Northwestern University Feinberg School of Medicine

Northwestern University Prosthetics-Orthotics Center

Modeling the Effects of Hip Joint Stiffness on RGO-Assisted Gait

Principal Investigator: Steven Gard, PhD

Student Investigator: William Brett Johnson, PhD

Co-Investigator: Stefania Fatone, PhD

Funded by: National Institute on Disability Research and Rehabilitation (NIDRR)

Status: Completed


The Reciprocating Gait Orthosis (RGO) is an assistive device for persons with lower limb paralysis (LLP) that enables them to walk upright with crutches or a walker. However, walking with RGOs is slow and exhausting, which limits their use. Improving the efficiency of RGO-assisted gait could encourage persons with LLP to use their RGOs more often. The results of several studies indirectly suggest that increasing the hip joint stiffness of RGOs may improve their efficiency; however, further research is needed. One of the challenges of RGO research is subject recruitment because the population of RGO users is small and diverse, and recruiting a large, homogenous sample population requires time and resources. A model of RGO-assisted gait would allow for the initial testing and development of hypotheses without investing resources into subject recruitment. So, we designed a Lower Limb Paralysis Simulator (LLPS) that altered the gait of able-bodied persons such that it could model RGO-assisted gait. The goal of this project was to evaluate the LLPS's efficacy in modeling RGO-assisted gait while simultaneously investigating how changes in hip joint stiffness in different planes can improve RGO-assisted gait efficiency. We hypothesized that increasing hip joint stiffness in either plane would decrease arm loading. We also hypothesized that there was an optimal stiffness in the sagittal plane where oxygen cost would be minimized.


Five able-bodied persons were trained to ambulate with the LLPS, and a motion capture system and force plates were used to quantify their gait dynamics as they walked with the LLPS. The resulting data were compared to preexisting gait data from RGO users. Then, the subjects walked with the LLPS under eight conditions where the LLPS's hip joint stiffness was varied in the coronal and sagittal planes, and a motion capture system, force plates, and spirometer were used to measure their gait dynamics and energy expenditure. To verify the results of the LLPS model, two RGO users walked under three conditions where their hip joint stiffness was varied in the sagittal plane. An inclinometer, goniometer, spirometer, and instrumented crutches were used to quantify their gait and energy expenditure.

ResultsLLPS Walking Speed

The study showed that LLPS users naturally demonstrated distinguishing features of RGO-assisted gait, such as perpetual trunk flexion, intermittent hip flexion, and large forces borne through the arms. The study also showed that decreasing the hip joint stiffness in the coronal plane increased the LLPS users' oxygen cost by decreasing their walking speed and increasing the crutch force integral. LLPS users were also shown to have an optimal hip joint stiffness in the sagittal plane that minimized their oxygen cost by maximizing their walking speed. However, no changes in arm loading were detected in response to changes in hip joint stiffness in the sagittal plane. The RGO users responded to increased hip joint stiffness in the sagittal plane much like LLPS users. In fact, one of the subject's oxygen cost decreased 75% when the hip joint stiffness increased. In conclusion, the LLPS was found to be a useful tool in researching RGO-assisted gait, and increasing hip joint stiffness in the sagittal plane may help RGO users walk faster and more efficiently.

Related Presentations and Publications

Johnson W, Fatone S, Gard S. Investigating the Effects of Hip Joint Stiffness on RGO Assisted Gait with a Lower Limb Paralysis Simulator. Midwest Chapter of the American Academy of Orthotists and Prosthetists; June 3-4, 2011; Grand Geneva Resort, WI.

Johnson WB, Fatone S, Gard SA. Modeling the Walking Patterns of Reciprocating Gait Orthosis Users with a Novel Lower Limb Paralysis Simulator. 33rd Annual IEEE Engineering in Medicine and Biology Society Conference (2011:7841-4. doi: 10.1109/IEMBS.2011.6091932); 2011 August 30-September 3; Boston, MA.

Johnson W. Modeling the Effects of Hip Joint Stiffness on RGO-Assisted Gait. PhD Dissertation, Chicago IL: Northwestern University; 2011.

Johnson W. Portable Equipment and Clinical Collaboration Enable Collection of Off-Site RGO Gait DataCapabilities, 20(1):1-3, 2012. Northwestern University, Chicago, Illinois.

Johnson WB, Fatone S, Gard SA (2013) Modeling the Effects of Sagittal-plane Hip Joint Stiffness on Reciprocating Gait Orthosis Assisted Gait. Journal of Rehabilitation Research and Development, 50(10):1449-1456.