- Open Access
Exoskeletons and orthoses: classification, design challenges and future directions
© Herr; licensee BioMed Central Ltd. 2009
- Received: 11 May 2009
- Accepted: 18 June 2009
- Published: 18 June 2009
For over a century, technologists and scientists have actively sought the development of exoskeletons and orthoses designed to augment human economy, strength, and endurance. While there are still many challenges associated with exoskeletal and orthotic design that have yet to be perfected, the advances in the field have been truly impressive. In this commentary, I first classify exoskeletons and orthoses into devices that act in series and in parallel to a human limb, providing a few examples within each category. This classification is then followed by a discussion of major design challenges and future research directions critical to the field of exoskeletons and orthoses.
- Wearable Device
- Leaf Spring
- Artificial Muscle
- Residual Limb
- Orthotic Device
The current series of the Journal of NeuroEngineering and Rehabilitation (JNER) is dedicated to recent advances in robotic exoskeletons and powered orthoses. The articles in this special issue cover a broad spectrum of embodiments, from orthotic devices to assist individuals suffering from limb pathology to limb exoskeletons designed to augment normal, intact limb function.
To set the stage for this special issue, I classify exoskeletons and orthoses into four categories and provide design examples within each of these. I discuss devices that act in series with a human limb to increase limb length and displacement, and devices that act in parallel with a human limb to increase human locomotory economy, augment joint strength, and increase endurance or strength. For each exoskeletal type, I provide a design overview of hardware, actuation, sensory, and control systems for a few characteristic devices that have been described in the literature, and when available, describe the results of any quantitative evaluation of the effectiveness of the devices in performing their intended tasks. Finally, I end with a discussion of the major design challenges that have yet to be overcome, and possible future directions that may provide resolutions to these design difficulties.
For the purposes of this commentary, exoskeletons and orthoses are defined as mechanical devices that are essentially anthropomorphic in nature, are 'worn' by an operator and fit closely to the body, and work in concert with the operator's movements. In general, the term 'exoskeleton' is used to describe a device that augments the performance of an able-bodied wearer, whereas the term 'orthosis' is typically used to describe a device that is used to assist a person with a limb pathology.
It is perhaps worth noting that the term "exoskeleton" has come to describe systems that are comprised of more than just a passive protective and supporting shell, as its usage in biology would suggest. "Exoskeleton" within our research community is taken to include mechanical structures, as well as associated actuators, visco-elastic components, sensors and control elements.
Elastic elements in the body, such as ligaments and tendons, have long been known to play a critical role in the economy and stability of movement [1–7]. Humans and other animals use these tissues to reduce impact losses while storing substantial quantities of energy when striking the ground, and to provide propulsion during terminal stance in walking, running and jumping. Such biological strategies have inspired designers of running track surfaces and wearable devices such as shoes and exoskeletons.
Parallel-limb exoskeletons for load transfer
Here we discuss exoskeletons that act in parallel with the human lower limb for load transfer to the ground. Perhaps an in-series leg exoskeleton like the SpringWalker (Figure 1c) increases the metabolic cost of running because the limb length of the human plus machine is substantially increased, thereby increasing both the work at the hip to protract the leg during the aerial phase and the overall energetic demand to stabilize movement, overcoming any potential advantage of extending limb length. Additionally, with an in-series leg exoskeleton device, the ground reaction forces are still borne by the human leg. In contrast, with a parallel mechanism, body weight could be transferred through the exoskeleton directly to the ground, decreasing the loads borne by the biological limbs and lowering the metabolic demands to walk, run, and hop. Furthermore, such a parallel exoskeleton would not increase limb length, thereby not increasing the overall energetic demand to stabilize movement.
