Stable operation of an elastic three-segment leg

Quasi-elastic operation of joints in multisegmented systems as they occur i n the legs of humans, animals, and robots requires a careful tuning of leg properties and geometry if catastrophic counteracting operation of the join ts is to be avoided. A simple three-segment model has been used to investig ate the segmental organization of the leg during repulsive tasks like human running and jumping. The effective operation of the muscles crossing the k nee and ankle joints is described in terms of rotational springs. The follo wing issues were addressed in this study: (1) how can the joint torques be controlled to result in a spring-like leg operation? (2) how can rotational stiffnesses be adjusted to leg-segment geometry? and (3) to what extend ca n unequal segment lengths and orientations be advantageous? It was found th at: (I) the three-segment leg tends to become unstable at a certain amount of bending expressed by a counterrotation of the joints; (2) homogeneous be nding requires adaptation of the rotational stiffnesses to the outer segmen t lengths; (3) nonlinear joint torque-displacement behaviour extends the ra nge of stable leg bending and may result in an almost constant leg stiffnes s; (4) biarticular structures (like human gastrocnemius muscle) and geometr ical constraints (like heel strike) support homogeneous bending in both joi nts; (5) unequal segment lengths enable homogeneous bending if asymmetric n ominal angles meet the asymmetry in leg geometry; and (6) a short foot supp orts the elastic control of almost stretched knee positions. Furthermore, g eneral leg design strategies for animals and robots are discussed with resp ect to the range of safe leg operation.