Control of limb loading during active horizontal perturbations at moderate and fast trots in rats

To understand how small animals cope with complex, unstructured, and unpredictable substrates, we analysed the kinetics of female rats ([Formula: see text]) moving at a fast and at a moderate trot over an unperturbed substrate and a substrate subjected to active horizontal perturbations. Perturbations were active single forwards or backwards displacements of an instrumented platform by 5 mm or 10 mm amplitudes in 0.05 s. Single leg ground reaction forces (SLGRF) were collected for unperturbed and perturbed locomotion (hindlimbs: 50/102, forelimbs: 45/130, respectively). When negotiating horizontal perturbations, rats displayed gait resetting (braking, accelerating) and non-resetting behaviours. Feedforward strategies differed between the fore- and hindlimbs. In circa 60% of the perturbed trials, forelimbs started the step in acceleration mode, while hindlimbs began the stance mostly in non-resetting mode (∼45%). In about 50% of all perturbed steps, the impulse provided by the SLGRF displayed a change in behaviour according to the expected response to the perturbation. The remaining 50% retained the feedforward strategy. Still, most perturbed trials displayed changes in SLGRF patterns that indicated passive and active reactions to platform shifts. Our results indicate that rats' sensorimotor control system tunes fore- and hindlimbs differently in expectation of a perturbation. In addition, the tendon-muscle systems of the limbs are recruited to prevent leg collapse at the beginning and end of the stance. At lower speeds, spinal and/or higher centre commands have enough time to re-adapt limb behaviour. At higher locomotion speeds, rats rely more on their limbs' intrinsic stability and feedforward control.