The MIT Biomechatronics Group recently built an elastic exoskeleton similar to Yagn's design. However, its intended application was not for running augmentation, but for lowering the metabolic demands of continuous hopping [15, 16]. The exoskeleton, shown in Figure 2b, comprises fiberglass leaf springs that span the entire leg, and is capable of transferring body weight directly to the ground during the stance period. In distinction to Yagn's exoskeleton, the MIT device does not include a clutch to disengage the exoskeletal leaf spring during the aerial phase since such a clutching control was deemed unnecessary for hopping. Without accounting for the added weight of each exoskeleton, wearing the exoskeleton reduced net metabolic power for continuous hopping by an average of 24% compared to normal hopping . When hoppers utilized external parallel springs, they decreased the mechanical work performed by the legs and substantially reduced metabolic demand compared to hopping without wearing an exoskeleton. Since the biomechanics of hopping are similar to that of running, it seems plausible that the effects of wearing an exoskeleton during hopping could predict the biomechanical and metabolic effects of wearing an exoskeleton during running, and that substantial energetic advantages might be achieved while running with a highly elastic, parallel leg exoskeleton. Clearly, for the goal of augmenting human running performance, lightweight and highly elastic leg exoskeletons that act in parallel with the human leg provide a research area of critical importance.
Parallel-limb exoskeletons have also been advanced to augment the load-carrying capacity of humans [17–32]. This type of leg exoskeleton could benefit people who engage in load carrying by increasing load capacity, lessening the likelihood of leg or back injury, improving metabolic locomotory economy, and/or reducing the perceived level of difficulty. One such exoskeletal design is shown in Figure 2c, or the Berkeley Lower Extremity Exoskeleton (BLEEX) developed by Professor Kazerooni. One of the distinguishing features of this exoskeleton is that it is energetically autonomous, or carries its own power source. Indeed, its developers claim it as the first "load-bearing and energetically autonomous" exoskeleton .
BLEEX features three degrees of freedom (DOF) at the hip, one at the knee, and three at the ankle. Of these, four are actuated: hip flexion/extension, hip abduction/adduction, knee flexion/extension, and ankle flexion/extension. Of the non-actuated joints, the ankle inversion/eversion and hip rotation joints are spring-loaded, and the ankle rotation joint is free-spinning . The kinematics and actuation requirements of the exoskeleton were designed by assuming behavior similar to that of a 75 kg human and utilizing clinical gait analysis data for walking [18, 19].
Interesting features of the kinematic design of the exoskeleton include a hip "rotation" joint that is shared between the two legs of the exoskeleton, and therefore, does not intersect with the wearer's hip joints. Similarly, the inversion/eversion joint at the ankle is not co-located with the human joint, but is set to the lateral side of the foot for simplicity. The other five rotational DOF's of the exoskeleton coincide with the joints of the wearer .
The exoskeleton is actuated via bidirectional linear hydraulic cylinders mounted in a triangular configuration with the rotary joints, resulting in an effective moment arm that varies with joint angle. BLEEX consumes an average of 1143 Watts of hydraulic power during level-ground walking, as well as 200 Watts of electrical power for the electronics and control. In contrast, a similarly sized, 75 kg human consumes approximately 165 W of metabolic power during level-ground walking [18, 19].
BLEEX was designed with linear hydraulic actuators since they were the "smallest actuation option available" based on their "high specific power (ratio of actuator power to actuator weight)" . However, a further study determined that electric motor actuation significantly decreased power consumption during level walking in comparison to hydraulic actuation . The weight of the implementation of the electrically-actuated joint, however, was approximately twice that of their hydraulically-actuated joint (4.1 kg vs. 2.1 kg).
The control scheme of the BLEEX seeks to minimize the use of sensory information from the human/exoskeleton interaction, and instead, utilizes mainly sensory information from the exoskeleton. Similarly to a bipedal robot, the exoskeleton can balance on its own, but the human wearer must provide a forward guiding force to direct the system during walking. The control system utilizes the information from eight encoders and sixteen linear accelerometers to determine angle, angular velocity, and angular acceleration of each of the eight actuated joints, a foot switch, and load distribution sensor per foot to determine ground contact and force distribution between the feet during double stance, eight single-axis force sensors for use in force control of each of the actuators, and an inclinometer to determine the orientation of the backpack with respect to gravity .
In order to achieve their goal of being energetically autonomous with such an actuator selection, significant effort was invested in developing a hybrid hydraulic-electric portable power supply .
