Wattage Wasted While Walking

10 February 2008

Interview with 

Professor Max Donelan, Simon Fraser University


The biomechanical energy harvester comprises an aluminium chassis and generator (cylindrical component at the top of the chassis with green plastic attached) mounted on a customized orthopaedic knee brace (black), totalling 1.6 kg mass.


Chris - We're now going to be joined by Professor Max Donelan.  He's a researcher at Simon Fraser University in Canada and he's found a way to turn people into the human equivalent of a Hybrid car.  Max, tell us a little bit more.  How does this work?

The Biomechanical Energy HarvesterMax - What we've done is take advantage of the inherent uneconomical nature of walking.  So, as you mentioned, we can take advantage of walking like a hybrid car takes advantage of stop and go driving.  That is, within the walking stride the muscles are first accelerating and then decelerating the body.  A hybrid car takes advantage of the decelerating, braking by using a generator to brake rather than a traditional brake which just dissipates as heat.  The generator produces electricity which we can then use in a productive way.  We've done a similar thing for walking where we've tried to use generators help the muscles slow the legs down.  In doing so the generator can assist the muscles and it also creates electricity at the same time.

Chris - It sounds a bit counter-intuitive to think that your legs would be so inefficient, that they need help in slowing down and that you can actually get useful energy out of this.

Max - Yes, I think it is a little counter intuitive but the main point: if you think about walking at a constant speed on the level then there's no net change in your mechanical energy so on average you're not moving any faster or any slower.  You're not going uphill and downhill so that means for every little bit of energy that your muscles put in, something also has to take it away.  It's mostly your muscles that take it away.  You don't need them to do it, you could use something like a generator instead.  The generator can take the mechanical energy away from the body and put it through the generator and produce electricity at the same time.

Chris - So in other words, as your leg's swinging to return to the starting position so you can take the next step you would normally need a muscle to kick in and stop the leg from moving at that point.  Your generator exploits that effect and instead of the muscle doing that work you're diverting that effort through your generator and it recovers the energy the body would otherwise have to expend.

Max - That's exactly right. So where we do it is at the knee and towards the end of the swing phase - so when you're moving your foot forwards to begin another step.  Your hamstring muscles, which are the muscles down the back of your leg, they turn on to slow down your knee extension.  Our device mounts at the knee that generator engages at that period and helps those hamstring muscles in slowing down the extension of the knee.  We can measure this through the fact that when we use the generator it actually allows those muscles to turn down by about 15%.  The muscle activity decreases.

Chris - The difference in the energy expenditure is what you're generating with your device.

Max - The idea is that people don't have a difference in energy expenditure, so generating electricity or not generating it is about the same effort you put in, but you still get the electricity out. So you're getting 5W of electricity without increasing their effort by a meaningful amount.

Chris - So what does this device look like when it's in situ on a person? Is it very ungainly, is it going to get in the way?

Max - In the current version I would say it is a bit ungainly so it looks like a normal orthopaedic knee brace with a chassis mounted to the side of it.  It's a bit bulky and a bit big but that's because it's really designed for convenient experimentation so you can pull gears out and generators out.  The final version will be less than a kilogram and it will easily sit underneath a pair of pants.  Currently too big but no problem getting it smaller.

Chris - When someone's wearing this thing how much energy physically are they able to produce with it?

Max - There's two modes. One mode gives 5W of electric and for context 5W is enough to charge ten mobile phones at the same time or to produce ten minutes of call time for one minute of walking.  That's not to say I think we're going to charge mobile phones in the near future but it gives you some context of how much power.

Chris - I'm not being flippant, Max, if I want to charge my mobile phone I plug it in.  So why would I want this?

Max - I think you're absolutely right.  For us plugging in a mobile phone is just a matter of convenience, but for many people their lives depend upon resources for power.  Consider for example if you had a powered prosthetic leg or a power device that helped you walk every stroke for you it's not always as simple as plugging it in.  For example, some of these prosthetic legs might run out of power within four hours or so.  If you can use your healthy leg to generate some electricity to power your artificial leg then you can walk farther.

Dave - I was wondering whether you could put it into a mode so that when you're walking downhill or down a big mountain you can make it absorb lots and lots of energy. I find that very, very uncomfortable when walking out in mountains.

Max - That's a great question so, you've hit on it exactly.  Walking downhill, there's a lot more energy available because essentially what your muscles are doing is making sure that you don't arrive at the bottom at the speed that gravity wants you to.  They're taking the potential energy you had at the top of the hill and your muscles are dissipating it all as heat so you walk down it in a controlled way.  If you can use a generator to do that instead you can produce lots and lots of electricity.  We find that if you walk down a hill you produce more electricity or if you walk faster.

Chris - Thank you very much, Max.Max Donnelan watching the Biomechanical Energy Harvester at work


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