back to article Thigh-drive phone charger put through its paces

North American boffins have produced a knee brace which can generate several watts of power as the wearer's leg flexes while walking. The inventors believe the device could be useful for powering medical equipment - and even mobile phones. In a paper for the boffinry journal Science, the researchers reveal their device's test …


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  1. Christoph
    Paris Hilton

    Not just walking

    This will of course also work for other activities which flex the legs. See icon.

  2. Surur


    Whats a neurotransmitter and why does it need external power? Is it time to get my tinfoil hat on?

  3. Tim Ling

    Wrist mounted?

    One for IT managers there I think

  4. Alan Parsons
    Thumb Up

    Fidgeters to charge laptops..?

    I'm sure I once read somewhere about an idea that would allow "knee jigglers" and "foot tappers" to charge their laptops by means of a device under the desk.. Wonder if something like that's available..? The last office I worked at I knew a chap that could power the whole bloody floor.

  5. Anonymous Coward
    Anonymous Coward

    Other activities

    Like... Cycling! Yes - cycling. Well what /else/ could it be?


  6. Anonymous Coward

    And then when we load up with other gizmos. .

    Recharge battery, Ipod, camera, PDA.

    We are Borg, resistance is futile (pronounced fyoo dill)

  7. Anonymous Coward

    @Lewis Page

    Power neurotransmitters? Funny that, mine don't need charging every night (well, not in a socket anyway). But pray tell us, how does this device allow us to recharge our nervous system in chemicals?

  8. bluesxman


    Forgive me for adding an air of practicality to the proceedings, but unless you're wearing two of these, wouldn't this make for unevenly toned leg muscles, by reducing the impact of the exercise on the leg it's attached to?

    And by extraction, even if you were to wear two (attractive as they are), aren't you going to have to walk/run further to get the same aerobic effect you used to due to it "helping the leg somewhat with its task"? Doesn't that defeat the object of the *cough* exercise of exercising?

  9. Trevor Watt

    At 5W output.....

    It has military uses, personal communications, charging NODs and battlefield equipment etc. Expect the patent to be swallowed up by the Big Green Machine.....

  10. Richard


    "aren't you going to have to walk/run further to get the same aerobic effect you used to due to it "helping the leg somewhat with its task"?"

    No - the device is extracting power from the system, so can only increase the amount of energy required, and thus the aerobic load. It may reduce the energy absorbed by the knees, but only because it absorbs that energy itself, rather than because there is less energy to absorb.

  11. James


    As far as I understand, your body is using energy to both 'push off' and take a step, as well as to stop your body collapsing to the floor at the end of a step. This device 'helps' the leg with this second part, absorbing some of the energy for itself.

    You are still putting in the same energy when taking the step, but you are doing less work 'removing' that energy at the end. Kinda like adding gravitational potential and removing it each time your body takes a step; this device assists the removal of it. IANA expert in gait analysis, however.

  12. Matt


    I think neuro-stimulators run off small long life batteries which need replacing every seven years or so. I don't think having one which recharges is going to help because the battery won't keep recharging beyond a few years anyway.

    I'm also a bit curious why this was invented in the US, I'm not sure how much electricity's going to be generated walking from the house to the garage anyway.....

  13. Register Reader


    "It may reduce the energy absorbed by the knees, but only because it absorbs that energy itself, rather than because there is less energy to absorb."

    From the sounds of it, it really would make you use less energy, the energy it's extracting is from the power of gravity pulling against you, which you usually have to counteract yourself, but this assists in the counteracting. Walking is hardly a very efficient system for travelling compared to other methods such as wheeled vehicles which don't directly fight against gravity (unless they're going up a slope).

  14. Pascal Monett Silver badge

    Charging battlefield equipment

    Yeah, I'm sure the thing will work fine while half-submerged in a puddle of mud, or encased in the sands of some desert country. Another fine gadget for soldiers to cuss about.

    Then again, the ruggedized version just might save some young man's kneecap when walking past an IED.

  15. Law
    Paris Hilton

    RE: Fidgeters to charge laptops..?

    "The last office I worked at I knew a chap that could power the whole bloody floor."

    Sorry! :)

    I'm also the annoying guy in the cinema..... my wife hates it.... wonder if paris is a knee jiggler

  16. Sam


    Just thinking (dangerous, I know) about energy recovery..If the millitary get this right, could they not extended the yomping power/range of a grunt in the same way that braking recovery is being looked at in vehicles?