In terms of performance, users wearing BLEEX can reportedly support a load of up to 75 kg while walking at 0.9 m/s, and can walk at speeds of up to 1.3 m/s without the load. A second generation of the Berkeley exoskeleton is currently in testing. The new device is approximately half the weight of the original exoskeleton (~14 kg ), in part due to the implementation of electric actuation with a hydraulic transmission system. A laboratory spin-off company called Berkeley Bionics (Berkeley, CA) has been formed in order to market the exoskeleton technology.
Here we discuss exoskeletons that act in parallel with the human joint(s) for torque and work augmentation. Many parallel-limb exoskeletons have been developed to augment joint torque and work [33–58]. In distinction to the load-carrying exoskeletons mentioned in the last section, this type of exoskeletal and orthotic device does not transfer substantial load to the ground, but simply augments joint torque and work. This type of leg exoskeleton could improve walking and running metabolic economy, or might be used to reduce joint pain or increase joint strength in paralyzed or weak joints.
The HAL-5 system utilizes a number of sensing modalities for control: skin-surface EMG electrodes placed below the hip and above the knee on both the anterior (front) and posterior (back) sides of the wearer's body, potentiometers for joint angle measurement, ground reaction force sensors, a gyroscope and accelerometer mounted on the backpack for torso posture estimation. These sensing modalities are used in two control systems that together determine user intent and operate the suit: an EMG-based system and a walking pattern-based system. Reportedly, it takes two months to optimally calibrate the exoskeleton for a specific user .
HAL-5 is currently in the process of being readied for commercialization. Modifications from previous versions include upper-body limbs, lighter and more compact power units, longer battery life (approximately 160 minutes continuous operating time), and a more cosmetic shell. The total weight of the full-body device is 21 kg. Cyberdyne (Tsukuba, Japan, http://www.cyberdyne.jp), a company spun off from Sankai's lab, is responsible for the commercialization of the product.
The ability of HAL to improve performance by increasing the user's capacity to lift and press large loads has been demonstrated http://www.cyberdyne.jp. An operator wearing HAL can lift up to 40 kg more than they can manage unaided. Additionally, the device increases the user's 'leg press' capability from 100 to 180 kg. However, to date no peer-reviewed, quantitative results have been published highlighting the effectiveness of the exoskeleton's lower-limb components for the improvement of locomotory function.
A second example of a parallel-limb orthosis that augments joint torque and work is shown in Figure 3b. The MIT Biomechatronics Group developed a powered ankle-foot orthosis  to assist drop-foot gait, a deficit affecting many persons who have experienced a stroke, or with multiple sclerosis or cerebral palsy, among others. The device consists of a modified passive ankle-foot orthosis with the addition of a series elastic actuator (SEA) that is controlled based on ground force and angle sensory information. Using the SEA, the device varies the impedance of the ankle during controlled plantar flexion in stance, and assists with dorsiflexion during the swing phase of walking.
In clinical trials, the MIT active ankle-foot orthosis (AFO) was shown to improve the gait of drop-foot patients by increasing walking speed, reducing the instances of "foot slap", creating better symmetry with the unaffected leg, and providing assistance during powered plantar flexion. Subjects' feedback was also favorable. The AFO is relatively compact and consumes a small amount of power (10W average electrical power consumption), and current work at iWalk, LLC http://www.iwalkpro.com, a spin-off company from MIT, is focused on developing an energetically autonomous, portable version of the device.
Parallel-limb exoskeletons that increase human endurance
Throughout the human body hundreds of muscles exert forces to stiffen and move the limbs and torso. During exhaustive exercise, only a small portion of these muscles fatigue. For a repetitive anaerobic activity, a parallel-limb exoskeleton could be designed to redistribute the cyclic work load over a greater number of muscles for the purpose of delaying the onset of fatigue. In such a strategy, springs within the exoskeleton could be stretched by muscles that would not normally fatigue if the exercise were conducted without the mechanism. The energy stored by the exoskeleton could then be used to assist those muscles that would typically fatigue, possibly improving endurance capacity.
