Case Study: Dynamic Analysis of an ARGO User

Brett Johnson, BE^1,2
Stefania Fatone, PhD, BPO(Hons)²

¹Biomedical Engineering Department, McCormick School of Engineering, Northwestern University - Evanston, Illinois

²Northwestern University Prosthetics Research Laboratory & Rehabilitation Engineering Research Program Department of Physical Medicine & Rehabilitation, Feinberg School of Medicine, Northwestern University - Chicago, Illinois

INTRODUCTION

The reciprocating gait orthosis (RGO) was greeted with much fanfare when it was introduced in the 1980s. It was hoped that this orthosis was the long-sought-after solution to enabling people with lower-limb paralysis to functionally walk. Unfortunately, these expectations were never fully realized despite many RGO designs and variants. While using RGOs, people with lower-limb paralysis walk seven times slower than able-bodied individuals while consuming seven times as much oxygen.^1,2 This form of ambulation is far from functional. In addition, RGOs have a high abandonment rate with some studies reporting rates as high as 70 percent.³ Adults who continue using their RGOs mostly use them for a limited amount of time a few sessions a week.⁴ In these cases, the RGO is more of an exercise device than a tool for daily living. Many researchers have attributed the lack of use to the RGOs high energy cost and have performed numerous studies measuring the oxygen consumption of RGO users.^1,5-9

Considerably fewer studies have studied the dynamics of RGO gait. The purpose of this case study was to quantify the dynamics of an RGO user's gait to identify possible mechanisms contributing to the high consumption of energy during ambulation.

METHODS

Figure 1: Reflective marker
set placed on the subject while
wearing his ARGO. The markers are
used to track the positions
of body segments during walking.

A 21-year-old male (6 ft., 2 in., 152 lb.) with a T11 traumatic spinal cord lesion was recruited to participate in this case study. Northwestern University's Institutional Review Board approved all procedures and informed consent was obtained from the participant prior to the experiment. Manual muscle testing indicated that the subject had grade zero strength bilaterally at the hips, knees, and ankles. The subject used an Advanced Reciprocating Gait Orthosis (ARGO) in conjunction with a walker. Reflective markers were placed on the subject's body segments and equivalent locations on the ARGO according to the Helen Hayes Marker set (Figure 1).¹⁰ Data collection was conducted at the VA Chicago Motion Analysis Research Laboratory. An eight-camera motion capture system (Eagle Digital RealTime, Motion Analysis Corporation, Santa Rosa, California) was used to record kinematic data, while six force platforms (Advanced Mechanical Technology Incorporated, Watertown, Massachusetts) embedded in a 10m walkway were used to measure the ground reaction forces acting on the subject's feet and walker. We collected and averaged five walking trials. The marker data were used to calculate joint angles and to estimate the trajectories of the center of mass of the subject's body segments. These trajectories were used in conjunction with the force plate data to calculate the forces and moments acting on the hip and shoulder, as well as the potential and kinetic energy of the body center of mass.

RESULTS

During stance phase, we observed that the vertical ground reaction force acting on the subject's foot decreased from an average of 70 percent to an average of 21 percent of the subject's body weight (Figure 2). We also observed exaggerated trunk flexion throughout the entire gait cycle (Figure 3). Our data analysis revealed that the joint forces acting at the hip during stance phase encouraged trunk flexion, while the forces acting at the shoulders encouraged trunk extension (Figure 4). The change in potential energy was found to be nearly ten times greater than the change in kinetic energy (Figure 5).


Figure 2: Normalized vertical ground reaction forces acting on the subject's foot during a particular gait cycle. The stance phase portion of the gait cycle is highlighted in gray.	Figure 3: Average trunk angle with respect to the vertical during a gait cycle. The stance phase portion of the gait cycle is highlighted in gray.

Figure 4: Average torques acting on the trunk, which were created by the joint forces at the hip and shoulders, during stance.	Figure 5: Potential and kinetic energy of the body center of mass. The potential energy is plotted on the left axis, while the kinetic energy is plotted on the right.

DISCUSSION

Although the subject was wearing a device that was, in part, designed to stabilize his leg so that he could bear weight on it, there are portions of the stance phase where he loads only 21 percent of his body weight through the leg. This may be explained by the motion of the trunk. Even though the subject's trunk is predominantly flexed throughout the gait cycle, he does extend it a little during stance, presumably to advance the swing leg. Bearing weight through the leg actually would oppose this extension because of the trunk flexion torque created by the hip joint force. Therefore, in order to extend his trunk during stance, the subject may be unloading his stance leg to reduce this flexion moment caused at the hips and loading his arms to increase the extension moment. This increased loading of the arms may help explain the high energy cost of ambulating with RGOs. These data also suggest that a walker placed anterior to the body may not encourage optimal use of an RGO.

Analysis of the changes in potential and kinetic energies of the subject over a gait cycle indicated that the subject was unable to conserve mechanical energy. Potential energy is a measure of how much work gravity can perform on a mass. If a mass is allowed to fall, gravity performs work on it by accelerating it. The farther the mass falls, the faster it goes; therefore, potential energy is a function of height. Kinetic energy is associated with a mass's speed. The faster a mass is traveling, the more kinetic energy it has. So when a mass falls, it loses potential energy as it loses height, but gains kinetic energy as it speeds up. When the loss in potential energy is equal to the gain in kinetic energy or vice versa, the sum of the potential and kinetic energies remains constant and the total energy is said to be conserved. Our data showed that the subject's total energy was poorly conserved: The change in potential energy was almost ten times larger than the change in kinetic energy. It appears that a large portion of potential energy that could have been used to increase the subject's speed was wasted. This energy may have been lost through friction, impacts with the ground, or maybe even muscle activity that opposes forward progression. Such inefficient use of potential energy also may help explain the high energy expenditure of ambulating with RGOs.

REFERENCES

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Acknowledgments

This work was supported by the National Institute on Disability and Rehabilitation Research of the United States Department of Education under grant H133E030030. The opinions in this publication are those of the grantee and do not necessarily reflect those of the Department of Education. The authors acknowledge the use of the VA Chicago Motion Analysis Research Laboratory of the Jesse Brown VA Medical Center, Chicago, Illinois.

Address correspondence to: BJ Johnson, NUPRL & RERP, 345 East Superior Street, RIC 1441; Chicago, IL 60611; Phone: 312.238.6500; Fax: 312.238.6510; E-mail: