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Original Research Articles

Feasibility of a Powered Knee Joint in Providing Stance and Swing Phase Knee Flexion when Using a Knee-Ankle-Foot Orthosis

Rafiaei, Masoud PhD; Arazpour, Mokhtar PhD; Bahramizadeh, Mahmood PhD; Farahmand, Farzam PhD; Jamshidi, Nima PhD; Samadian, Mohammad MD

Author Information
Journal of Prosthetics and Orthotics: October 2017 - Volume 29 - Issue 4 - p 177-182
doi: 10.1097/JPO.0000000000000149
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Knee-ankle-foot orthoses (KAFOs) have been used in subjects with lower-limb weakness, for example, poliomyelitis or incomplete spinal cord injury (SCI) to ambulate.1,2

Conventional KAFOs that use a locked knee joint, stance control KAFOs (SCKAFOs), or KAFOs with powered knee joints are used in these patient groups. Conventional KAFOs use a locked knee joint in full extension to prevent knee flexion during walking.3 Using a KAFO with the knee locked in full extension can cause abnormal gait pattern such as circumduction, vaulting, and hip hicking that can result soft tissue fatigue, pain, and increased vertical center of mass (COM) displacement, which can result in increased energy consumption.4 Walking with a fully extended fixed knee decreases gait efficiency by 24% and increases vertical displacement of the COM by 65%, which can result in rejection rates of between 60% and 100% by users.5,6 Stance control (SC) KAFOs were developed to solve these problems by permitting free knee motion in swing and locking the knee in full extension during stance phase of gait. Using SCKAFOs in subjects with poliomyelitis and postpolio syndrome has demonstrated an improvement in the kinematic parameters and energy consumption by reduction of the vertical displacement of the COM.1 However, these orthoses do not facilitate initial knee flexion during stance phase of gait.

Initial knee flexion in stance enhances shock absorption, reduces compensatory motions, and causes a smooth walking motion. Knee flexion in stance phase of gait is typically 15°.7 Walking with a stiff leg consumes more energy than walking with normal knee flexion.3 Knee flexion in stance with pelvic tilt can decrease the magnitude of the vertical displacement of the COM.3,7,8 Changes in determinants of the gait can help to understand and assess pathological gait,9 as pathological gait may induce exaggerating motion at unaffected level to preserve energy consumption, and it is therefore important to maintain a smooth and low amplitude displacement of the COG, where possible, to minimize energy expenditure during locomotion.9

The main limitation of current SCKAFOs is a lack of knee flexion in early stance phase. Stance control knee joints have historically provided a locked knee position in early stance phase. Allowing some early stance phase knee flexion during pathological gait could therefore be beneficial. During midstance, an immobilized knee causes excessive vertical COM motion that requires excessive trunk and upper-limb effort to carry the body over the stance limb.10

One of main needs in the design of novel SC orthotic knee joints is therefore allowing controlled knee flexion during stance to provide smooth progression of the body COM and shock absorption. Therefore, the aims of this present study were to design and develop a new SCKAFO and determine this efficacy on specific spatiotemporal and kinematic parameters in healthy subjects.




The new knee joint was motorized by adding a powered actuator (model Faulhaber Hubble, Germany) and consisted of a gearbox, in which two spur gears are engaged with each other that coupled with a screw bush and moved by a motor unit. This joint aims to improve movement and the kinematic characteristics of gait in subjects with quadriceps weakness by providing initial knee flexion with stability in stance and active knee flexion and extension in swing phase of gait. The crucial part of this design was to enable a synchronation pattern of the knee joint using foot pressure sensors. The new design contains a foot switch (a magnetic switch) with a rubber bed under the heel. The “initial knee flexion in stance phase” was provided by an elastic rubber that was used in the knee joint.


