Gait activity depends on limb extension and phasing in spinal cord contused rodents: implications for robotic gait training and assessment

The purpose of this study was to investigate the locomotor activity of spinal cord contused rats in response to robot-assisted extension of their hindlimbs. Nineteen rats received a contusion injury to the mid-thoracic spinal cord. We used a robotic gait-training device ("the rat stepper") and a robotic paw platform (the "slide") to pull the animals\´ hindlimbs into extension. The injured rats initiated swing with a significantly greater probability (p < 0.001) when the rat stepper pulled their hindlimbs into extension on a conventional treadmill (75+/-16.9%) as compared to the treadmill pulling their hindlimbs into extension with no robotic assistance (38.9+/-16.6%). Furthermore, using the rat stepper to extend one hindlimb and hold the other in stance while on the slide resulted in more unilateral stepping, rather than bilateral hopping activity, when compared to the extension of both hindlimbs simultaneously (81 ±0.24% vs. 43±0.34%, p < 0.001). Continuous training of one animal from each injury group with robot-assisted extension and appropriate interlimb phasing using the rat stepper and slide yielded substantially more steps in a two-minute training period when compared to training with a conventional treadmill (84 steps on average vs 12 steps on average). These results indicate that a greater amount of alternating stepping activity can be elicited by appropriately-phased, robot-assisted extension of an animal\´s hindlimb, thereby providing possible benefits to evaluation and training of gait following SCI.