>In their experiment, they compared how a muscle and a traditional direct-current (DC) motor would lift a weight up from the ground. The DC motor will yank the object with the maximum amount of force it can apply at that moment and will continue doing so as it lifts the weight.

>"The construction of muscle, however, allows for a more fluid, continuous and gradual movement," McGrath said. "Muscle can smoothly lift the object that does not require it to continuously yank on the weight as with all of its power."

>To have their motor behave like the fluid, energy-efficient muscle, the researchers used a device called a proportional-integral-derivative controller (PID), which works like cruise control in a car. The PID can recognize an error between the current speed and the set speed of cruise control and corrects by increasing or decreasing the force.

>Muscles have been shown to provide performance advantages useful for robotic systems, such as energy efficiency, stability, or increased range in motion. How muscles create these performance advantages, however, still remains largely unknown.