The previous attempt using servos had the advantage of being a direct mechano-electrical converter, meaning the servos would translate the battery energy directly to the needed mechanical linear movement. It would have still been impractical even if it worked, because of the many servos needed. Not exactly a good addon to fit on the lower legs.

This new idea of using an electromagnet promised a linear design, very similar to a piston. The shaft, made of iron, would be moving up towards the knee rigid support using an electromagnet.

I based this idea on my previous experience with coilguns. I knew that higher voltage would allow for a longer movement interval, while higher current would increase the kgf applied to the shaft.

One interesting idea to compensate for the inverse square law of the electromagnetic field was to use multi-stage coils, exactly like in a multistage coilgun design. 

Unfortunately the generated force was in the order of grams-force. Going up to 10kgf with a 10cm displacement was just a nice dream, totally unsuited for this design.

Something that might have worked was a combination of an electromagnet and a lever, to translate the strong electromagnetic field right next to the coil into a longer movement interval. Not sure about this, but I'm not very optimistic about it.