In this thesis the mechanisms and advantages of spring-like leg operation were investigated. By examining the long jump the general dynamic was described using a hierarchy of simple models taking salient mechanical and muscle-physiological properties into account. 1. Global system properties and the time course of the ground reaction force. The shape of the ground reaction force in the long jump is characterised by two clearly separated peaks (Seyfarth et al., 1999). The first passive peak takes about 30 - 40 ms. A comparison of models including distal masses (chapter II and V) and taking muscle properties (stretch enhancement etc., chapter IV and V) into account revealed that this peak is largely generated by deceleration of distal leg masses (soft and bony tissues) during heel strike. Contributions of muscle forces were only minor. The lumped parameters belonging to the distal mass are the result of an adequate description of the time course of the ground reaction force. Nonlinear visco-elastic coupling of distal masses to the skeleton proved to be necessary and represented passive muscle properties, the properties of the heel pad and the deformation of the foot and joints. The active peak (30 - 90 % of contact time) is characterised by a surprisingly constant leg stiffness with variations of merely 7%. Constant leg stiffness is achieved by synchronous bending of ankle and knee joint. At the joint level, during leg shortening an increase in force of the muscle-tendon complex and during lengthening a decrease is required. 2. Contributions of muscle properties to the leg operation. The continuous increase in muscle force can be attributed to an increase of activation level, the increased force due to muscle lengthening (force-length dependency) and the consequent continuos loading of the tendon and aponeurosis in series. During unloading decrease in ground reaction force was achieved by reducing muscle force due to increasing shortening velocity (force-velocity relationship) and muscle shortening (force-length dependency). Thereby, the shortening serial elastic element prolonged the phase of eccentric muscle operation and allowed the highest muscle forces to occur at about midstance. Performance depends on the ability of eccentric force generation. The elastic behaviour of the system is a result of fast loading of the muscle-tendon complex and is largely limited by muscle properties (force-length and force-velocity curve). It does not require a sophisticated neural program. In the case of the four-segment model elastic behaviour originated from internal properties and emerged during muscle activation optimised for maximum jumping distances. 3. Jumping performance and techniques. Taking internal system properties into account a quasi-elastic operation is the optimal strategy for long jumping distances. The elastic behaviour is achieved by synchronous loading of knee and ankle joint (chapter V). To achieve optimal jumping distance at given run-up speed a minimal leg stiffness had to be exceeded. Similar results can be obtained by compensating a lower stiffness with a smaller angle of attack (chapter II). The observed strategies (angle of attack and of take-off; Friedrichs et al., in prep.) can only be understood by considering the included muscle properties (chapter IV and V). For such a system the optimal angle of attack is independent of running speed. 4. Adjustment and stability of a dynamically loaded three segmented leg. Adding a third leg segment (like a foot) to a leg consisting of shank and thigh reduces the torque requirements at joint level and the kinetic energy associated with transverse leg segment movements. Simultaneously, it imposes the problems of kinematic redundancy, potential instability and muscular coordination. Optimised leg operation with respect to jumping performance (chapter V), leg stiffness or stability requires a homogeneous bending of both leg joints achieved by rotational stiffnesses adapted to the outer segment lengths (foot and thigh length; chapter III). Nonlinear otational stiffness behaviour and biarticular structures are alternative (replaceable) strategies to fulfill a safe leg operation for a wide range of initial joint configurations. A short foot with the option of heel contact is a powerful construction to control almost stretched knee positions if elastic joint behaviour is present. By using more flexed ankle joints and an adapted stiffness design the range of safe leg flexion can be extended.