Structured variability of steady-speed locomotion in rats.

By examining key locomotor parameters during terrestrial locomotion on a substrate without irregularities, we show that rats frequently accelerate and decelerate between two consecutive steps while maintaining an overall steady speed and that the touchdown order of contralateral limbs significantly influences those speed adjustments. The latter highly correlates with significant adjustments in relative forelimb protraction at touchdown and hindlimb extension at lift-off. We conclude that this remarkable level of variability in limb coordination would clearly be advantageous for the functional flexibility needed during terrestrial locomotion on much more irregular (rough) natural terrain. In addition, its occurrence on a substrate lacking irregularities suggests that much of stable, terrestrial steady-speed locomotion in rats is mechanically controlled.