The experimental results are shown in Figure 4b. The endurance was maximized around K ~0.25 for each subject. Further, the endurance with an exoskeleton increased by 1.5-fold to 2.5-fold compared to the endurance when no exoskeleton assist was employed. Using a mathematical model of the human arm and exoskeleton, researchers  related overall muscle efficiency to exoskeletal stiffness. The model predicted that muscle efficiency was maximized at the same dimensionless stiffness where endurance reached its maximum (K~0.25 in Figure 4b), suggesting that the endurance changes were a consequence of changes in the efficiency with which the body performed the required work for each cycle.
There are many applications for this class of exoskeleton. For example, a crutch was constructed with an orthotic elbow spring to maximize the endurance of physically-challenged persons in climbing stairs and slopes . When the crutch user flexes both elbows to place the crutch tips onto the next stair tread, orthotic elbow springs compress and store energy. This stored energy then assists the crutch user during elbow extension, helping to lift the body up the next step, and delaying the onset of bicep and tricep muscle fatigue. In future developments, robotic exoskeletons and powered orthoses could be put forth that actively vary impedance to optimally redistribute the body's work load over a greater muscle volume, maximizing the efficiency with which the body is able to perform mechanical work and significantly augmenting human endurance.
Although great progress has been made in the century-long effort to design and implement robotic exoskeletons and powered orthoses, many design challenges still remain. Remarkably, a portable leg exoskeleton has yet to be developed that demonstrates a significant decrease in the metabolic demands of walking or running. Many complicated devices have been developed that increase consumption, such as the SpringWalker  and the MIT load-carrying exoskeleton [27–29].
There are many factors that continue to limit the performance of exoskeletons and orthoses. Today's powered devices are often heavy with limited torque and power, making the wearer's movements difficult to augment. Current devices are often both unnatural in shape and noisy, factors that negatively influence device cosmesis. Given current limitations in actuator technology, continued research and development in artificial muscle actuators is of critical importance to the field of wearable devices. Electroactive polymers have shown considerable promise as artificial muscles, but technical challenges still remain for their implementation [61, 62]. These challenges include improving the actuator's durability and lifetime at high levels of performance, scaling up the actuator size to meet the force and stroke needs of exoskeletal/orthotic devices, and advancing efficient and compact driving electronics. Although difficulties remain, electroactive polymer muscles may offer considerable advantages to wearable robotic devices, allowing for integrated joint impedance and motive force controllability, noise-free operation, and anthropomorphic device morphologies. An improved understanding of muscle and tendon function during human movement tasks may shed light on how artificial muscles should ideally attach to the exoskeletal frame (monoarticular vs. polyarticular actuation) and be controlled to produce enhanced biomimetic limb dynamics. For example, neuromechanical models that capture the major features of human walking (e.g. [63, 64]) may improve understanding of musculoskeletal morphology and neural control and lead to analogous improvements in the design of economical, stable and low-mass exoskeletons for human walking augmentation.
Another factor limiting today's exoskeletons and orthoses is the lack of direct information exchange between the human wearer's nervous system and the wearable device. Continued advancements in neural technology will be of critical importance to the field of wearable robotics. Peripheral sensors placed inside muscle to measure the electromyographic signal, or centrally-placed sensors into the motor cortex, may be used to assess motor intent by future exoskeletal control systems [65, 66]. Neural implants may have the potential to be used for sensory feedback to the nerves or brain, thus allowing the exoskeletal wearer to have some form of kinetic and kinematic sensory information from the wearable device .
Current exoskeletal/orthotic devices are also limited by their mechanical interface. Today's interface designs often cause discomfort to the wearer, limiting the length of time that a device can be worn. It is certainly an achievable goal to provide comfortable and effective mechanical interfaces with the human body. Contemporary external prosthetic limbs attach to the human body most commonly via a prosthetic socket that is custom fabricated to an individual's own contours and anatomical needs. Although not a perfectly comfortable interface, today's prosthetic sockets nonetheless allow amputee athletes to run marathons, compete in the Ironman Triathlons, and even climb Mount Everest. One strategy employed in the fabrication of modern prostheses is to digitize the surface of the residual limb, creating a three dimensional digital description of the residual limb contours. Once the amputee's limb has been scanned, their geometric data are sent to a computer aided manufacturing (CAM) facility where a new prosthetic socket is fabricated rapidly and at relatively low cost.
