The global design of the hindlimb in quadrupeds

In the hindlimb of quadrupeds three major segments can be distinguished which can rotate with respect to each other in the hip, knee and ankle joints. Movements in a sagittal plane appear to be controlled by three sets of antagonistic mono-articular muscle groups (one set per joint) and two sets of antagonistic bi-articular muscles. This design allows co-activations of mono-articular agonists and their bi-articular antagonists. These co-contractions occur for example in ballistic tasks such as jumping where the transfer of angular acceleration in the joints into the translational acceleration of the hip decreases as a function of joint extension. Well-trained humans appear to solve this problem by a distinct proximodistal sequence in the timing of the joint extensions and a transport of energy via bi-articular muscles. This can also be observed in a number of animals. A second example has to do with the necessity for a certain distribution of net moments in the joints which is required to control the direction and magnitude of the external force on the ground. The control of these moments can to a large extent be judged as a requirement which is independent of the joint displacements required in the same task. Many walking, running or pulling tasks, for example, require knee extension combined with a net knee flexing moment. Co-activation of mono-articular agonists and their bi-articular antagonists appear to solve these problems in an effective and efficient way.