These Scientists Successfully Tested a Robotic Third Arm

Cognitive strategies for augmenting the body with a wearable, robotic arm

For some activities, having a third arm would be extremely practical. Besides the question of where it would be logically placed on a perfect cyborg, there is also the uncertainty: Is our brain even capable of controlling more than two arms?

“When challenging the brain to do something completely new, you can find out if it has the capacity for it,” says Silvestro Micera from the École Polytechnique Fédérale de Lausanne (EPFL). As part of the “Third-Arm Project,” he conducted a study to determine just that. It discovered confirmation when the team discovered that they could control a third robotic arm through breathing without affecting the other two arms. The results were presented in the journal “Science Robotics.”

Virtual Arm with Six Fingers

Cognitive strategies for augmenting the body with a wearable, robotic arm.

The goal is not a utopia of three-armed individuals. The main motivation behind this study was to better understand the nervous system, according to study leader Micera. However, there are practical applications: “This knowledge can then be used, for example, to design aids for people with disabilities or protocols for stroke rehabilitation.” Individuals could be equipped with a wearable robot arm to assist them in everyday tasks or aid in rescue operations.

To see if the human brain is capable of operating a third arm, the scientists initially built a virtual environment. The answer to where to position the additional arm was found in the golden mean: through virtual reality glasses, study participants saw a third hand between their left and right hands.

This third hand, to be symmetrical, had a thumb on both sides, totaling six fingers. “We drew a symmetrical hand to avoid bias towards the left or right hand,” explained Giulia Dominijanni. Each participant was strapped with a belt around the chest, measuring diaphragmatic movements. They were then asked to attack specific targets with their middle arm. The movement of the diaphragm—crucial for respiratory muscles—controlled the hand. According to the EPFL, users picked up control of the additional limb quickly. This occurred simultaneously with the two natural hands.

Control Through the Diaphragm or Ears

A test person tests the mobility of the virtual third hand.
A test person tests the mobility of the virtual third hand.

The system was tested on 61 individuals in over 150 sessions. Control of the third arm through the diaphragm did not impair the ability of participants to speak coherently. In a second step, researchers tested diaphragm control on an actual robotic arm. This simplified arm, likely not to win any beauty contests, consists of an extendable rod with a green circle at the end. According to researchers, participants quickly learned to control it. At least simple tasks, such as extending and moving the rod in the right direction, could be accomplished.

According to the EPFL, experts intend to investigate more sophisticated robotic devices using various control strategies in the following step. In the past, attempts have been made, for example, to control a third arm with ear muscles. Which muscle group is best suited for this purpose is likely still under exploration.