Physics Involved with Prosthetic Legs

During class, we spent a lot of time learning forces and vectors. As we were learning all this, I never really considered how the scope of applicability for force and vectors was way more than just using it to calculate the forces of a simple object on a pulley or ramp.

In reality, those simple objects can also be translated to actual humans. Recently, I went to The Virtual Reality and Motion Analysis Rehabilitation Laboratory where studies are done with veterans. One particular study was involving a veteran who had lost his leg. The study strived for finding an efficient way to better a prosthetic. During the process, there were a bunch of motion capture items placed on his body. There are certain cameras that use infrared to track the motion capture as seen in this picture.

The image below shows the simulation shown through a software called Qualisys. The colored dots are the motion capture items on the person and mimics what the camera is capturing. The dots are then identified and connected to form a rough skeleton of the body.

The veteran was asked to walk on a force place and everything he was doing in real life was playing in this simulation. In the image above, you can notice the red arrows pointing upwards. Those arrows are the force vectors. As the veteran kept on walking and moving, the force vectors were changing direction and length which signified the direction and magnitude of a force. For example, when he was leaning more on his right foot for stabilization, the force vector coming upwards from the right foot grew in length due to a higher magnitude. That vector would be the normal force. In addition to forces being studied, torque is also a major concept in learning more about prosthetics. For example, if a force does not act directly on the center of mass of an object it will rotate the object. If a force does not act through the middle of a joint with another segment it will change the angle of the joint and that can be determined by the data collected from the software and inertial sensors. All this data is used for better understanding what a person is doing and provides for a better diagnosis. Thus, I didn’t realize until then how much physics was involved in many fields involving prosthetics.

In summary:

When building a prosthetic leg, the following forces need to be examined-

Force of Gravity - force being applied to the prosthetic leg by the earth.

Normal Force - force being applied to the prosthetic leg by the ground that the leg is walking on

Force of Friction - force being applied to the prosthetic leg by the surface the leg is walking on as well