This article is part of a series on Robotic Lower Limb Exoskeletons, edited by Dr Ferris.

Research

A pneumatically powered knee-ankle-foot orthosis (KAFO) with myoelectric activation and inhibition

Gregory S Sawicki^1,2 and Daniel P Ferris^1,3,4

¹  Human Neuromechanics Laboratory, School of Kinesiology, University of Michigan, 401 Washtenaw Avenue, Ann Arbor, Michigan, 48109-2214, USA

²  Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, USA

³  Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, USA

⁴  Department of Physical Medicine and Rehabilitation, University of Michigan, Michigan, Ann Arbor, USA

author email corresponding author email

Journal of NeuroEngineering and Rehabilitation 2009, 6:23doi:10.1186/1743-0003-6-23

Published:	23 June 2009

Abstract

Background

The goal of this study was to test the mechanical performance of a prototype knee-ankle-foot orthosis (KAFO) powered by artificial pneumatic muscles during human walking. We had previously built a powered ankle-foot orthosis (AFO) and used it effectively in studies on human motor adaptation, locomotion energetics, and gait rehabilitation. Extending the previous AFO to a KAFO presented additional challenges related to the force-length properties of the artificial pneumatic muscles and the presence of multiple antagonistic artificial pneumatic muscle pairs.

Methods

Three healthy males were fitted with custom KAFOs equipped with artificial pneumatic muscles to power ankle plantar flexion/dorsiflexion and knee extension/flexion. Subjects walked over ground at 1.25 m/s under four conditions without extensive practice: 1) without wearing the orthosis, 2) wearing the orthosis with artificial muscles turned off, 3) wearing the orthosis activated under direct proportional myoelectric control, and 4) wearing the orthosis activated under proportional myoelectric control with flexor inhibition produced by leg extensor muscle activation. We collected joint kinematics, ground reaction forces, electromyography, and orthosis kinetics.

Results

The KAFO produced ~22%–33% of the peak knee flexor moment, ~15%–33% of the peak extensor moment, ~42%–46% of the peak plantar flexor moment, and ~83%–129% of the peak dorsiflexor moment during normal walking. With flexor inhibition produced by leg extensor muscle activation, ankle (Pearson r-value = 0.74 ± 0.04) and knee ( r = 0.95 ± 0.04) joint kinematic profiles were more similar to the without orthosis condition compared to when there was no flexor inhibition (r = 0.49 ± 0.13 for ankle, p = 0.05, and r = 0.90 ± 0.03 for knee, p = 0.17).

Conclusion

The proportional myoelectric control with flexor inhibition allowed for a more normal gait than direct proportional myoelectric control. The current orthosis design provided knee torques smaller than the ankle torques due to the trade-off in torque and range of motion that occurs with artificial pneumatic muscles. Future KAFO designs could incorporate cams, gears, or different actuators to transmit greater torque to the knee.