The new knee joint components included a motor driver (12 V, 450 rpm), two gear boxes, a screw bush, optical encoders, a programmable controller, plus a body and cap. The programmable control was based on models optimized by software specifically designed and built for this purpose. The pattern of motion in orthotic knee joint could then be programmed into the microcontroller to the main system. Using this approach, the actuator was capable of generating appropriate movement pattern at specified intervals during the complete gait cycle. The two gears moved in opposite directions and moved the shaft connecting the bars of joint that resulted in flexion and extension of the joint during swing phase. Two foot pressure sensors were installed under the footplate to detect stance and swing phase of gait. One rechargeable 12 V battery (Lipo Battery, Thunder Power RC G6 Pro Lite 25C, 5400 mAh, 6-Cell/6S) was used in this orthosis and was operated using a joystick during ambulation. Figure 1 shows an overall view of the new prototype device.

Figure 1
Figure 1:
Overall view of the new orthosis prototype that was used in this study.


Seven healthy volunteer subjects (mean age of 26–34 years, mean height of 170 ± 2.2 cm, and mean weight of 68 ± 4.0 kg) participated in this study. Exclusion criteria included any known history of orthopedic disease, poor balance, cardiac arrhythmias, vestibular dysfunction, peripheral neuropathy, or any other malformation in alignment of the lower limbs, which affect their ambulation.11 The experimental protocol for the study was approved by the ethical committee of the University of Social Welfare and Rehabilitation Sciences, Iran. The study protocol was explained to all volunteer subjects, and a consent form was signed by all participants. Table 1 demonstrates the characteristics of the subjects.

Table 1
Table 1:
Characteristics of participant subjects in this study


The subjects were trained to walk with the SCKAFO to enable them to adapt to wearing it before being tested. Subjects received 2 weeks of gait training after construction of the orthosis, which comprised five sessions per week for a 2-hour period with the orthosis. The gait training program enabled familiarity with the orthotic mechanism during standing and walking with the orthosis.


After orthotic gait training, subjects participated in a four-part data collection session that consisted of gait evaluation with normal walking, SCKAFO with locked knee joint, SCKAFO without initial flexion (IF) mode, and SCKAFO with IF mode. The same thermoplastic sections were used for each KAFO for each subject. A calibrated seven-camera three-dimensional video-based motion analysis system (Oxford Metrics, Inc., Oxford, UK) with a capture frequency of 200 Hz was used to gather data while walking with the orthoses in the motion analysis laboratory. Reflective markers were placed on the orthosis and the skin of the subjects in the following positions: over the jugular notch, the spinous process of the seventh cervical vertebrae, bilaterally over the acromioclavicular joints, the anterior superior iliac spine, the greater trochanter, the lateral condyle of the femur, the head and lateral malleolus of the fibula, calcaneus, and on the dorsum of the foot over the second metatarsal head.

Retroreflective markers were applied directly to the skin on the non-affected side in the positions described and left in place during the whole testing session. On the affected side, markers were again placed on the dorsum of the foot over the 2nd metatarsal head, and on the calcaneus, but this time on the lateral aspect of the mechanical ankle joint axis of rotation, on the lateral bar over the position of the fibular head, and on the mechanical orthotic knee joint over its axis of rotation.

Data were processed at 100 Hz with VICON Body Builder (Oxford Metrics, Oxford, UK) using the standard lower-limb model included in the software. The data were then analyzed using MATLAB (Math Works, Natick, MA, USA). The mean walking speed, step length, cadence, maximum knee flexion angle during swing, initial knee flexion in stance phase, stance phase (% gait cycle), stride length (m), hip flexion (degrees), and hip abduction (degrees) on the orthotic side were calculated for each subject. Walking speed was measured from the marker placed on the calcaneus. Hip hiking was analyzed from the trajectory of the markers put on each ASIS.


Normality of data was confirmed by using the Kolmogorov-Smirnov technique. One-way analysis of variance was used to compare the four test conditions. SPSS statistical software (JMP IN software, SAS Institute, Inc.) was used for analysis of the data. The level of significance was set at 0.05.



Table 2 indicates the spatiotemporal parameters of walking that measured in subjects in four test conditions.

