrees of freedom instead of 6 or 8? It might be because the designer knew robotics well. Let’s briefly explore this.
A theorem states that a 6-degree-of-freedom manipulator cannot change its configuration without altering the position of the end-effector in space. This is difficult to grasp at first, so let’s simplify it.
Imagine a robot arm with a base, two joints, and two links—like opening a compass and pinching one end with your fingers. Can you move the arm from a "lefty" position to a "righty" position while keeping the end point in the same place on the plane? No, no matter how you move the joints, the position will change during the movement. You can try this with two pens on a table.
Similarly, a 6-DOF robot can't switch between two configurations without changing the position of the end-effector. That’s why industrial robots often twist around when welding in the same spot—they have to reposition slightly to change orientation.
But with 7 degrees of freedom, like our arms, we can turn a key without moving our hand away from the keyhole. Our hand remains in the same position, but we can rotate it freely. This is possible because we have that extra degree of freedom.
In short, the human arm has 7 degrees of freedom: 6 for positioning in 3D space, and 1 for rotation. If we only had 6, turning a key would require awkward, exaggerated movements. More degrees of freedom mean more flexibility, but also more complexity and potential for injury. So 7 strikes the perfect balance.
I don’t know if 7 is a sacred number in the Bible—seven days a week, seven colors of the rainbow—but it seems fitting that our arms have seven joints as well. This answer wasn’t just professional, it was clear, insightful, and easy to understand. I almost want to bow down and thank the person who shared it.