Assessment of Hindlimb Locomotor Strength in Spinal Cord Transected Rats through Animal-Robot Contact Force

Spinal cord injured animal stepping in the instrumented robotic device. Load cells were placed at the interface between the robotic linkage and the animal’s hindlimb, as well as beneath the sliding support platforms. Animals stepped bipedally in the device, and had approximately 60–90% of their body weight supported for the duration of the study.

Sample data collected from a trained female rat (mass = 156 g, age = 50 days) at week 4 for the right limb only. One trial consisted of 60 s of automated step training, while the robotic device recorded ankle trajectory and interaction forces. For this trial, BWS was set as low as possible while still facilitating true steps by the animal (approximately 60% BWS). Sagittal plane ankle trajectory for the entire trial is shown in the upper left. The remaining figures only depict measurements for actual steps taken during the trial. Gray lines indicate each individual step, black lines indicate the mean of all steps.

Diagram of robotic sliding platforms (replacing treadmill belt) and hindlimb manipulator robotic linkages. Hindlimb manipulators were used to successively pull the animal’s hindlimb into extension to elicit swing, while the sliding platforms remained directly beneath the animal’s hind paw at all times.

Ground reaction and forward swing force data for animals in the Trained group while stepping at both 60% and 90% BWS across the four weeks analyzed. All forces are normalized to animal body weight – raw forces are listed in Tables  2, 3, 4, 5. Both mean and peak values for forces are presented. Bars represent median absolute deviation. ρ denotes significant decrease across four weeks for 60% BWS peak. α; denotes significant decrease across four weeks for 60% BWS peak and mean. * denotes significant decrease across four weeks for 90% peak.

Definition of forces measured using the robotic device. Only sagittal plane forces were analyzed. FSwingX : Horizontal Swing Force, FSwingY : Vertical Swing Force, GRFX : Horizontal Ground Reaction Force, GRFY : Vertical Ground Reaction Force.

Data for a representative animal illustrating the result that sagittal plane projection angle of ground reaction force during push-off became more vertical with recovery. Red: all steps, black: mean. See Fig. 3 for definition of GRF projection angle (i.e. θ Link).

Differences in ground reaction and forward swing force data between the Trained (black bars) and Control (white bars) groups while stepping at both 60% and 90% BWS across the four weeks analyzed. All forces are normalized to animal body weight and only peak forces are presented here. Raw forces are listed in Tables  2, 2, 3, 4, 5. Bars represent median absolute deviation. * denotes significant differences between Trained and Control groups.