In the future such file-to-factory rapid processes may be employed for the design and construction of exoskeletal and orthotic devices. In this framework, a three dimensional scanning procedure would produce a digital record of the human body's outer shape. This geometric data along with other anatomical information, such as data on tissue compliance and anatomically-sensitive areas, would be combined with strength and endurance information from a physical fitness diagnostic examination. Such anatomical and fitness data, combined with the wearer's augmentation requirements, would provide an individual's design specification profile. An exoskeleton, customized to fit the wearer's outer anatomical features and physiological demands, would then be designed as a 'second skin'. Such a skin would be made compliant in body regions having bony protuberances, and more rigid in areas of high tissue compliance. The exoskeletal skin would be so intimate with the human body that external shear forces applied to the exoskeleton would not produce relative movement between the exoskeletal inner surface and the wearer's own skin, eliminating skin sores resulting from device rubbing. Compliant artificial muscles, sensors, electronics and power supply would be embedded within the three dimensional construct, offering full protection of these components from environmental disturbances such as dust and moisture. Once designed, device construction would unite additive and subtractive fabrication processes to deposit materials with varied properties (stiffness and density variations) across the entire exoskeletal volume using large scale 3-D printers and robotic arms.
During the 20th century, investments in human-mobility technology primarily focused on wheeled devices. Relatively little investment was focused on the advancement of anthropomorphic exoskeletal technologies that allow humans to move bipedally at enhanced speeds and with reduced effort and metabolic cost. It seems likely that in the 21st century more investments will be made to drive innovation in this important area. The fact that large automobile companies, such as Honda and Toyota, have recently begun exoskeletal research programs is an indication of this technological shift. Perhaps in the latter half of this century, exoskeletons and orthoses will be as pervasive in society as wheeled vehicles are today. That would allow the elderly, the physically challenged and persons with normal intact physiologies to achieve a level of mobility not yet achieved. That would be a day in which the automobile – that large, metal box with four wheels – is replaced with wearable, all-terrain exoskeletal devices, allowing city streets to be transformed from 20th century pavement to dirt, trees and rocks. One can only hope.
This work was supported in part by the MIT Media Lab Consortia.
- Cavagna GA, Heglund NC, Taylor CR: Mechanical work in terrestrial locomotion: two basic mechanisms for minimizing energy expenditure. Am J Physiol 1977, 233: R243-R261.PubMedGoogle Scholar
- Ker RF, Bennett MB, Bibby SR, Kester RC, Alexander RM: The spring in the arch of the human foot. Nature 1987, 325: 147-149.View ArticlePubMedGoogle Scholar
- Alexander RM: Elastic Mechanisms in Animal Movement. Cambridge: Cambridge University Press; 1988.Google Scholar
- Blickhan R: The spring-mass model for running and hopping. J Biomech 1989, 22: 1217-1227.View ArticlePubMedGoogle Scholar
- McMahon TA, Cheng GC: The mechanics of running: how does stiffness couple with speed? J Biomech 1990,23(suppl 1):65-78.View ArticlePubMedGoogle Scholar
- Farley CT, Glasheen J, McMahon TA: Running springs: speed and animal size. J Exp Biol 1993, 185: 71-86.PubMedGoogle Scholar
- Hogan N: Skeletal muscle impedance in the control of motor actions. Journal of Mechanics in Medicine an Biology 2002,2(3):359-373.View ArticleGoogle Scholar
