Engineering
The Biomechanics of a Prosthetic Knee Joint
Quick fact
The most advanced prosthetic knees use microprocessors to adjust hydraulic resistance 50–100 times per second, allowing the knee to automatically adapt to walking speed and terrain, something even the best mechanical knees cannot do.
Why this is interesting
A modern prosthetic knee can let an amputee run, climb stairs, and even dance — but it isn't simply a joint on a stick. How does a piece of metal and plastic replicate the complex, dynamic behavior of your own knee?
Read the full explanation
Understanding The Biomechanics of a Prosthetic Knee Joint
Think of your natural knee as a sophisticated hinge that must be both stable and agile. During the stance phase (when your foot is on the ground), the knee must lock or resist bending to support your weight. During the swing phase (when your foot is in the air), it must bend to allow your foot to clear the ground and then extend to prepare for the next step. A prosthetic knee must perform these same functions, but it has no muscles or ligaments to coordinate the movement. Instead, it relies on mechanical or electronic systems. A basic prosthetic knee consists of a hinge mechanism that provides stability and a damping system that controls the speed of bending (flexion) and straightening (extension). In a simple mechanical knee, friction pads or a hydraulic cylinder provide resistance. The hydraulic cylinder contains fluid that must flow through a small opening when the knee bends; by changing the size of that opening, the resistance to bending is increased or decreased. This damping mimics the function of the hamstring muscles that control knee flexion during swing. Some knees have a stance-phase lock that engages when weight is applied, preventing the knee from folding during the stance phase. Others use a polycentric design with multiple axes of rotation, which provides a variable center of rotation that can improve stability and more closely mimic the natural knee's changing geometry. So, the art of prosthetic knee design is balancing stability (preventing unintentional buckling) with mobility (allowing smooth, natural swing). The user must learn to use their hip muscles and body movements to initiate walking, and the knee's mechanical response completes the cycle.
A deeper explanation
The biomechanics of a prosthetic knee are grounded in the mechanics of the gait cycle. During walking, the knee experiences a range of forces and motions. At heel strike, the knee is almost fully extended to accept weight. The ground reaction force passes behind the knee joint, generating a flexion moment that would collapse the knee if not resisted. In the natural knee, this is countered by active contraction of the quadriceps and the tension in the posterior capsule. In a prosthetic knee, this is achieved through a stance-phase stability mechanism. Mechanical knees often use a weight-activated locking mechanism: a brake that locks the knee when weight is applied. In contrast, hydraulic knees use a fluid-based system where resistance is proportional to the velocity of knee flexion. A small orifice in the hydraulic cylinder restricts the flow of fluid, creating a damping force. By using an adjustable valve, the stiffness of the knee can be tuned. This damping is crucial during swing phase: when the foot lifts off, the knee must flex quickly to shorten the leg and allow ground clearance, but not too quickly to cause the heel to kick up excessively. As the leg swings forward, the knee must extend to prepare for heel strike. A controlled extension is necessary to avoid an abrupt stop, which is achieved by increasing resistance at the end of extension—a concept called terminal impact damping. Advanced microprocessor knees replace or augment mechanical damping with an electronically controlled valve. Sensors measure joint angle and load, and a microcontroller adjusts the valve in real time, allowing the knee to distinguish between level walking, stair descent, and other tasks. For example, during stair descent, the knee needs to allow a controlled flexion while the user lowers their weight. The microprocessor detects the load pattern and opens the valve to absorb energy, preventing a free fall. Thus, the core biomechanical principle is the need to manage energy: absorb energy during weight acceptance and control the pendulum-like swing of the shank. The knee's damping system dissipates energy as heat, mimicking the eccentric contractions of the natural hamstrings. This is why prosthetic walking requires more energy than normal walking—the prosthetic cannot actively contribute energy; it can only dissipate and store (in some energy-return feet) but not generate. The design of the knee joint is a delicate balance between stability, damping, and alignment, and even the slightest misalignment can lead to inefficient gait and joint pain.