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Research Papers

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

[+] Author and Article Information
Jeff A. Nessler1

Department of Kinesiology,  California State University, San Marcos, CA 92096 e-mail: jnessler@csusm.eduDepartment of Kinesiology and Health Promotion,  California State Polytechnic University, Pomona, CA 91768 e-mail: mmoustafa@csupomona.eduDepartment of Kinesiology,  California State University, San Marcos, CA 92096

Moustafa Moustafa-Bayoumi

Department of Kinesiology,  California State University, San Marcos, CA 92096 e-mail: jnessler@csusm.eduDepartment of Kinesiology and Health Promotion,  California State Polytechnic University, Pomona, CA 91768 e-mail: mmoustafa@csupomona.eduDepartment of Kinesiology,  California State University, San Marcos, CA 92096

Dalziel Soto

Department of Kinesiology,  California State University, San Marcos, CA 92096 e-mail: jnessler@csusm.edudalziel.s@hotmail.comDepartment of Kinesiology and Health Promotion,  California State Polytechnic University, Pomona, CA 91768 e-mail: mmoustafa@csupomona.edudalziel.s@hotmail.comDepartment of Kinesiology,  California State University, San Marcos, CA 92096dalziel.s@hotmail.com

Jessica Duhon

Department of Kinesiology,  California State University, San Marcos, CA 92096 e-mail: jnessler@csusm.eduduhon001@cougars.csusm.eduDepartment of Kinesiology and Health Promotion,  California State Polytechnic University, Pomona, CA 91768 e-mail: mmoustafa@csupomona.eduduhon001@cougars.csusm.eduDepartment of Kinesiology,  California State University, San Marcos, CA 92096duhon001@cougars.csusm.edu

Ryan Schmitt

Department of Kinesiology,  California State University, San Marcos, CA 92096 e-mail: jnessler@csusm.eduschmi082@cougars.csusm.eduDepartment of Kinesiology and Health Promotion,  California State Polytechnic University, Pomona, CA 91768 e-mail: mmoustafa@csupomona.eduschmi082@cougars.csusm.eduDepartment of Kinesiology,  California State University, San Marcos, CA 92096schmi082@cougars.csusm.edu

1

Corresponding author.

J Biomech Eng 133(12), 121007 (Dec 23, 2011) (12 pages) doi:10.1115/1.4005408 History: Received January 24, 2011; Revised October 27, 2011; Posted October 31, 2011; Published December 23, 2011; Online December 23, 2011

Robotic locomotor training devices have gained popularity in recent years, yet little has been reported regarding contact forces experienced by the subject performing automated locomotor training, particularly in animal models of neurological injury. The purpose of this study was to develop a means for acquiring contact forces between a robotic device and a rodent model of spinal cord injury through instrumentation of a robotic gait training device (the rat stepper) with miniature force/torque sensors. Sensors were placed at each interface between the robot arm and animal’s hindlimb and underneath the stepping surface of both hindpaws (four sensors total). Twenty four female, Sprague-Dawley rats received mid-thoracic spinal cord transections as neonates and were included in the study. Of these 24 animals, training began for 18 animals at 21 days of age and continued for four weeks at five min/day, five days/week. The remaining six animals were untrained. Animal-robot contact forces were acquired for trained animals weekly and untrained animals every two weeks while stepping in the robotic device with both 60 and 90% of their body weight supported (BWS). Animals that received training significantly increased the number of weight supported steps over the four week training period. Analysis of raw contact forces revealed significant increases in forward swing and ground reaction forces during this time, and multiple aspects of animal-robot contact forces were significantly correlated with weight bearing stepping. However, when contact forces were normalized to animal body weight, these increasing trends were no longer present. Comparison of trained and untrained animals revealed significant differences in normalized ground reaction forces (both horizontal and vertical) and normalized forward swing force. Finally, both forward swing and ground reaction forces were significantly reduced at 90% BWS when compared to the 60% condition. These results suggest that measurement of animal-robot contact forces using the instrumented rat stepper can provide a sensitive and reliable measure of hindlimb locomotor strength and control of flexor and extensor muscle activity in neurologically impaired animals. Additionally, these measures may be useful as a means to quantify training intensity or dose-related functional outcomes of automated training.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

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.

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Figure 2

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.

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Figure 3

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.

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Figure 4

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.

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Figure 5

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  2345. 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.

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Figure 6

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).

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Figure 7

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  22345. Bars represent median absolute deviation. * denotes significant differences between Trained and Control groups.

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