- McMahon TA, Greene PR: The influence of track compliance on running. J Biomech 1979, 12: 893-904.View ArticlePubMedGoogle Scholar
- Kerdok AE, Biewener AA, McMahon TA, Weyand PG, Herr H: Energetics and mechanics of human running on surfaces of different stiffnesses. J Appl Physiol 2002, 92: 469-478.View ArticlePubMedGoogle Scholar
- Herr H, Gamow RI: Shoe and foot prosthesis with bending beam spring structures. U.S. Patent 5,701,686 1997.Google Scholar
- Herr H, Gamow RI: Shoe and foot prosthesis with bending beam spring structures. U.S. Patent 6,029,374 2000.Google Scholar
- Dick J, Edwards E: Human Bipedal Locomotion Device. U.S. Patent 5,016,869 1991.Google Scholar
- Yagn N: Apparatus for facilitating walking, running, and jumping. U.S. Patent 420179 1890.Google Scholar
- Yagn N: Apparatus for facilitating walking, running, and jumping. U.S. Patent 438830 1890.Google Scholar
- Herr H, Walsh C, Valiente A, Pasch K: Exoskeletons for running and walking. U.S. Patent Application 60/736,929 2006.Google Scholar
- Grabowski AM, Herr H: Leg exoskeleton reduces the metabolic cost of human hopping. J Appl Physiol 2009, in press.Google Scholar
- Kazerooni H, Steger R: The Berkeley lower extremity exoskeleton. Transactions of the ASME, Journal of Dynamic Systems, Measurements, and Control 2006, 128: 14-25.View ArticleGoogle Scholar
- Zoss AB, Kazerooni H, Chu A: Biomechanical design of the Berkeley lower extremity exoskeleton (BLEEX). IEEE ASME Trans Mechatron 2006,11(2):128-138.View ArticleGoogle Scholar
- Chu A, Kazerooni H, Zozz A: On the biomimetic design of the berkeley lower extremity exoskeleton (BLEEX). Proc. of the 2006 IEEE International Conference on Robotics and Automation: Barcelona, Spain 2005, 4345-4352.Google Scholar
- Zoss A, Kazerooni H: Design of an electrically actuated lower extremity exoskeleton. Advanced Robotics 2006,20(9):967-988.View ArticleGoogle Scholar
- Amundson K, Raade J, Harding N, Kazerooni H: Hybrid hydraulic-electric power unit for field and service robots. Proceedings of the IEEE IRS/RSJ International Conference on Intelligent Robots and Systems: August 2–6, 2005; Alberta, Canada 2005, 3453-3458.View ArticleGoogle Scholar
- Guizzo E, Goldstein H: The rise of the body bots. IEEE Spectrum 2005,42(10):50-56.View ArticleGoogle Scholar
- Huang GT: Wearable robots. Technology Review 2004.Google Scholar
- 2006 ARO in Review U.S. Army Research Laboratory, U.S. Army Research Office, Adelphi, MD; 2006.Google Scholar
- Walsh CJ, Pasch K, Herr H: An autonomous, under-actuated exoskeleton for load-carrying augmentation. Proc. IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS). Beijing, China 2006, 1410-1415.Google Scholar
- Walsh CJ, Paluska D, Pasch K, Grand W, Valiente A, Herr H: Development of a lightweight, underactuated exoskeleton for load-carrying augmentation. Proc. IEEE International Conference on Robotics and Automation. 2006; Orlando, FL, USA 2006, 3485-3491.Google Scholar
- Valiente A: Design of a quasi-passive parallel leg exoskeleton to augment load carrying for walking. In Master's Thesis. Massachusetts Institute of Technology, Department of Mechanical Engineering, MA, USA; 2005.Google Scholar
- Walsh CJ: Biomimetic design of an underactuated leg exoskeleton for load-carrying augmentation. In Master's Thesis. Massachusetts Institute of Technology, Department of Mechanical Engineering, MA, USA; 2006.Google Scholar
- Walsh C, Endo K, Herr H: A quasi-passive leg exoskeleton for load-carrying augmentation. Int J HR 2007,4(3):487-506.Google Scholar
- Gregorczyk KN, Obusek JP, Hasselquist L, Schiffman JM, Bensel CK, Gutekunst D, Frykman P: The Effects of a Lower Body Exoskeleton Load Carriage Assistive Device on Oxygen Consumption and Kinematics during Walking with Loads. 25th Army Science Conference. 2006; Orlando, FL, USA, Nov. 27–30Google Scholar