    Said grunt could then yomp further, and help someone meet <INSERT DEITY HERE> sooner than expected.

  17. Anonymous Coward
    Anonymous Coward

    That's stylish

    You'll look great walking down to the juice bar to toss back a few beet-and-leak smoothies, while listening to to some new indie rock tunes on your ipod, carrying your macbook air in your organic hemp fibre backpack, offsetting your carbon output, and wearing shorts and Birkenstocks all year.

  18. Highlander

    5W is not enough for what exactly?

    5W is more than enough for light. the light output per watt that LEDs can achieve is astonishing. it this reliably puts out 5 watts, then hook up 5 1W high flux LEDs and see what happens next.

  19. Anonymous Coward

    All we need now ...

    is for somebody to invent a pair of electric legs, so that less-active people can make use of this device too.

    Of course, those legs will have to consume less than 5 watts to be REALLY useful.

  20. Geoff Mackenzie

    Neurotransmitter charging

    Is it just me, or is this idea making anyone else think of LSD?

  21. Ishkandar

    All that is needed...

    ... is to connect this to a vibrating phone and you can smile as you jog !!

  22. Josh


    "I'm also a bit curious why this was invented in the US, I'm not sure how much electricity's going to be generated walking from the house to the garage anyway....."

    We have to start working on things like this now, while we still have some cheap energy source to power and produce all the crap we consume, like the car in said garage. When that source is gone, this technology will have been (hopefully) much more developed to power the future version of what devices you would bring in your car today.

    Do you think it would be possible to generate more power from larger animals, like camels or horses?

  23. Rick Brasche
    Thumb Up


    no worries about my Bushnell Holosight quitting when I'm in the field. If I get two, that's ten watts, enough to keep a GPS and maybe even comms gear charged.

    Beats carrying a few more pounds of batteries-more room for ammo!

  24. Anonymous Coward

    Idea has promise but not completely new

    My team has worked on similar things back to 03-04. We disclosed a very similar idea, but it is cool to see it put into practice.

    If this thing is patented, then the patent is probably easily invalidated due to prior art. A problem is that people that work on energy harvesting come from all different fields and don't read the literature outside their own field.

    It is in fact true that the device can improve the efficiency of the knee, but the real problem is weight. Adding weight to your body will increase your load regardless and probably more so than the "braking" gain in efficiency. And yes, you'd probably want one on each knee to keep it balanced.

    Even at 5 W (which I doubt is a long-term, continuous power), it would take 10-12 hours of brisk walking to recharge a laptop. Forget about foot tapping to recharge.

  25. Sandra Greer

    Great for the absent-minded

    Just having something to recharge the cell phone would be super. I generally forget, but need it for work.

  26. John Ellin

    Perfect for captain cyborg

    This is exactly what the nutter needs to keep his sub-dermal GPS devices charged up. Can't wait to see the press release....

  27. markp
    Thumb Up

    5 watts... pretty useful

    I mean, in eco mode, my laptop is clocked at an average of about 9 watts (i have a proggie that measures battery discharge in mWh, and as a byproduct tells you the net power drain), and upto a little under 30w with every last possible gun blazing. This could be a nice topper-upper if, say, I was to travel somewhere and had it stashed in a backpack. Eight hours of walking around the city on & off, with the generated power also partially shared to phone, AA-trickle charger and mp3-cum-USBHDD could leave a nice two hours of photo backing up / uploading and the like without having to grovel to someone for use of their mains sockets or scrabble around in the murky, spider-infested depths behind the bunks in a hostel room.

    Double them up and everything could get a guaranteed brimfull charge by the end of the day, and you'd get a tad more extra exercise into the bargain (maybe only 10% or so, but it's something).

    Would it also work with cycling, to power the (white/red LED) lights without having to nob around with dynamos and stuff?

    (Yeah, ok, my laptop is a low-drain ultraportable and was bought as such, so that it could both be used in a cramped train set (i measured; a 14" would be physically unusable on the birmingham commuter line, this 12" proved to be *just* usable), and so it could potentially last a workday of approx 50% use if I forgot/didn't have carrying space for/anywhere to plug in the charger, without needing a costly and cumbersome high-capacity battery (instead of it's 48Wh supplied compact one... now dipping below 30Wh after 2 years but still good for more than two hours off-charge). But, really, wouldn't that also be the machine of choice for a foot-travelling city rover, keeping the weight on their shoulders and bulk in their backpack to a minimum?)

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