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Research News at Vanderbilt

New ‘bionic’ leg gives amputees a natural gait

by | Aug. 17, 2011, 12:14 PM | Want more research news? Subscribe to our weekly newsletter »

A new lower-limb prosthetic developed at Vanderbilt University allows amputees to walk without the leg-dragging gait characteristic of conventional artificial legs.

The device uses the latest advances in computer, sensor, electric motor and battery technology to give it bionic capabilities: It is the first prosthetic with powered knee and ankle joints that operate in unison. It comes equipped with sensors that monitor its user’s motion. It has microprocessors programmed to use this data to predict what the person is trying to do and operate the device in ways that facilitate these movements.

“When it’s working, it’s totally different from my current prosthetic,” said Craig Hutto, the 23-year-old amputee who has been testing the leg for several years. “A passive leg is always a step behind me. The Vanderbilt leg is only a split-second behind.”

The bionic leg is the result of a seven-year research effort at the Vanderbilt Center for Intelligent Mechatronics, directed by Michael Goldfarb, the H. Fort Flowers Professor of Mechanical Engineering. The project was initially funded by a seed grant from the National Science Foundation, followed by a development grant from the National Institutes of Health. Key aspects of the design have been patented by the university, which has granted exclusive rights to develop the prosthesis to Freedom Innovations, a leading developer and manufacturer of lower limb prosthetic devices.

Goldfarb and Hutto

Professor Michael Goldfarb, right, with amputee Craig Hutto who is wearing the new bionic leg developed at Vanderbilt. (John Russell, Vanderbilt University)

“With our latest model, we have validated our hypothesis that the right technology was available to make a lower-limb prosthetic with powered knee and ankle joints,” said Goldfarb. “Our device illustrates the progress we are making at integrating man and machine.”

The Vanderbilt prosthesis is designed for daily life. It makes it substantially easier for an amputee to walk, sit, stand, and go up and down stairs and ramps. Studies have shown that users equipped with the device naturally walk 25 percent faster on level surfaces than when they use passive lower-limb prosthetics. That is because it takes users 30 to 40 percent less of their own energy to operate.

“Going up and down slopes is one of the hardest things to do with a conventional leg,” said Hutto. “So I have to be conscious of where I go because I can get very tired walking up and down slopes. But that won’t be a problem with the powered leg because it goes up and down slopes almost like a natural leg.”

Recent technological advances have allowed the Vanderbilt engineers to produce a device that weighs about nine pounds – less than most human lower legs – and can operate for three days of normal activity, or 13 to 14 kilometers of continuous walking, on a single charge. They have also dramatically reduced the amount of noise that the latest model makes, although it is slightly louder than they would like.

Read more about Hutto's story in The Vanderbilt View: Out of the Deep: A shark attack leads to a collaboration that could transform the lives of amputees.

One of the latest capabilities that the engineers have added is an anti-stumble routine. If the leg senses that its user is starting to stumble, it will lift up the leg to clear any obstruction and plant the foot on the floor.

In order to incorporate all the improvements, the prosthetic’s hardware design has gone through seven versions and its electronics board has been redone 15 times.

According to Goldfarb, it was tough to make the prosthetic light and quiet enough. In particular, it was difficult to fit the powerful motors and drive train that they needed into the volume available. The biggest technical challenge, however, was to develop the control system.

“As you add greater capability, you are also adding greater liability,” he said. “Not only does the controller have to perform individual operations reliability, but it has to perform several operations at the same time and not get confused.”

The Center for Intelligent Mechatronics is also developing an anthropomorphic prosthetic arm project and an advanced exoskeleton to aid in physical therapy.

Media Inquiries:
David Salisbury, (615) 322-NEWS

  • Wayne Renardson

    I’ve been posting Dr. Goldfarb’s research to AMP-L, a listserv I moderate out of University of Washington. AMP-L was created in December 1995 by fourteen people who wanted a forum to discuss ideas relevant to amputation. Originally hosted by the University of Pittsburgh, it moved to the University of Washington in December 1998.

    To SUBSCRIBE to AMP-L visit

    Subscribe to Amp-l by filling out the form. You must include your FULL name. Failure to do so will result in removal. You will be sent e-mail requesting confirmation which prevents others from gratuitously subscribing you. AMP-L is a hidden list, which means that the list of members/subscribers is available only to the list administrator for the sake of  privacy.

    All subscribers can keep abreast of the valuable research being conducted by Dr. Goldfarb and Craig Hutto.

    Wayne Renardson
    Nashville, TN 37204

    • Readers, please email Mr. Renardson to subscribe to his list serv.

  • Anonymous

    Why don’t we see them using EAPs (electro-active polymers also known as artificial muscles)? They would be noise free. I believe that where the future of prosthetics is.

    • Please see response below from Professor Goldfarb:

      “Although EAP’s would be essentially silent, they do not currently have the power density required for lower limb prosthetics, and most require high voltage, which is a concern in a human-centered device.”

  • Is the bionic leg heavier than typical original leg?
    I could have sworn after watching the video, seem like he’s trying a bit hard to lift the leg’s weight up a bit to pass the foot switching.

    • Nacon,
      The prosthetic leg is about the same weight as a natural leg. It is heavier than a passive prosthetic leg. In an interview, Craig Hutto said that he was initially concerned about the device’s added weight but that it hasn’t proven to be a problem.

    • bearfoot

      read the article!

