Having little success, despite the many ideas I tried, I decided to check out existing similar technologies to get an idea on what I could try next. I saw a military design and a research university project on assistive technologies in the form of an exoskeleton:

Looking carefully, it became obvious they both were using a small winch and steel wires to emulate muscles and tendons.

I had a few unused electrical screwdrivers in my toolbox, perfect for making a DIY winch and keeping size low. The only issue was they had low RPM, and ran on 3.7V. I planned to increase both the speed and the working voltage up to 12V. So I had to rework the motor's windings with thiner wire for more turns and make Ohm's law happy (more turns, more resistance, less current, more voltage).

The new rotor coils came out nice, with about 40 turns per coil (not enough for 12V, but good for a few tests):

Load test

So here's 4L of water (4Kg) in a bucket, connected to the geared motor using a strong nylon wire (to be replaced with steel):

The diameter of the wire roll could be increased for higher speed.

This design is probably the best way to go, but I have to build a better winch. However the design is going to be more complicated, since the rotation axis need to be parallel to ground and not perpendicular. It would need to be fixed to the knee rigid support, and it might not be very comfortable. Also