Locomotor ability in spinal rats is dependent on the amount of activity imposed on the hindlimbs during treadmill training

Studies have shown that treadmill training with body weight support is effective for enhancing locomotor recovery following a complete spinal cord transection (ST) in animals. However, there have been no studies that have investigated the extent that functional recovery in ST animals is dependent on the amount of activity imposed on the hindlimbs during training. In rats transected as neonates (P5), we used a robotic device to impose either a high or a low amount of hindlimb activity during; treadmill training starting 23 days after transection. The rats were trained 5 days per week for 4 weeks. One group (n = 13) received 1000 steps/training session and a second group (n = 13) received 100 steps/training session. During training, the robotic device imposed the maximum amount of weight that each rat could bear on the hindlimbs, and counted the number of stepping movements during each session. After 4 weeks of training, the number of steps performed during treadmill testing was not significantly different between the two groups. However, the quality of stepping in the group that received 1000 steps/training session improved over a range of levels of weight bearing on the hindlimbs and at different treadmill speeds. In contrast, little improvement in the quality of stepping was observed in the group that received only 100 steps/training session. These findings indicate that the ability of the lumbar spinal cord to adjust to load- and speed-related sensory stimuli associated with stepping is dependent on the number of repetitions of the same activity that is imposed on the spinal circuits during treadmill training.