to be difficult, if not impossible, to design.Until recently, such a devicerequired electricity from an external power supply.Now, American scientists have built a small,wearable addition to normal shoes.Their new invention eases the load on muscles in the legand makes walking easier.Steven Collins and Greg Sawicki are biomedical engineers.They both studied how human beings walk.They found that our ankles and calvesperform motions similar to a spring joined to a clutch.In an automobile,a clutch is used to connect and disconnecta driving or driven part of an engineor other piece of equipment.The researchers found that the human body partsare able to both store and release energy,just like a spring.Steven Collins works with Carnegie Mellon Universityin Pennsylvania.Greg Sawicki is with North Carolina State University.He spoke to VOA on Skype."We found in basic science experiments that that system,your calf and Achilles tendon, works a lot like a catapult.So, the muscle holds on to the tendon and your bodyactually stretches your Achilles tendon quite a bitand then stores the energy in the tissueand then it's given back to propel you forward in the world."Mr. Sawicki says he and Mr. Collins designed a mechanical devicethat performs the same kind of energy ‘give-and-take' outside the body.Their new ankle exoskeleton is made of carbon fiber and metal.It connects to ordinary shoesand takes over part of the work of walking.Mr. Sawicki says the device reduces the amount of energyrequired for walking by as much as seven percent.The unpowered ankle exoskeleton can help peoplewalk farther with the same amount of energy.It also can restore normal movementfor individuals who have trouble walking.Mr. Sawicki says it takes only a few minutesfor someone to get used to the ankle exoskeleton.He says the wearer quickly learns to use less muscle energy,letting the device do much of the work."You really don't notice it until when you take it off.And when you take it off you realizethat it was there and giving you the boost."Mr. Sawicki says the device is mainly meant for peoplerecovering from a surgery operation or a stroke.He adds that anyone who spends a lot of time walking,like hospital workers and policemen, could use the device.There are no plans to make the ankle exoskeletonavailable to the general public.But the inventors say some manufacturershave expressed interest in the device..