Table 2
Table 2:
Mean and standard deviation (SD) of the spatiotemporal parameters and kinematics during walking in healthy subjects in four test conditions

Walking with any of the KAFOs produced a significant reduction in walking speed compared with normal walking. However, there was no significant difference in walking speed between the SCKAFO without IF and SCKAFO with IF, although walking speed was significantly slower (p = 0.001), with the SCKAFO (with and without IF) compared with walking with the locked knee KAFO.


There was significantly higher initial knee flexion in stance phase when walking with the SCKAFO with IF compared with the other types of KAFOs in this study, but there was no significant difference between normal walking and SCKAFO with IF (Table 2 and 3) (Figure 2). The mean of the knee flexion in swing phase was 37.57° ± 1.02° and 36.41° ± 1.02° when using SCKAFO with and without IF, respectively.

Table 3
Table 3:
Comparison of the spatiotemporal parameters and kinematics during walking in healthy subjects in four test conditions
Figure 2
Figure 2:
There was significantly higher initial knee flexion in stance phase when walking with the SCKAFO with IF compared with the other types of KAFO in this study, but there was no significant difference between normal walking and SCKAFO with IF (Table 2 and 3) (Figure 2).

Maximum hip flexion on the orthotic side was increased when walking with the SCKAFO without the IF mode compared with normal walking (Table 2), whereas when wearing SCKAFO with IF, the mean of the hip flexion was not significantly altered compared with normal walking. Hip abduction was also significantly reduced during walking with SC mode compared with KAFO with locked knee joint (Table 2 and 3). There was no significant difference noted between walking with the SC knee joint with or without IF in hip abduction parameter. Hip abduction increased in the locked knee position in comparison with the other test conditions.


There are many orthoses that provide stability and walking in subjects with lower-limb weakness.1 Stance control KAFOs provide a smoother gait pattern and reduced energy consumption compared with traditional orthoses.1,12 Stance control KAFOs do not routinely provide knee flexion in stance phase. Initial knee flexion provides shock absorption and reduced stress in proximal segments and vertical displacement of the COM.7 In this study, we present a new SC orthosis (SCO) that provided initial knee flexion in stance and knee flexion in the swing phase of gait for walking. The prototype of the new orthosis was investigated with user involvement. A knee flexion and extension movement in swing was produced by the actuator installed on the joint, and 10° of initial knee flexion in stance was produced by compression of the exchangeable bumper that a simulated walking pattern more similar to able-bodied gait.

Mean walking speed was reduced by 34%, 44%, and 40% when wearing the KAFO with the knee joint locked, with SC without IF, and SC with IF, respectively, compared with normal walking. In KAFO wearing conditions, using the KAFO in the locked knee position had little effect on the reduction of this parameter. It seems that in this condition, the subjects used compensatory motions during ambulation to compensate for the lack of knee flexion. The maximum hip abduction also occurred in this test condition. Using the SC knee joint with IF had no statistically significant influence on improvement in walking speed compared with the SC knee joint without IF. Compared with previous studies in this field, the mean of this parameter in using powered SCKAFO was reported to be 0.61 in the poliomyelitis subject and 0.57 m/s in able-bodied subjects. Speed of walking data resulted from alteration to stride length. In this study, the rate of step length increased with wearing an SC knee joint with IF compared with KAFO with locked and SC without IF. Compared with previous studies in this field, the new knee joint had better results in all parameters in healthy subjects, but their values were reduced compared with normal walking. In regard to rates of the analyzed parameters, the gait training time with the new orthosis maybe was a retainable reason for these results.

Maximum knee flexion during swing phase in normal healthy subjects has been quoted as being 67°.11 In this study, the mean of this parameter was 61°. Maximum knee flexion was 44° and 42° when using the SCKAFO in KAFO users.13 The mean value of knee flexion in swing phase with a SCKAFO has been reported to be 44°.6,14,15 A 32° and 42.5° average range of knee motion during swing with new SCKAFO and with powered KAFO, respectively, has been reported.15,16 By utilizing knee flexion in swing, SCKAFO users were able have toe clearance and move the leg forward.

