The study published in the latest edition of Science Robotics described the robot resembling a machined torso and two legs and a human operator wearing a vest that transmits information about the human's motion and ground reaction forces to the robot.

The human operator can both direct the robot's locomotion and feel the robot's motions, thanks to the vest. For example, if the robot is starting to tip over, the human feels a corresponding pull on the vest and can adjust in a way to rebalance the robot synchronously.

"Now if you want to open a heavy door, the human can command the robot to throw its body at the door and push it open, without losing balance," said Joao Ramos, who developed the approach as a postdoc at Massachusetts Institute of Technology (MIT).

Ramos found that balance could be decided by a person's center of mass in relation to their center of pressure, a point on the ground where a force equivalent to all supporting forces is exerted.

The position of these two ingredients could be physically represented as an upside-down pendulum, with the top end representing a human's center of mass (usually in the torso) and the bottom representing their center of pressure on the ground, according to the study.

The researchers then developed a control algorithm based on the pendulum model, linking feedback between human and robot.

In the experiment, when the robot was struck with a hammer from various directions, Ramos wearing the vest could feel the jerk in the direction the robot moved and he instinctively resisted the tug, which the robot translated into a subtle shift in the center of mass in relation to center of pressure and in turn kept from tipping over.

The researchers are planning to develop a full-body humanoid with similar balance control, that one day is able to gallop through a disaster zone and rise up to push away barriers as part of rescue missions.