The invention in some embodiments relates to a dynamic seal for a prosthetic knee. The dynamic seal in one embodiment is utilized to seal a magnetorheological fluid comprising a liquid and solid particles within a chamber of the knee. The dynamic seal embodiments are specially configured with a pre-loaded tensioned garter spring which has a coil spacing that is at least as large as the size of the particles or maximum size of the particles in the magnetorheological fluid. Desirably, this allows the magnetorheological fluid particles to flow in and out of the dynamic seal without clogging the seal and advantageously provides for a reliable dynamic seal.