Prosthetic Knee Systems Overview

source: Bill Dupes’s original post published on this website


Prosthetic knees have evolved greatly over time, from the simple pendulum of the 1600s to those regulated by rubber knees01bands and springs or pneumatic or hydraulic components. Now, some knee units have advanced motion control modulated through microprocessors. For the transfemoral (above-knee, including hip and knee disarticulation) amputee, successful function depends on selecting the correct knee to fit the person’s age, health, activity level and lifestyle. The latest or advanced knee is not necessarily the best choice for everyone. For some amputees, safety and stability are more important than functional performance. Active amputees, on the other hand, prefer a knee that will give them a higher level of function even if it requires greater control.

Given the wide variety of choices and consumer needs, prosthetists and rehabilitation specialists can help amputees choose the best prosthetic knees for their individual requirements. They can also teach amputees how to use their new knees properly, which is critical for avoiding discomfort, stumbling and falling. A key way to evaluate an individual’s prosthetic needs is to observe his or her walking cycle, which can be divided into two parts: the “stance phase” (when the leg is on the ground supporting the body) and the “swing phase” (when the leg is off the ground, also referred to as “extension”). The happy medium between these two extremes (stance, or stability, versus ease of swing, or flexion) is different for each individual.

Although over 100 individual knee mechanisms are commercially available, they can be divided into two major classifications: mechanical and computerized. Mechanical knees can be further separated into two groups: single-axis knees and polycentric, or multiaxis, knees. All knee units, regardless of their level of complexity, require additional mechanisms for stability (manual or weight-activated locking systems) and additional mechanisms for control of motion (constant or variable friction and “fluid” pneumatic or hydraulic control).

Single-Axis Vs. Polycentric Knees

The single-axis knee, essentially a simple hinge, is generally considered the “workhorse” of the basic knee classes due to its knees02relative simplicity, which makes it the most economical, most durable, and lightest option available. Single-axis knees do have limitations, however. By virtue of their simplicity, amputees must use their own muscle power to keep them stable when standing. To compensate for this, the single-axis knee often incorporates a constant friction control and a manual lock. The friction keeps the leg from swinging forward too quickly as it swings through to the next step.

knees03Polycentric knees, also referred to as “fourbar” knees, are more complex in design and have multiple axes of rotation. Their versatility is the primary reason for their popularity. They can be set up to be very stable during early stance phase, yet easy to bend to initiate the swing phase or to sit down. Another popular feature of the knee’s design is that the leg’s overall length shortens when a step is initiated, reducing the risk of stumbling. Polycentric knees are suitable for a wide range of amputees. Various versions are ideal for amputees who can’t walk securely with other knees, have knee disarticulation or bilateral leg amputations, or have long residual limbs. A standard polycentric knee has a simple mechanical swing control that provides an optimal single walking speed; however, many polycentric knees incorporate fluid (pneumatic or hydraulic) swing control to permit variable walking speeds. The most common limitation of the polycentric design is that the range of motion about the knee may be restricted to some degree, though usually not enough to pose a significant problem. Polycentric knees are also heavier and contain parts that may need to be serviced or replaced more often than those of other types of prosthetic knees.

Microprocessor Knees

Microprocessor knees are a relatively new development in prosthetic technology. Onboard sensors detect movement and timing and then knees04adjust a fluid /air control cylinder accordingly. These microprocessor-controlled knees lower the amount of effort amputees must use to control their timing, resulting in a more natural gait. In spite of all of the amazing inventions and constant tweaks and improvements, the perfect prosthetic knee has yet to be invented; otherwise, there wouldn’t be over 100 different designs on the market. As advanced as the technology seems today compared to the earliest designs of the 1600s, one can only imagine the developments that will eventually result as researchers further explore the potential of mechanical, hydraulic, computerized and “bionic,” or neuroprosthetic, technology.

– by the way, my CV is finally up-to-date! –

assistive devices comparison

what-are-crutch-alternativesTwo weeks ago I stumbled upon this interesting website, which provides useful information about different assistive devices. I tried to make a list of Pros & Cons of crutches, walkers and exoskeletons (as for the latter category, in very general terms) in order to compare them from a more global point of view. The result is the table below here, that you can zoom by a simple click.

Would you help me complete it? For sure it contains mistakes and inaccuracies, and further details should be added 🙂


bike, brakes, forces, moments, friction…

brutally copy-pasted from this website, to which all rights belong

What happens on bikes when we brake? To make a simple model, consider a bike that contacts the ground in two places and has a center of mass. The bike is gray, the ground is black, and the forces on the bike are red. The front is to the left; the bicycle is moving left.


As the bike goes along, the normal forces on the wheels counter the force of gravity on the center of mass. We’ve drawn the center of mass equidistant from the supports, so to make the net torque zero, the two normal forces are equal. What happens when we brake using the front wheel?


We’ve added in the horizontal braking force slowing the bike down. There is now a torque about the center of mass. This torque acts to rotate the bicycle up.
However, as long as the braking force is fairly small, we don’t actually lift the bicycle up off the ground. Instead, it will rise a very small distance. As it rises, the normal forces re-adjust to cancel the torque about the center of mass, like this:


There is no longer torque about the center of mass, so the bike no longer rotates. It has gained some very small gravitational potential energy, but too slight to notice. However, the weight on the front wheel is now much greater. Since there is more weight on that wheel, we can apply even more braking force if we want. The braking force is limited by a constant coefficient of friction times the normal force, so a bigger normal force allows a bigger braking force.

If we brake with the back wheel, the braking forces causes precisely the same torque about the center of mass. The normal force on the front wheel will still increase and the normal force on the rear wheel will still decrease. That means we can’t brake as well because we’re using the wheel with less weight on it. We can’t flip over because as the bike starts to rise (and it will, even using the back brake), the braking force gets weaker and weaker and ceases to provide enough torque to continue rotating the bike.

The condition for the bike to stay on the ground is that the torque from the braking force (about the center of mass) needs to be less than that of the maximum normal force on the front wheel. This gives:

( Fb / m )  <  ( g * d / h )

where Fb is the braking force, m the mass, g gravitational acceleration, d the horizontal distance from the front wheel to the center of mass, and h the height of the center of mass. This puts a limiting acceleration on the bicycle while braking. To be able to brake harder, get lower and further back.

welcome to Japan

Tokyo-University-of-Agriculture-and-TechnologyHere I am, in Tokyo for the next seven months. I’ll work as Project Assistant Professor at the Tokyo University of Agriculture and Technology. It is actually a postdoc, which means that I’m here to learn. I have my research project and the opportunity to give some classes to BSc students. I’ll do my best to succeed 🙂

The previous scientific experience I lived, which was also the first one after obtaining my PhD degree, was not exactly as I expected… But I’m happy about it since I learnt useful things (basically, how to program in C# and how to create a software interface in Unity 3D). I’ll update my CV accordingly as soon as possible.