Lower Extremity Exoskeletons

By: Patrick Anderson

In the late 1960s, researchers started to address the prospect of wearable robotic technologies for humans. The majority of these first attempts were intended for enhancing the physical capabilities of able-bodied people, particularly those serving in the military (3). Despite the failure of these initial powered prototypes to attain widespread usage either for safety reasons or sheer size and cost inefficiencies—exoskeletons have remained an active area of research to date. Over the past decade, researchers and biotech companies have managed to continuously improve upon existing human exoskeleton technologies at an astounding rate, aiming to make them lighter and more cost-efficient. The targeted buyers of robotic exoskeletons have also extended to include workers in physically demanding professions and individuals suffering from pathologies affecting mobility. This article seeks to illuminate advances in robotics for lower-extremities as they pertain to rehabilitative treatment for individuals with impaired mobility.

Conditions and Applications

According to a 2014 CDC survey, 14% of Americans above the age of 18 suffer from conditions seriously impeding their mobility (2). Neurological lesions, such as spinal cord injury (SCI), motor cortex damage following stroke or degeneration caused by diseases like Multiple Sclerosis result in losses in synaptic connections between higher processing centers and efferent nerve fibers, ultimately reducing motor output. In addition, a multitude of deleterious secondary health complications are associated with impaired mobility. These complications include high blood pressure, bowel problems, and autonomic dysreflexia—health issues that are especially severe for aging individuals (4). In the case of SCI, depending on the degree of impairment—ranging from incomplete paraplegia to complete tetraplegia—the yearly expenses directly attributable to these conditions range from $300,000 to $1,000,000 in the first year post-injury and $40,000 to $180,000 in each subsequent year8. Individuals with these and similar pathologies suffer an overwhelming physical, emotional and financial burden.

Fortunately, it has been proven that therapy involving lower-extremity training exercises strengthens muscle organization and improves overall coordination among healthy and impaired individuals (11). While implementation of such programs used to require formidable manual assistance, the integration of mechatronic exoskeletons into current therapeutic practices has gradually freed physical therapists of former labor-intensive practices (11).

Components of Gait

Before delving into how the exoskeletons are able to simulate normal bipedal locomotion, it is necessary to discuss briefly the process of human gait so as to understand the basic paradigm that these technologies seek to mimic. The gait process is measured from heel strike to heel strike of one of the legs in motion. Following heel strike, the foot flattens, the heel rises, the toe is lifted, and the leg is propelled forward during the so-called “swing phase.6” Gait involves various transfers of energy derived from ground forces, gravitational forces, spring storage and muscle torque. In order to stabilize properly during walking, there are numerous control mechanisms within the body that are employed to minimize displacement of the center of gravity. To achieve this, various muscles and tendons act in tandem to rotate, tilt, and flex the joints of the lower extremities at precisely timed intervals6. In sum, human gait is a complex model with a total of seven degrees of freedom (three at the hip for rotation, one at the knee and three at the ankle) for which powered exoskeletons must account (3).

Basic Mechanical Function

While there are many differences in overall design, most powered orthoses share similar features that actively work to replicate gait. These devices involve a DC power supply that is either worn in a backpack by the user of the device or located nearby (in the case of treadmill training orthoses) (3). The DC motor supplies power to actuators located at the three joints of the lower extremities, providing flexion, extension, abduction, and adduction of these joints as needed6. In addition, there are sensors located at pivot points to measure angular displacement and angular acceleration as well as on the flat portion of the foot to measure load and force distribution (3). Almost all devices also include some form of body weight support that allows the user to leverage their torso and arms to stabilize themselves more efficiently (11). In order to prevent over dependence on machine power, many exoskeletons employ methods to maximize human-motor learning by providing varying degrees of assistance. In some newer models, the foot trajectory travels through a tunnel and depending on the foot’s location relative to the tunnel, normal forces act on the foot to return it to its place. However, if the foot is traveling through the tunnel, no machine forces will act on the foot, permitting the user exercise greater autonomy (1).

Types of Lower Extremity Exoskeletons and Active Orthoses

One of the lower-extremity exoskeletons currently in use is the LOKOMAT, which is used for treadmill gait training and consists of body weight support and a powered leg orthosis (7). The LOKOMAT contains position, adaptability and impedance controllers that allow the user to walk on a treadmill with more dynamic settings. This exoskeleton is also able to activate leg muscles that are not utilized due to paralysis through functional electrical stimulation (7). In order to provide the physiotherapist with feedback on the effectiveness of the therapy, the LOKOMAT also has a graphical user interface that provides biofeedback during training sessions (5). Another powered exoskeleton that has proved highly effective is the ReoAmbulator. This device is another treadmill gait trainer that utilizes an “assist as needed control algorithm” to allow the user to increment the level of voluntary control throughout training (7). Finally, the ReWalk is an exoskeleton that has proven to be in high demand among individuals with thoracic-level spinal chord injury. This wearable robotic technology is an over-the-ground walker that allows users to stand, walk and even ascend and descend steps over a limited distance. The ReWalk is currently the only FDA approved technology that is available for personal use outside of a clinical setting (9).

Areas of Improvement for Future Research

While current exoskeleton technologies are much more advanced than their older counterparts, there is still further research that needs to be conducted to reduce costs, increase locomotive capacities and produce lighter products.

Recent research has focused on greater use of pneumatic muscle-type actuators, which are air-pressurized systems that act as artificial muscles to produce extension and contractile motions similar to those in the muscles of the body. Researchers in this field are seeking to create perfected control algorithms to apply to these actuators so that locomotion more closely mirrors gait (7). Nevertheless, in conjunction

with further robotic research, it is highly important that the exact functions of muscles and tendons during gait are more fully understood. If researchers understand these functionalities, it could lead to novel developments in producing assistive exoskeletons that are more suited toward leg architecture and can remove unnecessary elements in design that contribute weight to the products (3). Additionally, with this improved understanding, exoskeletons can also be altered to be more accommodating for different leg shapes and sizes (3).

Patrick Anderson ‘17 is a junior in Cabot House.

WORKS CITED

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[5] Jezernik, S., Colombo, G., Keller, T., Frueh, H., & Morari, M. (2003). Robotic orthosis lokomat: a rehabilitation and research tool. Neuromodulation: Technology at the neural interface, 6(2), 108-115.

[6] Lieberman, D. (2015, November 11). Human Gaits. Lecture presented at Life Sciences 2, Cambridge, MA.

[7] M.A.M. Dzahir, and S.I. Yamamoto (2014): Recent Trends in Lower-Limb Robotic Rehabilitation Orthosis: Control Scheme and Strategy for Pneumatic Muscle Actuated Gait Trainers, Robotics, Vol.3, No.2, p.120-148

[8] National Spinal Cord Injury Statistical Center. (2013). Facts and Figures at a Glance. Birmingham, AL: University of Alabama at Birmingham.

[9] ReWalk Robotics. (2014, June 26). ReWalk™ Personal Exoskeleton System Cleared by FDA for Home Use [Press release].

[10]Sale, P., Franceschini, M., Waldner, A., & Hesse, S. 2012. Use of the robot assisted gait therapy in rehabilitation of patients with stroke and spinal cord injury. European Journal of Physical and Rehabilitation Medicine, 48(1), 111-121.

[11]Tkach D, Reimer J, Hatsopoulos NG. 2007. Congruent Activity during Action and Action Observation in Motor Cortex. Journal of Neuroscience 27:13241–13250.