Knee Flexion Strategy Enables Obstacle Avoidance in Lower Limb Amputees

Individuals with a lower limb amputation experience a deficiency in sensory input and in the absence of muscles and joints, adapt to a prosthesis to become functionally independent. Conditions to enable stepping over an obstacle using a knee flexion strategy can be satisfied when an amputee maintains enough distance between the distal tip of the foot and the obstacle during stance, producing sufficiently high, yet feasible, hip torques and using static ground friction, according to recent study results.

“In our study, conditions that enable a prosthetic knee flexion strategy in transfemoral amputee subjects during obstacle avoidance were investigated,” Helco G. van Keeken, of the Center for Human Movement Sciences at the University Medical Center Groningen part of the University of Groningen, Groningen, The Netherlands, told O&P Business News. “This study explored the hip torque principle and the static ground principle as object avoidance strategies.”

Results for clinical practice

Researchers used a prosthetic limb simulator device for able-bodied patients in the first part of the study to look at the influence of applied hip torques and static ground friction on the prosthetic foot trajectory, using inverse dynamics to calculate the energy produced by the hip joint. In the second part of the study, a two-dimensional forward dynamics model was used to investigate the relation between obstacle-foot distance and the necessary hip torques used during obstacle avoidance.

“We hypothesized that the foot should move forward when applying hip torque because the whole limb swings forward. However, because of the flexible knee and the inertial properties, the foot moves relatively more in an upward direction than in a forward direction when applying large hip torques and using ground friction,” van Keeken said.

Overall, the results showed that it should be feasible to step over an obstacle with the knee flexion strategy. The hip torques and the static ground friction influenced the trajectory of the prosthetic foot, requiring more energy to be produced by the hip, compared with a knee extension strategy. The researchers used a mechanical model to confirm these findings.

These findings have consequences for therapy, van Keeken said.

“During therapy, in many occasions, patients are instructed to walk toward an obstacle and then step over it. Because of these instructions, the patient will probably stop in front of the object, and therefore will be standing close to the obstacle as they think they were told. The distance between the prosthetic foot and the object will then be too small for safe clearance during obstacle avoidance,” he said. “In that situation, the only possible solution for safe obstacle clearance is the knee extension strategy with an externally rotated and abducted limb.”

During therapy there is the risk that patients may learn to use circumduction at the hip on the prosthesis side with the prosthetic knee in extension and plantar flexion on the non-affected side for safe obstacle clearance.

“In that case, the patients should be taught to keep on walking and use the prosthetic limb as the leading limb during the obstacle clearance, which would result in a sufficient distance between the foot and the object,” van Keeken said.

Integrated research

The results of this study are part of a project that identified which strategies amputee patients use when engaging a lower extremity prosthesis and how they develop these strategies during their rehabilitation period. Twenty patients who just started using their transfemoral or transtibial prosthesis during difficult motor tasks were compared with 15 experienced patients and 15 matched able-bodied patients. An applied science approach was performed by the Center for Human Movement Sciences Groningen, in which simple experiments and modeling were emphasized, whereas the Center for Rehabilitation Groningen performed a more classical approach, in which functionality of the three groups was measured and compared. The final goal of this project is to provide guidelines for rehabilitation and prosthetics adjustment and design.


Findings and insights from this project are used in the Healthy Ageing Network Northern Netherlands (HANNN) Smart Mobility Devices with Improved Patient Prosthesis Interaction (SPRINT) project. According to van Keeken, “SPRINT contributes to the plans of the high-tech health farm by developing new rehabilitation techniques and devices that restore patient mobility and shift intramural rehabilitation to extramural care.

“SPRINT includes a unique multidisciplinary combination of fundamental researchers, applied researchers, health care institutes and industries,” he said. “This makes it possible to cover the entire chain of innovation, from fundamental research on mobility to market introduction of products.” — by Casey Murphy

For more information:
Postural control after lower limb amputation; Changes in body representation and the recovery of postural control. Available at: Accessed Oct. 10, 2012.
van Keeken HG. Med Eng Phys. 2012;34:1109-1116.

Disclosure: This project was financially supported by the OIM Foundation, the Beatrixoord Foundation and the Anna Foundation.

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