This system functions as a constant-friction knee during leg swing but is held in extension by a braking mechanism as weight is applied during stance phase. This knee is a common choice for older and relatively inactive patients. Typically provided by a spring, this enhancement releases energy stored during knee flexion to help the amputee propel the knee to full extension in late swing phase. Extension assist both prepares the prosthesis for heel strike and weight-bearing and helps reduce energy expenditure and fatigue during ambulation. Advanced swing control enhancements for various prosthetic knee designs utilize fluid dynamics to provide variable resistance to leg swing velocity and thus enable the amputee to vary walking speed.
Pneumatic control is achieved by a cylinder housed in the shank.
With knee flexion, a piston rod attached to the thigh portion of the prosthesis is pushed into the cylinder, forcing air through an adjustable port. The smaller the port opening, the greater the degree of swing control. Because air is compressible, it provides extension assist as flexion begins to decrease. A spring coil assist can also be added. Pneumatic systems are generally considered superior to friction knees but less-effective than hydraulic components.