A Crafty Bugger

When Schooner Mark II arrives (due sometime in the next week or so), then I not only have the challenge of learning to walk on the new leg (which shouldn't take too long) but of also rigging up and adjusting a new cycling leg.

As mentioned before, the new prosthetic leg has been designed to enable a fluid pedalling motion by allowing sufficient freedom of movement around the knee joint (especially behind the knee). It is that very freedom which means I will take a little time to re-adapt to walking on it - my knees, hips and all the associated muscles, tendons etc need to have the ability to cope with the new level of freedom. That will mostly be sorted out simply through using it and maybe a few supplemental exercises.

The prosthetic is also designed so that my walking foot extension can be readily removed from the leg socket and a cycling leg extension attached. Undo/redo a couple of allen bolts to drop one out and put the other in.

Getting it right requires a few planets to align, with one of them just being put into the right orbit today.



At the moment the problem is being tackled from each end of the prosthetic - firstly the end where the remainder of my left leg attaches (fits into) to the prosthetic socket. That's been the job of my prosthetics specialist, George. I'm going to write more about that when I get the new leg.

The other end is the interface with the bicycle pedal. I still want to be able to use the standard clipless pedal system for all the normal advantages that provides (for the non-cyclists amongst you - that's a pedal system whereby a cleat on the bottom of your cycling shoe enables you to "click" into a pedal, which then holds your foot securely onto the pedal, similar to how a ski boot is secured onto a ski).

I also did some research which indicated that riders using prosthetic legs have had two main approaches to using pedals. One has been to use their normal prothetic foot (or a proxy prosthetic foot) and wear a cycling shoe on that. This has some inherent problems, as there no longer exists much of the lower leg required to stablise the foot section and the lever it creates puts non-radial forces through your leg to socket interface. The other approach is simply to place a cleat directly under the end of the pipe section that extends down from the leg socket.

This latter arrangement is effectively like pedalling with a cleat attached under your heel. That means that forces are applied in a line that goes directly down through your prosthetic socket, straight down the pipe section and into the pedal. This is radial alignment is also the strongest arrangement, and since I have no foot on that leg, there are no toes to overlap the front wheel (which is why you could not position a cleat as such with a normal foot).

But hang on - isn't that going to do some weird things with my pedalling action and balance considering my other foot will use the standard shoe/cleat and pedal position?

Well of course it will, but not as bad as one might at first think. The one advantage of a prosthetic leg is that I can make it as long or short as I want. It can be made to extend out at any angle I desire. The cleat can be moved fore/aft, side to side and rotated in its own plane, so there are significant types of adjustment should it be required. Indeed there are also shims that would enable the cleat to be slightly rotated perpendicular to its plane if that were necessary.

OK, so how about shifting that planet's orbit to make it happen?

In steps my good buddy Peter Barnard and all his crafty machining and cycling nouse on how to design and manufacture a solution for me.

The challenge was to fashion an adapter so that a cycling cleat with a standard 3-hole fit like this slightly worn Campag cleat:



could be attached to a standard attachment block used for prosthetic legs:



Here are some pics of the solution. This is the adapter plate Pete fashioned up:




Note the overall shape and how it is designed to be flat on one side for the prosthetic attachment block and curved on the other to match the curve of the cycling cleat. Also the attachment of the prosthetic block requires four screws in a square arrangement and the cycling cleat requires three screws in a triangular arrangement.

The way the design is made is so that two of the prosthetic block screw holes were able to be lined up with two of the cleat attachment holes (which was simply fortuitous), so that enables all three to be securely fitted together with two nuts and bolts and reduces the overall number of screws needed by two.

So, attaching the prosthetic block on one side:



and attaching the cleat to the other side:



and it all looks like this when together. From side on:



and head on:



So that's either end of the planetary orbits being sorted out. Now I need to work on some of the middle planets. There are quite a few, not least of which is the limited flexion of my knee joint at the moment!