Coordination and integration in the hindleg step cycle of the rat: kinematic synergies.

The kinematics of the hindleg step cycle of the rat in the vertical domain is composed of 7 synergies. Additionally, a global spatiotemporal principle ensures that each segment of the leg is never displaced backwards. The concepts of flexion and extension are inadequate for the description of step kinematics. As a limb segment changes its orientation, it does so in relation to the next serially connected limb segment or else in relation to gravitation. We call these two aspects of kinematics 'movements' and 'displacements' respectively, and describe segment kinematics in these terms. Of the 7 kinematic synergies, 5 involve a specific invariant interplay between 'movements' and 'displacements'. Together with the two additional parts they form the skeleton around which the step is organized. The flexible and regulatory nature of the step is obtained by the superposition of biasable properties on top of this skeleton. These include the durations, amplitudes, and initial and final positions of movements-displacements. The formalization of the step cycle kinematics represents explicitly intralimb coordination and integration. It also specifies the demand made upon the muscular and neural background organization that mediates the kinematics in a language which is appropriate for neurophysiological investigation.