Abstract— Knee-ankle-foot orthoses (KAFOs) are full leg braces for individuals with knee extensor weakness, designed to support the person during weight bearing activities by preventing knee flexion. KAFOs typically result in an unnatural gait pattern and are primarily used for level ground walking. A novel variable resistance orthotic knee joint, the Ottawalk-Variable Speed (OWVS), was designed to address these limitations. This paper presents a pilot test to evaluate the OWVS functional performance during walking and stair descent. A carbon-fiber KAFO was adjusted for an able-bodied participant by a certified orthotist, with a standard orthotic single axis knee joint on the medial side and the OWVS on the lateral side. The participant performed level ground walking (stance-control, open, closed) and stair descent tests. The operator was able to manually switch between closed mode in terminal swing to open mode in terminal stance for stance- control walking. Knee angle kinematics were similar between open and stance control modes. For stair descent, resistance settings supported the participant as they lowered their body to the next step, but with smaller range of motion compared to the open setting. The Ottawalk-Variable Speed design successfully controls knee flexion during stance and stair descent, with one lateral control joint. Mode switching was fast and appropriate. This microprocessor controlled SCKAFO has a low profile that fits beneath clothing and the variable resistance design will allow people to negotiate different terrain types. I. INTRODUCTION If the knee extensor muscles are not sufficiently strong, the knee will collapse due to the external knee flexion moment [1], [2]. Muscular weakness can result from peripheral neurological diseases, muscular diseases, central neurological diseases, spinal cord injury, osteoarthritis, or severe injury [2]–[6]. Consequences can range from abnormal gait patterns to complete instability [3], [7]. Even if the extensor muscles are only slightly weakened, walking with an abnormal gait pattern is highly energy-consuming and can cause soft tissue damage [2]. Knee-ankle-foot orthoses (KAFO) are full leg braces designed for individuals with knee extensor weakness. Conventional KAFO designs can lock the knee in an extended position, which keeps the individual stable throughout stance. However, the resulting stiff-legged gait is This research was partially funded by the Natural Sciences and Engineering Research Council of Canada (NSERC). Elan valve supplied by The Blatchford Group. Andrew Herbert-Copley is with The Ottawa Hospital Research Institute, Centre for Rehabilitation Research and Development, Ottawa, ON, K1H 8M2, Canada (e-mail: aherbert@ohri.ca). Edward D. Lemaire is with The Ottawa Hospital Research Institute, Centre for Rehabilitation Research and Development, Ottawa, ON, K1H 8M2, Canada; and the Faculty of Medicine, University of Ottawa, Ottawa, ON, K1H 8M5 (e-mail: elemaire@toh.ca). Natalie Baddour is with the Department of Mechanical Engineering, University of Ottawa, Ottawa, ON, K1N 6N5, Canada, (e-mail: nbaddour@uottawa.ca). an unnatural and inefficient gait pattern that can lead to soft tissue injury [2], [7], [8]. Stance control KAFO (SCKAFO) support the leg during weight bearing and permit free knee motion during swing. If the user has sufficient hip muscle strength for forward progression, SCKAFO improve mobility compared to fixed knee KAFO and provide many clinical benefits [7]. SCKAFOs on the market use mechanical or microprocessor control. Mechanical devices have the advantage of not requiring external power and are generally smaller, more aesthetically pleasing, and less expensive. Microprocessor controlled SCKAFOs are the most functionally advanced, have more reliable mode switching, and can have multiple settings for different terrain types. Many SCKAFO devices require a fully extended knee to engage and disengage the locking mechanism, which may not be possible for all individuals with knee extensor weakness. For people who lack strength and/or control to fully extend their knee consistently, the orthosis will not catch them after stumbles that land on a flexed knee. To address these difficulties, a variable resistance orthotic knee joint was designed, based on the Ottawalk-Speed (OWS) [9] and a variable flow hydraulic valve adapted from the Elan prosthetic ankle (Blatchford Group, Basingstoke, UK). The new knee joint, known as the Ottawalk-Variable Speed (OWVS), was designed to provide variable knee flexion resistance to improve mobility on multiple surfaces and in daily activities. This paper presents a pilot test to evaluate the OWVS functional performance during walking and stair descent. II. METHODS A. Ottawalk-Variable Speed (OWVS) The OWVS design (Fig. 1) uses a linear hydraulic cylinder to guide knee joint motion. A variable flow hydraulic valve (modified from Elan prosthetic ankle) controls hydraulic flow from the cylinder and can be adjusted to change knee joint flexion resistance. The new orthotic knee joint will allow individuals with knee extensor weakness to safely walk on level ground with a more natural gait pattern, descend stairs smoothly, and sit in a controlled manner. This low profile design will fit beneath normal clothing, unlike other orthotic options in the same class. The OWVS has 11 custom settings: open (no resistance to knee joint flexion); closed (inhibit knee joint flexion); and 9 additional resistance settings designed to support and smoothly lower users of different weights during stair descents or stand-to-sit motions. A key design feature is uninhibited knee joint extension at any setting, so the user can extend his or her leg to recover from a stumble. Evaluation of a Variable Resistance Orthotic Knee Joint Andrew Herbert-Copley, Edward D. Lemaire, and Natalie Baddour 978-1-4577-0220-4/16/$31.00 ©2016 IEEE 2210