octopus joint

Oddly, Octopuses Have Ephemeral Elbows

By Bjorn Carey 17 April 2006

You might never expect to tell a wobbly armed octopus to keep its elbows off the dinner table, but new research reveals the creatures stiffen their arms to form human-like joints to guide food to their mouths.

A three-jointed human arm has only seven degrees of freedom (DOFs), which are defined as the types of movements each joint can perform. Your shoulder and wrist each have three DOFs—each can tilt up and down, turn left and right, and can roll in a circular motion. Your elbow, however, only has one DOF, which is tilting up and down.

Scientists consider each of an octopus’ eight arms to possess a virtually infinite number of degrees of freedom, allowing them to bend and twist freely. But when it’s time to eat, octopuses use their flexible muscles to form temporary, quasi-articulated joints that work similarly to how human joints function.

Researchers recorded muscle activity in octopus limbs, and found that an arm generates two waves of muscle contractions that propagate toward each other. When the waves collide, they form a part-time joint.

This process occurs three times, forming a shoulder where the arm meets the body, a wrist where the suckers have grasped their food, and an “elbow” somewhere in between. The elbow typically exhibits the most movement during food retrieval.

The researchers say this is a remarkably simple and apparently optimal mechanism for adjusting the length of arm segments according to where the food item is grasped along the arm.

The similarity of structural features and control strategies between jointed vertebrate arms and flexible octopus limbs suggests that these configurations evolved separately in octopuses and vertebrates, a result scientists call an example of convergent evolution.

The research is detailed in the April 18 issue of the journal Current Biology.

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Controlled muscle contractions allow an octopus to form quasi-joints similar to a human’s.

Octopus joint

We have, through the development of ETP-OCTOPUS, explored the many positive qualities of our hydraulic hub-shaft connections. The most important features are compact design, quick locking, accurate positioning and good runout. The joint will not damage the contact surfaces and must be easy to dismantle.

For ETP-OCTOPUS the pressure setting is done with an external pressure source separate from the clamping joint. This makes the pressure setting extremely fast and makes it possible to pressurize several joints simultaneously and remotely.


ETP-OCTOPUS is a double-walled hardened steel sleeve with a flange. In the flange there are bores for mounting of a hub and a scraper with a screw joint, threaded connection for hydraulic oil and an air relief screw. The inner diameter has spiral tracks in order to get an even friction and a defined locking force.


The assembly is done with the screw joint. When hydraulically pressurized the inner sleeve expands uniformly against the shaft and creates a rigid joint. When the pressure is released the sleeve returns to its original dimension and can easily be moved along the shaft to its new position and be repressurized.


The unique hydraulic principle, the solid and simple design of the sleeve gives a great number of advantages:

  • High torsional stiffness.
  • Both axial force and torque can be transmitted.
  • Easy to build in, only space for the screw joint is needed.
  • Can be used independent of the operating temperature.
  • No axial movement during pressurizing.
  • Low risk of leakage.

ETP Transmission AB – Leader in hydraulic clamping and centering products. We specialize in Hub-Shaft connections, toolholders for metalworking and sleves for woodworking. | hub-shaft connections, hydraulic tool- and workpiece holders, clamping system