The patterns of knee and hip motion were similar to that in normal motion. Therefore, it could feasibly be used by patients with lower-limb weakness, who yet have a stable lower limb in walking. This is similar to results were reported by Bernhardt et al., Davis et al., and Hebert et al. for patients with polio, spina bifida, incomplete SCI, and multiple sclerosis.15,17,18 However, the most important advantage of the new SCO is initial knee flexion with stability in stance and knee flexion in swing that could provide a smoother and more normal pattern of gait for users, plus decreasing brace rigidity and possibly increasing patient acceptance. In this study, subjects produced 10° knee flexion in stance and 40° knee flexion in swing. This is more similar to the sagittal plane kinematic pattern of the knee in normal gait.

The power of the electrical actuators in the new knee joint could be altered according to the individual's characteristics and could synchronize knee and foot component activation. The key findings were that simulation and replication of knee and foot component sagittal plane rotations similar to that in normal human locomotion may be feasible when adapting a KAFO with actuators. These findings are worthy of further development. It is therefore the intention to evaluate the efficacy of the new SCO in providing appropriate walking parameters for lower-limb paralysis, and a further study is therefore planned to evaluate the effect of this type of orthosis on walking by lower-limb paralysis patients.

Wearing the SC knee joint with IF provided similar hip flexion and abduction compared to normal walking. Wearing the SC knee joint increased speed of walking and stride length and knee flexion in stance and swing phases as compared to SC knee joint without IF. These increases were not statistically significant, but these were important findings of this study.


The disadvantages of KAFO with drop-locked knee joints have been decreased with the development of SCOs. The development of SCOs has been under way since the 1980s, including mechanical and powered approaches.19–28 The UTX, SCOKJ, and SPL were the first-generation SCKAFOs. The Becker E-Knee and Ottobock Sensor Walk microprocessor-controlled devices were the next generation and were activated by ankle range of motion or limb inclination during the gait cycle. A reduction in compensatory movements and an associated reduction of joint loads and energy expenditure during level walking were reported in using SCO in this field.1,14,15,29–31 In addition, the most important functional limitation of all of these orthoses is that no dampened knee flexion is possible in the weight-bearing condition. This means that movement patterns that are important for everyday activities such as a nearly natural step-over-step descent of ramps and stairs or sitting down while loading the orthosis are not possible. The orthosis in this study has been developed to cover the functional limitations of SCOs. During level walking, the orthotic mechanism should allow for natural knee flexion in the stance phase.


There were some limitations in this study. First, this study was performed on healthy volunteer subjects. However, evaluation of the feasibility of using a new powered knee joint for these subjects was initially needed. A further study of new knee joint should be analyzed on patients with lower-limb weakness. In this study, energy consumption was not considered, and indeed, subjects were only acclimatized to the powered SCKAFO. The test method did not utilize randomized sampling; this is a limitation of the study. However, there was an approximate hour of rest allowed between testing sessions for the different orthoses, so carryover effects would be minimal.


This study provides evidence that a KAFO with a new powered knee joint has benefits over existing assistive KAFOs with locked and SC knee joints. First, a new powered knee joint allows for a flexion in stance and swing phase, reducing pelvic tilt and hip hiking. The new powered knee joint may also remove the instabilities caused by lateral trunk movements found in stiff-legged walking. Second, longer walking distances might be achieved with the KAFO with SC with IF because a longer stride length and increased speed of walking may be achieved with this mode compared with other modes (locking knee joint and SC without IF). Further investigation is currently under way to assess its effect on gait parameters and energy consumption in subjects with lower-limb weakness.


We thank the University of Social Welfare and Rehabilitation Sciences for financial support for this research.


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knee-ankle-foot orthoses; stance control knee joint; walking; knee flexion in stance phase

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