- Jansen JF, Birdwell JF, Boynton AC, Crowell HP III, Durfee WK, Gongola JD, Killough SM, Leo DJ, Lind RF, Love LJ, Mungiole M, Pin FG, Richardson BS, Rowe JC, Velev OA, Zambrano T: Phase I Report: DARPA Exoskeleton Program. Oak Ridge National Laboratory Report number ORNL/TM-2003/216 2003.Google Scholar
- Marks P: Power dressing. New Sci 2001.Google Scholar
- Vukobratovic M, Hristic D, Stojiljkovic Z: Development of active anthropomorphic exoskeletons. Med Biol Eng 1974,12(1):66-80.View ArticlePubMedGoogle Scholar
- Vukobratovic M, Borovac B, Surla D, Stokic D: Scientific Fundamentals of Robotics 7, Biped Locomotion: Dynamics Stability, Control, and Application. New York: Springer-Verlag; 1990.View ArticleGoogle Scholar
- Hristic D, Vukobratovic M, Timotijevic M: New model of autonomous 'active suit' for distrophic patients. Proceedings of the International Symposium on External Control of Human Extremities 1981, 33-42.Google Scholar
- Grundmann J, Seireg A: Computer Control of Multi-Task Exoskeleton for Paraplegics. Proceedings of the Second CISM/IFTOMM International Symposium on the Theory and Practice of Robots and Manipulators 1977, 233-240.Google Scholar
- Seireg A, Grundmann JG: Design of a Multitask Exoskeletal Walking Device for Paraplegics. In Biomechanics of Medical Devices. Marcel Dekker, Inc, New York; 1981:569-644.Google Scholar
- Tomovic R, Popovic D, Gracanin F: A technology for self-fitting of orthoses. Proceedings of the International Symposium on External Control of Human Extremities 1978, 15-25.Google Scholar
- Hristic D, Vukobratovic M, Gracanin F: Development and evaluation of modular active orthosis. Proceedings of the International Symposium on External Control of Human Extremities 1978, 137-146.Google Scholar
- Schwirtlich L, Kovacevic S, Popovic D: Clinical evaluation of the self-fitting modular orthoses by spastic paraplegics. Proceedings of the International Symposium on External Control of Human Extremities 1981, 21-32.Google Scholar
- Jaukovic ND: Active peroneal orthosis. Proceedings of the International Symposium on External Control of Human Extremities 1981, 13-20.Google Scholar
- Schwirtlich L, Popovic D: Hybrid orthoses for deficient locomotion. Proceedings of the International Symposium on External Control of Human Extremities 1984, 23-32.Google Scholar
- Popovic D, Schwirtlich L: Hybrid powered orthoses. Proceedings of the International Symposium on External Control of Human Extremities 1987, 95-104.Google Scholar
- Popovic D, Schwirtlich L, Radosavijevic S: Powered hybrid assistive system. Proceedings of the International Symposium on External Control of Human Extremities 1990, 177-186.Google Scholar
- Durfee WK, Hausdorff JM: Regulating knee joint position by combining electrical stimulation with a controllable friction brake. Ann Biomed Eng 1990, 18: 575-596.View ArticlePubMedGoogle Scholar
- Goldfarb M, Durfee WK: Design of a controlled brake orthosis for FES-aided gait. IEEE Trans Rehabil Eng 1996,4(1):13-24.View ArticlePubMedGoogle Scholar
- Yamamoto K, Hyodo K, Ishii M, Matsuo T: Development of power assisting suit for assisting nurse labor. JSME International Journal. Series C 2002,45(3):703-711.View ArticleGoogle Scholar
- Yamamoto K, Ishii M, Hyodo K, Yoshimitsu T, Matsuo T: Development of power assisting suit (miniaturization of supply system to realize wearable suit). JSME International Journal Series C 2003,46(3):923-930.View ArticleGoogle Scholar
- Kawamoto H, Sankai Y: Power assist system HAL-3 for gait disorder person. Lecture Notes on Computer Science (LNCS), vol. 2398/2002. Proceedings of the International Conference on Computers Helping People with Special Needs (ICCHP), Berlin, Germany 2002.Google Scholar
