Out on a Limb
Outcome
Sketch of prosthetic foot
(click to enlarge)
e.nz magazine, March/April 2003, p. 6 (click to enlarge)
Following the paralympics Mark Inglis decided the time had come to tidy up some unfinished business and return to Mount Cook. However unnatural, cycling is an extremely energy-efficient way of getting around; climbing, on the other hand, is sheer hard work – even more so for a person who must continuously fight for balance and who must bodily lift and throw his or her leg forward to take a step. (It is estimated that a person with an above-the-knee prosthesis uses about 80 percent more energy than an able-bodied person just to walk on a flat surface.) Something special in the way of prosthetics was clearly required.
Lightness was critical. Carbon fibre is light and immensely strong and has good energy-storing properties, but it becomes brittle at low temperatures. For this reason Wayne opted for a mixture of spring steel and titanium for the working components of what became known as the alpeds. A single carbon-fibre spar connected the foot to the socket.
To allow Mark to ascend and descend the steep route, it was important to find some way of placing his knees over his feet or behind them, depending on whether he was going uphill or down. It's a subtle adjustment, which normally isn't noticed, but a critical one.
COP Outcome development and evaluation
After some trials and breakages ("If it doesn't break during design then it's over-engineered") an 8mm spring was used to capture energy on heelstrike.
The energy stored was released as the front of the foot left the ground and effectively "kicked" the prosthesis through itself, saving Mark energy. During slope trials Inglis found the action so active that "they almost levitated me up the hill, with the spring off the toe almost throwing my feet up." The heel had a soft action and narrow profile, which gave great stability and cushioning on downhill sections, always the hardest on the stumps.
At 1pm on 7 January 2002, in near-perfect conditions, Mark Inglis reached the summit of Aoraki/Mt Cook. Wayne Alexander accompanied him on the climb.
Post script: Later this year Mark Inglis hopes to climb the 8000m Mount Shishapangma in Tibet, and the following year Mount Everest.
Wayne Alexander is busy reinventing the Solex and is negotiating with American interests over something he believes will make jetskis passé.
