The octopus stiffens its flexible arms, imitating human joints to guide food to its mouth, say scientists who've discovered how they form an "elbow."

Octopus arms are able to move in nearly any direction, or what scientists call a virtually infinite number of "degrees of freedom."

In contrast, the human arm is limited to seven degrees of freedom:

• Three each for the shoulder and wrist (up and down, left and right, and circular).
• One for the elbow, which tilts up and down.

An octopus forms a temporary elbow to feed. (Courtesy of Binyamin Hochner)

Israeli and Italian researchers, led by biologist Binyamin Hochner of Hebrew University in Jerusalem, found last year that when it's feeding time, the eight-armed octopus forms a temporary elbow-like joint to grasp the food and tip it into the mouth.

Now, the researchers say they've figured out how the muscles create the joint-like bends.

By recording muscle activity from the limbs of five octopuses, they discovered that the creatures produce two waves of muscle contractions that move toward each other.

When the two waves hit, a part-time joint is formed, the researchers report in the April 18 issue of the journal Current Biology.

The octopus uses the technique to form a three-jointed limb, similar to the human shoulder, elbow and wrist, that allow its tentacles to grasp a morsel of food.

"This is a remarkably simple mechanism for adjusting the length of the segments according to where the object is grasped," the team wrote.

Such articulated limbs controlled by joints is the best way to achieve precise point-to-point movements, the researchers conclude.