On technomorphic modelling and classification of biological joints

Summary Biological motion systems are of particular interest to engineers in robotics, prosthetics and micromechanics. Since biological motion systems show a high degree of mobility, smooth movements and minimal deployment of material, the analysis of such systems might help to invent or optimize technical motion systems. To enable the transfer of explanatory techniques, biomechanics and engineering need a shared terminology. Generally, a reference limb, muscles, tendons, a joint and a driven limb are forming two closed mechanisms with different transfer functions. The direction of the forces applied to links is influenced by guiding structures. Movable connections can be constructed by form closure, force closure, and compliance of an anisotropic segment between two rigid segments. Eleven different basic variants of rigid-body joint structures can be classified by the possible relative translatory and rotatory movements in orthogonal co-ordinates. If classified by the form of their rigid parts, biological rigid-body joints (diarthroses) show some similarities to technical joints, but occur in fewer basic variants. The functioning of exoskeletal joints can involve hydrostatic forces, depending on the structure of the rigid elements and the type of linkage between them. Structural classification of biological joints should include at least: 1) degree of freedom (d.o.f.), 2) possible relative movements, 3) variability of d.o.f., 4) restrictions of range of motion, 5) stress in reaction to external forces. From an engineer’s viewpoint, additional features are necessary for structural characterization, including the maintenance of closure and guidance, the geometry and behaviour of contact, geometrical parameters and material properties.