  • Rachel McMahan

    That’s really nice! Great work Vanderbilt. It’ll be exciting to see the finished product.

  • Anonymous

    “Needs a little WD-40”. Fantastic device. Hope the guy gets to keep it and that they refine it. Decent battery life and little maintenance beside great comfort would be wonderful. It is unfortunate that they have not been able to mass produce something that would assist most amputees. However I realize that each person is different as are the injuries-not to mention the cost of such.
    Keep up the good work.

  • Anonymous

    Most Impressive


  • My question is the cost for this something the average person could ever afford?

    • Anon

      Well like any new technology it will probably be very expensive at first, but once production costs run down then the technology will become cheaper and easily accesible to us. 

    • Mr. Benson, please see response below from Professor Goldfarb:

      “Our goal is to have this sold at a cost that is in line with the current cost of lower limb prosthetics (which admittedly is high). Despite that, assuming the cost is aligned with other technology on the market, our prosthesis has the potential to decrease falls and decrease stress on intact joints, and therefore should recover health care costs rather than increase them.”

  • This is very interesting.  I wonder if something like that can be made for stroke victims who’s leg doesn’t work correctly?

    • Ms. Marchbanks,
      Please see response below from Professor Goldfarb.
      Melanie Moran

      “Actually, we are working on that exact issue, although we’re in relatively early stages at this time.”

  • Anonymous

    Can you make the leg look more robotic than Human?

  • PDC

    1)Rather than comparison to  “passsive” prostheses, why not consider comparison to other microprocessor controlled limbs?
    2) How much advantage is gained relative to energy expenditure with this unit being powered. 9 lbs is on the heavy side for a lower limb prosthesis.

    • Please see response below from Professor Goldfarb:

      “1) By passive prostheses, we are talking about microprocessor-controlled limbs. Passive means that they cannot contribute power to mobility. All of our comparisons have generally been to the Otto Bock C-leg, which is a passive prosthesis, but is also a microprocessor-controlled.
      2) You’re right that the leg is heavier than most passive prostheses. However, the leg is powered, such that it pushes off and contributes power to gait. Our subject feels that walking with the (heavier) powered leg is easier than walking with his C-leg. At this point, however, we only have one data point.”

  • Paul

    I am a
    72 year old AKA who currently wears Freedom’s PLIE 20 knee.  I switched to the PLIE from the Otto Bock
    when its warranty period ended for three reasons:

    a.  greater flexibility for charging;  PLIE just switch batteries but Otto Bock
    requires daily charging.  Hard to back
    pack if need charge overnight.

    b.  greater flexibility with choices of feet

    c.  more water resistant; with Otto Bock, if it
    rains one better have a good watertight seal or 
    leg stops working


    All of
    those have been realized; however PLIE leg has proven to be unreliable. Twice,
    in less than one year, I have had to send the leg back for major rework.  First time, there was a problem with the diaphragm,
    second time with the charging system itself. 


    It is
    why I am writing after seeing the video of the new leg being developed
    and licensed to Freedom innovations.  From
    my experience, Freedom’s focus appears to be on function not reliability and
    certainly not customer service.  For most
    amputees, reliability has to be priority number one.  We expect it to work day in and day out since
    we expect to live a normal life walking day in and day out.  All vendor legs can be used to walk down
    steps leg over leg.  They don’t work for
    going up stairs, but having that function at expense of reliability is not
    desirable.  With help from a good
    physical therapist, with legs from all three manufacturers one can walk up and
    down hill, and not have to drag the prosthetic leg. 


    experience as a marine engineer has been that adding complexity, frequently
    adversely impacts reliability.  Don’t
    know if Freedom or Vanderbilt have done a failure modes and effects analysis on features of
    the new leg, but do know that if it decreases availability (mean time between
    failure plus mean time to repair/replace) it’s not what an amputee needs.




  • I’m assuming this would not be highly effective in a martial arts style heavily dependent on speed, agility and lower body strength. However, this does seem like it might be a step towards that direction. That is the type of leg I seek, so that I will be able to resume my style of combat prior to losing my right leg (above knee) in 2008.

  • Medo

    The weight of the second generation is about 9lb. What is the weight of the first generation? Also, what is the difference between both generations? I see that there is difference in the design as the first generation’s motor is connected directly to ball screw and almost the knee joint and ankle joint are on the same line. However, the second generation’s motor is perpendicular to the knee frame and the knee joint is shifted relative to the ankle joint. Does this change in the design provide with more stability and better performance?

  • tiger

    my left leg was amputated almost 5 years ago now , I was suppose to be the first aka amputee to receive osseointegration surgery in the USA , it never happened … have been looking for better devices ever since … is there anyway to get involved with this study ? i’m a very fit 56 old man who has always led a very active life , an athelete my intire life and i work construction still … you wanna test a leg on someone who will put it through the mill ??? you want me !!!!!! the legs i have know can’t keep up and are constantly being repaired , wearing a loaner right now … i’m serious about living my life to the fullest , maybe your bionic leg will help me do that better . thank you for any considerations on my behalf .

  • Ebony Strebig

    What percentage of ‘on’ usage of above knee prosthesis is actually in walking gate mode per day versus shuffling, short stepping, standing, sitting? Where are the studies? Are electronic legs worth the 4 pounds and $$$?

  • Betty Cook

    I have been an amputee my whole life, I have a below the knee amputation, from a birth defect and would like to know if this technology would be able to be made for a below the knee amputee like myself.