- Kawamoto H, Lee S, Kanbe S, Sankai Y: Power assist method for HAL-3 using EMG-based feedback controller. Proceedings of the IEEE International Conference on Systems, Man, and Cybernetics 2003, 1648-1653.Google Scholar
- Pratt JE, Krupp BT, Morse CJ, Collins SH: The RoboKnee: an exoskeleton for enhancing strength and endurance during walking. Proc. IEEE International Conference on Robotics and Automation, New Orleans, LA, USA 2004, 2430-2435.Google Scholar
- Blaya JA, Herr H: Control of a variable-impedance ankle-foot orthosis to assist drop-foot gait. IEEE Trans Neural Syst Rehabil Eng 2004,12(1):24-31.View ArticlePubMedGoogle Scholar
- Ferris DP, Czerniecki JM, Hannaford B: An ankle-foot orthosis powered by artificial muscles. J Appl Biomech 2005, 21: 189-197.PubMed CentralPubMedGoogle Scholar
- Ferris DP, Gordon KE, Sawicki GS, Peethambaran A: An improved powered ankle-foot orthosis using proportional myoelectric control. Gait Posture 2006, 23: 425-428.View ArticlePubMedGoogle Scholar
- Sawicki GS, Gordon KE, Ferris DP: Powered lower limb orthoses: applications in motor adaptation and rehabilitation. Proceedings of the 2005 IEEE International Conference on Rehabilitation Robotics (ICORR) 2005, 206-211.Google Scholar
- Nikitczuk J, Weinberg B, Mavroidis C: Rehabilitative knee orthosis driven by electro-rheological fluid based actuators. Proceedings of the 2005 IEEE International Conference on Robotics and Automation 2005, 2283-2289.View ArticleGoogle Scholar
- Banala SK, Agrawal SK, Fattah A, Krishnamoorthy V, Hsu W, Scholz J, Rudolph K: Gravity-balancing leg orthosis and its performance evaluation. IEEE Transactions on Robotics 2006,22(6):1228-1239.View ArticleGoogle Scholar
- Kong K, Jeon D: Design and control of an exoskeleton for the elderly and patients. IEEE Trans Neural Syst Rehabil Eng. 2006,15(3):367-378.Google Scholar
- Herr H, Langman N: Optimization of human-powered elastic mechanisms for endurance amplification. Journal of the International Society for Structural and Multidisciplinary Optimization (ISSMO) 1997, 13: 65-67.View ArticleGoogle Scholar
- Herr H: Crutch with Elbow and Shank Springs. US Patent 5,458,143 1995.Google Scholar
- Herr H, Kornbluh R: New horizons for orthotic and prosthetic technology: artificial muscle for ambulation. In Smart Structures and Materials 2004: Electroactive Polymer Actuators and Devices (EAPAD): San Diego, CA 5385(1):1-9.View ArticleGoogle Scholar
- Mulgaonkar A, Kornbluh R, Herr H: A new frontier for orthotics and prosthetics: application of dielectric elastomer actuators to bionics. In Dielectric Elastomers as Electromechanical Transducers: Fundamentals, Materials, Devices, Models and Applications of an Emerging Electroactive Polymer Technology. Edited by: Carpi F, De Rossi D, Kornbluh R, Pelrine R, Sommer-Larsen P. Elsevier; 2008.Google Scholar
- Endo K, Paluska D, Herr H: A quasi-passive model of human leg function in level-ground walking. Proc. IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS). 2006; Beijing, China 2006, 4935-4939.Google Scholar
- Geyer H, Herr H: A muscle-reflex model that encodes principles of legged mechanics predicts human walking dynamics and muscle activities. IEEE Trans Neural Syst Rehabil Eng 2009, in press.Google Scholar
- Loeb GE: Neural prosthetics. In The Handbook of Brain Theory and Neural Networks. 2nd edition. Edited by: Arbib MA. Cambridge, Massachusetts: MIT Press;Google Scholar
- Truccolo W, Friehs GM, Donoghue JP, Hochberg LR: Primary motor cortex tuning to intended movement kinematics in humans with tetraplegia. J Neurosci 2008,28(5):1163-1178.View ArticlePubMedGoogle Scholar
- Kuiken T, Li G, Lock BA, Lipschutz RD, Miller LA, Stubblefield KA, Englehart KB: Targeted muscle reinnervation for real-time myoelectric control of multifunction artificial arms. JAMA 2009,301(6):619-628.PubMed CentralView ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.